# Venus, Phosphine and the Possibility of Life

David L Clementsa CONTACT David L. Clements Email<EMAIL_ADDRESS>aDepartment of Physics, Imperial College, Prince Consort Road, London, SW7
2AZ, UK

###### Abstract

The search for life elsewhere in the universe is one of the central aims of
science in the 21st century. While most of this work is aimed at planets
orbiting other stars, the search for life in our own Solar System is an
important part of this endeavour. Venus is often thought to have too harsh an
environment for life, but it may have been a more hospitable place in the
distant past. If life evolved there in the past then the cloud decks of Venus
are the only remaining niche where life as we know it might survive today. The
discovery of the molecule phosphine, PH3, in these clouds has reinvigorated
research looking into the possibility of life in the clouds. In this review we
examine the background to studies of the possibility of life on Venus, discuss
the discovery of phosphine, review conflicting and confirming observations and
analyses, and then look forward to future observations and space missions that
will hopefully provide definitive answers as to the origin of phosphine on
Venus and to the question of whether life might exist there.

###### keywords:

Venus; astrobiology; search for life;

##  1 Introduction

The search for life elsewhere in the universe is one of the major driving
forces for astronomy and astrophysics in the 21st century [1] with extremely
large telescopes on the ground and in space being designed and built to this
end. The main goal of these facilities is to study the atmospheres of rocky,
terrestrial planets in orbit around other stars - so-called exo-planets - and
follows the explosive development of exoplanet studies since their first
discovery in 1995 [2]. We now know of over 5000 exoplanets, and more are
announced all the time (for the latest information on exoplanet discoveries
see `www.exoplanet.eu`). However, while convincing evidence for life on
exoplanets may arrive in the next 20 years, there are many questions that such
observations will not be able to answer, including the biochemical processes
exoplanet life uses, how and when it originated, and how it evolved into
whatever form it has today - a form that will also be unknowable.

If we want answers to these further questions the only places they may be
answered is in our own Solar System. While still very distant, planets such as
Mars, the Jovian moon Europa, and Saturn’s moons Enceladus and Titan, which
might harbour signs of life, are accessible to direct in situ studies that can
answer these questions. And any answers inevitably lead back to the question
of our own origins on Earth, as well as the more general issue of the
prevalence of life in the universe.

The search for signs of life on Mars is well underway, with orbiting
spacecraft looking at its atmosphere (eg. the ExoMars Trace Gas Orbiter (TGO)
[3]) and an ever-increasing number of rovers scouring its surface [4]. Some of
these are already preparing samples of rock to be returned to Earth for
laboratory analysis. Further out in the Solar System, the first stages of the
exploration of Jupiter’s moons Ganymede and Europa, in part for signs of
habitable environments, are already under development with the European Space
Agency’s (ESA’s) JUpiter ICy moons Explorer (JUICE) [5], aimed primarily at
Ganymede, due for launch in 2023, and NASA’s Europa Express, aimed squarely at
Europa, due for launch in 2024. Plans are at an earlier stage for the
exploration of the moons of Saturn that might harbour life, including Titan
with its thick atmosphere and Enceladus with its plumes of water vapour
spewing into space, but it is clear that these too will be visited sometime in
the next few decades.

Until very recently this list of targets - Mars, Europa, Ganymede, Titan and
Enceladus - would have been considered the most likely places to find signs of
life in the Solar System. It was thus rather surprising to find Venus added to
this list in late 2020 with the discovery of an unusual gas, phosphine,
chemical formula PH3, in its atmosphere [6]. Venus, as we will see below, has
a surface that is completely hostile to life and so had been largely
discounted from these considerations. But, as we will also see, there have
long been thoughts that there might be niches in the atmosphere of this planet
that might be more favourable to the existence of life than its deeply
unpleasant surface [7, 8]. In this article we discuss how life is sought using
atmospheric observations, why phosphine is a potential signature of biological
activity, how it was detected on Venus, and what its discovery might mean for
our understanding of the history of Venus and of life in the Solar System. We
also look at the prospects for future studies of Venus in search of further
possible signs of life.

##  2 What is Life?

The formal definition of life adopted by NASA for searches for life elsewhere
is that ‘Life is a self-sustaining chemical system capable of Darwinian
evolution’ [9]. This definition clearly applies to most things that we would
consider alive on Earth, but it does leave some things out. Viruses, for
example, cannot reproduce on their own, but instead require a host cell, whose
reproductive machinery they take over. Alternative definitions are available
(eg. [10]), but the NASA definition seems a useful starting point to begin any
discussion of where life might exist in the Solar System or elsewhere. Given
this definition we can start to examine what the requirements might be for
life to exist.

