figure table

# Star and Planet Formation with the Single Aperture Large Telescope for
Universe Studies (SALTUS) Space Observatory

Kamber R. Schwarz  Max-Planck-Institut für Astronomie (MPIA), Königstuhl 17,
69117 Heidelberg, Germany  Alexander Tielens  Astronomy Department, University
of Maryland, 296 Stadium Dr. PSC, College Park, USA, 20742  Joan Najita
National Optical Astronomy Observatory (NOAO), 950 N. Cherry Ave, Tucson, AZ
85719, USA  Jennifer Bergner  University of California, Berkeley, Berkeley,
CA, 94720, USA  Quentin Kral  LESIA, Observatoire de Paris, Université PSL,
CNRS, Sorbonne Université, Université Paris Cité, 5 place Jules Janssen, 92195
Meudon, France  Carrie Anderson  NASA Goddard Space Flight Center, Planetary
Systems Laboratory, 8800 Greenbelt Road, Greenbelt, USA, 20771  Gordon Chin
NASA Goddard Space Flight Center, Planetary Systems Laboratory, 8800 Greenbelt
Road, Greenbelt, USA, 20771  David Leisawitz  NASA Goddard Space Flight
Center, Observational Cosmology Laboratory, 8800 Greenbelt Road, Greenbelt, MD
USA 20771  David Wilner  Center for Astrophysics Harvard Smithsonian, 60
Garden St., Cambridge, MA 92138, USA  Peter Roelfsema  Netherlands Institute
for Space Research (SRON), Landleven 12, 9747 AD Groningen, The Netherlands
Floris van der Tak  Netherlands Institute for Space Research (SRON), Landleven
12, 9747 AD Groningen, The Netherlands  Erick Young  Universities Space
Research Association, 425 3rd Street SW, Suite 950, Washington DC, 20024, USA
Chris Walker  University of Arizona, Department of Astronomy and Steward
Observatory, Tucson, USA, 85719

###### Abstract

The Single Aperture Large Telescope for Universe Studies (_SALTUS_) is a far-
infrared space mission concept with unprecedented spatial and spectral
resolution. _SALTUS_ consists of a 14-m inflatable primary, providing
$16\times$ the sensitivity and $4\times$ the angular resolution of _Herschel_
, and two cryogenic detectors spanning a wavelength range of 34-660
$\mathrm{\mu}$m and spectral resolving power of $300-10^{7}$. Spectroscopic
observations in the far-infrared offer many unique windows into the processes
of star and planet formation. These include observations of low energy water
transitions, the H2 mass tracer HD, many CHONS constraining molecules such as
NH3 and H2S, and emission lines from the phonon modes of molecular ices.
Observing these species will allow us to build a statistical sample of
protoplanetary disk masses, characterize the water snowline, identify Kuiper
Belt like debris rings around other stars, and trace the evolution CHONS from
prestellar cores, through to protoplanetary disks and debris disks. This paper
details details several key star and planet formation science goals achievable
with _SALTUS_.

###### keywords:

star formation, protoplanetary disks, astrochemistry, terahertz spectroscopy,
far-infrared, space telescopes

*Kamber R. Schwarz<EMAIL_ADDRESS>
##  1 Introduction

The Single Aperture Large Telescope for Universe Studies (_SALTUS_) is a far-
infrared space mission concept proposed to NASA under the Astrophysics Probe
Explorer (APEX) Announcement of Opportunity in November 2023. _SALTUS_ covers
the far-infrared wavelength range $\approx 30-700$ $\mathrm{\mu}$m, most of
which is not covered by any current observatory. The design of _SALTUS_
consists of a 14m off-axis inflatable primary aperture and two cryogenic
instruments: SAFARI-Lite and the High Resolution Receiver (HiRX). The large
aperture size allows for unprecedented sensitivity and a spatial resolution of
$\sim 1$′′ at 50 $\mu$m”. The full technical details of the _SALTUS_
observatory can be found in Arenberg et al., “Design, Implementation and
Performance of the Primary Reflector for SALTUS,” Kim et al, “SALTUS
Observatory Optical Design and Performance,” and Harding, Arenberg, Donovan et
al.,“SALTUS Probe Class Space Mission: Observatory Architecture & Mission
Design,” J. Astron. Telesc. Instrum. Syst. (this issue). SAFARI-Lite is a
direct-detection grating spectrometer providing simultaneous 35–230
$\mathrm{\mu}$m spectroscopy with a resolving power of $R=300$. The full
technical details can be found in Roelfsema et al., “The SAFARI-Lite Imaging
Spectrometer for the _SALTUS_ Space Observatory,” J. Astron. Telesc. Instrum.
Syst. (this issue). HiRX is a multi-pixel, multi-band heterodyne receiver
system spanning wavelength ranges 522–659 $\mu$m, 136–273 $\mu$m, 111.9–112.4
$\mu$m, 63.1–63.4 $\mu$m, and 56.1–56.4 $\mu$m with a resolving power of
$R=1\times 10^{5}-1\times 10^{7}$. The full techincal details can be found in
Walker et al., “The High Resolution Receiver (HiRX) for the Single Aperture
Large Telescope for Universe Studies (SALTUS),” J. Astron. Telesc. Instrum.
Syst. (this issue).

This paper provides an overview of the promise of _SALTUS_ for understanding
star and planet formation, including molecular clouds, protostellar cores,
protoplanetary disks, and debris disks. Accompanying papers in this issue
describe the plans for guaranteed-time (GTO) and guest observing (Chin et al.,
“Single Aperture Large Telescope for Universe Studies (SALTUS): Probe Mission
and Science Overview,” J. Astron. Telesc. Instrum. Syst.), SALTUS’
contributions to High-Redshift Science (Spilker et al., “Distant Galaxy
Observations”J. Astron. Telesc. Instrum. Syst.), Milky Way and nearby galaxies
science (Levy et al., “Nearby Galaxy Observations” J. Astron. Telesc. Instrum.
Syst.), and solar system observations (Anderson et al., “Solar System Science”
J. Astron. Telesc. Instrum. Syst.). Additionally, some of _SALTUS’s_ key
science cases build on the _OASIS_ MIDEX-class mission concept, which used a
similar large inflatable aperture for terahertz frequency observations [1].

###  1.1 Programmatic Motivation

The _SALTUS_ star and planet formation science programs presented here address
multiple high-priority science questions as identified by Astro2020 [2],
detailed below.

  * •

Question E-Q1c: _How Common Is Planetary Migration, How Does It Affect the
Rest of the Planetary System, and What Are the Observable Signatures?_
_SALTUS_ will provide the disk water measurements, including measurements of
the water snowline location, needed to connect atmosphere compositions to the
water distribution of planet-forming disks and thereby to connect JWST
observations of exoplanet atmospheres to a formation time and location. (§2.2)

  * •

Question E-Q1d: _How Does the Distribution of Dust and Small Bodies in Mature
Systems Connect to the Current and Past Dynamical States Within Planetary
Systems?_ _SALTUS_ SAFARI-Lite will determine the occurance of exo-Kuiper
belts around the nearest 30 G and K stars known to host debris disks,
characterizing the commonality of dust in mature planet systems (§2.6.1).

  * •

Question E-Q3a: _How Are Potentially Habitable Environments Formed?_ _SALTUS_
will answer this question by observing [CII] at 157 $\mu$m and [OI] at 63 and
145 $\mu$m in debris disks, tracing the C/O ratio of material available for
accretion onto terrestrial planets (§2.6.2).

  * •

Question E-Q3b: _What Processes Influence the Habitability of Environments?_
and Question F-Q4b: _What Is the Range of Physical Environments Available for
Planet Formation?_ _SALTUS_ will determine the mass and temperature structure,
as well as the abundance of CHONS bearing species, in roughly 1000
protoplanetary disks across evolutionary stages. (§2.1.1,§2.2,§2.4.1)

##  2 Star and Planet Formation Science with SALTUS

###  2.1 Protoplanetary Disk Mass

One of the most fundamental properties of planet formation is the mass of a
planet-forming disk, which determines the total amount of material available
to forming planets and the mechanisms through which planets can form, e.g,
through gravitational instability vs. via core accretion [3]. The main
contributor to the disk mass is H2, which does not emit for the majority of
disk regions since the molecule has no permanent dipole moment, with large
energy spacings not well matched to the local temperatures. The ground-state
transition is the quadrupole J=2-0 with an energy spacing of 510 K. Thus,
exciting an H2 molecule to the J=2 state requires high gas temperatures and H2
emission originates only from the illuminated surface layers of the disk
within a fraction of an au of the central star. Since most of the gas is at
larger radii and is much colder, alternate tracers must be used to determine
the total gas mass.

The most commonly used gas mass tracers in protoplanetary disks are continuum
emission from dust and emission from rotational transitions of CO. Each method
relies on different problematic assumptions. Uncertainties in the dust grain
optical properties and the grain size distribution lead to significant
uncertainty in the derived dust mass from observed emission. Then, to convert
from dust mass to gas mass, a gas-to-dust mass ratio must be assumed. This
value is typically assumed to be 100, as has been measured in the interstellar
medium (ISM). However, several factors can change this ratio in disks,
including loss of gas due to disk winds and accretion onto the central star,
which will decrease the gas-to-dust ratio, and growth of dust grains beyond cm
sizes, at which point the dust emission is no longer observable. Additionally,
assuming a constant gas-to-dust ratio across the disk is not appropriate since
high spatial resolution observations at millimeter wavelengths demonstrate
that the outer radius of the dust disk is often much smaller than the outer
radius of the gas disk [4, 5].

The CO abundance relative to H2 in the ISM is well constrained to be
5${}\times 10^{-5}$ to 2${}\times 10^{-4}$ [6]. However, when converting from
CO abundance to H2 in a protoplanetary disk, additional corrections must be
made to account for the reduced abundance of CO relative to H2 in the surface
layer, where CO is photo-dissociated, and near the cold midplane, where CO is
frozen out onto dust grains [7]. Additional chemical reactions in the gas and
on dust grains can also destroy CO [8]. The resulting reduction in CO gas
abundance, whatever the cause, varies not only across sources but also as a
function of radius within a single disk [9]. Thus, there are large
uncertainties when converting CO flux to total gas mass [10].

Given the myriad assumptions that go into each technique, it is not surprising
that the two methods of determining disk mass rarely agree. Alternative mass
probes, preferably requiring fewer assumptions, are needed to determine the
true disk gas mass. One possibility is to use the disk rotation curve to
constrain the enclosed mass [11]. However, because disks must always be less
massive than the central star in order to remain gravitationally stable, the
contribution of the disk to the rotation curve is small. This technique is
only feasible for a small number of the most massive disks [12].

