# A retrospective analysis of mid-infrared observations of the Comet
D/Shoemaker-Levy 9 and Wesley impacts on Jupiter

J. A. Sinclair<EMAIL_ADDRESS>C. M. Lisse  G. S. Orton  M.
Krishnamoorthy  L. N. Fletcher  J. Hora  C. Palotai  T. Hayward  Jet
Propulsion Laboratory/California Institute of Technology, 48Oak Grove Dr,
Pasadena, CA 91109, United States  Applied Physics Laboratory, Johns Hopkins
University, Baltimore, MD, United States  University of Michigan, Ann Arbor,
MI, United States  School of Physics & Astronomy, University of Leicester,
University Road, Leicester, LE1 7RH, United Kingdom  Center for Astrophysics,
Harvard & Smithsonian, Cambridge, MA, United States  Florida Institute of
Technology, Melbourne, FL, United States  Gemini Observatory, NOIRLab, La
Serena, Chile

###### Abstract

We present a retrospective analysis of Earth-based mid-infrared observations
of Jupiter capturing the aftermath of the impacts by Comet D/Shoemaker-Levy 9
(henceforth SL9) in July 1994 and the unknown object previously termed the
“Wesley impactor” in July 2009. While the effects of both impacts on Jupiter’s
atmosphere have been reported previously, we were motivated to re-examine both
events using consistent data reduction and analysis methods to enable robust,
quantitative comparisons. To study the aftermath of the SL9 impacts, we
examined infrared (7.8 - 20.5 $\upmu$m) spectrophotometry of Jupiter measured
by the MIRAC (Mid-Infrared Camera) on NASA’s Infrared Telescope Facility on
July 20, 21 1994 and low-resolution (R = 100) N-band (7 – 13 $\upmu$m)
spectroscopy measured by SpectroCam-10 on the Palomar Telescope on July 22
1994. To study the aftermath of the Wesley impact, we examined low-resolution
(R = 100) N- and Q-band (R = 80, 17 – 25 $\upmu$m) spectroscopy recorded by
Gemini South Telescope’s T-ReCS (Thermal-Region Camera Spectrograph). We
analyzed the observations with two independent analyses: 1) a least-squares
search over a grid of candidate mineral species to determine the composition
of impact residue and 2) a radiative transfer analysis to derive atmospheric
information. We observe that the SL9 impact sites are enhanced in
stratospheric CH4 emissions at 7.9 $\upmu$m, due to shock heating and
adiabatic compression from plume re-entry, and from 8.5 - 11.5 $\upmu$m due to
stratospheric NH3 emission and non-gaseous cometary material, in agreement
with previous work. In the G impact site, we derive NH3 concentrations of
$5.7^{+4.5}_{-2.8}$ ppmv at 30 mbar. In new findings, we find that the SL9
impact sites also exhibit a non-gaseous emission feature at 18 - 19 $\upmu$m.
The non-gaseous emission at 8.5 - 11.5 $\upmu$m and 18 - 19 $\upmu$m emission
result is best reproduced by predominantly amorphous olivine and (obsidian)
silica at similar abundances. The Wesley impact site exhibits enhanced
emissions from 8.8 - 11.5 $\upmu$m and 18 - 19 $\upmu$m. We found this could
be reproduced by predominantly amorphous olivine and stratospheric gaseous NH3
at concentrations of 150${}^{+338}_{-121}$ ppbv retrieved at 30 mbar.
Stratospheric abundances of NH3 are a factor of $\sim$40 higher in the SL9
impacts compared to the Wesley impact, which confirms the former reached
deeper, NH3-richer altitudes of Jupiter’s atmosphere compared to the latter.
The absence of silicas in the Wesley impact would place an upper limit of 10
km/s on the incident angle and $\sim$9∘ on the entry angle of the impactor
such that shock heating associated with the impact did not reach temperatures
required for silicates to be converted.

###### keywords:

††journal: Icarus

##  1 Introduction

Jupiter has the second largest gravitational sink in our solar system (after
the Sun) and therefore experiences a relatively high impact rate compared to
the other planets. In recent decades, several impacts have been detected in
Jupiter. Most were bolide flashes that left no detectable residual influence
on Jupiter including the September 10, 2012 impact [Hueso et al., 2013] and
the April 2020 impact [Giles et al., 2021] detected by Juno’s ultraviolet
instrument (UVS, Gladstone et al. 2017). However, two impact events left
prominent perturbations to Jupiter’s atmosphere. One was the series of impacts
by fragments of Comet D/Shoemaker-Levy 9 (hereafter SL9) in July 1994 (c.f.
the review by Harrington et al. 2004). The other was an impact by an unknown
body into the nightside of Jupiter on July 19, 2009, the aftermath of which
was first detected by amateur astronomer Anthony Wesley (hereafter the Wesley
impact, c.f. Sánchez-Lavega et al. 2010). Using spectroscopic and imaging
observations from Earth-based telescopes, the effects of both impact events on
the atmosphere of Jupiter were investigated intensely.

SL9 was tidally disrupted into at least 16 fragments, which then impacted
Jupiter’s atmosphere at $\sim$40∘S between July 16 and 22, 1994. The impacts
occured at relative velocities similar to Jupiter’s escape velocity ($\sim$65
km/sec), shock-heated gas to extreme temperatures (30000 - 40000 K, e.g.
Zahnle and Mac Low 1994) and vaporized the cometary material. The residue then
rose in a plume along the initial entry path, outside of the atmosphere,
before re-entering the atmosphere at near-horizontal trajectories (e.g.
Griffith et al. [1997], Figure 14). Upon re-entry, the material re-compressed
and began sinking in atmosphere, converting kinetic to thermal energy and
thereby heating the atmosphere as deep as the lower stratosphere and enhancing
hydrocarbon emissions at mid-infrared wavelengths (e.g. Orton et al. 1995,
Bézard et al. 1997). Species such as NH3, normally cold-trapped below
Jupiter’s tropopause, were excavated to stratospheric altitudes and observed
in emission at mid-infrared wavelengths (e.g. Kostiuk et al. 1996, Griffith et
al. 1997). Shock-induced chemistry and/or delivery from the comet itself
enhanced the abundances of existing species, such as HCN (e.g. Bézard et al.
1997, Noll et al. 1995) and introduced new chemical species into the
stratosphere including H2O (e.g. Bjoraker et al. 1996, Encrenaz et al. 1997)
and CO (e.g. Lellouch et al. 1997, Kim et al. 1999). In particular,
stratospheric H2O and CO continue to remain detectable in Jupiter’s
stratosphere at the time of writing (e.g. Lellouch et al. 2002, Cavalié et al.
2013, Benmahi et al. 2020).

The aftermath of the Wesley impact was first noticed on July 19 2009 at
$\sim$55∘S as a dark “bruise” at visible wavelengths [Sánchez-Lavega et al.,
2010] but also bright at near-infrared wavelengths [Hammel et al., 2010, de
Pater et al., 2010, Orton et al., 2011] indicating the presence of material at
millibar pressure levels. The same region exhibited stratospheric NH3 emission
at mid-infrared wavelengths (e.g. Fletcher et al. 2011, de Pater et al. 2010),
which indicated an impactor had excavated NH3 from below the tropopause to
stratospheric altitudes, similar to the SL9 impacts. By dynamical arguments,
impacts on Jupiter are orders of magnitude more likely to be comets than
asteroids since Jupiter should have cleared the latter from its orbit
$\sim$Gyr ago [Schenk et al., 2004]. Nevertheless, there was a convergence of
evidence suggesting the Wesley impactor was an asteroid. Several studies
inferred the material from the Wesley impactor was more “rocky” and less icy
than material from the previous SL9 impacts. Using ultraviolet observations
measured by the Hubble Space Telescope (HST), Hammel et al. [2010] noted that
the 2009 debris field was less diffuse and shrank faster than comparable-sized
SL9 fields, which implied the Wesley impactor material was heavier and denser
and therefore sank faster in the atmosphere than the SL9 material. While
strong, stratospheric heating was observed for several days following the SL9
impacts (see above), no such heating occured in the aftermath of the Wesley
impact, as evidenced by the lack of any enhanced, stratospheric CH4 emissions
(e.g. Fletcher et al. 2010, Orton et al. 2011). This was interpreted as the
impactor material being sufficiently heavier/denser such that the plume did
not rise as high as the upper stratosphere. Enhanced, stratospheric C2H6
emissions were observed [Fletcher et al., 2010], which indicated an increase
in its abundance within the impact region. An enhancement of C2H6, in contrast
to oxidized products such as H2O and CO, was more consistent with a “dry”
impactor with a higher C/O ratio [Zahnle, 1996], as expected for an asteroid.
From the discovery images of the 2009 impact, Sánchez-Lavega et al. [2010]
inferred an impact trajectory elevation angle of 20∘ $\pm$ 5∘, much shallower
than the 45∘ angle of SL9. Assuming ballistic trajectories, similar to the
approach of Pankine and Ingersoll [1999], and comparing the visible debris
field with the “intermediate” SL9 fragments, they estimated the impactor size
as 0.5-1 km. These measurements left few constraints on the trajectory of the
impactor, but Orton et al. [2011] examined a suite of possible trajectories
and determined that (a) an asteroidal origin for the impactor was possible,
and (b) of the most likely sources of asteroids ejected into Jupiter-
encountering orbits - Trojans (Levison et al. 1997, Duncan et al. [2004]) or
Main-Belt Hildas [Di Sisto et al., 2005] \- that the Hilda family was more
likely.

A further contrast between the two impact events was the detection of a
9.1-$\upmu$m non-gaseous feature in the Wesley impact region, attributed to
crystalline silicas such as quartz and cristobalite [Orton et al., 2011,
Fletcher et al., 2011] and its (apparent) absence from the SL9 impact.
Stringent upper limits of silica of 0.5% in abundance have been derived from
high signal-to-noise ratio spectra of comets, including comets 9P/Tempel 1,
C/Hale-Bopp 1995 O1 [Lisse et al., 2007a] and 17P/Holmes [Reach et al., 2010].
Similarly, there is no spectroscopic evidence of silica on asteroids or
Trojans (e.g. Emery et al. 2006, Vernazza et al. 2010). The presence of
silicas in the residue of the Wesley object was therefore interpreted to have
been formed during the impact by the high-pressure ($>$104 bar) and/or high-
temperature ($>$ 1500 K) alteration of ferromagnesian silicates, as seen in
terrestrial studies of tektites, silica magmas and rock fulgurites [Lisse et
al., 2009]. However, such temperatures and pressures were achieved during the
SL9 impacts (e.g. Mac Low and Zahnle 1994, Deming and Harrington 2001,
Korycansky et al. 2006). Amorphous olivine and amorphous pyroxene (typically
found in equal abundances in comets) would have been readily converted to
silicas and so the apparent absence of silicas from the SL9 impacts was
puzzling.