From a physicist’s perspective the key thing that life needs to be able to
support itself is a source of energy. For most of the life we are familiar
with on the Earth that source of energy is the Sun - plants photosynthesise
using sunlight, while animals and other organisms consume plants in various
ways, including eating things that have eaten plants. However, sunlight is not
the only source of energy used by life on Earth. In the depths of Earth’s
oceans, where sunlight never shines, there are thriving communities of
organisms surrounding and fed by hydrothermal vents [11]. These arise where
ocean water can enter the Earth’s crust and be heated by magma. The heated
water dissolves minerals from the rocks and circulates back into the ocean
through the vents. The primary producers of energy in these vents are
chemosynthetic bacteria that use a variety of processes to derive energy from
the chemicals emerging from the vents. These then support a diverse community
of other organisms around the vents, including giant tube worms that can be up
to 3 metres long. Hydrothermal vents in the young Earth are a possible site
for the first emergence of life on our planet [12]. Similar hydrothermal vent
structures are thought to exist beneath the ice-covered surfaces of moons like
Europa and Enceladus [13]. However, life without light on Earth is not limited
to hydrothermal vents. It has even been suggested that most ecosystems on
Earth exist in the dark, deriving their energy from chemical processes
separate from, and independent of, photosynthesis [14]. This clearly has
implications for the search for life elsewhere.

What requirements for life are there beyond a source of energy? Revisiting
NASA’s definition of life we see that it is defined as a chemical system. The
chemical basis for all the life we know on Earth is the element carbon. This
element is exceptional in the Periodic Table as the lightest element in Group
IV. It thus has a half-filled electron shell giving it a valence of 4 so it
can donate or receive up to 4 electrons, allowing it to bind with itself to
form chains, and with numerous other elements. Some of the commonest are
hydrogen, oxygen and nitrogen, contributing to the wide variety of complex
organic compounds found in living organisms. The next heaviest Group IV
element is silicon. It has similar high valence and has been proposed as an
alternative building block for life [15], but there are a number of issues
which mean that carbon is a better choice.

Given a chemical basis for life, there must be a way for the necessary
chemical reactions to take place so that the processes of life can operate.
The best way to achieve this is for the reacting chemicals to be dissolved by
some solvent so that they can easily combine and interact. On Earth the
solvent behind all biology is water. While other solvents have been suggested,
especially in environments too hot or too cold for liquid water to be
available [16], water remains the only solvent which we know is associated
with life. The bulk of our searches for life elsewhere have thus been focussed
on places where liquid water might, or is known to, exist.

Our consideration of the nature of life thus leads us to the conclusion that
we should be looking for life on an astronomical body where there is a ready
supply of energy, where complex carbon-based chemistry can take place, and
where liquid water can exist. In our own Solar System this leads us to focus
on the planet Mars and the icy moons of gas giants, such as Europa and
Ganymede orbiting Jupiter, and Titan and Enceladus orbiting Saturn. Venus does
not appear on this list because, as we will see below, its current surface
conditions are far too hostile for life as we know it, but, as we will also
see, this may not always have been the case, and there may be loopholes that
could allow any life that might have emerged on the surface of this planet to
persist today in the dense clouds above its surface.

##  3 Looking for Life

Life is easy to find on Earth - we are surrounded by it - but looking for life
on astronomical bodies, even our closest neighbours, is a rather more
difficult task, especially since we do not expect to find something as
obviously alive as a tree or a cow. In the absence of complex macroscopic life
we have to look for signs of activity from microbial life. What might these
be? Using Earth as an example, the clearest sign that life exists is the
presence of oxygen in our atmosphere. This is generated by photosynthesising
organisms. Today we would associate this with plants, but in the ancient
history of our planet, the first oxygenating organisms were single-celled
microbes known as blue-green algae. These were responsible for the Great
Oxygenation Event which took place about 2.5 billion years ago [17]. Before
this event oxygen was not a major constituent of Earth’s atmosphere. After it,
the Earth had an atmosphere much closer to what we see today, with abundant
free oxygen available across the planet. In principle, the presence of oxygen
in the atmosphere of Earthlike exoplanets will be detectable by future
instruments through the absorption of ozone, O3, at mid-infrared wavelengths
[18].

Molecules that potentially indicate the presence of life, such as oxygen and
ozone, are known as biosignatures (see [19] and references therein). They
include oxygen, ozone, methane (CH4), N2O, C2H6 CH3Cl, CH3SH and more. A wider
variety of biosignatures than just oxygen and ozone are needed to cope with
potentially different biospheres than the one we currently inhabit. In fact,
for much of the history of life on Earth, there would not have been sufficient
oxygen or ozone for life to be detectable since the Earth was dominated by
anaerobic life (ie. life that does not require or produce oxygen). Searches
for other small molecules that might be additional biosignatures are also
underway [20]. The common factor among all of these biosignature molecules is
that they should not exist in the abundances seen unless biological processes
are maintaining their abundance ie. their abundance is out of equilibrium with
their environment. However, biological processes are not the only way of
maintaining an out-of-equlibrium abundance of some of these biosignatures. For
example, the splitting of water into hydrogen and oxygen by stellar
ultraviolet radiation, and the subsequent escape of hydrogen from the planet’s
atmosphere, can produce a significant partial pressure of oxygen on an abiotic
(ie. lifeless) planet given certain other conditions [21]. Care must therefore
be taken in interpreting any unusual abundance of a single molecule as an
unambiguous biosignature without a good understanding of the broader
environment in which it is found.