####  2.1.1 Tracing Mass with HD

_SALTUS_ will use HD to measure the gas mass in hundreds of disks,
establishing the variation in this fundamental parameter across systems.
Observations of the H2 isotopologue HD are unique to the far-IR, with the
ground state 1-0 rotation transition at 112.07 $\mu$m (2.675 THz; SAFARI-Lite
LW Band, HiRX-Band 3) and the 2-1 transition at 56.24 $\mu$m (5.331 THz;
SAFARI- Lite MW Band, HiRX-Band 4b). HD is the main reservoir of deuterium and
its abundance relative to H2 will be close to the elemental D/H abundance;
thus, HD can be used to trace disk mass while avoiding many of the limitations
of other mass tracers. For example, HD emission is optically thin and not
subject to chemical processing that can change abundances of other tracers
relative to H2 [13]. Observing of only the HD 1-0 are capable of constraining
the disk mass to within a factor of 2-10 depending on disk mass, while the
additional observation of the HD 2-1 line decreases this uncertainty to no
more than a factor of 3 [14, 15]. _SALTUS_ HiRX is designed to observe both
lines simultaneously.

There is currently no observatory capable of detecting HD. Near the end of its
lifetime, Herschel targeted HD in seven massive disk systems, resulting in
three detections [4, 5], with HD-derived disk gas masses of 30-210 MJ [16].
Crucially, these mass measurements revealed that both CO and H2O gas are
depleted in these disks relative to the ISM [6, 17]. _SALTUS_ SAFARI-Lite will
measure the total gas mass in hundreds of protoplanetary systems over its
5-year baseline mission, down to masses as low as 0.1 MJ (Figure 1). When
combined with observations of cold water vapor, this determines the amount of
water removed from the outer disk and transformed into water ice in the
planet-forming midplane [18].

Converting the HD detections into an accurate total gas mass requires
knowledge of the disk temperature structure, as HD does not emit appreciably
below 20 K. The _SALTUS_ design allows for full spectral coverage with SAFARI-
Lite or simultaneous observations in the four HiRX bands. While integrating on
the HD 1-0 and 2-1 lines in HiRX-3,4b, _SALTUS_ is able to observe multiple
optically thick H2O and CO lines in HiRX-1,2 spanning 55-1729 K in excitation
energy, compared to 128.49 K for the HD J=1 excited state. These optically
thick lines provide direct measurements of the gas temperature throughout the
disk.

The high spectral resolution of HiRX can then be used to map emission to
different physical locations in the disk using a technique known as Doppler
tomography or tomographic mapping. Because disk rotation follows a Keplerian
velocity profile, the radius at which gas emission originates can be
determined from the line profile. Thus, high spectral resolution observations
of molecular lines in disks can be used to determine the radial location of
the emission without having to spatially resolve the disk. As shown in Figure
2, the velocity offset for emission originating in the inner disk is of order
several km s-1 assuming a disk inclination of 45 degrees, while in the outer
disk the velocity offset is much smaller. The velocity resolution ($\Delta v$)
of SALTUS HiRX is $<1$ km s-1, sufficient to distinguish emission originating
in the inner versus outer disk.

Taking the expected HD fluxes and line-to-continuum values into account from
Figure 1 [14], SAFARI-Lite measures the J=1-0 and 2-1 lines at the
5$\mathrm{\sigma}$ level in 1 hour for the limits provided in Figure 1,
enabling reliable disk gas mass estimates. For a survey of disk mass across
systems, which requires only the total HD flux, spectrally unresolved
observations with SAFARI-Lite are able to quickly build a catalog of HD
detections. The expected continuum flux from a $3\mbox{${}\times 10^{-5}$}$ M⊙
disk at 140 pc, where many young stars are found, is 0.02 Jy [14]. SNR of 300
requires a sensitivity of 66 $\mathrm{\mu}$Jy at 112$\mathrm{\mu}$m, the
wavelength of the HD 1-0 transition. Based on the modeled grating sensitivity
of SAFARI-Lite (Roelfsema et al., this issue), this can be achieved in less
than an hour on source. SAFARI-Lite’s greater sensitivity at 54
$\mathrm{\mu}$m, the wavelength of the HD 2-1 transition, achieves SNR 300 in
even less time.

For a subset of the brightest disks, HiRX spectrally resolves the strong HD
lines at a 1 km s-1 velocity resolution to measure the line profile in detail
and use Doppler tomography to constrain the disk structures. As an example, TW
Hya has a peak flux of 0.49 Jy. _SALTUS_ HiRX Band 3 yields a $5\sigma$
detection at $\Delta v=1$ km s-1 in 20 hours. We can expect to observe five
targets per year in the tomographic mode if we allocate 100 hours per year to
these observations. These deep HiRX observations of sources spanning several
arcseconds on the sky will also provide constraints on the spatial extent of
the line emission for these disks, important for validating the models used
for interpretation of surveys. In total, _SALTUS_ will obtain the disk gas
masses in hundreds of protoplanetary systems during its nominal five year
mission without the need for ancillary data.

---
Figure 1: Model predictions of disk gas (D=140 pc), as a function of HD line
flux. The width of the blue band represents the range of model results for
different disk structures. The three red points represent the only known HD
detections from Herschel- PACS. _SALTUS_ SAFARI-Lite will make observations
for the full range of fluxes plotted, with 5$\sigma$, 1-hr sensitivities
between 0.5 and 2${}\times 10^{-20}$ W m-2 across the 4 bands. SALTUS HiRX
will be sensitive enough to provide high spectral resolution observations,
with 5$\sigma$, 1-hr sensitivities $>5\mbox{${}\times 10^{-19}$}$ W m-2 in
J=2-1 and $>2\mbox{${}\times 10^{-19}$}$ W m-2 in J=1-0. Figure modified from
[14].

###  2.2 The Spatial Distribution of Water in Protoplanetary Disks

SALTUS will be the first mission with the sensitivity to measure the
distribution and physical properties of water in a large sample of
protoplanetary disks. These measurements are key for understanding planet
formation and how terrestrial planets acquire water. The _SALTUS_ instruments
are designed to probe both the gas and the solid H2O reservoirs and relate
them to the characteristics of the central protostar (luminosity, spectral
type) and of the planet-forming disk (evolutionary state, mass, structure,
temperature). The large frequency range of HiRX and SAFARI-Lite provide access
to many H2O lines with a wide range of excitation energies, tracing the cold-
to-warm water vapor in disks, addressing Decadal Questions E-Q1c, E-Q3b, and
F-Q4b.

---
Figure 2: A simulated disk-integrated spectrum of HD 1-0 for a $10^{-3}$ M⊙
disk at a distance of 140 pc, inclined by 45 degrees. The spectral resolution
is $\Delta v$ = 0.9 km s-1. Disk emission is Doppler shifted due to Keplerian
rotation. Emission with a high velocity offset from the line center originates
from small radii, while emission close to the systemic velocity originates
from large radii. The HiRX high spectral resolution enables differentiation
between emission from different radii in protoplanetary disks.

The first part of this program focuses on water vapor. _Herschel_ revealed
tantalizing but tentative and limited evidence of water removal from the
surface layers of outer disks [19, 20]. _SALTUS’s_ large improvement in
sensitivity relative to _Herschel_ makes observations of water in disks
routine and enables a complete survey of water in all protoplanetary disks
within 200 pc. This large number of observations will allow _SALTUS_ users to
conclusively identify trends between the distribution of water in disks and
other properties, e.g., dust disk size and the presence of substructure [21].

Question E-Q1c from the 2020 Decedal asks: “How common is planetary migration,
how does it affect the rest of the planetary system, and what are the
observable signatures?” The composition of a planet’s atmosphere is related to
the composition of the disk where and when it accreted its material and can be
used to determine if a planet could have formed at its current location or
must have migrated. The chemical composition of the disk at a given location
evolves over time due to both chemical and dynamical processes [22]. Current
observational studies aim to use atmospheric C/O to differentiate between
early and late migration of Hot Jupiters [22]. Models of how migration changes
C/O in a planet’s atmosphere make simplifying assumptions for C/O in the disk
[23]. The main volatile oxygen reservoir in disks, water, is virtually
unconstrained by observations. _JWST_ is already significantly improving our
understanding of water in the mid-IR [24, 25, 26, 27]. However, as noted by
Decadal Question E-Q1c, additional longer wavelength observations of cooler
regions of the disk are needed to understand disk composition. HiRX will map
the radial distribution of cold water vapor in hundreds of protoplanetary
disks. These disks will span a wide range of stellar mass and mass accretion
rate, disk dust mass, and disk radial extent, and span multiple starforming
regions, covering a variety of evolutionary stages [28] HiRX will observe the
cold water vapor (not probed by _JWST_) by targeting the ground state ortho
and para transitions (Figure 3), allowing users to collect statistics on the
cold water abundance in disks across evolutionary stages. _SALTUS_ will
provide the disk water measurements needed to connect _JWST_ observations of
exoplanet atmospheres to a formation time and location.

---
Figure 3:  The HiRX and SAFARI-Lite instruments will measure the gaseous
emission spectrum of planet-forming disks, thereby determining unambiguously
the disk gas mass, the water content, and the HDO/H2Oabundance ratios; all
critical aspects of disk/planet formation models. The far-IR provides access
to low energy transitions that probe the cold, outer parts of the disk of the
planet-forming reservoir.

_SALTUS_ will tomographically map the water vapor distribution toward a wide
variety of disks, answering the question “Where is the water?” Mapping the
location of water in protoplanetary disks is crucial for understanding the
transport of water during planet formation [2]. Current planet formation
models predict that most planets form beyond the snowline (gas/ice giants and
possibly smaller planets) and then experience radial migration and dynamical
scattering [29]. Confirming this prediction requires observational constraints
on the water snowline location as functions of e.g., disk mass, stellar mass,
and evolutionary state, to compare to the orbital radii of exoplanets in
mature systems.

The ice snowline, the H2O desorption front located at $\sim$150-170 K in the
disk, controls the radial distribution of the C/O ratio in the gas and solid
phase [23], implying that the spectral characteristics of planets are linked
to their formation location. Water on terrestrial planets which formed within
the snow line (including Earth) is thought to have been at least partially
delivered by comet and asteroid impacts originating from cold disk reservoirs
beyond the snow line [30]. As the mass of solids is expected to be the largest
near the snow line, the formation of giant planets, such as Jupiter, is
generally linked to the location of the snow line in the solar nebula. Giant
planet formation may be aided by the increased “stickiness” of H2O ice grains
relative to minerals, which greatly enhances the coagulation of small dust
grains [31] – the first step in planet formation – in the colder regions of
these disks.

_SALTUS_ will probe the midplane water snowline location by observing multiple
high upper-state energy ($E_{u}\sim 1000$ K) water lines, which emit mostly
from inside the midplane water snowline [32]. Using tomographic mapping,
_SALTUS_ will determine if water is returning to the gas with the inward drift
of icy dust grains, enriching the water content of the terrestrial planet-
forming region. In contrast, _JWST_ primarily probes higher energy emission
lines, with upper-state energies of several thousand Kelvin [27, 25].
Additionally, the dust continuum in the inner disk at mid-IR wavelengths is
optically thin, such that the H2O emission observable by _JWST_ originates in
the photosphere. The continuum is less optically thick at the sub-millimeter
wavelengths observed by _SALTUS_ , allowing us to probe deeper in the disk.
_SALTUS_ will make the first measurements of the disk midplane water snowline
location in non-outbursting disks, an important landmark in the core accretion
picture. _SALTUS_ enables us to assess the role of the water snowline in
determining the architecture of planetary systems [33, 34] and the extent to
which processes (e.g., migration, dynamical scattering) alter exoplanetary
orbital radii; this addresses Decadal Question E-Q1c.