Overall, contrasts between the SL9 and Wesley impacts led to the inference
that the Wesley impactor must be different in composition and/or origin from
SL9. However, previous observations of the SL9 and Wesley impacts employed
different methods for data reduction, calibration and radiative transfer
calculations and so previous comparisons between the events were not
immediately robust. We were therefore motivated to re-examine and
quantitatively compare both impacts using consistent methods. In this work, we
perform an analysis of Earth-based, mid-infrared observations recorded
following the impacts of SL9’s fragments in July 1994 and the Wesley impact in
July 2009.

##  2 Observations

###  2.1 SL9 impact observations in 1994

####  2.1.1 IRTF-MIRAC

Figure 1: IRTF-MIRAC images recorded on 1994 July 20. The filter wavelength,
time of observation (UT) and central meridian longitude (System III) are
included in each panel. The aftermath of SL9 impacts are evident as bright
regions on the disk, particularly at 7.85, 10.50 – 11.25 and 17.20 $\upmu$m.
Figure 2: As in Figure 1 but using images recorded on 1994 July 21.  Figure 3:
a) IRTF-MIRAC spectrophotometry of the SL9 impact site L (red points) and
background atmosphere (blue points) on 1994 July 20, b) the fractional
residual between the impact site and background (Equation 1). Similar results
are shown for SL9 impact G on the same date (c, d) and SL9 impact K on 1994
July 21 (e, f). Horizontal error bars represent the full width at half maximum
(FWHM) of the given filter. No Q band images were recorded on 1994 July 21.

Spectrophotometric images from 7.85 to 20.5 $\upmu$m were measured using the
Mid-Infrared Array Camera, MIRAC [Hoffmann et al., 1998] on NASA’s 3-m
Infrared Telescope Facility (IRTF). The data were reduced, using standard
object-minus-sky (A-B) subtraction and flat-fielding steps. Absolute
calibration was performed by scaling the images to convolved, mid-infrared
spectra recorded by the Voyager IRIS and Cassini CIRS instruments, following
the approach used by Fletcher et al. [2009b]. The noise-equivalent radiances
(NESR) of the images were calculated by determining the standard deviation of
sky pixels more than 5” outside of Jupiter’s limb.

We concentrated on observations of the impact sites of the G, K and L
fragments, as they were bright and had the best wavelength coverage. Images
recorded on 1994 July 20 captured the G and L impacts approximately 41 - 43
hours and 2 - 4 hours after their predicted times of impact, respectively.
Images on July 21 captured the K impact 18 - 20 hours after the predicted time
of impact. Figures 1 and 2 show the images recorded on July 20 and 21 and
Table 1 further details each image and the time elapsed since the G, K and L
impacts.

Date |  Instrument |  Time |  $\lambda$ |  Airmass |  Longitude |  $\Delta t_{G}$ |  $\Delta t_{K}$ |  $\Delta t_{L}$
---|---|---|---|---|---|---|---|---
1994-Jul-20 |  IRTF/MIRAC |  (UT) |  ($\upmu$m) |  |  range |  (hrs) |  (hrs) |  (hrs)
|  |  00:56 |  17.2 |  2.34 |  264 - 64 |  41.34 |  - |  2.57
|  |  00:59 |  17.9 |  2.29 |  265 - 65 |  41.39 |  - |  2.62
|  |  01:00 |  20.80 |  2.27 |  266 - 66 |  41.41 |  - |  2.63
|  |  01:15 |  10.5 |  2.05 |  275 - 75 |  41.66 |  - |  2.88
|  |  01:18 |  11.25 |  2. |  277 - 77 |  41.71 |  - |  2.93
|  |  01:20 |  12.2 |  1.98 |  278 - 78 |  41.74 |  - |  2.97
|  |  01:22 |  12.5 |  1.96 |  279 - 79 |  41.78 |  - |  3.
|  |  01:26 |  13.3 |  1.90 |  78 - 278 |  41.84 |  - |  3.07
|  |  01:28 |  13.6 |  1.89 |  77 - 277 |  41.88 |  - |  3.10
|  |  01:33 |  13.0 |  2.15 |  74 - 274 |  41.96 |  - |  3.18
|  |  01:51 |  8.57 |  2.11 |  63 - 263 |  42.26 |  - |  3.48
|  |  01:55 |  10.74 |  2.02 |  61 - 261 |  42.33 |  - |  3.55
|  |  02:18 |  7.85 |  2.19 |  47 - 247 |  42.71 |  - |  3.93
1994-Jul-21 |  IRTF/MIRAC |  01:48 |  10.74 |  2.44 |  86 - 246 |  - |  15.28 |  -
|  |  02:07 |  8.57 |  2.26 |  97 - 257 |  - |  15.6 |  -
|  |  03:09 |  10.50 |  1.30 |  135 - 295 |  - |  16.63 |  -
|  |  03:19 |  11.25 |  1.26 |  141 - 301 |  - |  16.8 |  -
|  |  03:23 |  12.50 |  1.25 |  143 - 303 |  - |  16.86 |  -
|  |  03:24 |  13. |  1.57 |  143 - 303 |  - |  16.88 |  -
|  |  03:43 |  8.00 |  1.22 |  155 - 315 |  - |  17.2 |  -
|  |  03:47 |  8.20 |  1.21 |  157 - 317 |  - |  17.26 |  -
|  |  03:50 |  9.00 |  1.21 |  159 - 319 |  - |  17.31 |  -
|  |  03:52 |  9.30 |  1.20 |  160 - 320 |  - |  17.35 |  -
|  |  03:55 |  9.70 |  1.20 |  162 - 322 |  - |  17.4 |  -
|  |  03:58 |  10.20 |  1.20 |  164 - 324 |  - |  17.45 |  -
|  |  04:01 |  11.70 |  1.20 |  166 - 326 |  - |  17.5 |  -
|  |  04:05 |  13.30 |  1.19 |  168 - 328 |  - |  17.56 |  -
|  |  04:09 |  13.60 |  1.19 |  171 - 331 |  - |  17.63 |  -
|  |  05:27 |  12.20 |  1.19 |  218 - 378 |  - |  18.93 |  -
|  |  05:31 |  10.30 |  1.20 |  220 - 380 |  - |  19.0 |  -
|  |  06:06 |  7.85 |  1.28 |  241 - 401 |  - |  19.58 |  -
1994-Jul-22 |  Palomar/SC-10 |  01:49 |  8 - 13.5 |  1.64 |  308 - 35 |  90.23 |  - |  -
Table 1: Details of the observations examining the SL9 G, K and L impact
sites. $\Delta t_{G}$, $\Delta t_{K}$ and $\Delta t_{L}$ denote the time
elapsed (in hours) since impacts G, K and L.

From each image recorded on July 20, a broadband spectrum of the L impact was
derived by calculating the mean radiance and emission angle between 35-45∘S
(planetocentric) and 330-343∘W (System III). The uncertainty on the mean
radiance was calculated as the larger of: 1) the standard deviation on the
mean or 2) the NESR scaled by n${}_{pixel}^{0.5}$, where npixel is the number
of diffraction-resolved pixels averaged. A similar broadband spectrum of the G
impact site was extracted from the July 20 images at same latitude range but
longitudes from 23 – 32∘W. Using the images recorded on July 21, a broadband
spectrum of the K impact was similarly extracted using longitudes from
260-275∘W in the same latitude range.

We considered several methods for calculating the radiance of the unperturbed
“background” atmosphere from each image in order to quantify the enhancement
in emission due to the impacts. Ultimately, we chose to derive this background
“off-impact” radiance by calculating the mean radiance within the same
longitude range of the impacts (see above) but over a latitude range of
28-38∘N. The chosen latitude range was determined such that the radiances of
the impact site and the “background” unperturbed site were as similar as
possible in emission angle, such that both observations sound a similar
altitude level in Jupiter’s atmosphere and such that foreshortening effects
are removed when computing a ratio between the two. We chose not to calculate
the background atmosphere using unperturbed areas within the same 35-45∘S
latitude band. By July 20 and July 21, over 9 individual fragments of SL9 had
impacted the atmosphere and it was near-impossible to extract radiances of an
unperturbed region at a similar emission angle as the impact site. While MIRAC
images were recorded on July 14 before the SL9 impacts, we chose not to use
these images to calculate the background atmosphere for the following reasons.
Firstly, the images on July 14, July 20 and 21 captured different longitude
ranges of Jupiter and thus the sites of the future impacts were either not
sampled on July 14 or recorded at very different emission angles compared to
the July 20 - 21 images. Secondly, images on July 19/20 were recorded in
several CVF (circular-variable filter) positions between 8 and 12 $\upmu$m,
which were not recorded on July 14 images. Limiting the study of the K and L
impact sites in July 19-20 images only to the filters used on July 14 would
remove measurements at key wavelengths where non-gaseous species have spectral
features.

In order to quantify the enhancement in emission due to impact, we calculated
the fractional residual, $f_{r}$, between radiances measured over the impact
region, $R_{i}$, and the off-impact/background region, $R_{b}$, as defined in
Equation 1.

$f_{r}=\frac{R_{i}-R_{b}}{R_{b}}$ (1)

Figure 3 shows the broadband spectra of the G, L and K impact sites, the
background atmosphere as detailed above, and the fractional residual between
them (Equation 1). As noted in previous work, the site of the L impact is
enhanced at 7.85 $\upmu$m due to CH4 emissions enhanced by stratospheric
fallback heating. Marginal enhancements between 12 and 13 $\upmu$m are noted
and are attributed to emissions of C2H6 and C2H2, which are also enhanced by
stratospheric heating and possibly by enhancements to their abundances. In
both the K and L impacts, the 8.5 – 11.5 $\upmu$m range is enhanced due, in
part, to emissions from gaseous NH3 lofted into the stratosphere as well as
non-gaseous compounds produced from the cometary debris upon impact.
Significant enhancements in emission are also observed at 18 - 19 $\upmu$m for
the G and L impacts, whereas impact and background radiances at 20.5 $\upmu$m
agree within uncertainty. As far as we are aware, there are no gases in
Jupiter’s atmosphere that have significant spectral features at this
wavelength and so we attribute this feature to non-gaseous compounds produced
from the cometary debris. For the G and K impact, radiances at 7.85 $\upmu$m
and 12 – 14 $\upmu$m agree with background radiances within uncertainty, which
indicates less stratospheric heating or chemical alteration relative to the L
impact site. This suggests that the G and K fragments were smaller than the L
impactor fragment and therefore produced a smaller atmospheric response.
Alternatively, the G and K impact sites had evolved and cooled in the 18-20
and 41-43 hours between the predicted times of their impacts and their
respective measurements. In contrast, the site of the L impact was observed
only 2 – 4 hours after the predicted impact time.