Phosphine, PH3, is one of the small molecules suggested as a potential
biosignature by the team investigating novel biosignature molecules [22]. On
Earth it is exclusively associated with anaerobic ecosystems, or with human
industrial chemistry. Sources have been found associated with anaerobic
environments in ponds, marshes and sludges, and specifically with piles of
penguin guano and in the faeces of European badgers. While it is clearly
associated with anaerobic biology, the specific biochemical pathway for
phosphine production in anaerobic systems remains unclear. Its use as a
biosignature was originally envisioned in the context of a significant
concentration of the gas within the atmosphere of a terrestrial exoplanet with
a large anaerobic biosphere. While phosphine has been detected in the vast
reducing (ie. hydrogen rich) atmosphere of the gas giant Jupiter [23], where
it is produced by normal chemical processes in the very dense, hot inner
regions then brought to the surface by convection, its presence in the
oxidised atmosphere of a terrestrial planet would be difficult to explain by
equilibrium chemistry, making it a good candidate biosignature. Observational
facilities able to find phosphine in the atmosphere of a distant exoplanet are
some way in the future, but, as we see below, a surprise was waiting for us
much closer to home.

##  4 Venus - an unlikely candidate for astrobiology

---
Figure 1: Left: Venus as seen in the optical by the Messenger mission. In the
optical the planet appears almost featureless because of the highly reflective
clouds that cover the entire planet. Credit: NASA/Johns Hopkins University
Applied Physics Laboratory/Carnegie Institution of Washington Right: Venus as
seen in the ultraviolet by the Japanese space mission AKATSUKI. This image is
produced by observations in two ultraviolet bands, at 365 and 283 nm. The
colours are the result of unexplained ultraviolet absorption by small
particles in the cloud layer. Credit: JAXA/ISAS/DARTS/Kevin M. Gill

###  4.1 Venus Today

Venus has often been described as Earth’s evil twin since the two planets have
very similar sizes and masses, with Venus having a radius about 95%, and a
mass about 82% of the Earth’s. It is also a little under 30% closer to the Sun
than the Earth. Viewed from a telescope, Venus appears as a featureless pale
disk because it has a thick atmosphere containing a permanent cloud layer,
opaque to optical observations, that reflects over 70% of the sunlight that
falls on it (see Figure 1). Venus’ orbit closer to the Sun, combined with the
effects of these reflective clouds, might naively suggest a surface
temperature not too different from the Earth. Because of this, in the early
20th century it was thought that the clouds might be water vapour and that the
surface of Venus could be habitable, inspiring visions of steamy tropical
jungles filled with alien life. Once more detailed observations became
available, and space probes started to visit in the 1960s, it became apparent
that Venus was very different from these initial speculations.

Rather than having an Earthlike atmosphere, early 1960s space probes like
NASA’s Mariner 2 and the Soviet Union’s Venera 4 found that Venus has an
extremely dense atmosphere dominated by carbon dioxide, CO2, with a small
amount of nitrogen (3.5%) and traces of other gases such as sulphur dioxide
(SO2). The surface atmospheric pressure on Venus is about 93 times higher than
sea level pressure on the Earth. This huge blanket of CO2 absorbs thermal
radiation from the surface of the planet which would otherwise be radiated
away into space. The Sun’s radiation is thus trapped beneath the clouds,
leading to a runaway greenhouse effect and surface temperatures of about 735 K
(462 C), making it hotter than the maximum surface temperature of Mercury
[24]111It is worth noting that the discovery of the runaway greenhouse effect
on Venus prompted early discussions about the dangers of CO2 emissions from
fossil fuel use on Earth and their possible impact on climate.. To make things
even more unpleasant, the thick clouds are largely made up of sulphuric acid
as a result of atmospheric SO2 dissolving into droplets of water that condense
at altitudes of about 55 km above the surface. All of this combines to make
the surface of Venus today an utterly inhospitable place for anything we might
consider a biological system. The true nature of the surface of Venus in fact
led leading astronomer Carl Sagan to describe the planet as hell.

Figure 2: A 360 degree panorama of the surface of Venus captured by the Venera
9 probe during its 53 minutes of operation on the harsh surface of the planet.
This is the first image ever obtained from the surface of Venus. Part of the
probe can be seen at the bottom of the image.