The second part of the program will target water ice directly. Water ice is
the most abundant non-refractory solid-state component of planet-forming
disks, locking up a major fraction of the elemental oxygen. The water ice
distribution in protoplanetary disks is of fundamental importance for our
understanding of planet formation and their characteristics. As a result of
its simultaneous spectral coverage of the full 34-230 $\mu$m range and its
high sensitivity, SAFARI-Lite is uniquely suited to study emission in the the
diagnostic lattice modes of ices in protoplanetary disks (Figure 4), providing
temperature, mass, and structure of the emitting ices. Previous far-IR space
missions (S _pitzer, Herschel, ISO_) lacked the wavelength coverage or
sensitivity for a systematic study of far-IR ices, especially in planet-
forming disks. While the NIRSpec and MIRI instruments on _JWST_ cover the
near- and mid-IR region, home to ice fundamental modes, these shorter
wavelengths require a very favorable viewing angle – almost edge-on – and
cannot perform a systematic study of the role of ices in planet-forming disks.
Further, because these features are seen in absorption, they provide only a
lower limit on the absorbing column, as a photon’s path as it is scattered
through the disk is uncertain [35, 26]. The far-IR features have the advantage
of being seen in emission, and are therefore not subject to same constraints
due to viewing angle and scattering.

Figure 4: The far-IR spectrum of the Herbig star, HD 142527, measured by
ISO/SWS+LWS and Herschel/ PACS (red points), reveals the presence of the
lattice modes of crystalline H2O ice at 43 and 62 $\mu$m [36]. This is
compared to emission models at different ice/rock ratios (grey lines). The
tell-tale signature of crystalline water ice is recognizable to ice/rock
ratios as small as 0.1. For comparison, the estimated ice/rock ratio in the
solar nebula during planet formation is 1-2.

The large wavelength coverage and moderate spectral resolution of SAFARI-Lite
are well matched to the expected profile variations in the lattice modes of
H2O ice, measuring the temperature history of the ice grains. This is linked
to a physical location through models of disk temperature structure,
constrained by the H2O gas observations. The gas and solid reservoirs interact
through sublimation and condensation as icy grains drift inwards from the cold
outer disk to the warm inner disk and through turbulent cycling between the
colder mid-plane and the warmer disk photosphere. _SALTUS_ will quantify the
mass of the gaseous H2O and ice reservoirs in a large sample of protostellar
and protoplanetary sources, assess the interrelationship of these reservoirs,
and connect them to physical characteristics of the stars and their disks and
thereby address the importance of the physical processes that link them.

###  2.3 Water in Prestellar Cores

Prestellar cores are the gravitationally bound phase of star formation
immediately prior to the protostar formation[37, 38, 39], with cold (T$<10$
K), dense ($n>10^{5}$ cm-3) central regions that are well shielded from the
surrounding interstellar radiation field. During this phase, the initial
chemical conditions are set for the disk and subsequent planet formation. The
direct chemical inheritance from the prestellar phase to the protostellar disk
has been established, e.g., reflected in the D/H ratio from ALMA observations
of deuterated water [40].

Although most of the water in prestellar cores resides in the solid state on
the dust grain icy surfaces [41], photodesorption by UV photons can liberate
water molecules into the gas phase at abundances that are typically
$<\mbox{$10^{-9}$}$ with respect to H2 [42]. Two main sources of UV photons
exist: the surrounding interstellar radiation field is the dominant heating
component of dust grains [43] and a low intensity UV radiation field from H2
excitation due to collisions with electrons that come from cosmic ray
ionizations of H2 and He [44].

The $1_{10}-1_{01}$ ground state rotational transition of ortho-H2O at 538.2
$\mu$m (557 GHz; HiRX 1) can be observed in absorption against the continuum
of the prestellar core [42]. The line can also be seen in emission if the
central density of the prestellar cores is $>10^{7}$ cm-3, although only a few
prestellar cores are known that have this extreme central density [45]. The
gas phase water in the outer part of the core at low AV has a photodesorption
rate that depends on the strength of the interstellar radiation field
($G_{0}$), and a constraint on $G_{0}$ is needed to determine the dust
temperature and the gas temperature profiles in the outer part of prestellar
cores [46]. Accurate temperature profiles are crucial for radiative transfer
modeling of molecular emission and absorption observed toward prestellar
cores, and _SALTUS_ water vapor observations of prestellar cores will play an
important role in constraining the temperature profile in the outer part of
the cores.

###  2.4 Astrochemistry: CHONS from cores to disks

Hot cores are hot molecular line emission regions within massive star-forming
regions, typically characterized by high temperatures (100s of K) and
densities ($\sim$$10^{7}$ cm-2) [47]. Originally identified from the detection
of hot NH3 towards Orion-KL [48], hot cores were subsequently found to host an
incredibly rich gas-phase organic chemistry [49]. Ice mantles are the main
sites of astrochemical complex organic molecule formation, and ice sublimation
is the source of the chemical complexity detected in hot cores [50, 51].
Observing molecular line emission from hot cores provides powerful constraints
on their physical and chemical conditions [52].

_SALTUS’s_ high sensitivity at far-IR wavelengths will open a new window into
studying complex organic molecules in hot cores. Figure 5 illustrates how a
massive star-forming region can appear line-poor with _Herschel_ but harbor
hundreds of spectral lines when observed with a higher sensitivity and
resolution observatory (ALMA Band 10). With _SALTUS_ , we similarly expect
higher line densities of organics compared to Herschel. While the sensitivity
increase with _SALTUS_ will be more modest than with ALMA, we note that ALMA
Band 9 and 10 observations require exceptional weather conditions and do not
extend to wavelengths shortward of 315 $\mathrm{\mu}$m, whereas _SALTUS_ will
provide access to wavelengths as short as 34 $\mathrm{\mu}$m.

While many complex organics can be detected at longer wavelengths, there are
several advantages to obtaining far-IR observations. First, the lines covered
by _SALTUS_ typically probe higher upper state energies than millimeter-
wavelength lines, which can better constrain excitation conditions. This is
especially important for high-mass hot cores, in which organics often have
excitation temperatures of a few 100 K [53, 54]. Constraints on organic
molecule excitation temperatures are required to interpret the physical
conditions of the emitting regions, as well as the chemical relationships
between different classes of molecules, see also[55].

The early Class 0 and I stages of low-mass protostellar evolution,
characterized by an infalling envelope of gas and dust, are often accompanied
by an outflow, which promotes accretion onto the protostar by carrying away
angular momentum. Encounters between the outflow and the ambient envelope
material produce shocks, which can alter the local chemistry through heating
and grain sputtering. In some “chemically rich” outflows, the gas-phase
abundances of molecules associated with the ice phase (H2CO, CH3OH, CH3OCHO)
are enhanced due to shock-induced ice sputtering [56, 57, 58, 59]. Thus, these
chemically rich outflows offer a valuable window to probe the organic
composition of interstellar ices. Moreover, studies of outflow shock physics
and chemistry inform our understanding of the same processes that take place
on smaller, disk-forming scales within the protostellar core.

---
Figure 5:  Comparison of observations of the high-mass star formation region
NGC 6334I taken with ALMA (red) and Herschel (black and inverted). The inset
shows the beam size for Herschel, ALMA, and _SALTUS_ overlayed on a methyl
formate intensity map of the region. The effects of beam dilution will be much
less severe with SALTUS than with Herschel. Figure adapted from [60].

The archetypical chemically rich outflow shock, L1157-B1, was observed as part
of the _Herschel_ CHESS survey [61]. The 471-540 $\mu$m spectrum contained
emission lines from high-excitation transitions of grain chemistry tracers
like NH3, H2CO, and CH3OH [62]. An excitation analysis revealed that these
lines emit with temperatures $\geq 200$ K, intermediate between the cold
emission observed by longer-wavelength transitions and the very hot gas traced
by H2 emission. Thus, observations of higher-excitation organics towards
outflow shocks can help link these different emission regimes and disentangle
how the shock chemistry and physics progresses (Figure 6). These insights can
in turn be used to refine models of shock astrochemistry, which are needed to
connect observed gas-phase abundances to the underlying grain compositions
[63].

---
Figure 6:  Observations of an outflow from a low mass protostar. Top left: H2O
distribution as measured by _Herschel_ in the $2_{12}-1_{01}$ transition. Top
center: SiO contours overlayed on the H2Omap. Top right: Pure rotational H2
emission from the $S(2)$ transition. Bottom left: Blue-shifted outflow lobe in
CO J=2-1. Bottom right: Blow up of the B1 shock at $1^{\prime\prime}$ spatial
resolution in acetaldehyde with HDCO contours (left) and methyl cyanide with
methanol contours (right). At high spatial resolution the outflow resolved
into many small dense clumps with very distinct and highly variable chemical
composition. Figure from [64].

Lastly, chemically rich outflow shocks are the only low-mass star forming
regions where phosphorus carriers have been detected [65, 66]. In shock
chemistry models, PH3 and smaller P-bearing hydrides are predicted to be at
least as abundant as the P carriers PN and PO [67]. PH3 has only one strong
transition observable longward of 600 $\mu$m, and remains undetected in star-
forming regions. _SALTUS’s_ broad spectral coverage measurements allow for a
more complete inventory of the volatile phosphorus carriers in star-forming
regions.

####  2.4.1 Astrochemistry: CHONS in disks

While molecules observable at millimeter wavelengths have been extensively
studied in disks, there are almost no constraints on the inventories of light
hydrides in disks, many of which are observable only at submillimeter/far-IR
wavelengths. Perhaps the most exciting observations of light hydrides enabled
by _SALTUS_ are observations of NH3. Indeed, the N budget in disks is poorly
constrained given that the dominant N carrier, N2, cannot be directly observed
in the gas. Ice spectroscopy towards low-mass protostars, the evolutionary
progenitors of disks, has revealed that NH3 is an important N carrier in the
ice, with relative abundances of $\sim 5\%$ with respect to H2O compared to
$<1\%$ in nitriles, or XCN [68]. While nitriles are commonly detected towards
disks [69, 70, 71, 72, 73], NH3 has only been detected towards two disks. The
524.1 $\mu$m transition of o-NH3 was first detected by Herschel towards the
nearby TW Hya disk [74], and NH3 was also detected towards the embedded (Class
I) disk GV Tau N at mid-IR wavelengths tracing hot emission from the inner few
au [75]. HiRX Bands 1 and 2 will cover multiple strong transitions tracing
cool NH3 (upper state energies 27-170 K).