####  2.1.2 Palomar/SpectroCam-10

Figure 4: The spectral-spatial image of the Palomar/SC-10 observation recorded
on 1994 July 22. Spatial pixels 0 – 50 capture the unperturbed atmosphere
west-to-east in the $\sim$44∘S latitude band , the enhanced emissions of SL9
impact G are evident near the limb at spatial pixels 55-60 and dark sky off
planet is captured from spatial pixels 62 – 70.

We also examined spectroscopic measurements recorded on 1994 July 22 by the
SpectroCam-10 (henceforth ‘SC-10’) instrument [McGhee, 2000] at the 200-inch
(5.1 m) Hale telescope at the Palomar Observatory. These spectra capture the
site of the SL9 G impact approximately 90 hours after the predicted time of
impact. Table 1 provides further details of the Palomar/SC-10 observations
recorded.

We did not examine spectra from the R-impact site, as they were largely
dominated by blackbody emission with fewer identifiable spectral features and
have already been presented by Nicholson et al. [1995]. Spectra were obtained
using the 1x15” slit with the slit length oriented parallel to Jupiter’s
equator and covering the region of peak brightness due to the impact. The slit
position also captured 5 – 6 sky pixels off Jupiter from which the standard
deviation was calculated to derive the noise-equivalent radiance. Each
exposure was reduced by subtracting an off-Jupiter exposure. Bad pixels in the
image were removed by interpolation. A slight tilt of the spectral lines in
the images was corrected such that they were aligned vertically in each image.
The wavelength grid was derived using the known wavelength of telluric ozone.
Figure 4 shows the resulting spectral-spatial image.

Spatial registration and viewing geometries of each pixel were calculated by
identifying the location of Jupiter’s limb in the spectral-spatial image
(Figure 4), the known latitude of the G impact (44∘S), the 0.25” pixel scale
of the instrument and the sub-observer latitude and longitude at the time of
observation determined by JPL Horizons.

Initially, the absolute calibration was performed using Callisto, just as for
the R-impact observations reported by Nicholson et al. [1995]. However, this
resulted in radiances (even away from the impact) that were factors of 3 – 4
higher than typical values for Jupiter. We suggest differences in atmospheric
seeing between the measurements of Jupiter and Callisto could account for
these unphysically-large radiances. In order to calibrate the Palomar/SC-10
data in absolute radiance, we instead scaled the observed radiances to CIRS
(Composite Infrared Spectrometer, Kunde et al. 1996) spectra of Jupiter
recorded during the 2000-2001 flyby. Using the 2.5 cm-1 resolution (0.025
$\upmu$m resolution at 10 $\upmu$m) CIRS data, we computed a mean spectrum
between 40-45∘S over all sampled longitudes. The CIRS spectrum was then
convolved down to the coarser 0.1 $\upmu$m (10 cm-1) resolution of
Palomar/SC-10. We coadded Palomar/SC-10 spectra over 10 spatial pixels
centered over the central meridian, and away from the impact region, which
resulted in a mean emission angle similar to the CIRS spectrum. A calibration
scale factor was derived by computing a ratio of the mean CIRS radiance to the
mean Palomar count from 8.9 to 13 $\upmu$m, and subsequently applied to all
Palomar radiances.

Figure 5: The top panel shows Palomar/SpectroCam-10 spectra of the SL9 G
impact site (red) and an unperturbed region in the same latitude band (blue).
The lower panel shows the fractional residual between the two spectra. Note
the difference in emission angles of the two observations.

A coadded spectrum of the G impact site was computed by averaging the spectra
obtained in spatial pixels 53 – 56 (approximately longitudes of 23-32∘W),
which resulted in a mean emission angle of 73∘ ($\upmu$ $\sim$ 0.3, where
$\upmu$ is the cosine of the emission angle). The 15” slit of SC-10 samples
dark sky off the eastern limb and the 44∘S latitude band east of the central
meridian. Unfortunately, no spectra were measured at $\upmu$ $\sim$ 0.3 west
of the central meridian, which would allow a spectrum of the background
atmosphere to be computed at a similar viewing geometry as the G impact site
for comparison. We coadded spectra over spatial pixels 40 – 50 (or longitudes
of 0 – 18∘W) as a compromise between averaging a sufficient number of spectra
to increase the effective signal-to-noise ratio and achieving a mean emission
angle ($\upmu$ $\sim$ 0.5) as similar as possible to the impact spectrum. We
anticipate the difference in emission angle between the impact and non-impact
region may introduce an offset in computing the fractional residual since
different emission angles sound slightly different levels of the atmosphere
(at different temperatures). Nevertheless, we continued to analyze the spectra
such that a spectroscopic analysis of both SL9 and Wesley impacts could be
performed.

Figure 5 shows the Palomar spectra of the SL9 G impact site, background
atmosphere and the fractional residual (Equation 1) between the two locations.
As observed by MIRAC at other SL9 sites (Figure 3), the G impact site is
enhanced in CH4 emission at $\sim$8 due to stratospheric heating and between 9
– 11 $\upmu$m due to NH3 gas lofted in to the stratosphere and non-gaseous
compounds produced from the cometary material.

###  2.2 Wesley impact observations in 2009

Date |  Instrument |  Time |  Spectrum |  Airmass |  CML
---|---|---|---|---|---
2009-Jul-24 |  Gemini-S/T-ReCS |  04:25 |  N1 |  1.176 |  278
|  |  04:31 |  N2 |  1.160 |  281
|  |  04:38 |  N3 |  1.145 |  285
|  |  04:45 |  N4 |  1.132 |  290
|  |  04:51 |  N5 |  1.119 |  293
|  |  04:57 |  N6 |  1.108 |  297
|  |  05:05 |  N7 |  1.097 |  302
|  |  06:35 |  Q1 |  1.041 |  356
|  |  06:41 |  Q2 |  1.042 |  360
|  |  06:48 |  Q3 |  1.045 |  4
|  |  06:55 |  Q4 |  1.048 |  8
|  |  07:01 |  Q5 |  1.052 |  12
|  |  07:08 |  Q6 |  1.057 |  16
Table 2: Details of the observations capturing the aftermath of the Wesley
impact. Spectra beginning with ‘N’ and ‘Q’ are N-band (7.5 - 13 $\upmu$m) and
Q-band (17 - 25 $\upmu$m) spectra, respectively. ‘CML’ denotes the central
meridian longitude at the time of observation in System III.  Figure 6: T-ReCS
N band (a) and Q band (b) coadded spectra of the Wesley impact (red spectra)
and a background region for comparison (blue) as recorded on 2009 Jul 24. The
cosine of emission angle of the spectra ($\upmu$) are shown. Panels (c) and
(d) show similar spectra except where individual observations were omitted
such that the coadded spectra of the impact and background were similar as
possible in emission angle. Panels (e) and (f) show the fractional residual
(Equation 1) between the spectra shown in panels (c) and (d).  Figure 7:
Comparison of the fractional residuals of the SL9 and Wesley impact sites from
all datasets. The top panels shows the 7.5 - 25 $\upmu$m range, the bottom
panel focuses on the 7.5 - 13 $\upmu$m range. The fractional residuals have
been normalized to unity for ease of comparison.

Low-resolution (R = 100) N-band (7.5-13 $\upmu$m) and (R = 80) Q-band (17 – 25
$\upmu$m) spectra were measured using the Thermal-Region Camera Spectrograph,
T-ReCS, on the 8-m Gemini South Telescope on 2009 July 24, approximately four
(Earth) days or $\sim$10 Jupiter rotations after the effects of the impact
were first observed. The slit of the spectrograph was aligned parallel to
Jupiter’s equator, centered over the latitude of the Wesley impact
($\sim$60∘S), and measurements were repeated while Jupiter rotated. 7 N-band
and 6 Q-band measurements were recorded over the course of the night,
providing multiple emission angle spectra of the impact site (with the core at
$\sim$304∘W) as it rotated across Jupiter’s disk as well spectra of the
unperturbed atmosphere in the same latitude band. The measurements are
detailed in Table 2. Spectra were also measured of the Cohen standard HD216032
[Cohen et al., 1999], which were used to perform the nominal, absolute
calibration of the Jupiter spectra. These spectra were previously analyzed and
presented by Fletcher et al. [2011] and Orton et al. [2011]. Initially, they
found that the N- and Q-band spectra could not be fit simultaneously with a
sensible atmospheric model, which was attributed to the wavelength-dependent
effects of telluric contamination. In order to resolve this, Fletcher et al.
[2011] first calculated the ratio between spectra of the impact and background
atmosphere such that telluric contamination is removed. Second, they computed
a series of synthetic spectra from 7 - 25 $\upmu$m over a range of emission
angles using an atmospheric model derived from Cassini-CIRS (Composite
Infrared Spectrometer, Kunde et al. 1996) measurements during the 2001 flyby
[Fletcher et al., 2009a]. The impact-to-background ratios were then applied to
synthetic spectra (at the appropriate emission angle) to compute absolute
spectra in radiance units without telluric artefacts. Further details of this
technique are provided in Section 4.1 of Fletcher et al. [2011]. We likewise
adopt and re-analyze this corrected version of the T-ReCS spectra in this
work.

In inverting the T-ReCS spectra, Fletcher et al. [2011] used the multi
emission angle observations of the same location to improve the vertical
sensitivity of retrieved atmospheric parameters. In this work, we instead
adopt a single, average spectrum such that the treatment of spectra for the
SL9 and Wesley impacts are similar as possible. Nominally, spectra of the
impact and background atmosphere were computed by coadding individual spectra
recorded at respective longitude ranges of 299 -307∘W and 316-325∘W, as shown
in Figure 6a-b. While using all available spectra within these longitude
ranges results in a higher signal-to-noise ratio, they capture the impact site
and background atmosphere at very different mean emission angles. For a direct
comparison of the spectra between impact site and background atmosphere, we
repeated the coaddition of spectra over the same longitude ranges but omitted
a subset of individual spectra such that the impact and background spectra
were similar in mean emission angle. These are shown in Figure 6c-d and the
fractional residual between them are shown in Figure 6e-f.

The atmosphere at the site of the Wesley impact is enhanced in emission from
8.8 to 12 $\upmu$m and 17 – 19 $\upmu$m in comparison to the unperturbed
atmosphere. Again, the 8.8-to-12 $\upmu$m feature was nominally attributed to
some mixture of stratospheric NH3 gas (in emission) and non-gaseous material.
The 17 – 19 $\upmu$m feature was also attributed to non-gaseous material. In
Section 3.1, we adopt the fractional residual computed from spectra similar in
emission angle (Figure 6c-f) as an emissivity spectrum of the impact region.