Spacecraft continued to visit Venus after the early missions, including a long
series of Venera probes launched by the Soviet Union. Several of these, as
well as the Pioneer Venus mission from NASA, sent probes into the atmosphere
of Venus to better understand its chemistry and constituents. The Venera and
Vega projects also attempted to land probes on the hostile surface of Venus.
After a number of failures they were eventually successful and conducted a
number of studies. None of the landers lasted very long, with the longest
lived surviving only about 2 hours. Nevertheless, some of these landers
managed to send back images of the Venusian surface, one of which can be seen
in Figure 2.

While the surface of Venus is undoubtedly hostile to life at the present time,
there is a potential niche for biological processes in the clouds that obscure
the planet since they are cool enough for liquid water to be present, and at
an atmospheric pressure that matches that of the Earth at sea level [25, 26]
(see Figure 3). Speculation about the possibility of life in the clouds of
Venus dates back to the very earliest days of our understanding of the true
conditions on the planet [27] and continues to the present day (eg. [28, 29]).

Figure 3: The atmospheric structure of Venus, showing how temperature and
pressure vary with height. The cloud layer at around 55km altitude has
temperature and atmospheric pressure levels that are comparable to those on
the surface of the Earth. [29]

###  4.2 Venus in the past

While the surface of Venus is undoubtedly hostile today, this might not have
been the case in the distant past. If the surface of Venus was warm and wet -
in the sense that liquid water could flow on the surface - then it is possible
that life might have emerged there. When conditions later become increasingly
hostile to life on the surface it might have evolved to seek sanctuary in
clouds, the last habitable ecological niche on the planet. But is there any
evidence to suggest that Venus was ever less hostile than it is today?

Sadly, we do not have direct access to information about the conditions on
Venus billions of years ago, but there are hints that suggest that Venus may
once have had much more water on its surface, and in its atmosphere, than it
does today. Principal among this evidence is the deuterium to hydrogen ratio
(D/H ratio). Venus currently has a D/H ratio that is 150$\pm$30 times that of
the Earth [30] 222Earth’s D/H ratio is $\sim$ 1.6 $\times$ 10-4. This suggests
that Venus has lost a substantial fraction of its water through the escape of
hydrogen, which is favoured over the escape of deuterium since the latter has
a higher mass. Hydrogen is driven off Venus through interactions with the
solar wind which can directly interact with the upper layers of its atmosphere
since, unlike the Earth, Venus lacks a protective magnetic field. However, it
is possible that Venus may have retained a magnetic field, and thus the
possibility of liquid water oceans, for several billion years after its
formation [31].

Computer simulations have been used to asses whether a warm wet Venus in the
first billion years of the Solar System would have a stable climate that could
be conducive to the emergence of life [32]. While the conclusions of this work
are still not agreed - some suggest that even a wet Venus would never have
been able to condense its water into oceans, leading to a so-called ‘steam
Earth’ scenario [33] \- it is intriguing to consider the possibility that
Venus may in fact have been the first habitable planet in the Solar System. A
warm wet Venus (see Figure 4) might have remained habitable until as recently
as about 700 million years ago, allowing substantial time for life to evolve
and propagate across its surface given that life seems to have emerged on
Earth somewhere between 3.7 and 4.3 billion years ago [12]. If this is
correct, it may only be in the most recent 15% of the age of the Solar System
that massive volcanic activity on Venus established a runaway greenhouse
effect on the planet, leading to water stripping and the hot, dry, hostile
surface that we see today.

Figure 4: An artist conception of what a warm, wet Venus might have looked
like during earlier stages in the evolution of the Solar System. Credit: NASA

##  5 Atmospheric mysteries

As well as uncertainty about the role of water in Venus’ past, there are also
some significant mysteries about the planet’s atmosphere today even before we
get to the detection of phosphine. Long-standing issues include ([34] and
references therein):

  * •

The variation of the abundance of water vapour and SO2 with altitude in and
above the cloud layers [35]. Water, H2O, persists throughout the atmosphere
but SO2 levels drop from parts per million abundances below the clouds to
parts per billion above. This is not what is expected given that both gases
are thought to be released by volcanism at the surface and to be well mixed
throughout the atmosphere until both are destroyed by solar UV at altitudes of
70 km or higher. The apparent depletion of SO2 in the clouds is currently not
understood.

  * •

Oxygen, O2, is present in the clouds of Venus where it was detected by gas
chromatographs on board Pioneer Venus Probe [36] and the Venera 13 and 14
descent modules [37]. There is currently no known process by which oxygen can
be formed in the cloud layers so its origin is something of a mystery.