_SALTUS_ observations of multiple NH3 lines will allow for the first NH3
excitation analysis in the outer disk. Additionally, _SALTUS’s_ high spectral
resolution will enable a kinematic analysis of the NH3 line profiles in
sources with high SNR, providing constraints on the spatial origin of the
emission and the location of the NH3 snowline. Auxiliary constraints on the
disk structures, provided by _SALTUS_ observations of CO isotopologues, HD,
and H2O, will permit robust NH3 abundance retrievals. The NH3/H2O abundance
ratio is of particular interest, as it can be directly compared with the ratio
measured in comets to provide insights into how N is inherited by solar system
bodies.

Another promising avenue for disk science with _SALTUS_ is S-bearing hydrides.
Sulfur is commonly very depleted from the gas in dense star-forming regions,
though several S carriers (CS, SO, H2S, H2CS) have been detected in disks [76,
77, 78]. H2S was only recently detected in Class II disks: first towards GG
Tau A [79], followed by UY Aur and AB Aur[80, 81]. Towards other well-known
disks, deep searches for H2S have only produced upper limits [76]. To date,
only the $1_{10}-1_{01}$ line at 168.73 GHz has been targeted, which is
readily observable by ground-based telescopes but also intrinsically weak
compared to the higher-frequency lines covered by SALTUS. The H2S lines at
160.7 $\mu$m and 233.9 $\mu$m appear particularly promising for detection in
disks with _SALTUS_ , particularly if the emission originates in a somewhat
warm environment.

In addition to the H2O ice phonon modes discussed above, SAFARI-Lite’s
broadband coverage spanning 30 to 230 $\mu$m will cover unique spectral
signatures from a large number of volatile ice species, most notably N2, O2,
CO2, CO, CH3OH, CH4, H2S, NH3, and HCN. The uniqueness of the lattice modes
enables us to clearly distinguish between the amorphous and crystalline ice
phases, opening up a window to phase transition temperatures, which ultimately
informs on the thermal evolution of the ice. Ice lattice modes are also the
best viable way to determine the presence of homo-nuclear molecules such as O2
and N2, whose fundamental modes are IR inactive. The possibility to quantify
the abundance of N2 ice in protoplanetary disks is particularly interesting as
N2 is likely a major carrier of nitrogen [82, 83].

###  2.5 D/H Ratios as a Probe of Interstellar Heritage

Water is a key ingredient in the emergence of life and is, therefore, a key
aspect in the assessment of the habitability of (exo)planets. Yet, the origin
and delivery of water to habitable planets and notably Earth remains unclear.
Terrestrial water could have been delivered by water-rich asteroids driven by
the migration of Jupiter in the solar nebula and/or by the late heavy
bombardment during a solar system-wide rearrangement [84, 30]. Outgassing from
the deep mantel likely also contributed to Earth’s surface water [85]. The
enhanced D/H ratio in standard mean ocean water (SMOW) of 1.5${}\times
10^{-4}$ [86] relative to the interstellar elemental D/H ratio [1.5${}\times
10^{-5}$; [87]] provides support for this view as deuterium fractionation is a
chemical signature indicating that a fraction of water formed under cold
conditions, likely at the surface of interstellar grains (Figure 7) [88].

This anomaly would reflect the effects of chemistry at low temperatures in
cold prestellar cores where the small zero-point energy difference between D-
and H-bearing species can create large deuterium fractionations [88, 89].
However, the observed D/H ratio in deeply embedded protostars (hot corinos) –
tracing the inherited water content – is higher than the D/H ratio in Earth’s
water (VSMOW) by factors of 2 to 6 (purple symbols in Figure 7). Hence,
chemical processing must have occurred in warm gas, reducing the deuterium
fractionation. Likely, this reprocessing of the water occurred in the warm
surface layers of protoplanetary disks – on a disk-wide scale – where
radiation from the young star photo-desorbs H2O from preexisting ices and
reforms water through gas phase reactions. The variation in measured D/H
ratios for various astronomical objects provides important clues to the
formation conditions at different locations in nascent planetary systems.
_SALTUS_ will help to unravel the following questions: What is the HDO/H2O
ratio in protoplanetary disks and how does that depend on the characteristics
of the protostar, the conditions in the protoplanetary disk, and the molecular
core environment? What processes play a role in the water cycle of
protoplanetary disks?

---
Figure 7:  Measured D/H ratios in various Galactic and solar system sources.
D/H values are based on measurements of HDO and H2O (left axis) for all
objects with the exception of the solar system gas giants, the protosolar
nebula, and the ISM, where the D/H values are based on HD/H2. D/H from HD/H2
trace the dominant reservoir of deuterium, while H2O based measurements
provide insight into the chemical history of H2O. The D/H ratio of Earth’s
water, as measured by VSMOW is shown by the navy rectangle. Figure patterned
after [90], in turn adapted from[91, 92, 93]. Values are taken from [94, 95,
96, 97, 98, 99, 100, 101, 102, 103, 104, 105, 106, 42, 107, 108, 109, 110,
111].

_SALTUS_ will detect deuterated isotopologues of complex organics in hot
corino and non-hot corino sources. While a 1-hour HiRX integration will
provide $>5\sigma$ detections of single deuterated $\mathrm{CH_{3}OCH_{3}}$,
double deuterated $\mathrm{CH_{3}OCH_{3}}$, and single deuterated
$\mathrm{C_{2}H_{5}OH}$ in hot corinos, a 10-minute integration will yield a
robust detection of the strongest lines of deuterated molecules in
protostellar envelopes.

The D/H ratio in protoplanetary disks has been probed primarily through trace
species such as DCO+ and DCN [112, 113, 114]. ALMA observations have
constrained the D/H ratio in water for one disk, V883 Ori, based on detections
of HDO and H$\mathrm{{}_{2}^{18}}$O at 200 GHz [115]. In this system, the D/H
ratio was found to be similar to that for water in the ISM. However, V883 Ori
is an exceptionally warm disk currently undergoing an accretion burst, thus
increasing the observable water column. The main isotopologues of water are
difficult to observe from ground-based facilities, even at high altitude
[116]. Such observatories are often limited to the much weaker
H$\mathrm{{}_{2}^{18}}$O lines, which are still impacted by the low
atmospheric transmission at the relevant frequencies [105]. We identify the
strongest transitions of HDO using the physical/chemical disk model of [16]
(Figure 8). This model of the nearby disk TW Hya reproduces the resolved ALMA
observations of multiple CO transitions as well as the total HD 1-0 flux from
Herschel and the upper limits on the HDO 225 GHz line from the Submillimeter
Array (SMA) [117, 4]. The strongest HDO transitions in protoplanetary disks
are at 71.4 $\mu$m, and inaccessible from the ground. These observations will
provide the link between water in the ISM to water in planetary systems,
providing a definitive answer to whether water on terrestrial planets is
commonly inherited from the ISM.

---
Figure 8:  Predicted HDO line emission in TW Hya, based on the best fit
physcial/chemical disk model of [16] and assuming HDO/H2O= 2${}\times
10^{-3}$.

###  2.6 Debris Disks

The debris disk phase follows the protoplanetary disk phase. Debris disks are
gas-poor, with broad disks or rings of second-generation dust thought to be
influenced by the presence of planets [118]. Debris disk observations allow us
to study populations of small bodies around other stars and infer the presence
of planets that otherwise evade detection [119, 120]. They also provide
insight into the composition of solid bodies in other planetary systems [121,
122]. Debris disks may play a role in planet formation because of the gas
(with total masses up to $\sim$1 M⊕) that is now observed in these disks
[123]. Indeed, this gas could spread and accrete onto planets, thus changing
their initial atmospheric compositions between 10-100 Myr [124]. This
secondary gas component may also be important to understand our own Solar
System and find out whether the Kuiper belt can still release gas today or
whether it may have contained gas in its youth [125]. If this gas could got
accrete onto the giants, it may explain, e.g., the high metallicity of Uranus
and Neptune [124]. _SALTUS_ has the potential to observe Kuiper Belt
analogues, which is be necessary to be able to explore these questions.

####  2.6.1 Kuiper Belt Analogues

Debris disks with the same intrinsic luminosity as the Solar System’s Kuiper
Belt have yet to be observed [126]. These exo-Kuiper Belts have typical
temperatures of $\sim$50 K, corresponding to a black-body emission peak in the
far-IR. Updating sensitivity estimates from the original _SPICA_ SAFARI [127]
to _SALTUS’s_ SAFARI-Lite, _SALTUS_ will reach the $5\sigma$ sensitivity
threshold to detect exo-Kuiper belts around the nearest 30 G and K stars with
known debris disks in 1 hour of integration. _SALTUS_ can determine the
frequency of exo-Kuiper Belts, characterizing how common dust is in mature
planetary systems, thus addressing Decadal Question E-Q1d. Additionally, the
angular resolution of _SALTUS_ should enable mapping of the dust in some of
these systems, thereby serving as a Rosetta Stone between the far-IR and
longer wavelength observations of ALMA. It is possible that the massive exo-
Kuiper belts detected to date prevent the development of life in the habitable
zone due to an excessively high bombardment rate. In this case, targeting
systems with belts similar to ours (i.e. with low masses) could help optimize
the search for life on another planet.

####  2.6.2 Gas in Debris Disks

_SALTUS_ can observe the gas content in debris disks, which are expected to
contain detectable levels of carbon and oxygen; based on the gas production
model developed by [127, 128]. These observations, focusing on ionized carbon
and neutral oxygen, will complement those made by ALMA, which targets CO and
neutral carbon [129, 130]. By doing so, _SALTUS_ can gather valuable
information about the carbon ionization fraction, a crucial factor in
understanding the dynamics of gas, including determining the dominant
mechanism of angular momentum transport. Possibilities include the magneto-
rotational instability (MRI) [131], or MHD winds or even some hydrodynamic
instabilities such as vertical shear instability (VSI) or Rossby Wave
Instability (RWI) [132]. These different mechanisms operate at different
ionization fractions, densities, and depend on the magnetic field
configuration as well. Only new data for a large variety of systems will allow
us to pinpoint the dominant mechanism. One can also use the spatial
information (extracted from high spectral resolution) to rule out some
mechanisms, as for instance, MHD winds are expected to only produce viscous
expansion inwards. In contrast, turbulence will also allow the gas to extend
further outwards than its production source.

The low surface density in debris disks allows penetration of a high photon
flux from the central star, converting molecules like CO, CO2, and H2O into
ionized atomic carbon and oxygen via photodissociation and photoionization. By
targeting [CII] and [OI] in the far-IR, _SALTUS_ users gain insights into the
initial species released from planetesimals by examining the C/O ratio, e.g.,
to investigate whether CO, CO2, or H2O is released, which could have strong
connections with TNOs in the Solar System for which we can now probe the
composition with the _JWST_ [133]. These observations provide a comprehensive
understanding of the gas disk composition at different radii from the central
star. The accretion of carbon and oxygen by young planets may play a pivotal
role in the formation of the building blocks of life [134, 135] or affect the
temperature through greenhouse effects, thus influencing their habitability.
Currently, debris disk studies have been mainly with A stars [15]. _SALTUS_
has the sensitivity to detect [CII] and [OI] in the more common FGK stars.
These observations determine the C/O ratio across spectral type during the
late stages of planet formation, when volatile gasses are delivered to
terrestrial planets, and address Decadal question E-Q3a “How are potentially
habitable environments formed?” _SALTUS_ can particularly look at this
question around solar mass stars.