For the purpose of comparing relative wavelength dependence between the
impacts, the fractional residuals for each dataset were normalized, as shown
in Figure 7. This higlights two main contrasts between the SL9 impacts and the
Wesley impact. First, the SL9 impact sites exhibited enhanced 7 – 8 $\upmu$m
stratospheric emissions whereas the Wesley impact site was not enhanced in CH4
emissions outside of uncertainty, which is in agreement with previous work
(e.g. Orton et al. 2011,Fletcher et al. 2011). Second, the broad N-band
feature extends from 8.8 to 13 $\upmu$m whereas for SL9, the broad feature
extends to shorter wavelengths of $\sim$8.5 $\upmu$m. As we demonstrate in
Section 3.1, the N-band feature extends to shorter wavelengths in the SL9
impact due to the presence of obsidian (glassy silica) and its absence from
the Wesley impact spectrum.

##  3 Analysis & Results

We performed two separate analyses to determine the species responsible for
the observed emission features in the SL9 and Wesley impact sites. Each
analysis has advantages and disadvantages over the other. First, the
fractional residual between the spectra of the impact region and a region away
from the impact was calculated and then normalized. The normalized fractional
residual spectrum was then modelled by performing a least-squares search over
a grid of candidate mineral species with varying abundances. In this part of
the analysis, gaseous NH3 in Jupiter’s stratosphere was also treated as a
“mineral” and included in the model grid search. The advantage of this method
is that gaseous NH3 and non-gaseous mineral species are constrained
simultaneously. The disadvantage is that the calculation of the fractional
residual assumes the temperature of the impact region and off-impact region
are similar and were measured at a similar emission angle. In the Wesley
impact, negligible stratospheric heating was observed over the impact region
(Figure 6, Fletcher et al. 2011) and both regions were sampled at a range of
emission angles, which allowed spectra similar in emission angle to be
calculated. However, for SL9, strong stratospheric heating was observed over
the impact regions and it was impossible to sample the impact and off-impact
region at similar emission angles. Thus, the assumption of similar temperature
and emission angle is valid for the Wesley impact but is a poor assumption for
the SL9 impact. This analysis is detailed further in Section 3.1.

In the second approach, we inverted the spectra using the NEMESIS radiative
transfer code [Irwin et al., 2008]. The vertical profiles of temperature and
stratospheric hydrocarbons were allowed to vary and retrievals were performed
over a grid of NH3 vertical profiles. The combination of parameters producing
a synthetic spectrum that minimized the goodness-of-fit was interpreted to
represent the gaseous component of the spectrum. Wavelengths where the
synthetic spectra did not adequately fit the observed spectra were interpreted
to result from non-gaseous emission from impactor material and the wavelength
dependence of the non-gaseous emission was compared with the absorptance
spectra of several candidate mineral species. Unlike Fletcher et al. [2011],
we did not attempt to include mineral species in the radiative transfer
inversion to avoid introducing degenerate parameters such as aerosol size
distribution and vertical profiles to an already large parameter space. The
advantage of this method is that the effects of temperature and emission angle
on the planet are fully characterized, a range of vertical profiles of NH3 can
be tested, and absolute temperatures and NH3 concentrations are constrained.
The disadvantage is that the inversion could misinterpret non-gaseous emission
as gaseous emission. This analysis is detailed further in Section 3.2.

###  3.1 Mineralogical dust modelling

Mineralogical spectral modeling was performed by adopting a collection of
candidate mineralogical species, performing a grid search to find a
combination of species that produced a model emissivity spectrum that
minimized the goodness-of-fit ($\chi^{2}/n$, where $n$ is the number of
degrees of freedom) to the emergent emissivity spectra of both impacts (Figure
5 and 6). For an optically thin refractory residue, a composite spectrum can
be written as the combination of the contributions from individual species
[Lisse et al., 2009], as in Equation 2

$F_{v}=\frac{B_{\nu}[T_{atm}]}{\Delta^{2}}\sum_{i}\int_{a_{min}}^{a_{max}}Q_{\mbox{abs},i}(a,\lambda)\pi
a^{2}\frac{dn_{i}(r)}{da}da$ (2)

where $\Delta$ is the distance between the observer and the dust, $B_{\nu}$ is
the Planck function at wavelength $\lambda$ and atmospheric temperature
$T_{atm}$. $a$ is the particle radius, $r$ is the distance from the Sun,
$Q_{abs,i}$ is the emission efficiency of the ith species and $dn/da$ is the
differential particle size distribution as a function of radius, $a$. The
emitted flux depends on the composition (location of spectral features) and
the particle size (feature to continuum contrast). The particle size
distribution was assumed to be a power law or power law reduced at small
particle sizes to model radiation-pressure blowout effects. For the candidate
mineralogical species, we nominally considered $\sim$100 species that have
previously been used to fit the mid-infrared spectra of dust produced by
comets, asteroids and jovian trojans in our solar system and by exocomets,
exoasteroids, and aggregating planetesimals in other nearby star systems
[Lisse et al., 2006, 2007a, 2007b, 2008, 2009, 2012, 2017], including silica-
rich hyper velocity impact systems like HD172555 [Lisse et al., 2009, 2020].
These species include multiple amorphous silicates with olivine-like and
pyroxene-like composition; multiple ferromagnesian silicates (forsterite,
fayalite, clino- and ortho-pyroxene, augite, anorthite, bronzite, diopside,
and ferrosilite); silicas, both amorphous and crystalline (obsidian, tektites,
quartz, cristobalite, tridymite); phyllosilicates (such as saponite,
serpentine, smectite, montmorillonite, and chlorite); sulfates (such as
gypsum, ferrosulfate, and magnesium sulfate); oxides (including various
aluminas, spinels, hibonite, magnetite, and hematite); Mg/ Fe sulfides
(including pyrrohtite, troilite, pyrite, and ningerite); carbonate minerals
(including calcite, aragonite, dolomite, magnesite, and siderite); water-ice,
clean and with carbon dioxide, carbon monoxide, methane, and ammonia
clathrates; carbon dioxide ice; graphitic and amorphous carbon; and the
neutral and ionized polycyclic aromatic hydrocarbon (PAH) emission models of
Draine and Li [2007]. The Supplementary material of Lisse et al. [2006]
details the sources of absorption spectra of the aforementioned species.
However, for both impacts in this study, the wavelengths at which non-gaseous
spectral features were and were not observed (Figure 7) ruled out the large
majority of these nominal species. Ultimately, we only considered the
mineralogical species listed in Table 3.

Figure 8: (a) A family of NH3 vertical profiles with varying fractional scale
heights, as indicated in the legend, to parameterize stratospheric NH3. Panel
(b) shows the corresponding forward model spectra at T-ReCS/SC-10 spectral
resolution using the same color conversion. Panel (c) shows seven NH3
emissivity spectra computed by finding the fractional residual between two
forward model spectra, as detailed by the legend. For example, spectrum ‘v1’
was computed by finding the residual between the forward model spectra using
fractional scale heights of 0.9 and 0.2.

Since gaseous NH3 lofted into the stratosphere has a significant contribution
to the observed spectral features, we also included and treated gaseous NH3 as
a “mineral” in our spectral fitting. In order to compute a emissivity spectra
of NH3, we forward modelled spectra of Jupiter with a range of vertical NH3
profiles and computed their fractional difference. Section 3.2 provides
further details of the radiative transfer model and atmospheric model of
Jupiter used to compute forward model spectra. The nominal, vertical NH3
profile was retrieved from Cassini-CIRS spectra at 45∘S on Jupiter and assumes
NH3 is well mixed with a volume mixing ratio of 360 ppmv at pressures higher
than 890 mbar, and then decreases in abundance at lower pressures according to
a fractional scale height of 0.21 (see A for further details). In order to
parameterize NH3 gas lofted into the stratosphere, we adopted the approach
presented by Fletcher et al. [2011], where the NH3 profile, $q$, is set to the
CIRS-derived profile, $q_{CIRS}(p)$ at pressures higher than a cutoff
pressure, $p_{0}$, and then a decrease in abundance with altitude quantified
by a fractional scale height, $f$. This is quantified in Equation 3 below.

$\begin{split}q(p\geq p_{0})&=q_{CIRS}(p)\\\
q(p<p_{0})&=q_{0}\left(\frac{p}{p_{0}}\right)^{\frac{1-f}{f}}\end{split}$ (3)

Species | SL9 G (1994) | Wesley (2009)
---|---|---
|  % |  $\Delta\chi^{2}$ |  % |  $\Delta\chi^{2}$
Amorphous Olivine |  0.3 |  16.6 |  0.65 |  27.3
Amorphous Pyroxene |  0 |  0 |  0 |  0
Obsidian |  0.32 |  1.3 |  0 |  0
Fosterite |  0.03 |  0.04 |  0.01 |  0
Fayalite |  0 |  0 |  0 |  0
Diopside |  0.08 |  0.1 |  0 |  0
Ferrosite |  0 |  0 |  0.07 |  0.06
Ortho Enstatite |  0 |  0 |  0.02 |  0.01
Australite Tektite |  0 |  0 |  0 |  0
Bediasite Tektite |  0.03 |  0.2 |  0 |  0
MgFeS |  0 |  0 |  0 |  0
PAH |  0 |  0 |  0 |  0
NH3 (g) |  0.2 |  1.0 |  0.24 |  0.7
Amorphous Carbon |  0 |  0 |  0 |  0
H2O (s) |  0.02 |  0.1 |  0 |  0
$\chi^{2}/n$ | 1.1 | 0.91
Table 3: The list of species included in the spectral fitting, the derived
proportion of each mineral (%) and the change in the reduced $\chi^{2}$
($\Delta\chi^{2}$) when that species is omitted from the spectral fitting.
Results for the SL9 G impact site are shown in the middle column and for the
Wesley impact in the right-hand column.  Figure 9: The observed fractional
residual of SL9 Impact G using Palomar/SC-10 spectroscopy and the best-fitting
total mineralogical spectrum (solid grey). The top panel shows the full
wavelength range, the lower panel focuses on the 8 - 13 $\upmu$m range. The
individual species that contribute to the total mineralogical curve (see Table
3) are shown and colored according to the legend. Areas covered by 45∘-lines
denote the wavelengths omitted in performing the mineralogical fitting due to
contamination by telluric O3 or jovian, stratospheric (gaseous) CH4 emission).
Figure 10: The observed fractional residual of the Wesley impact using
Gemini-S/T-ReCS spectroscopy and the best-fitting total mineralogical spectrum
(solid grey). The top panel shows the full wavelength range, the lower panel
focuses on the 8 - 13 $\upmu$m range. The individual species that contribute
to the total mineralogical curve (see Table 3) are shown and colored according
to the legend. Areas covered by 45∘-lines denote the wavelengths omitted in
performing the mineralogical fitting due to contamination by telluric O3 or
jovian, stratospheric (gaseous) CH4 emission).

$q_{0}$ is the abundance of NH3 at pressure, $p_{0}$, $f$ is a fractional
scale height (between 0 and 1). For the purposes of computing emissivity
spectra of NH3, we assumed $p_{0}$ = 330 mbar and varied $f$ between 0.2 and
1.0 in increments of 0.05. However, we note to readers that the vertical
profile of NH3 in the SL9 impact regions may be more complex than the power
law parameterization we adopted here.