  * •

Observations of the clouds of Venus in the ultraviolet reveal complex spatial
and temporal changes in absorption and reflection [8]. This is in contrast to
observations in the optical and near-infrared where the clouds of Venus appear
nearly featureless on the dayside (see eg. Figure 1). Significant cloud
contrasts are only seen in reflected sunlight at wavelengths shorter than
about 400 nm, and at near-infrared wavelengths (1.7 to 2.4 $\mu$m) in emission
on the nightside. These variations in ultraviolet absorption were first
observed in photographic observations in 1928 [38], but, despite all the
ground-based observations, spacecraft observations from orbit, and descent
probes sampling the atmosphere, the chemical and physical origin of the
absorber responsible remains unknown.

  * •

The clouds of Venus contain a variety of constituents. Based on size analysis
from the Pioneer Venus Probe particle size spectrometer [39] they can be
divided up into three particle sizes. These correspond to aerosols, of size
$\sim$0.4 $\mu$m, droplets of size $\sim$2 $\mu$m, and larger particles of
size $\sim$7 $\mu$m. The larger particles are present only in the middle and
lower cloud layers, at altitudes from 47.5 to 56.5 km above the surface. The
nature of these largest particles, which may have a substantial solid
component, and be non-spherical in shape, is currently unclear.

  * •

Recent reanalysis of the chemistry of Venus’ atmosphere based on measurements
by mass spectrometers on descent probes suggest the possibility of chemical
disequilibrium in the middle cloud layers [40]. This result is based on the
presence of several species in the mass spectrometer data, including hydrogen
sulphide, nitrous, nitric & hydrochloric acids, carbon monoxide, ethane, and
hydrogen cyanide as well as phosphine (this reanalysis was conducted after the
detection of phosphine by ground based observations, of which more later) and
possibly ammonia. This chemical mix indicates that reducing chemistry is
taking place in the clouds. If so, then the processes behind this activity
would be out of equilibrium with the oxidising chemistry of Venus. In the
context of the search for life elsewhere, chemical disequilibrium is a
potential biosignature, making these results very interesting. More recently
still, it seems that ammonia, NH3, may have been independently detected in
Venus’ atmosphere by ground based observations (Greaves et al., private
communication), confirming the tentative results from the Pioneer Venus Probe
mass spectrometer reanalysis, and adding an extra piece of evidence in favour
of chemical disequilibrium in the clouds of Venus.

These problems with our understanding of the atmosphere of Venus, and
specifically its clouds, make the case for renewed interest in the planet as a
possible astrobiology target. Even if biological processes do not provide the
explanation for these poorly understood phenomena, it is clear that there are
chemical and physical processes underway in the clouds of Venus that we do not
currently understand. Further observations to explore the chemistry of Venus
and its atmosphere are thus needed. It is in this context that a team of
astronomers proposed to look for phosphine, PH3, on Venus333The author of the
current paper was part of this team and is part of the ongoing work to study
phosphine on Venus..

##  6 The Search for Phosphine

Phosphine, as we have seen above, has been proposed as a potential
biosignature gas which might be present in significant quantities on inhabited
planets orbiting other stars [22]. Current observational facilities, however,
are not yet able to make phosphine observations of terrestrial exoplanets.
While we know that phosphine is present in the Earth’s atmosphere in small
amounts, thanks to industrial processes and anaerobic organisms, there were no
limits on the amount of phosphine present on other Solar System terrestrial
planets. Mercury has essentially no atmosphere so is an inappropriate target.
Mars has a very thin atmosphere so is unlikely to have much phosphine even if
there is biological activity underway there, and any phosphine present would
be rapidly destroyed by solar UV radiation. This leaves Venus as the only
reasonable terrestrial planet in the Solar System where some test observations
in search of phosphine might be conducted.

Therefore in 2016 a team of astronomers led by Prof Jane Greaves put together
a proposal to the James Clerk Maxwell Telescope (JCMT) to conduct test
observations of Venus’ atmosphere to look for absorption from the J=1-0
rotational transition of phosphine, which would produce an absorption line at
a wavelength of 1.123 mm ($\sim$ 267 GHz). There are other transitions of
phosphine at other wavelengths, notably in the far-IR and in the mid-IR, but
this particular transition has some advantages, Firstly, observations can be
conducted from the ground. Observations of the next highest rotational
transition, J=2-1, would require observations from the stratosphere (see
below). Mid-IR observations of other transitions can be conducted from the
ground, of which more later, but the Greaves team did not have easy access to
the necessary mid-IR facilities.

The initial idea for the observations was to acquire a few hours of data to
better understand the observational issues with looking for a weak absorption
line against a very bright continuum source, Venus, with the eventual intent
to propose a longer series of observations to set a stringent upper limit,
since phosphine was not expected to be found. That is not, however, how things
turned out.