###  2.7 References

## References

  * [1] C. K. Walker, G. Chin, S. Aalto, et al., “Orbiting Astronomical Satellite for Investigating Stellar Systems (OASIS): following the water trail from the interstellar medium to oceans,” in Astronomical Optics: Design, Manufacture, and Test of Space and Ground Systems III, T. B. Hull, D. Kim, P. Hallibert, et al., Eds., Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series 11820, 118200O (2021).
  * [2] C. Kouveliotou, E. Agol, N. Batalha, et al., “Enduring quests-daring visions (nasa astrophysics in the next three decades),” (2014).
  * [3] P. J. Armitage, Astrophysics of planet formation, Second Edition (2020).
  * [4] E. A. Bergin, L. I. Cleeves, U. Gorti, et al., “An old disk still capable of forming a planetary system,” Nature 493, 644–646 (2013).
  * [5] M. K. McClure, E. A. Bergin, L. I. Cleeves, et al., “Mass Measurements in Protoplanetary Disks from Hydrogen Deuteride,” Astrophys. J. 831, 167 (2016).
  * [6] F. Du, E. A. Bergin, and M. R. Hogerheijde, “Volatile depletion in the TW Hydrae disk atmosphere,” Astrophys. J. letters 807, L32 (2015).
  * [7] A. Miotello, S. Bruderer, and E. F. van Dishoeck, “Protoplanetary disk masses from CO isotopologue line emission,” Astron. Astrophys. 572, A96 (2014).
  * [8] K. R. Schwarz, E. A. Bergin, L. I. Cleeves, et al., “Unlocking CO Depletion in Protoplanetary Disks. I. The Warm Molecular Layer,” Astrophys. J. 856, 85 (2018).
  * [9] K. Zhang, E. A. Bergin, K. Schwarz, et al., “Systematic Variations of CO Gas Abundance with Radius in Gas-rich Protoplanetary Disks,” Astrophys. J. 883, 98 (2019).
  * [10] A. Miotello, I. Kamp, T. Birnstiel, et al., “Setting the Stage for Planet Formation: Measurements and Implications of the Fundamental Disk Properties,” in Protostars and Planets VII, S. Inutsuka, Y. Aikawa, T. Muto, et al., Eds., Astronomical Society of the Pacific Conference Series 534, 501 (2023).
  * [11] B. Veronesi, T. Paneque-Carreño, G. Lodato, et al., “A Dynamical Measurement of the Disk Mass in Elias 227,” Astrophys. J. letters 914, L27 (2021).
  * [12] S. M. Andrews, R. Teague, C. P. Wirth, et al., “On Kinematic Measurements of Self-Gravity in Protoplanetary Disks,” arXiv e-prints , arXiv:2405.19574 (2024).
  * [13] E. A. Bergin and J. P. Williams, The Determination of Protoplanetary Disk Masses, vol. 445, Springer (2017).
  * [14] L. Trapman, A. Miotello, M. Kama, et al., “Far-infrared HD emission as a measure of protoplanetary disk mass,” Astron. Astrophys. 605, A69 (2017).
  * [15] I. Kamp, M. Honda, H. Nomura, et al., “The formation of planetary systems with SPICA,” 38, e055 (2021).
  * [16] J. K. Calahan, E. Bergin, K. Zhang, et al., “The TW Hya Rosetta Stone Project. III. Resolving the Gaseous Thermal Profile of the Disk,” Astrophys. J. 908, 8 (2021).
  * [17] K. R. Schwarz, E. A. Bergin, L. I. Cleeves, et al., “The Radial Distribution of H2 and CO in TW Hya as Revealed by Resolved ALMA Observations of CO Isotopologues,” Astrophys. J. 823, 91 (2016).
  * [18] K. R. Schwarz, J. Najita, J. Bergner, et al., “Protoplanetary Disk Science with the Orbiting Astronomical Satellite Investigating Stellar Systems (OASIS) Observatory,” Space Sci. Rev. 219, 12 (2023).
  * [19] M. R. Hogerheijde, E. A. Bergin, C. Brinch, et al., “Detection of the Water Reservoir in a Forming Planetary System,” Science 334, 338 (2011).
  * [20] F. Du, E. A. Bergin, M. Hogerheijde, et al., “Survey of Cold Water Lines in Protoplanetary Disks: Indications of Systematic Volatile Depletion,” Astrophys. J. 842, 98 (2017).
  * [21] A. Banzatti, I. Pascucci, A. D. Bosman, et al., “Hints for Icy Pebble Migration Feeding an Oxygen-rich Chemistry in the Inner Planet-forming Region of Disks,” Astrophys. J. 903, 124 (2020).
  * [22] P. Molliere, T. Molyarova, B. Bitsch, et al., “Interpreting the Atmospheric Composition of Exoplanets: Sensitivity to Planet Formation Assumptions,” Astrophys. J. 934, 74 (2022).
  * [23] K. I. Öberg, A. C. A. Boogert, K. M. Pontoppidan, et al., “The Spitzer Ice Legacy: Ice Evolution from Cores to Protostars,” Astrophys. J. 740, 109 (2011).
  * [24] G. Perotti, V. Christiaens, T. Henning, et al., “Water in the terrestrial planet-forming zone of the PDS 70 disk,” Nature 620, 516–520 (2023).
  * [25] D. Gasman, E. F. van Dishoeck, S. L. Grant, et al., “MINDS. Abundant water and varying C/O across the disk of Sz 98 as seen by JWST/MIRI,” Astron. Astrophys. 679, A117 (2023).
  * [26] J. A. Sturm, M. K. McClure, T. L. Beck, et al., “A JWST inventory of protoplanetary disk ices. The edge-on protoplanetary disk HH 48 NE, seen with the Ice Age ERS program,” Astron. Astrophys. 679, A138 (2023).
  * [27] A. Banzatti, K. M. Pontoppidan, J. S. Carr, et al., “JWST Reveals Excess Cool Water near the Snow Line in Compact Disks, Consistent with Pebble Drift,” Astrophys. J. letters 957, L22 (2023).
  * [28] C. F. Manara, M. Ansdell, G. P. Rosotti, et al., “Demographics of Young Stars and their Protoplanetary Disks: Lessons Learned on Disk Evolution and its Connection to Planet Formation,” in Protostars and Planets VII, S. Inutsuka, Y. Aikawa, T. Muto, et al., Eds., Astronomical Society of the Pacific Conference Series 534, 539 (2023).
  * [29] D. N. C. Lin, P. Bodenheimer, and D. C. Richardson, “Orbital migration of the planetary companion of 51 Pegasi to its present location,” Nature 380, 606–607 (1996).
  * [30] S. N. Raymond and A. Izidoro, “Origin of water in the inner Solar System: Planetesimals scattered inward during Jupiter and Saturn’s rapid gas accretion,” Icarus 297, 134–148 (2017).
  * [31] C. W. Ormel, D. Paszun, C. Dominik, et al., “Dust coagulation and fragmentation in molecular clouds. I. How collisions between dust aggregates alter the dust size distribution,” Astron. Astrophys. 502, 845–869 (2009).
  * [32] S. Notsu, H. Nomura, D. Ishimoto, et al., “Candidate Water Vapor Lines to Locate the H2O Snowline through High-dispersion Spectroscopic Observations. I. The Case of a T Tauri Star,” Astrophys. J. 827, 113 (2016).
  * [33] G. M. Kennedy and S. J. Kenyon, “Planet Formation around Stars of Various Masses: The Snow Line and the Frequency of Giant Planets,” Astrophys. J. 673, 502–512 (2008).
  * [34] R. B. Fernandes, G. D. Mulders, I. Pascucci, et al., “Hints for a Turnover at the Snow Line in the Giant Planet Occurrence Rate,” Astrophys. J. 874, 81 (2019).
  * [35] J. A. Sturm, M. K. McClure, J. B. Bergner, et al., “The edge-on protoplanetary disk HH 48 NE. II. Modeling ices and silicates,” Astron. Astrophys. 677, A18 (2023).
  * [36] M. Min, J. Bouwman, C. Dominik, et al., “The abundance and thermal history of water ice in the disk surrounding HD 142527 from the DIGIT Herschel Key Program,” Astron. Astrophys. 593, A11 (2016).
  * [37] E. A. Bergin and M. Tafalla, “Cold Dark Clouds: The Initial Conditions for Star Formation,” Annual Review of Astron and Astrophys 45, 339–396 (2007).
  * [38] P. André, J. Di Francesco, D. Ward-Thompson, et al., “From Filamentary Networks to Dense Cores in Molecular Clouds: Toward a New Paradigm for Star Formation,” in Protostars and Planets VI, H. Beuther, R. S. Klessen, C. P. Dullemond, et al., Eds., 27–51 (2014).
  * [39] J. E. Pineda, D. Arzoumanian, P. Andre, et al., “From Bubbles and Filaments to Cores and Disks: Gas Gathering and Growth of Structure Leading to the Formation of Stellar Systems,” in Protostars and Planets VII, S. Inutsuka, Y. Aikawa, T. Muto, et al., Eds., Astronomical Society of the Pacific Conference Series 534, 233 (2023).
  * [40] S. S. Jensen, J. K. Jørgensen, K. Furuya, et al., “Modeling chemistry during star formation: water deuteration in dynamic star-forming regions,” Astron. Astrophys. 649, A66 (2021).
  * [41] E. A. Bergin and R. L. Snell, “Sensitive Limits on the Water Abundance in Cold Low-Mass Molecular Cores,” Astrophys. J. letters 581, L105–L108 (2002).
  * [42] E. F. van Dishoeck, L. E. Kristensen, J. C. Mottram, et al., “Water in star-forming regions: physics and chemistry from clouds to disks as probed by Herschel spectroscopy,” Astron. Astrophys. 648, A24 (2021).
  * [43] I. Evans, Neal J., J. M. C. Rawlings, Y. L. Shirley, et al., “Tracing the Mass during Low-Mass Star Formation. II. Modeling the Submillimeter Emission from Preprotostellar Cores,” Astrophys. J. 557, 193–208 (2001).
  * [44] S. S. Prasad and S. P. Tarafdar, “UV radiation field inside dense clouds \- Its possible existence and chemical implications,” Astrophys. J. 267, 603–609 (1983).
  * [45] P. Caselli, E. Keto, E. A. Bergin, et al., “First Detection of Water Vapor in a Pre-stellar Core,” Astrophys. J. letters 759, L37 (2012).
  * [46] K. E. Young, J.-E. Lee, I. Evans, Neal J., et al., “Probing Pre-Protostellar Cores with Formaldehyde,” Astrophys. J. 614, 252–266 (2004).
  * [47] F. F. S. van der Tak, E. F. van Dishoeck, and P. Caselli, “Abundance profiles of CH3OH and H2CO toward massive young stars as tests of gas-grain chemical models,” Astron. Astrophys. 361, 327–339 (2000).
  * [48] M. Morris, P. Palmer, and B. Zuckerman, “Hot ammonia in Orion,” Astrophys. J. 237, 1–8 (1980).
  * [49] G. A. Blake, E. C. Sutton, C. R. Masson, et al., “Molecular Abundances in OMC-1: The Chemical Composition of Interstellar Molecular Clouds and the Influence of Massive Star Formation,” Astrophys. J. 315, 621 (1987).
  * [50] R. T. Garrod and E. Herbst, “Formation of methyl formate and other organic species in the warm-up phase of hot molecular cores,” Astron. Astrophys. 457, 927–936 (2006).
  * [51] E. Herbst and E. F. van Dishoeck, “Complex Organic Interstellar Molecules,” Annual Review of Astron and Astrophys 47, 427–480 (2009).
  * [52] R. T. Garrod and S. L. Widicus Weaver, “Simulations of Hot-Core Chemistry,” Chemical Reviews 113, 8939–8960 (2013).
  * [53] N. R. Crockett, E. A. Bergin, J. L. Neill, et al., “Herschel Observations of Extraordinary Sources: Analysis of the HIFI 1.2 THz Wide Spectral Survey toward Orion KL. I. Methods,” Astrophys. J. 787, 112 (2014).
  * [54] J. L. Neill, E. A. Bergin, D. C. Lis, et al., “Herschel Observations of Extraordinary Sources: Analysis of the Full Herschel/HIFI Molecular Line Survey of Sagittarius B2(N),” Astrophys. J. 789, 8 (2014).
  * [55] J. B. Bergner, Y. L. Shirley, J. K. Jørgensen, et al., “Astrochemistry with the Orbiting Astronomical Satellite for Investigating Stellar Systems (OASIS),” Frontiers in Astronomy and Space Sciences 8, 246 (2022).
  * [56] G. Garay, I. Köhnenkamp, T. L. Bourke, et al., “Molecular Abundance Enhancements in the Highly Collimated Bipolar Outflow BHR 71,” Astrophys. J. 509, 768–784 (1998).
  * [57] C. Codella and R. Bachiller, “Molecular outflows in intermediate-mass star forming regions: the case of CB3,” Astron. Astrophys. 350, 659–671 (1999).
  * [58] M. A. Requena-Torres, N. Marcelino, I. Jiménez-Serra, et al., “Organic Chemistry in the Dark Clouds L1448 and L183: A Unique Grain Mantle Composition,” Astrophys. J. letters 655, L37–L40 (2007).
  * [59] H. G. Arce, J. Santiago-García, J. K. Jørgensen, et al., “Complex Molecules in the L1157 Molecular Outflow,” Astrophys. J. letters 681, L21 (2008).
  * [60] B. A. McGuire, “2018 Census of Interstellar, Circumstellar, Extragalactic, Protoplanetary Disk, and Exoplanetary Molecules,” Astrophys. J. Supp. 239, 17 (2018).
  * [61] C. Ceccarelli, A. Bacmann, A. Boogert, et al., “Herschel spectral surveys of star-forming regions. Overview of the 555-636 GHz range,” Astron. Astrophys. 521, L22 (2010).
  * [62] C. Codella, B. Lefloch, C. Ceccarelli, et al., “The CHESS spectral survey of star forming regions: Peering into the protostellar shock L1157-B1. I. Shock chemical complexity,” Astron. Astrophys. 518, L112 (2010).
  * [63] A. M. Burkhardt, C. N. Shingledecker, R. Le Gal, et al., “Modeling C-shock Chemistry in Isolated Molecular Outflows,” Astrophys. J. 881, 32 (2019).
  * [64] A. Tielens, Molecular Astrophysics (2021).
  * [65] T. Yamaguchi, S. Takano, N. Sakai, et al., “Detection of Phosphorus Nitride in the Lynds 1157 B1 Shocked Region,” pasj 63, L37–L41 (2011).
  * [66] J. B. Bergner, K. I. Öberg, S. Walker, et al., “Detection of Phosphorus-bearing Molecules toward a Solar-type Protostar,” Astrophys. J. letters 884, L36 (2019).