Figure 8a, b show the resulting vertical profiles of NH3 and corresponding
forward model spectra at T-ReCS/SC-10 spectral resolution. We computed a range
of NH3 emissivity spectra by calculating the fractional residual between
different combinations of forward model spectra, as shown in Figure 8c. The
goal was to produce a range of spectra with varying strengths of the stronger,
10.3- and 10.8-$\upmu$m lines with respect to the weaker lines between 9 - 10
$\upmu$m and 11 - 12 $\upmu$m. In modeling the emission spectra of the SL9 and
Wesley impacts, each emissivity spectrum was adopted in turn, and scaled by a
factor at all wavelengths. We found that emissivity spectrum v2 (the
fractional residual between the forward models using fractional scale heights
of 1.0 and 0.55) yielded the best fits to the Palomar/SC-10 spectra of the SL9
G impact, whereas v6 yielded the best fits to the Gemini/T-ReCS spectra of the
Wesley impact.

After testing the spectral mineralogical fitting with several different size
distributions, we found that a single power law of $dn/da$ $\sim$ $a^{-4.20}$
optimized the fit to the Palomar/SC-10 spectra of the SL9 G impact whereas a
power law index of -4.30 best reproduced the Gemini-S/T-ReCS spectra of the
Wesley impact. Both are extremely steep, as expected for impact-produced dust
populations dominated by small, sub-micron- to micron-sized particles [Lisse
et al., 2009, Takasawa et al., 2011, Johnson et al., 2012]. While T-ReCS
spectra of the Wesley impact recorded both the N and Q band, Palomar spectra
of the SL9 impact only recorded the N band. In order for the analysis to be as
consistent as possible for both impacts, we included the IRTF-MIRAC broadband
17.2-, 17.9 and 20.8-$\upmu$m measurements of the SL9 G impact (Figures 3) in
the fitting of the Palomar spectra. The MIRAC measurements were scaled by a
factor of 4 such that the difference in fractional residual between the
Palomar N-band and MIRAC Q-band measurements was similar as the difference in
fractional residual between the MIRAC N- and Q-band measurements.

Finally, for both impacts, we omitted wavelengths 9.7 - 9.9 $\upmu$m from the
spectral fitting since these wavelengths are obscured by telluric ozone. For
Palomar/SC-10 spectra of SL9, wavelengths shorter than $\sim$8.5 $\upmu$m
contain strong stratospheric CH4 emission, which we did not want to be
interpreted as non-gaseous emission by the spectral fitting. For the
Gemini/T-ReCS spectrum of the Wesley impact, we found that including the 8.0 -
8.7 $\upmu$m drove unphysical solutions for mineralogical compositions. In
order to analyze both impacts/observations as consistently as possible, we
chose to omit wavelengths shorter than 8.7 $\upmu$m in the fitting of both
observations.

Table 3 details the best-fitting compositions of minerals for each impact and
Figures 9 and 10 compare the fractional residuals of both impacts with the
best-fitting mineralogical composition. For the SL9 G impact, we found that
the main components producing significant emissivities were amorphous olivine,
silica (in the form of obsidian), and gaseous NH3 in the stratosphere. A small
amount of the water ice, diopside, and forsterite typically found in comets
was present in the D/SL9 case, but their absence from the synthetic spectrum
had only a small, statistically-insignificant effect on the reduced
$\chi^{2}$. We found no evidence of amorphous pyroxene in the D/SL9 case,
which was initially unexpected since comets typically have a $\sim$1:1 ratio
of pyroxenaceous to olivinaceous species (e.g. Lisse et al. 2006, 2007b). As
we discuss in Section 4, we suggest that all the cometary pyroxene was
converted into silicas by the strong heating ($>$ 1000 K) associated with the
impact. A similar finding was seen in the HD172555 exodisk system by Lisse et
al. [2009].

By contrast, the Wesley impact emission appears composed of almost pure
amorphous olivine, with some ferromagnesian pyroxenes mixed in at an $\sim$8:1
ratio, together with gaseous NH3. No emission due to silica is apparent in our
new reduction and spectral analysis, a marked difference from the findings of
Fletcher et al. [2011]. Without any evidence of silica, it is impossible to
understand this mixture of materials being formed at high temperatures. In
order to explain the conversion of the usually dominant crystalline olivine
into amorphous olivine, we suggest that the Wesley residue was formed at low
temperatures and high pressures, as discussed further in Section 4.

Figures 9 and 10 also demonstrate the a posteriori fit of the best-fitting
mineralogical composition to the 8.0 - 8.7 $\upmu$m spectral region that was
omitted from the least-squares search. For SL9, the fit to the Palomar/SC-10
spectrum between 8.3 - 8.7 $\upmu$m is adequate (within uncertainty). The fit
is expectedly poorer from 8 - 8.3 $\upmu$m, which captures stratospheric CH4
emission that was not included in the grid search. For the Wesley impact, the
fit of the best-fitting mineralogical composition does not adequately fit the
8.3 - 8.7 $\upmu$m region. This suggests a missing (and currently unknown)
source of opacity from 8.3 - 8.7 $\upmu$m in the impact region, which is in
absorption rather than in emission.

###  3.2 Radiative transfer inversions

We adopt the NEMESIS (Non-linear optimal Estimator for MultivariatE spectral
analysis, Irwin et al. [2008]) radiative transfer code to perform inversions
of the data. This code has been used extensively in previous investigations
[Orton et al., 2011, Fletcher et al., 2009b, 2010, 2011]. Upper-tropospheric
temperatures were sensed using H2-related collision-induced absorption (CIA)
continuum that dominates wavelengths in the 17-25 $\upmu$m range, with
coefficients derived by ab initio models for H2-H2 [Fletcher et al., 2018a],
H2-He [Birnbaum et al., 1996], and H2-CH4 [Borysow and Frommhold, 1986]. The
spectroscopic line information for CH4 and its isotopologues, C2H2, C2H4,
C2H6, NH3 and PH3, which are the relevant and dominant gaseous species at the
wavelengths of this study, were adopted from Supplementary Table 1 of Fletcher
et al. [2018b].

For computational efficiency, we chose to perform forward models and
inversions using the correlated-k method, where absorption coefficients of all
relevant gases within a wavenumber are sorted in order of line strength and
the distributions of line strengths, or k distribution, is computed [Lacis and
Oinas, 1991, Rodgers, 2000]. In order to model both Gemini/ T-ReCS and
Palomar/SC-10 spectra, the aforementioned spectroscopic line information was
convolved with a triangular line function with a FWHM of $\Delta\nu$= 10 cm-1
and $\Delta\nu$ = 5.55 cm-1 to model the N-band (7 – 13 $\upmu$m) and Q-band
(17 – 25 $\upmu$m), respectively. K-distributions in each band were calculated
and concatenated such that the N- and Q-band spectra could be modelled
simultaneously.

####  3.2.1 Modeling Palomar/SC-10 spectra

Figure 11: Reduced $\chi^{2}$ values in fitting the Palomar/SC-10 spectra as a
function of deviation pressure, $p_{0}$ and fractional scale height of the NH3
(gas) profile. The left column shows results derived from inverting the full 8
– 13 $\upmu$m spectral range, the right column shows results derived from
inverting the 8 – 8.5 and 11.5 – 13 $\upmu$m spectral range (and radiances in
the 8.5 – 11.5 $\upmu$m range were forward modelled). are shown in the left
column, Results are shown at wavelengths of 8 – 8.5 $\upmu$m (R branch of CH4
emission, continuum), 8.5 – 11.5 $\upmu$m (NH3 and non-gaseous emission), 11.5
– 13 $\upmu$m (C2H6 emission) and all wavelengths. White lines denote
$\chi^{2}/n$ increments of 2. White crosses denote model results that fit the
spectra within absolute $\chi_{min}^{2}$ \+ 2.3 (the 1-$\sigma$ confidence
level) and reproduce the observed strength of NH3 emission at 10.36 and 10.75
$\upmu$m within uncertainty.

Adopting a initial guess or a priori atmosphere derived from Cassini-CIRS
(Composite Infrared Spectrometer) measurements of 40∘S in 2001 (see A), we fit
the observed Palomar/SC-10 8–13 $\upmu$m spectra of the G impact site (Figure
5) by allowing the vertical profiles of temperature, NH3, the abundance of
tropospheric aerosol and C2H4 to vary. The tropospheric aerosol and
stratospheric C2H4 abundances were parameterized by retrieving a scale factor
applied to all altitudes of their respective CIRS-derived profiles.
Temperature was allowed to vary continuously at all altitudes with wavelengths
8.2 – 8.5 $\upmu$m providing sensitivity to the upper troposphere (1– 5 mbar)
and CH4 (8 - 8.1 $\upmu$m) and C2H6 emission (11.3 – 13.0 $\upmu$m) providing
sensitivity to the lower stratosphere (50 mbar $<$ p $<$ 0.1 mbar). Outside of
the range of sensitivity, retrieved temperatures tend back to a priori (CIRS-
derived) values. As in Section 3.1, we parameterized the vertical profile of
NH3 using the approach presented in Fletcher et al. [2011]. A 2-dimensional
model grid was computed with 13 $p_{0}$ values ranging from 50 mbar to 7 mbar
in increments of 50 mbar and fractional scale heights from 0.2 to 1 in steps
of 0.05. The vertical profiles of NH3 of the model grid were adopted in turn,
fixed and the SC-10 spectrum of the SL9 G impact was inverted. In previous
work analyzing observations of the SL9 impacts, a range of parameterizations
of the stratospheric NH3 vertical profile were adopted, including a step
profile (e.g. Fast et al. 2002) and a double-peaked profile (e.g. Griffith et
al. 1997). However, the observations analyzed in those studies were of high
spectral resolving powers ($\sim$104 to 107) and therefore better resolve the
vertical structure in the impact regions compared to the $\sim$102 resolving
powers analyzed in this work. We therefore chose to parameterize the vertical
profile of NH3 using a simple power law index (two parameters) to avoid a
highly degenerate parameter space and to be consistent with our
parameterization of the NH3 vertical profile in analyzing T-ReCS spectra of
the Wesley impact.