###  6.1 JCMT Observations

The JCMT is a 15 m diameter mm/submm telescope on the mountain Mauna Kea on
the Big Island of Hawaii, at an altitude of about 4000 m. Mauna Kea is an
ideal site for mm/submm observations since it is both high and dry, and so
avoids much of the water vapour in the atmosphere of the Earth that would
otherwise absorb and contaminate observations at these wavelengths. It is
equipped with an array of instruments that operate both as continuum imagers
and as high resolution spectrometers. For the first set of JCMT phosphine
observations [6] an instrument called Receiver A3 (RxA3) was used. RxA3 was
one of the early instruments used on the JCMT and was delivered to the
telescope in 1998. It was retired not long after the phosphine detection
observations reported in [6].

Like many mm/submm spectroscopy receivers, RxA3 uses a heterodyne approach,
whereby the incoming astronomical signal, in this case at frequencies of
around 267 GHz, is multiplied by a pure sine wave signal at a nearby
frequency, the so-called local oscillator frequency, using a device called a
mixer. This results in the production of a signal at a frequency that
corresponds to the difference in frequencies between the received signal and
the local oscillator frequency, and allows signals at frequencies outside the
frequency range of interest to be removed. This lower frequency signal, at
what is called the intermediate frequency, or IF, is then measured and dealt
with by later stages of processing. The technology used in most mm/submm
receivers relies on SIS mixers (superconductor-insulator-superconductor) to
mix the astronomical and local oscillator frequencies. For more information on
how these operate, and on much else in radio astronomy, see [41].

The IF signal from RxA3 is then processed by the Auto-Correlation Spectral
Imaging System (ACSIS - this system is used by all spectral receivers at the
JCMT, including both RxA3 and its replacement ’Ū’ū). This digitises the input
IF signal, calculates the autocorrelation of the signal with itself -
essentially multiplying the signal by a time delayed version of itself - and
then calculates the Fourier transform of the autocorrelated signal. According
to the Wiener-Khinchin theorem [41], the autocorrelation of a signal is the
Fourier transform of the signal’s power spectral density ie. the amount power
received as a function of frequency. Fourier transforming the autocorrelation
of the IF signal thus gives us what we want - the spectrum of the source in
the frequency range of interest.

Figure 5: The James Clerk Maxwell Telescope, a 15 m diameter mm/submm
telescope on Mauna Kea in Hawaii, currently operated by the East Asian
Observatory. The 15 m primary mirror is protected from wind during
observations by a large gortex screen which is why it cannot be seen directly
even when the telescope is taking observations, as in this picture. Credit:
William Montgomerie/EAO/JCMT.

Venus was observed by the JCMT using RxA3 in search of phosphine on five
mornings in June 2017. These dates were chosen so that Venus appeared large
enough to fill the telescope beam, minimising any effects due to errors in
pointing the telescope. Venus is a strong continuum emitter at millimetre
wavelengths, so phosphine would be detected as a weak absorption line against
this strong continuum. This strong continuum, however, leads to a number of
problems with the quality of the data. A number of effects, including
reflections from the floor or roof of the telescope dome, or in the receiver
cabin itself, entering the beam, lead to strong, time varying baselines in the
output spectra. These have to be detected and removed. For the initial JCMT
detection of phosphine [6] these effects were removed by the usual method of
fitting polynomial functions to the data, excluding the region of the spectrum
where phosphine might lie. Once this process was applied to each of the 140
spectra that made up the observations, and despite the original assumption
that only an upper limit would be found, an absorption line ascribed to
phosphine was detected, corresponding to an abundance of about 20 to 25 parts
per billion (ppb). The JCMT spectrum of phosphine can be seen on the right
hand side in Figure 6.

Figure 6: The Phosphine 1.123mm J=1-0 line as detected by ALMA (left) [42] and
JCMT with RxA3 (right) [6]. The black lines indicate the level of SO2
absorption derived from simultaneous (ALMA) and near-simultaneous (JCMT)
observations. As can be seen the PH3 detections are clear and the SO2
contamination is minimal. These spectra are continuum subtracted, so zero on
the y-axis represents the continuum level. We use the standard astrophysics
approach for presenting high resolution spectra in this Figure, where the
spectrum is centred on the line of interest at zero velocity and frequencies
are indicated by the doppler velocity in km/s needed to shift from this
central value.

###  6.2 ALMA Observations

Figure 7: Some of the 64 antennae that make up the ALMA telescope. Credit:
ESO/C. Malin.

Following the rather surprising detection of phosphine at the JCMT some
further observations in search of independent confirmation of this discovery
were needed. To this end, observing time was granted on the Atacama Large
Millimetre/Submillimetre Array (ALMA) in March 2019. Despite operating at
similar mm/submm wavelengths, ALMA is a rather different telescope to the JCMT
because it is an interferometer. It is made up of 66 separate antennae, mostly
12m in diameter, the signals of which are combined together to produce the
final results. 43 of the 12 m antennae were used for the Venus phosphine
observations.