  * [67] I. Jiménez-Serra, S. Viti, D. Quénard, et al., “The Chemistry of Phosphorus-bearing Molecules under Energetic Phenomena,” Astrophys. J. 862, 128 (2018).
  * [68] K. I. Öberg, A. C. A. Boogert, K. M. Pontoppidan, et al., “The Spitzer Ice Legacy: Ice Evolution from Cores to Protostars,” Astrophys. J 740, 109 (2011).
  * [69] A. Dutrey, S. Guilloteau, and M. Guelin, “Chemistry of protosolar-like nebulae: The molecular content of the DM Tau and GG Tau disks.,” Astron. Astrophys. 317, L55–L58 (1997).
  * [70] K. I. Öberg, V. V. Guzmán, K. Furuya, et al., “The comet-like composition of a protoplanetary disk as revealed by complex cyanides,” Nature 520, 198–201 (2015).
  * [71] V. V. Guzmán, K. I. Öberg, J. Huang, et al., “Nitrogen Fractionation in Protoplanetary Disks from the H13CN/HC15N Ratio,” Astrophys. J. 836, 30 (2017).
  * [72] J. B. Bergner, K. I. Öberg, E. A. Bergin, et al., “A Survey of C2H, HCN, and C18O in Protoplanetary Disks,” Astrophys. J. 876, 25 (2019).
  * [73] S. E. van Terwisga, E. F. van Dishoeck, P. Cazzoletti, et al., “The ALMA Lupus protoplanetary disk survey: evidence for compact gas disks and molecular rings from CN,” Astron. Astrophys. 623, A150 (2019).
  * [74] V. N. Salinas, M. R. Hogerheijde, E. A. Bergin, et al., “First detection of gas-phase ammonia in a planet-forming disk. NH3, N2H+, and H2O in the disk around TW Hydrae,” Astron. Astrophys. 591, A122 (2016).
  * [75] J. R. Najita, J. S. Carr, S. D. Brittain, et al., “High-resolution Mid-infrared Spectroscopy of GV Tau N: Surface Accretion and Detection of NH3 in a Young Protoplanetary Disk,” Astrophys. J. 908, 171 (2021).
  * [76] A. Dutrey, V. Wakelam, Y. Boehler, et al., “Chemistry in disks. V. Sulfur-bearing molecules in the protoplanetary disks surrounding LkCa15, MWC480, DM Tauri, and GO Tauri,” Astron. Astrophys. 535, A104 (2011).
  * [77] S. Guilloteau, E. Di Folco, A. Dutrey, et al., “A sensitive survey for 13CO, CN, H2CO, and SO in the disks of T Tauri and Herbig Ae stars,” Astron. Astrophys. 549, A92 (2013).
  * [78] R. Le Gal, K. I. Öberg, R. A. Loomis, et al., “Sulfur Chemistry in Protoplanetary Disks: CS and H2CS,” Astrophys. J. 876, 72 (2019).
  * [79] N. T. Phuong, E. Chapillon, L. Majumdar, et al., “First detection of H2S in a protoplanetary disk. The dense GG Tauri A ring,” Astron. Astrophys. 616, L5 (2018).
  * [80] P. Rivière-Marichalar, A. Fuente, R. Le Gal, et al., “H2S observations in young stellar disks in Taurus,” Astron. Astrophys. 652, A46 (2021).
  * [81] P. Rivière-Marichalar, A. Fuente, G. Esplugues, et al., “AB Aur, a Rosetta stone for studies of planet formation. II. H2S detection and sulfur budget,” Astron. Astrophys. 665, A61 (2022).
  * [82] K. R. Schwarz and E. A. Bergin, “The Effects of Initial Abundances on Nitrogen in Protoplanetary Disks,” Astrophys. J 797, 113 (2014).
  * [83] K. M. Pontoppidan, C. Salyk, A. Banzatti, et al., “The Nitrogen Carrier in Inner Protoplanetary Disks,” Astrophys. J 874, 92 (2019).
  * [84] D. P. O’Brien, K. J. Walsh, A. Morbidelli, et al., “Water delivery and giant impacts in the ‘Grand Tack’ scenario,” Icarus 239, 74–84 (2014).
  * [85] M. W. Broadley, D. V. Bekaert, L. Piani, et al., “Origin of life-forming volatile elements in the inner Solar System,” Nature 611, 245–255 (2022).
  * [86] R. Hagemann, G. Nief, and E. Roth, “Absolute isotopic scale for deuterium analysis of natural waters. Absolute D/H ratio for SMOW,” Tellus 22, 712–715 (1970).
  * [87] T. Prodanović, G. Steigman, and B. D. Fields, “The deuterium abundance in the local interstellar medium,” Mon. Not. R. Astron. Soc. 406, 1108–1115 (2010).
  * [88] A. G. G. M. Tielens, “Surface chemistry of deuterated molecules,” Astron. Astrophys. 119, 177–184 (1983).
  * [89] C. Ceccarelli, P. Caselli, D. Bockelée-Morvan, et al., “Deuterium Fractionation: The Ariadne’s Thread from the Precollapse Phase to Meteorites and Comets Today,” in Protostars and Planets VI, H. Beuther, R. S. Klessen, C. P. Dullemond, et al., Eds., 859–882 (2014).
  * [90] C. M. Anderson, N. Biver, G. L. Bjoraker, et al., “Solar System Science with the Orbiting Astronomical Satellite Investigating Stellar Systems (OASIS) Observatory,” Space Sci. Rev. 218, 43 (2022).  doi:10.1007/s11214-022-00911-5.
  * [91] P. Hartogh, E. Lellouch, R. Moreno, et al., “Direct detection of the Enceladus water torus with Herschel,” Astron. Astrophys. 532, L2 (2011).
  * [92] D. C. Lis, N. Biver, D. Bockelée-Morvan, et al., “A Herschel Study of D/H in Water in the Jupiter-family Comet 45P/Honda-Mrkos-Pajdušáková and Prospects for D/H Measurements with CCAT,” Astrophys. J. letters 774, L3 (2013).
  * [93] D. Bockelée-Morvan, N. Biver, B. Swinyard, et al., “Herschel measurements of the D/H and 16O/18O ratios in water in the Oort-cloud comet C/2009 P1 (Garradd),” Astron. Astrophys. 544, L15 (2012).
  * [94] S. S. Jensen, J. K. Jørgensen, L. E. Kristensen, et al., “ALMA observations of water deuteration: a physical diagnostic of the formation of protostars,” Astron. Astrophys. 631, A25 (2019).
  * [95] N. Biver, R. Moreno, D. Bockelée-Morvan, et al., “Isotopic ratios of H, C, N, O, and S in comets C/2012 F6 (Lemmon) and C/2014 Q2 (Lovejoy),” Astron. Astrophys. 589, A78 (2016).
  * [96] G. L. Villanueva, M. J. Mumma, B. P. Bonev, et al., “A Sensitive Search for Deuterated Water in Comet 8p/Tuttle,” Astrophys. J. letters 690, L5–L9 (2009).
  * [97] L. Paganini, M. J. Mumma, E. L. Gibb, et al., “Ground-based Detection of Deuterated Water in Comet C/2014 Q2 (Lovejoy) at IR Wavelengths,” Astrophys. J. letters 836, L25 (2017).
  * [98] D. C. Lis, D. Bockelée-Morvan, R. Güsten, et al., “Terrestrial deuterium-to-hydrogen ratio in water in hyperactive comets,” Astron. Astrophys. 625, L5 (2019).
  * [99] A. Coutens, C. Vastel, E. Caux, et al., “A study of deuterated water in the low-mass protostar IRAS 16293-2422,” Astron. Astrophys. 539, A132 (2012).
  * [100] A. Coutens, C. Vastel, S. Cabrit, et al., “Deuterated water in the solar-type protostars NGC 1333 IRAS 4A and IRAS 4B,” Astron. Astrophys. 560, A39 (2013).
  * [101] A. Coutens, C. Vastel, U. Hincelin, et al., “Water deuterium fractionation in the high-mass star-forming region G34.26+0.15 based on Herschel/HIFI data,” Mon. Not. R. Astron. Soc. 445, 1299–1313 (2014).
  * [102] M. V. Persson, J. K. Jørgensen, E. F. van Dishoeck, et al., “The deuterium fractionation of water on solar-system scales in deeply-embedded low-mass protostars,” Astron. Astrophys. 563, A74 (2014).
  * [103] K. S. Wang, F. F. S. van der Tak, and M. R. Hogerheijde, “Kinematics of the inner thousand AU region around the young massive star $<$ASTROBJ$>$AFGL 2591-VLA3$<$/ASTROBJ$>$: a massive disk candidate?,” Astron. Astrophys. 543, A22 (2012).
  * [104] M. Emprechtinger, D. C. Lis, R. Rolffs, et al., “The Abundance, Ortho/Para Ratio, and Deuteration of Water in the High-mass Star-forming Region NGC 6334 I,” Astrophys. J. 765, 61 (2013).
  * [105] F. F. S. van der Tak, C. M. Walmsley, F. Herpin, et al., “Water in the envelopes and disks around young high-mass stars,” Astron. Astrophys. 447, 1011–1025 (2006).
  * [106] F. P. Helmich, E. F. van Dishoeck, and D. J. Jansen, “The excitation and abundance of HDO toward W3(OH)/(H2O).,” Astron. Astrophys. 313, 657–663 (1996).
  * [107] L. Bonal, C. M. O. D. Alexander, G. R. Huss, et al., “Hydrogen isotopic composition of the water in CR chondrites,” Geochim et Cosmochem Acta 106, 111–133 (2013).
  * [108] L. Yang, F. J. Ciesla, and C. M. O. D. Alexander, “The D/H ratio of water in the solar nebula during its formation and evolution,” Icarus 226, 256–267 (2013).
  * [109] E. Jacquet and F. Robert, “Water transport in protoplanetary disks and the hydrogen isotopic composition of chondrites,” Icarus 223, 722–732 (2013).
  * [110] E. L. Gibb, B. P. Bonev, M. A. DiSanti, et al., “An Infrared Search for HDO in Comet D/2012 S1 (ISON) and Implications for iSHELL,” Astrophys. J. 816, 101 (2016).
  * [111] N. Biver, R. Moreno, and D. Bockelée-Morvan, “Hdo in comet 46p/wirtanen from alma observations,” (2024).  In prep.
  * [112] J. Huang, K. I. Öberg, C. Qi, et al., “An ALMA Survey of DCN/H13CN and DCO+/H13CO+ in Protoplanetary Disks,” Astrophys. J 835, 231 (2017).
  * [113] V. N. Salinas, M. R. Hogerheijde, G. S. Mathews, et al., “DCO+, DCN, and N2D+ reveal three different deuteration regimes in the disk around the Herbig Ae star HD 163296,” Astron. Astrophys. 606, A125 (2017).
  * [114] C. E. Muñoz-Romero, K. I. Öberg, C. J. Law, et al., “Cold Deuterium Fractionation in the Nearest Planet-forming Disk,” Astrophys. J 943, 35 (2023).
  * [115] J. J. Tobin, M. L. R. van’t Hoff, M. Leemker, et al., “Deuterium-enriched water ties planet-forming disks to comets and protostars,” Nature 615, 227–230 (2023).
  * [116] S. Facchini, L. Testi, E. Humphreys, et al., “Resolved ALMA observations of water in the inner astronomical units of the HL Tau disk,” Nature Astronomy (2024).
  * [117] C. Qi, D. J. Wilner, Y. Aikawa, et al., “Resolving the Chemistry in the Disk of TW Hydrae. I. Deuterated Species,” Astrophys. J. 681, 1396–1407 (2008).
  * [118] T. D. Pearce, R. Launhardt, R. Ostermann, et al., “Planet populations inferred from debris discs. Insights from 178 debris systems in the ISPY, LEECH, and LIStEN planet-hunting surveys,” Astron. Astrophys. 659, A135 (2022).
  * [119] A. M. Lagrange, M. Bonnefoy, G. Chauvin, et al., “A Giant Planet Imaged in the Disk of the Young Star $\beta$ Pictoris,” Science 329, 57 (2010).
  * [120] N. Skaf, A. Boccaletti, E. Pantin, et al., “The $\beta$ Pictoris system: Setting constraints on the planet and the disk structures at mid-IR wavelengths with NEAR,” Astron. Astrophys. 675, A35 (2023).
  * [121] T. Mittal, C. H. Chen, H. Jang-Condell, et al., “The Spitzer Infrared Spectrograph Debris Disk Catalog. II. Silicate Feature Analysis of Unresolved Targets,” Astrophys. J 798, 87 (2015).
  * [122] T. J. Rodigas, C. C. Stark, A. Weinberger, et al., “On the Morphology and Chemical Composition of the HR 4796A Debris Disk,” Astrophys. J 798, 96 (2015).
  * [123] A. Moór, M. Curé, Á. Kóspál, et al., “Molecular Gas in Debris Disks around Young A-type Stars,” Astrophys. J. 849, 123 (2017).
  * [124] Q. Kral, J. Davoult, and B. Charnay, “Formation of secondary atmospheres on terrestrial planets by late disk accretion,” Nature Astronomy 4, 769–775 (2020).
  * [125] Q. Kral, J. E. Pringle, A. Guilbert-Lepoutre, et al., “A molecular wind blows out of the Kuiper belt,” Astron. Astrophys. 653, L11 (2021).
  * [126] C. Eiroa, J. P. Marshall, A. Mora, et al., “DUst around NEarby Stars. The survey observational results,” Astron. Astrophys. 555, A11 (2013).
  * [127] Q. Kral, L. Matrà, M. C. Wyatt, et al., “Predictions for the secondary CO, C and O gas content of debris discs from the destruction of volatile-rich planetesimals,” Mon. Not. R. Astron. Soc. 469, 521–550 (2017).
  * [128] Q. Kral, S. Marino, M. C. Wyatt, et al., “Imaging [CI] around HD 131835: reinterpreting young debris discs with protoplanetary disc levels of CO gas as shielded secondary discs,” Mon. Not. R. Astron. Soc. 489, 3670–3691 (2019).
  * [129] Q. Kral, M. Wyatt, R. F. Carswell, et al., “A self-consistent model for the evolution of the gas produced in the debris disc of $\beta$ Pictoris,” Mon. Not. R. Astron. Soc. 461, 845–858 (2016).
  * [130] G. Cataldi, Y. Wu, A. Brandeker, et al., “The Surprisingly Low Carbon Mass in the Debris Disk around HD 32297,” Astrophys. J. 892, 99 (2020).
  * [131] Q. Kral and H. Latter, “The magnetorotational instability in debris-disc gas,” Mon. Not. R. Astron. Soc. 461, 1614–1620 (2016).
  * [132] C. Cui, S. Marino, Q. Kral, et al., “Dynamics of cold circumstellar gas in debris discs,” Mon. Not. R. Astron. Soc. 530, 1766–1780 (2024).
  * [133] M. N. De Prá, E. Hénault, N. Pinilla-Alonso, et al., “Widespread CO2 and CO ices in the trans-Neptunian population revealed by JWST/DiSCo-TNOs,” Nature Astronomy (2024).
  * [134] R. S. Sutherland and M. A. Dopita, “Effects of Preionization in Radiative Shocks. I. Self-consistent Models,” Astrophys. J. Supp. 229, 34 (2017).
  * [135] C. H. M. Pabst, A. Hacar, J. R. Goicoechea, et al., “[C II] 158 $\mu$m line emission from Orion A I. A template for extragalactic studies?,” Astron. Astrophys. 651, A111 (2021).