At mid-infrared wavelengths, the magnitude of stratospheric hydrocarbon
emissions is modulated both by the vertical temperature profile and the
abundance of the emitting species. In previous work (e.g. Nixon et al. 2007,
2010, Fletcher et al. 2016, Sinclair et al. 2020), stratospheric temperatures
are normally derived from fitting CH4 emission at 7.2 – 8.5 $\upmu$m since the
vertical profile of CH4 is generally assumed to be horizontally homogeneous in
the lower stratosphere. However, in this work, there are only a few spectral
points capturing CH4 emission in this wavelength range and with a poorer
signal-to-noise ratio (SNR) due to telluric obscuration. As a result, the
vertical temperature profile is largely constrained from fitting C2H6
emission, since it is sampled by a greater number of spectral points and at a
higher SNR. Our analysis assumes the CIRS-derived C2H6 profile at 45∘S derived
from Cassini-CIRS measurements recorded during the 2001 Jupiter flyby,
however, stratospheric abundances of C2H6 do vary temporally due radiative
forcing and dynamics (e.g. Melin et al. 2018, Hue et al. 2018). Thus, absolute
stratospheric temperatures and abundances should be interpreted with caution.

Figure 12: The top panel shows the observed Palomar/SC-10 spectrum (black
points with error bars) and model spectra are shown as solid, colored lines.
The red spectrum corresponds to the “best-fitting” result when inverting the
full 8 – 13 $\upmu$m range, which produces too much NH3 emission at
wavelengths longer than 11 $\upmu$m. The blue spectrum is the best-fitting
result when inverting only the 8 – 8.5 and 11.5 – 13 $\upmu$m spectral range
and that produces NH3 emission features of a similar strength as observed
within uncertainty - see the text for further details. The blue synthetic
spectrum is considered to represent the component of the observed spectrum
formed by gaseous emission, as detailed further in the text. The bottom panel
shows the corresponding temperature and NH3 profiles (solid lines) associated
with the synthetic spectra. Dotted lines represent 1-$\sigma$ uncertainty. The
black, solid lines denote the a priori (CIRS-derived) temperature and NH3
profiles adopted in inverting the spectra. Hashed regions at pressures lower
than 0.1 mbar indicate where the observations have little/no sensitivity.

Figure 11a-d shows the reduced $\chi^{2}$ fits to the Palomar spectrum at 8.0
– 8.5 $\upmu$m (capturing continuum and stratospheric CH4 emission), 8.5 –
11.5 $\upmu$m (predominantly NH3 gaseous emission and non-gaseous impact
residue ), 11.5 – 13.0 $\upmu$m (continuum and stratospheric C2H6 emission)
and over all wavelengths. We find there is no model atmosphere within the
described model grid that fits all wavelengths adequately, i.e. with
$\chi^{2}/n$ $\sim$ 1\. While the 8.5 – 11.5 $\upmu$m spectral range is
relatively better fit ($\chi^{2}/n$ $\sim$ 20) with models assuming higher
values of $p_{0}$ and $f$ (i.e. higher NH3 abundances over a larger vertical
range of atmosphere), the same models produce too much NH3 emission and
therefore a poorer fit in the 11.5 – 13.0 $\upmu$m spectral range. This is
also demonstrated in Figure 12. We attribute the inability to fit all
wavelengths with the same temperature, aerosol and ammonia profile due to the
presence of significant non-gaseous emission predominantly in the 8.5 – 11.5
$\upmu$m spectral range.

Given the inability to fit all wavelengths of the spectrum due to non-gaseous
emission, we instead modelled the spectrum in the following way. The
retrievals of temperature and aerosol abundance were repeated across the same
model grid of NH3 profiles (see above) but using only the 8 – 8.5 and 11.5 –
13 $\upmu$m spectral ranges. These ranges were chosen because they capture CH4
and C2H6 emission features, and also correspond to wavelength ranges where the
candidate mineralogical species (see Section 3.1) have significantly weaker
absorption features. The retrieved atmosphere was then forward-modelled across
the entire 8 – 13 $\upmu$m spectral range and the reduced $\chi^{2}/n$ values
were calculated. The goodness-of-fit values for these results are shown in
11e-h. In omitting the 8.5 – 11.5 $\upmu$m spectral range, the retrieval is
able to adequately fit ($\chi^{2}/n\sim 1$) the 8 – 8.5 and 11.5 – 13 $\upmu$m
spectral range with the same atmosphere.

In order to calculate the component of the spectrum produced by gaseous
emission, we searched the model grid results for results that satisfied the
following criteria. First, the synthetic spectrum must fit the observed
spectrum in the 8.0 – 8.5 and 11.5 – 13 $\upmu$m ranges with an absolute
$\chi^{2}$ value of less than $\chi_{min}^{2}$ \+ 2.3, which denotes the
1-$\sigma$ confidence level [Press et al., 1992] when varying two parameters.
Second, the synthetic spectrum must reproduce the line-to-continuum radiance
difference of the strong NH3 emission features at 10.36 and 10.75 $\upmu$m
within uncertainty on the radiance. The models that satisfy these criteria are
shown as white crosses in Figure 11e-h. A model atmosphere with the NH3
profile deviating from the CIRS-derived profile at 650 mbar and with a
fractional scale height of 0.75 yields the best-fitting spectrum that satisfy
these criteria. The spectrum associated with this model was adopted as the
gaseous emission spectrum. The uncertainty of the gaseous component of the
spectrum was determined by calculating the standard deviation of the synthetic
spectra that satisfied the aforementioned criteria. Similarly, at each
atmospheric level, the standard deviation in NH3 values from the range of the
model atmosphere that satisfied the aforementioned criteria were calculated
and adopted as their respective uncertainties. We performed a similar
calculation to determine the uncertainty on temperature but found that the
standard deviation was smaller than the uncertainty on temperature from the
best-fitting retrieval and so the latter was adopted as the uncertainty on
temperature. The gaseous emission spectrum and the associated vertical
profiles of temperature and NH3 and their uncertainties are shown in Figure
12.

Figure 13: Comparison of the fractional residual between observed and modelled
SC-10 spectra of the SL9 G impact site, which denotes the non-gaseous
component of the spectrum, and the absorptance spectra of candidate
mineralogical species according to the legend. The fractional residual was
normalized to 1 and all mineral absorptance spectra have been normalized to
0.6 for ease of comparison. The 9.65 – 9.9 $\upmu$m range is hatched due to
telluric ozone and so we do not consider any apparent features in this range.

Relative to the CIRS-derived a priori temperature profile, heating of the
atmosphere is evident from $\sim$100 mbar to $\sim$0.1 mbar (with no
sensitivity to temperature at lower pressures). The warmest temperatures of
174.3 $\pm$ 0.2 K were retrieved at the 2-mbar level. The observations have
limited sensitivity to temperature at pressures lower than 0.1 mbar and so we
cannot rule out heating of the atmosphere at lower pressures. At 30 mbar and
0.1 mbar, we derive NH3 concentrations of $38^{+7}_{-19}$ ppmv and
$5.7^{+4.5}_{-2.8}$ ppmv, respectively. The non-gaseous emission spectrum was
derived by calculating the fractional residual (Equation 1) between the
observed spectrum and the best-fitting gaseous emission spectrum (Figure 12 in
blue). Figure 13 compares the non-gaseous fractional residual with the
features of candidate mineralogical species.

Figure 14: Reduced $\chi^{2}$ distributions calculated from the observed and
modelled T-ReCS spectra. Results in the left column are for retrievals where
the entire spectral range available (7.5 - 13, 17 - 25 $\upmu$m) was included
in the inversion. $\chi^{2}/n$ are shown at wavelengths from a) 7.5 – 13
$\upmu$m, b) 17 – 19 $\upmu$m, c) 19 – 25 $\upmu$m and d) all wavelengths.
Similar results are shown in panels e) – h) but where the 17 – 19 $\upmu$m
range was excluded from the inversion. White contours denote increments of
$\chi/n$ of 0.5. White crosses denote model combinations that allow the 7.5 -
13 $\upmu$m and 19 - 25 $\upmu$m spectral ranges to be fit within (absolute)
$\chi_{min}^{2}$ \+ 2.3, which corresponds to the 1-$\sigma$ confidence level.

The derived non-gaseous emission exhibits a broad feature from wavelengths of
8.5 $\upmu$m to 11.5 $\upmu$m. As in Section 3.1, the dominant opacity sources
responsible for the broad feature are a mixture of amorphous olivine,
obsidian, and tekt2. Superimposed on this broad feature are narrower features
at wavelengths of 9.6, 9.8, 10.0, 10.2, 10.6 and 10.9 $\upmu$m with
(normalized) fractional residuals of 0.56 $\pm$ 0.12, 0.74 $\pm$ 0.16, 0.57
$\pm$ 0.12, 1.$\pm$ 0.18, 0.29 $\pm$ 0.08 and 0.30 $\pm$ 0.15. We do not
consider the feature at 9.8 $\upmu$m real due to its proximity to telluric O3.
Although the observed feature at 10.0 $\upmu$m is consistent in wavelength
with forsterite, the mineral exhibits a much broader feature (FWHM $\sim$0.4
$\upmu$m) compared to what is observed (FWHM $\sim$ 0.2 $\upmu$m). In
addition, forsterite has a feature at 11.25 $\upmu$m, which is twice as strong
as its feature at 10.0 $\upmu$m, where no significant residual is observed. In
taking into account the 0.1 $\upmu$m (R = $\lambda/\Delta\lambda$ = 100)
accuracy of the wavelength grid, the features at 10.6 and 11.0 $\upmu$m are
near-coincident with those of fayalite at 10.5 and 10.9 $\upmu$m. However, in
acknowledging the 0.1-$\upmu$m accuracy of the wavelength grid, we also note
that the features at 9.6, 9.8 10.0 and 10.2 $\upmu$m are within 0.1 $\upmu$m
of NH3 lines. Thus, it remains inconclusive whether the identified narrow
features result from non-gaseous species or poor fitting of the NH3 emission
features.

####  3.2.2 Modeling Gemini/T-ReCS spectra

The atmosphere derived from Cassini-CIRS measurements of 60∘S in 2001 (see A)
was adopted as the background atmosphere and a priori. The T-ReCS spectra were
then inverted over the same model grid of NH3 profiles adopted in inverting
the SC-10 spectra (see Section 3.2.1). In order to optimize the signal-to-
noise ratio, we adopted the coadded spectrum in Figure 6, where all individual
spectra sampling the impact region were coadded. Initially, we inverted the
entire T-ReCS spectral range from 7.5 to 13 $\upmu$m and 17 to 25 $\upmu$m.
Figure 14a-d shows the reduced $\chi^{2}$ distributions of the model grid in
fitting the observations.

Figure 15:  The top row compares observed T-ReCS spectra (black points with
error bars) and the best-fitting synthetic spectra (solid, red lines) in the N
(left) and Q (right) band. The middle row shows the fractional residual
between the observed and modelled spectra . The bottom row shows the a priori
(solid, back) and retrieved temperature profile (solid red, with 1-$\sigma$
uncertainty shown as dotted lines) according the lower axis. The best-fitting
NH3 profile (solid black) is shown according the upper axis. Hashed regions at
pressures lower than 0.1 mbar indicate where the observations have little/no
sensitivity.