While the signals received by each ALMA antenna are dealt with in a manner
similar to RxA3 on the JCMT, using a heterodyne SIS mixer and local oscillator
in the receiver, these signals are then cross-correlated pairwise with those
from each of the other antennae in the array (where each pair of antennae
forms a ‘baseline’) to produce an interferometric map of the target.
Interferometry allows angular resolutions to be achieved that correspond to a
telescope whose diameter equals the longest baseline separating individual
antennae.

The cross-correlation of signals detected by each pair of antennae produces a
series of ‘visibilities’ which are a measure of the two-dimensional Fourier
transform of the sky distribution of brightness. The visibilities at each
observed frequency are then Fourier transformed to produce a series of images
at successive frequencies, i.e. a spectral cube. However, since only a finite
number of antennae pairs are available, even for an array with as many
antennae as ALMA, the Fourier plane is like a telescope with lots of holes.
Various methods are used in a process called ‘cleaning’ to derive the actual
image from the limited sampling in the Fourier plane. For more information on
interferometry see [41] or the ALMA
Primer444https://almascience.eso.org/documents-and-tools/cycle9/alma-science-
primer.

Processing interferometric data involves different challenges to those
encountered at the JCMT. For example, the angular size of Venus was so great
that even the shortest ALMA baselines could not provide good images on the
scale of the whole disc and the imperfect sampling led to strong ripples so
the data from the affected short baselines, all less than 33 m in length, were
removed. There were also strong spectral ripples on some parts of the planet,
such as the poles, which had to be excluded from further analysis otherwise
they would add noise to the spectra, reducing the sensitivity of the final
results. Further analysis of the ALMA data processing also found some errors
in the standard reduction script used, see Section 6.3.1, which improved on
the initial detection. The end result of the ALMA observations once all these
various effects are taken into account is shown in Figure 6 - a good detection
of phosphine absorption at a level of $\sim$20 ppb that matches what was seen
by the JCMT but with somewhat higher signal-to-noise.

###  6.3 The Detection of Phosphine from the Ground

The detection of phosphine in the atmosphere of Venus was, to say the least, a
surprise. The observations from JCMT and ALMA thus prompted a considerable
amount of debate and further observations using other facilities. In this
section we look at these various discussions, their conclusions, and counter-
arguments to the suggestion that phosphine has not been detected or that
whatever has been detected was not phosphine.

####  6.3.1 Reanalysis of the ALMA Data

One of the first responses to the Greaves et al. detection paper, [6], was a
reanalysis of the ALMA data by a separate group [43]. This analysis did not
reproduce the phosphine detection of [6], and instead found an upper limit to
the phosphine abundance of about 1 ppb. They identified some processes used in
the standard ALMA calibration scripts which were not adequate for a very
bright, time-varying, beam-filling target or indeed for the correspondingly
bright calibrator sources used. This led to reprocessing of the raw data by
the ALMA observatory and European Southern Observatory (ESO) staff
(independently of any of the research groups), who provided new scripts taking
these and additional problems into account. The reprocessing simplified the
basic removal of instrumental bandpass ripples using the moon Callisto as a
calibrator, and also avoided the chance of spectral averaging producing sharp
edges which could mimic an absorption line. The new scripts also accounted for
the non-linear instrumental response to the high intensity of Venus (the
brightest source in the sky after the Sun at these wavelengths) and its large
angular size, although, since this exceeds the extent of accurate models of
the response of individual ALMA dishes, this is thought to be a source of
residual error.

Greaves et al. responded to this reanalysis [44, 42] by employing the improved
observatory scripts and updating their own processing, using three different
independent methods to obtain final images and spectra. The first step after
observatory calibration is to remove the shortest baselines as explained
above, and then to make a simple, linear spectral fit to the visibility data
to remove the contribution of Venus. Next, residual spectral ripples can be
corrected either in the visibility data or after Fourier transforming to make
an image cube, and before or after cleaning. Spectra were extracted over
different portions of the planet; small residual errors meant that only those
spectra extracted from regions symmetric about the planet centre were
considered reliable. A range of parameters allowed the continued recovery of a
phosphine signal using all the updated methods, optimised at 7.7$\sigma$
significance by excluding the planetary poles [42]. They attributed the non-
recovery of the phosphine signal by [43] as a result of including baselines
shorter than 33 m in most of their analyses, as well as including parts of the
image of the planet that had significant spectral artefacts that raise the
noise in the final combined spectrum. They concluded that the phosphine
detection in the ALMA data remained robust.

####  6.3.2 Was it a real line?

A common feature of both the original ALMA and JCMT data analyses in [6] was
the use of fairly high order polynomials to allow the removal of varying
baselines. In doing this, it is necessary to mask out the region of the
spectrum around a suspected line otherwise the polynomial fitting method might
fit and remove a real line, mistaking it for a small scale baseline ripple.
Several authors suggested that this process can instead lead to the creation
of fake lines, and that this was in fact the origin of the claimed phosphine
detection [43, 45, 46]. There are two counter-arguments to this suggestion
that the detection is essentially a statistical false positive.