### Disclosures

The authors have no relevant financial interests in the manuscript and no
other potential conflicts of interest to disclose.

### Code, Data, and Materials Availability

This paper reviewer the science cases and potential observations for a future
space mission and so data sharing in not applicable at this time.

Kamber R. Schwarz holds a postdoctoral position at the Max Planck Institute
for Astronomy in Heidelberg. She was a NASA Sagan Postdoctoral Fellow at the
Lunar and Planetary Laboratory at the University of Arizona. She received a
PhD in Astronomy & Astrophysics at the University of Michigan in 2018. She
studies the evolution of volatile gas during planet formation, with the goal
of determining the amount of volatile carbon, nitrogen, and oxygen available
to form planets. Her research combines observations from the infrared to the
millimeter, using facilities such as ALMA, NOEMA, and JWST, with
physical/chemical modeling to constrain the timescales and mechanisms of
volatile reprocessing. She has authored about 100 publications.

Alexander Tielens is a professor of astronomy in the Astronomy Department of
the University of Maryland, College Park. He received his MS and PhD in
astronomy from Leiden University in 1982. He has authored over 500 papers in
refereed journals and has written two textbooks on the interstellar medium.
His scientific interests center on the physics and chemistry of the
interstellar medium, in particular in regions of star and planet formation.

Joan Najita is an Astronomer at NSF’s NOIRLab and its Head of Scientific Staff
for User Support. She was formerly the Chief Scientist at the National Optical
Astronomy Observatory (NOAO) and served on its scientific staff since 1998. In
1993 she received her PhD from University of California, Berkeley. Najita has
been responsible for strategic planning, science career development, science
communications, and the health of the scientific environment at the
Observatory. Her interests include traditional research topics (such as star
and planet formation, exoplanets, and the Milky Way), advocacy for the
development of new research capabilities (such as infrared spectroscopy and
massively multiplexed wide-field spectroscopy), as well as the sociological
context of astronomy (such as the nature of discovery in astronomy, and its
science sociology and resource allocation practices). She has a lifelong
interest in communicating science to the public and in the role of science in
society. Joan Najita has been named a 2021–2022 fellow at Harvard Radcliffe
Institute, joining artists, scientists, scholars, and practitioners in a year
of discovery and interdisciplinary exchange in Cambridge. She has authored
about 190 publications

Jennifer Bergner is an Assistant Professor of Chemistry at UC Berkeley. She
received her BS degree from University of Virginia and MA and PhD from Harvard
in 2019. Her astrochemistry group uses a variety of tools to explore the
chemistry at play in protostars and protoplanetary disks, the progenitors of
planetary systems. With cryogenic vacuum experiments she mimics the extremely
low temperatures and pressures of star-forming regions in the lab to explore
the chemical and microphysical behavior of volatile ices. She also uses state-
of-the-art telescope facilities like ALMA and JWST to observe the spectral
fingerprints of volatile molecules in protostars and protoplanetary disks,
providing insight into the chemical landscape of planet formation and the
underlying physical processes which drive astrochemical evolution. She has
about 128 publications.