Overall, the fit to the observations is optimized when the vertical profile of
NH3 deviates from the CIRS profile at 100 – 200 mbar and decreases in
abundance with a high fractional scale height. While we can adequately fit the
7.5 – 13 $\upmu$m spectrum with variations in temperature, NH3 and
tropospheric aerosol, we find that the 17 – 19 $\upmu$m feature cannot be
adequately fit with a physically-sensible atmospheric model and cannot be
adequately fit using an atmospheric model that simultaneously fits the 7.5 –
13 $\upmu$m range. This is further suggestive the 17 – 19 $\upmu$m enhancement
of the Wesley impact site, with respect to the background atmosphere, results
from a non-gaseous species in the atmosphere.

As shown in Figure 14b-c, when the 17 – 19 $\upmu$m was included in the
spectral inversion, we found that the overall fit to 17 – 25 $\upmu$m spectral
range was poor. We performed a second set of retrievals where the 17 – 19
$\upmu$m spectral range was omitted from the spectral inversion and found that
the fit to the 19 – 25 $\upmu$m range significantly improved (comparing
Figures 14c and g). In the remainder of this section, we will therefore
discuss results from retrievals omitting the 17 – 19 $\upmu$m range.

The best-fitting ($\chi^{2}/n$ = 0.629) NH3 profile deviates from the CIRS-
derived profile at $p_{0}$ = 200 mbar, and decreases with a fractional scale
height of 1. However, we note that a range of solutions with p0 ranging from
150 to 350 mbar and fractional scale heights from 0.75 to 1.0 also fit the
observed spectrum with (absolute) $\chi^{2}$ values less than $\chi_{min}^{2}$
\+ 2.3, which corresponds to the 1-$\sigma$ confidence level when varying two
parameters [Press et al., 1992]. These models are marked in Figure 14e – h. A
fractional scale height of 1 imposes a constant abundance of NH3 with altitude
from the 200-mbar level to the top of the atmosphere, which is physically
unrealistic. Instead, the vertical profile of NH3 is likely to be
approximately constant with altitude from $p_{0}$ = 2 mbar to $\sim$ 0.1 mbar,
over which the observations have sensitivity, but then decreases significantly
at higher altitudes outside the vertical range of sensitivity of the
observations. We adopted the best-fitting spectrum (see above) as the gaseous
component of the impact region. The uncertainty of the gaseous spectrum was
determined by calculating the the standard deviation of the synthetic spectra
that had (absolute) $\chi^{2}$ less than $\chi_{min}^{2}$ \+ 2.3. At each
atmospheric level, the standard deviation in NH3 values from the range of the
model atmosphere that satisfied the aforementioned criteria were calculated
and adopted as their respective uncertainties. We performed a similar
calculation to determine the temperature uncertainty but found that the
standard deviation was smaller than the temperature uncertainty from the best-
fitting retrieval and so the latter was adopted as the real temperature
uncertainty.

Figure 15 compares the observed T-ReCS spectra with the gaseous spectrum noted
above. This result was forward modelled at 17 – 19 $\upmu$m (omitted from the
inversion as noted above) so this spectral range could be included in the
comparison. At 30 mbar, we derive NH3 abundances of $150^{+338}_{-121}$ ppbv,
which is in agreement (within uncertainty) of the results derived by Fletcher
et al. [2011]. As also noted in previous work (e.g. Orton et al. 2011,
Fletcher et al. 2011), there is no evidence of stratospheric heating
associated with the Wesley impact. In fact, stratospheric temperatures are
$\sim$10 K cooler than those derived from Cassini measurements in 2001. In the
Q band, the model spectra cannot adequately fit the observed spectra from 17.5
– 19.5 $\upmu$m. For example, at 18.5 $\upmu$m, the fractional residual is
15.4 $\pm$ 4.3% and therefore greater than 3$\sigma$. However, we find we can
adequately fit the majority of the spectrum in the N band by varying only the
vertical profiles of temperature, NH3 and scaling the abundance of C2H4.

##  4 Discussion

Using methods as consistently as possible, our analysis of mid-infrared data
capturing the Jovian atmosphere in the aftermath of the SL9 and Wesley impacts
have revealed similarities and differences between the two events.

In both impacts, NH3, normally concentrated below Jupiter’s tropopause, was
transported to stratospheric altitudes by ballistic splashback, which produced
strong NH3 emission features in the N band with the strongest lines at
$\sim$10.4 and $\sim$10.8 $\upmu$m (see Figure 7, as seen in previous work
[Griffith et al., 1997, Fast et al., 2002, Orton et al., 2011, Fletcher et
al., 2011]. In inverting the spectra of both impacts, we parameterized the NH3
vertical profile by using the CIRS-derived profile (see A) at pressures higher
than a deviation pressure, $p_{0}$ and a decrease in abundance with altitude
according to a fractional scale height, $f$. The fit to the continua and the
strength of the NH3 emission lines in the Palomar/SC-10 spectra of SL9’s G
impact was optimized using $p_{0}$ = 650 mbar and a fractional scale height of
0.65. This results in NH3 concentrations of $38.4^{+6.8}_{-18.7}$ ppmv and
$5.66^{+4.54}_{-2.82}$ ppmv at 30 and 0.1 mbar, respectively. These are
broadly-consistent with the findings of Griffith et al. [1997], who derived
$\sim$1 ppmv NH3 concentrations at 0.1 mbar using data recorded $\sim$6 days
after K impact, and Fast et al. [2002], who derived $\sim$3 ppmv NH3
concentrations at pressures lower than 1 mbar using data recorded $\sim$4 days
after the G impact. The fit to Gemini-South/T-ReCS spectra of the Wesley
impact was optimized using $p_{0}$ = 200 mbar. At 30 mbar, we derive NH3
abundances of $150^{+338}_{-121}$ ppbv, which is in agreement within
uncertainty of the results derived by Fletcher et al. [2011]. However, we note
to the reader the our inversion of the spectra did not include the opacities
of non-gaseous features and so these concentrations represent an upper limit.
Nevertheless, the higher values of p0 and higher concentrations of NH3
required to fit the spectra of the SL9 impact, compared to the Wesley impact,
is consistent with the SL9 fragments entering the atmosphere at angles closer
to the local zenith, and reaching deeper, NH3-richer altitudes.

In both events, material introduced by the impactors into the atmosphere was
evident as non-gaseous emission features in the spectra of the impact. In
order to determine the mineral species responsible for these non-gaseous
emission features and their relative abundances, we performed a least-squares
search of the mineral species listed in Table 3. In the impact sites of SL9,
we would have expected a preponderance of amorphous olivine and pyroxene, plus
water ice and amorphous carbon as well as NH3 gas lofted to stratospheric
altitudes. The results of our mineralogical least-squares fitting indicate
that amorphous olivine and NH3 gas were significant opacity sources in the
impact site of SL9’s G fragment. However, our analysis detected silica (in the
form of obsidian) and an absence of amorphous pyroxene. This was initially
surprising since comets are expected to have similar abundances of
olivinaceous to pyroxenacous species (e.g. Lisse et al. 2006, 2007b). We
suggest that high temperatures and pressures produced by the cometary impacts
converted all the cometary pyroxene into silicas. While amorphous pyroxene is
stable up to temperatures of $\sim$1000 K, amorphous olivine is stable up to
$\sim$1500 K and so presumably, the material was heated to a temperature
between 1000 - 1500 K such that amorphous olivine remained unaltered. A
similar finding was seen in the HD172555 exodisk system by Lisse et al.
[2009]. As far as we are aware, silica was not detected or reported in
previous studies of the SL9 impact residue though this is unsurprising given
the majority of mid-infrared studies focused on high-resolution spectroscopy
of NH3 lines between 10 - 11 $\upmu$m (e.g. Griffith et al. 1997, Fast et al.
2002) and did not sample the 9 - 9.5 $\upmu$m region. Nicholson et al. [1995]
presented Palomar/SC-10 spectra (the same dataset used in this paper) of the R
impact, also observing a broad N-band feature, but this was interpreted as
silicate emission, not silica. However, their study did not attempt to derive
an emissivity spectrum of the impact (by computing a residual between the
impact and a non-impact region) or model the features using a least-squares
search of mineral species.

If the Wesley impactor was a comet, we would also have expected a composition
of predominantly amorphous olivine and pyroxene, plus water ice and amorphous
carbon. If the Wesley impactor was an asteroid, as suggested previously (e.g.
Hammel et al. 2010, Fletcher et al. 2010, Orton et al. 2011, we would have
expected a preponderance of crystalline olivine and phyllosilicates plus
impact-alteration-produced silicas. Instead, our analysis found that the
Wesley impact emission appears composed of almost pure amorphous olivine, with
some ferromagnesian pyroxenes mixed in at an $\sim$8:1 ratio. No emission due
to silicas (expected from 9.1 - 9.4 $\upmu$m) is apparent in our new reduction
and spectral analysis, a marked difference from the findings of Fletcher et
al. [2011] who used the same dataset. We believe our results differ due to
differences in how the spectra were coadded and modelled. Fletcher et al.
[2011] inverted spectra of the impact obtained over a range of emission angles
and found the 9.1-$\upmu$m signature was most evident in the higher emission-
angle observations, whereas, in this work, we averaged individual T-ReCS
spectra over a range of emission angles to compute a single, coadded spectrum.

Given the presence of silicas is analysis-dependent, we consider its non-
detection in this work a tentative result. If we assume the non-detection of
silica is correct, we must therefore assume residue from the Wesley impactor
did not reach the required temperatures of $\sim$1000-1500K in order to
convert rocky silicates into silicas. Instead, we must conclude from our new
study that the Wesley residue was formed at low temperatures and high
pressures such that the usually dominant crystalline olivine was readily
converted into amorphous olivine. Whether these pressures were achieved due to
the shock of impact, or due to the pressures achieved at depth inside a large
differentiated body, is not clear. We currently favor the impact induced shock
amorphization explanation, as all reports have the Wesley impactor as being
relatively small in size ($<$10 km radius), which would be too small to
thoroughly self-pressure alter its olivine throughout. In addition, such an
explanation would require removal of the body’s low pressure, near-surface
crystalline olivine without heating the high pressure phase enough to produce
silica. This still belabors the question of how a body impinging on Jupiter
with $\sim$65 km/sec (Jupiter’s escape speed) relative velocity and a huge
specific energy per kg of material can manage to NOT melt and transform its
matter. As noted above, SL9 was seen to create large amounts of glassy silica
(a material not found in comets) from amorphous pyroxene. A remote possibility
is that the Wesley 2009 impactor entered the atmosphere at an extremely
shallow angle, which would allow material to spall off the impactor at
relatively low temperatures. If we take the velocity of impact required to
convert silicates into silicas as $\sim$10 km/sec (Lisse et al. [2009] and
references therein), an impact angle of less than tan${}^{-1}(10/65)$ $\sim$
9∘ with respect to the local horizontal would be required in order to impart
the impactors kinetic energy slow enough to the body that it could have a
chance to retain all its silicates. This is outside the 20∘ $\pm$ 5∘ estimated
by Sánchez-Lavega et al. [2010] and Pond et al. [2012] in order to explain the
lack of stratospheric heating [Fletcher et al., 2010] and the observed debris
deposition between 3- 10 mbar [de Pater et al., 2010, Hammel et al., 2010].