Quentin Kral is an astronomer at the Paris Observatory (LESIA). His main
research interests are debris disks, the solar system, and planetary
formation. He is an expert on the new gas component that is now observed in
mature extrasolar systems once the young planet-forming disk has dissipated.
He mainly uses ALMA to test his models and investigate the gas and dust in
exoplanetary systems. He is the PI of the exoplanet.eu catalog of exoplanets.
He has published over 130 articles. He received his master’s degree from Ecole
Normale Supérieure (ENS Paris) and his PhD from Paris Observatory in 2014.

Carrie M. Anderson is a research scientist at NASA Goddard Space Flight Center
(GSFC). She received a BS in physics from Arizona State University in 2000,
and MS and PhD degrees in Astronomy from New Mexico State University in 2003
and 2006, respectively. She is the author of more than 45 papers in refereed
journals and has written one book chapter. Her research focuses on the remote
sensing of planetary atmospheres, primarily in the areas of thermal structure
and composition, using space- and ground-based data, in the visible, near-IR,
mid-IR, far-IR, and submillimeter spectral regions. Her research also includes
laboratory transmission spectroscopy measurements of ice films in a high-
vacuum cryo chamber located in her Spectroscopy for Planetary ICes
Environments (SPICE) laboratory at NASA GSFC.

Gordon Chin is a research scientist at NASA Goddard Space Flight Center
(GSFC). He received his B.A. in physics from Columbia College in 1970, and his
M.A., M. Phil., and PhD in physics from Columbia University in 1972, 1974, and
1977, respectively. He is the author of more than 50 refereed journal papers.
His current research interests includes the development of sub-millimeter
planetary flight spectrometers targeting planetary atmospheres, the lunar
exosphere, and ocean world plume environments in the solar system.

David T. Leisawitz is an astrophysicist and Chief of the Science Proposal
Support Office at NASA’s Goddard Space Flight Center. He received a Ph.D. in
Astronomy from the University of Texas at Austin in 1985. His primary research
interests are star and planetary system formation, infrared astrophysics,
wide-field spatio-spectral interferometry, and far-infrared space
interferometry. He is NASA Center Study Scientist for the Far-IR Surveyor,
Mission Scientist for the Wide-field Infrared Survey Explorer (WISE), and
earlier served as Deputy Project Scientist for the Cosmic Background Explorer
(COBE) under Project Scientist and mentor Dr. John Mather. He is Principal
Investigator for “Wide-field Imaging Interferometry,” a Co- Investigator on
the “Balloon Experimental Twin Telescope for Infrared Interferometry
(BETTII),” and member of a three-person External Advisory Panel for the “Far
Infrared Space Interferometer Critical Assessment (FISICA),” a European
Commission FP7 research program. In 2004-05, he served as Principal
Investigator and science team lead for the Space Infrared Interferometric
Telescope (SPIRIT) mission concept study. He has authored about 300
publications.

David J. Wilner is a Senior Astrophysicist at the Smithsonian Astrophysical
Observatory in the Radio and Geoastronomy Division at the Center for
Astrophysics, Harvard & Smithsonian. His main research interests are
circumstellar disks and the formation of planets, and the development of
aperture synthesis techniques. Much of his science program makes use of radio,
millimeter, and submillimeter interferometers, including the Submillimeter
Array, ALMA, and the VLA. He received an A.B. in Physics from Princeton
University and a Ph.D. in Astronomy from the University of California. He
frequently lectures on imaging and deconvolution in radio astronomy. He has
authored about 450 publications.

Peter Roelfsema is a senior scientist/project manager at SRON Netherlands
Institute for Space Research. He has been involved in several satellite
projects, currently as PM for the Dutch Athena/X-IFU contribution, and before
that as PI for SPICA’s SAFARI Far-IR spectrometer and as lead of the
international SPICA collaboration. He was PI and ad-interim PI for
Herschel/HIFI, and in the early Herschel development phase, he was one of the
lead system engineers developing the Herschel ground segment concept and
operational systems. Before Herschel he led the ISO/SWS operations team in
Villafranca/Spain and the SWS analysis software development team. He started
his scientific career as a radio astronomer, utilizing the WSRT, VLA and ATNF
to study radio recombination lines of galactic HII regions and nearby active
galaxies. With ISO and Herschel he did (Far)IR spectroscopic work on galactic
HII regions, studying e.g. PAH properties and metal abundance variations in
our galaxy. He has published over 150 papers in astronomical journals
conference proceedings and supervised a number of PhD students.

Floris van der Tak is a Senior Scientist in the Astrophysics program of the
Netherlands Institute for Space Research (SRON), where his research interests
include astrochemistry, the habitability of exoplanets, the physics of the
interstellar medium, star formation, molecular spectroscopy and radiative
transfer. He received a PhD from Leiden University in 2000. He was the Project
Scientist for the SPICA/SAFARI instrument. He has authored about 216
publications.

Erick Young is a Senior Science Advisor at Universities Space Research
Association. He is a widely recognized authority on infrared astronomy and the
former Science Mission Operations Director for SOFIA. He specializes in
designing science instruments and has participated in many NASA’s space
infrared astronomy missions. He was responsible for developing the far-
infrared detector arrays on the Spitzer Space Telescope’s Multiband Imaging
Photometer for Spitzer. As SOFIA Science Mission Operations Director, he
manages the airborne observatory’s equipment, instruments, support facilities,
and infrastructure. He was also responsible for the overall scientific
productivity of the facility, including the Guest Investigator program. He has
about 385 publications.

Christopher K. Walker is a Professor of Astronomy, Optical Sciences,
Electrical & Computer Engineering, Aerospace & Mechanical Engineering, and
Applied Mathematics at the University of Arizona (UofA). He received his
M.S.E.E. from Clemson University (1980), M.S.E.E. from Ohio State University
(1981), and Ph.D. in Astronomy from the University of Arizona (1988). He has
worked at TRW Aerospace and the Jet Propulsion Laboratory, was a Millikan
Fellow in Physics at Caltech, and has been a faculty member at the UofA since
1991, where he has worked to advance the field of terahertz astronomy. He has
supervised sixteen Ph.D. students, led numerous NASA and NSF projects,
authored/coauthored 130+ papers, and published two textbooks: ”Terahertz
Astronomy” and ”Investigating Life in the Universe”.

###### List of Figures

  1. 1 Model predictions of disk gas (D=140 pc), as a function of HD line flux. The width of the blue band represents the range of model results for different disk structures. The three red points represent the only known HD detections from Herschel- PACS. _SALTUS_ SAFARI-Lite will make observations for the full range of fluxes plotted, with 5$\sigma$, 1-hr sensitivities between 0.5 and 2${}\times 10^{-20}$ W m-2 across the 4 bands. SALTUS HiRX will be sensitive enough to provide high spectral resolution observations, with 5$\sigma$, 1-hr sensitivities $>5\mbox{${}\times 10^{-19}$}$ W m-2 in J=2-1 and $>2\mbox{${}\times 10^{-19}$}$ W m-2 in J=1-0. Figure modified from [14].
  2. 2 A simulated disk-integrated spectrum of HD 1-0 for a $10^{-3}$ M⊙ disk at a distance of 140 pc, inclined by 45 degrees. The spectral resolution is $\Delta v$ = 0.9 km s-1. Disk emission is Doppler shifted due to Keplerian rotation. Emission with a high velocity offset from the line center originates from small radii, while emission close to the systemic velocity originates from large radii. The HiRX high spectral resolution enables differentiation between emission from different radii in protoplanetary disks.
  3. 3  The HiRX and SAFARI-Lite instruments will measure the gaseous emission spectrum of planet-forming disks, thereby determining unambiguously the disk gas mass, the water content, and the HDO/H2Oabundance ratios; all critical aspects of disk/planet formation models. The far-IR provides access to low energy transitions that probe the cold, outer parts of the disk of the planet-forming reservoir.
  4. 4 The far-IR spectrum of the Herbig star, HD 142527, measured by ISO/SWS+LWS and Herschel/ PACS (red points), reveals the presence of the lattice modes of crystalline H2O ice at 43 and 62 $\mu$m [36]. This is compared to emission models at different ice/rock ratios (grey lines). The tell-tale signature of crystalline water ice is recognizable to ice/rock ratios as small as 0.1. For comparison, the estimated ice/rock ratio in the solar nebula during planet formation is 1-2.
  5. 5  Comparison of observations of the high-mass star formation region NGC 6334I taken with ALMA (red) and Herschel (black and inverted). The inset shows the beam size for Herschel, ALMA, and _SALTUS_ overlayed on a methyl formate intensity map of the region. The effects of beam dilution will be much less severe with SALTUS than with Herschel. Figure adapted from [60].
  6. 6  Observations of an outflow from a low mass protostar. Top left: H2O distribution as measured by _Herschel_ in the $2_{12}-1_{01}$ transition. Top center: SiO contours overlayed on the H2Omap. Top right: Pure rotational H2 emission from the $S(2)$ transition. Bottom left: Blue-shifted outflow lobe in CO J=2-1. Bottom right: Blow up of the B1 shock at $1^{\prime\prime}$ spatial resolution in acetaldehyde with HDCO contours (left) and methyl cyanide with methanol contours (right). At high spatial resolution the outflow resolved into many small dense clumps with very distinct and highly variable chemical composition. Figure from [64].
  7. 7  Measured D/H ratios in various Galactic and solar system sources. D/H values are based on measurements of HDO and H2O (left axis) for all objects with the exception of the solar system gas giants, the protosolar nebula, and the ISM, where the D/H values are based on HD/H2. D/H from HD/H2 trace the dominant reservoir of deuterium, while H2O based measurements provide insight into the chemical history of H2O. The D/H ratio of Earth’s water, as measured by VSMOW is shown by the navy rectangle. Figure patterned after [90], in turn adapted from[91, 92, 93]. Values are taken from [94, 95, 96, 97, 98, 99, 100, 101, 102, 103, 104, 105, 106, 42, 107, 108, 109, 110, 111].
  8. 8  Predicted HDO line emission in TW Hya, based on the best fit physcial/chemical disk model of [16] and assuming HDO/H2O= 2${}\times 10^{-3}$.