An impact on Jupiter by 0.5- to 1.5-km objects, similar in size to the
fragments of SL9 and the Wesley impactor, is likely in the near future. While
Schenk et al. [2004] inferred from the cratering record on the Galilean moons
that SL9/Wesley-sized impacts on Jupiter will occur once every $\sim$100
years, Sánchez-Lavega et al. [2010] instead calculated a rate of once every 10
- 20 years from the observed, recent rate. For future impact events, we
believe a mid-infrared characterization of the impact site would allow the
impactor and its effect on the atmosphere to be characterized in the following
ways. First, the relative proportions of amorphous pyroxanaceous and
olivinaceous species would be constrained from their features at 9, 10, and 19
$\upmu$m. Second, the detection or non-detection of silica would indicate
whether temperatures and pressures produced by the impact reached the
$\sim$1500 K or 4 - 6 GPa levels required to temperature alter silicate
species. Third, if stratospheric NH3 emission is detected, concentrations of
NH3 could be derived, which in turn would constrain the terminal depth of the
impactor material. While broadband imaging and low-resolution spectroscopy
readily allowed non-gaseous features from the impactor to be identified, the
coarse spectral resolving power (R = 100) presented in this work introduced
uncertainty in identifying gaseous vs. non-gaseous spectral features and
allowed only simple (two variable) parameterizations for the vertical profile
of NH3. Characterization of future impacts would therefore benefit from N- and
Q-band spectroscopy performed near-simultaneously and at higher spectral
resolving powers (1000 $<$ 15000). For Earth-based observations, R $>$ 1000
would also better allow CH4 emissions from the planet to be disentangled
relative to telluric CH4 features (assuming the Earth-Jupiter relative
velocity exceeds $\sim$ 15 km/s). This would improve the retrieval of the
stratospheric temperature field since the CH4 is generally considered to be
horizontally homogeneous in the lower stratosphere.

##  5 Conclusions

We performed a retrospective analysis of mid-infrared Earth-based observations
of Jupiter recorded in July 1994 and July 2009 in order to compare the effects
on Jupiter’s atmosphere by Comet D/Shoemaker-Levy 9 (SL9) and the Wesley
impactor quantitatively. Spectrophotometry by IRTF/MIRAC and
Palomar/Spectrocam-10 in 1994 and Gemini/T-ReCS in 2009 were reduced,
calibrated and analyzed using methods as consistently as possible in order to
facilitate robust comparisons between the two impacts. As presented in
previous work, we find the 8 – 11.5 $\upmu$m spectral range of both impact
sites is significantly enhanced due to stratospheric emission from NH3, having
been lofted from the upper troposphere to the stratosphere by ballistic
“blowout”, as well as non-gaseous impact material (see below). Also, in
agreement with previous work, we find that the SL9 impacts exhibited enhanced
stratospheric CH4 emissions whereas no significant enhancement was evident in
the site above the Wesley impact. In new findings, we determine that the sites
of the SL9 and Wesley impacts both exhibit enhanced emissions at 18 – 19
$\upmu$m. For example, at 17.9 $\upmu$m, the SL9 L impact site is enhanced by
41.2 $\pm$ 11.5% and the Wesley impact site is by 16.4 $\pm$ 3.7%. In order to
detect and quantify the species responsible for these observed spectral
features, we performed two separate but complementary analyses. First, we
performed mineralogical spectral modeling by adopting a collection of
candidate mineralogical species and performing a grid search to find a
combination of species that produced a model emissivity spectrum that
minimized the goodness-of-fit to the observations. For the impact site of
SL9’s G fragment, we find that stratospheric NH3 emission, amorphous olivine
and silica (in the form of obsidian) are the dominant opacity sources
responsible for the observed spectral features at 8.5 - 11 $\upmu$m and 18 -
19 $\upmu$m. We find no evidence of amorphous pyroxene in the site of SL9’s G
fragment, which was initially unexpected since comets typically exhibit a
$\sim$1:1 ratio of pyroxenaceous species to olivinaceous species. We suggest
that the high pressures and temperatures produced by the impact readily
converted cometary pyroxene into silicas. For the Wesley impact, we find that
stratospheric NH3 and amorphous olivine are the dominant opacity sources
responsible for the observed spectral features. We find no evidence of silicas
in the aftermath of the Wesley impacts. This is in contrast to previous work
[Fletcher et al., 2011] who used the same data but processed/analyzed
differently, which suggests the detection or non-detection of silica is
analyssis-dependent. We therefore consider the non-detection of silica a
tentative finding. If the non-detection of silica is correct, it would require
that material from the Wesley impactor was modified at relatively lower
temperatures such that species retain all their silicates. This would require
that the Wesley impactor entered the atmosphere at a very shallow angle of 9∘,
which is shallower than the 20 $\pm$ 5∘ estimated previously [Sánchez-Lavega
et al., 2010, Pond et al., 2012]. Our second analysis involved inverting the
spectra of both impacts using radiative transfer software in order to
constrain NH3 abundances. For SL9’s G impact, we constrain NH3 concentrations
of 38.4${}^{+6.8}_{-18.7}$ and 5.66${}^{+4.54}_{-2.82}$ ppmv at 30 and 0.1
mbar, respectively, which is broadly consistent with previous work [Griffith
et al., 1997, Fast et al., 2002] though our parameterization of NH3’s vertical
profile differed. For the Wesley impact, we derive an NH3 concentration of 150
${}^{+338}_{-121}$ ppbv at 30 mbar, which is consistent (within uncertainty)
with Fletcher et al. [2011]. Higher concentrations of NH3 in the SL9 impacts
is consistent with the fragments reaching a deeper terminal depth compared to
the Wesley impact.

##  6 Acknowledgements

Some of this research was carried out at the Jet Propulsion Laboratory,
California Institute of Technology, under a contract with the National
Aeronautics and Space Administration (80NM0018D0004). During the course of
this research, Meera Krishnamorthy was a student at the California Institute
of Technology and worked at JPL as an intern in the Caltech Summer
Undergraduate Research Fellowship (SURF) program, supported by these funds.
Fletcher was supported by a European Research Council Consolidator Grant
(under the European Union’s Horizon 2020 research and innovation programme,
grant agreement No 723890) at the University of Leicester. We thank Mike
Chaffin and Sonal Jain for creating IDL-compatible color
tables111https://github.com/planetarymike/IDL-Colorbars that are perceptually-
uniform, colorblind-friendly, and print correctly in black and white. Emma
Dahl was supported by an appointment to the NASA Postdoctoral Program at the
Jet Propulsion Laboratory, administered by Oak Ridge Associated Universities
under contract with NASA.

##  7 Data Availability

Archiving of the IRTF/MIRAC images and the Gemini-South/T-ReCS spectra at the
Planetary Data System (PDS) is in progress (at the time of writing). Until
they are available on the PDS, the data will be made available upon request.
The calibrated spatial-spectral Palomar/SC-10 image of Jupiter is available at
the Mendeley data archive222DOI:10.17632/ydt2ff9n83.1. Raw data files and the
software used to process the Palomar data would also be made available upon
request.

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##  Appendix A Cassini-CIRS retrievals

During Cassini’s 2000/2001 flyby of Jupiter, four maps of 2.5 cm-1 ‘MIRMAP’
observations - ‘ATMOS02A’, ‘ATMOS02B’, ‘ATMOS02C’ and ‘ATMOS02D’ - were
acquired from January 1st to Jan 11th. Spectral from all four maps were sorted
into 2∘ latitude bins, nyquist-sampled by 1∘S and coadded. The effective noise
on these spatially-binned observations was assumed to be the largest of
either: 1) the standard deviation of the mean or 2) the noise-equivalent
spectral radiance (NESR) spectrum of CIRS by a factor of
$(0.5/{\Delta\tilde{\nu}})(1/(\sqrt{N})+1/(\sqrt{M}))$, where
$\Delta{\tilde{\nu}}$ is the spectral resolution of the target spectra, N is
the number of averaged target spectra and M is the number of deep-space
spectra.

The spectra at 40∘S (the latitude of the SL9 impacts) and 60∘S (the latitude
of the Wesley impact) were inverted by varying temperature, NH3, tropospheric
aerosol, C2H2, C2H4 and C2H6. Inversions were performed using the NEMESIS
radiative transfer code [Irwin et al., 2008]. The sources of line data, the
atmospheric model are identical to those described in Section 3.2.

The vertical temperature profile was allowed to vary continuously at all
altitudes. The vertical profile of NH3 was parameterized with three variables:
1) a deep volume mixing ratio ($q_{deep}$) at pressures higher than a knee
pressure, 2) the knee pressure ($p_{knee}$) and 3) a fractional scale height
($f_{NH3}$). For tropospheric aerosol, a grey absorber was assumed with an a
priori base pressure ($p_{base}$) of 0.7 bar, an optical depth ($\tau_{1}$) of
1 and a fractional scale height ($f_{cloud}$) of 0.2, were assumed, and all
three parameters were varied. The vertical profiles of C2H2, C2H4 and C2H6
were adopted from a photochemical model of Jupiter [Moses and Poppe, 2017] and
varied by applying a scale factor to all altitudes.

Comparisons of the observed and synthetic spectra and the corresponding
retrieval parameters are shown in Figures 16 and 17. The retrieved atmospheric
parameters at 40∘S and 60∘S are adopted as a priori values for inversions of
the Palomar/SC-10 and Gemini-S/T-ReCS spectra, respectively.

Figure 16: Inversions of Cassini-CIRS spectra recorded during the 2001 Jupiter
flyby at 40∘S. The top and middle panels show observed spectra (black circles
with error bars) from 7 - 11 $\upmu$m and 11 - 17 $\upmu$m, and synthetic
spectra are shown as solid lines. The retrieved atmospheric parameters
corresponding to the synthetic spectra are shown in the bottom panel and the
legend. Black, solid lines represent a priori values, solid lines denote
retrieved values and dotted lines outline the 1-$\sigma$ uncertainty. Figure
17: Inversions of Cassini-CIRS spectra recorded during the 2001 Jupiter flyby
at 60∘S. The top and middle panels show observed spectra (black circles with
error bars) from 7 - 11 $\upmu$m and 11 - 17 $\upmu$m, and synthetic spectra
are shown as solid lines. The retrieved atmospheric parameters corresponding
to the synthetic spectra are shown in the bottom panel and the legend. Black,
solid lines represent a priori values, solid lines denote retrieved values and
dotted lines outline the 1-$\sigma$ uncertainty.