The argument that the claimed phosphine detection is a false positive is that
when you take the ripple-contaminated spectrum, block out a portion of it
where there might be a line, and use a sixth or higher order polynomial to fit
the baseline, then some noise spikes or contaminating ripples in the blocked
out section may end up looking like a line. This is in fact correct, and blind
searches for line candidates at random locations in the spectrum would indeed
suffer from this effect, significantly reducing confidence that any detections
are real. However, the detection of phosphine in [6] did not solely rely on
measuring the depth of an absorption line at a random position. Instead, it
also relied on the wavelength of the line seen coinciding with that of the
line being searched for, phosphine. This significantly reduces the chance of a
noise spike or residual masquerading as a phosphine detection. Analysis in
[47] shows that adding the additional constraint that a fake line must be at a
specific frequency reduces the chance of a false positive for line detection
to $<1.5\%$.

Furthermore, if the line was in fact a false positive then there would be no
reason for any such noise-generated feature to lie at exactly the same
frequency in both the ALMA and JCMT data. As pointed out in [6], the only
feature at matching wavelengths in both the ALMA and JCMT data lies at the
expected frequency of phosphine. This further bolsters our confidence that the
detected phosphine line is real, and not a statistical artefact resulting from
the data processing approach.

####  6.3.3 Is it really phosphine?

The foregoing analysis suggests that the line discovered is in fact real and
not a statistical false positive. However, can we be sure that it is in fact
phosphine and not some other molecular species that happens to have an
absorption feature at a similar frequency? Sulphur dioxide, SO2, a known
constituent of Venus’ atmosphere, has a transition due to the (J =
30${}_{9,21}-31_{8,24}$) transition at 266.943329 GHz, a frequency shift from
the PH3 J = 1-0 line at 266.944513 GHz that corresponds to a velocity
difference of just 1.3 km/s. The possibility that the claimed phosphine line
is actually a misidentification of this SO2 line was first suggested by [43]
and has been further explored by others [48, 49]. While they have concluded
that SO2 contamination or misidentification is a possibility, a number of
problems with this interpretation have been pointed out by [42]. Firstly,
while the line centres of PH3 J=1-0 and SO2 J = 30${}_{9,21}-31_{8,24}$ are
close, they are still 1.3 km/s apart, leading to a $\sim$ 3 $\sigma$
discrepancy between the measured line centre and that expected for the SO2
line. Furthermore, simultaneous (in the case of ALMA) and near-simultaneous
(in the case of JCMT) observations of a different and stronger SO2 line [42]
provide predictions of the relative strength of the SO2 transition that might
contaminate the phosphine line. They find that the level of contamination of
the phosphine line by SO2 is $\sim 10\%$ for the JCMT data and $<2\%$ for the
ALMA data. This level of contamination by SO2 is shown as a black line in
Figure 6. On this basis it seems likely that the detected line is indeed
phosphine, and that any contamination by the neighbouring SO2 line is
insignificant.

####  6.3.4 Other Observations

The phosphine J=1-0 line at 1.123 mm is not the only line of this molecule.
However, many of the other transitions are at wavelengths that are more
difficult to observe from the ground. Nevertheless, observations have been
attempted of other lines in search of independent confirmation of the presence
of phosphine.

The first of these used archival data from the TEXES (Texas Echelon Cross
Echelle Spectrograph) instrument, a 5 to 25$\mu$m high resolution mid-infrared
spectrometer, on the NASA Infrared Telescope Facility (IRTF) on Mauna Kea in
Hawaii [50]. These observations were part of a long term project to monitor
SO2 and H2O in the cloud tops of Venus, and involved observations at a range
of frequencies. One of these datasets, obtained in March 2015, fortuitously
included a range of wavelengths where there are some relatively strong
phosphine transitions, at a wavelength around 10.471 $\mu$m (corresponding to
a frequency of 28.65 THz). No phosphine absorption is detected, indicting an
upper limit of about 5 ppb, which is substantially lower than the claimed
millimetre wave phosphine detection.

Further infrared data, this time from the Venus Express spacecraft, were
analysed, looking for absorption from phosphine lines at wavelengths around
4.125 $\mu$m above the cloud layers [51]. This data was taken at various times
from June 2006 to December 2014, and measured absorption against the light of
the Sun as it rises or sets, rather than against the emission of Venus itself.
This means that only a small part of the atmosphere is studied rather than the
entire planetary disk as is the case, for example, for the JCMT or TEXES
observations. These Venus Express observations also failed to find any
phosphine absorption, setting limits on its abundance of 0.2 to 20 ppb
depending on the specific observations and the assumed altitude of the
absorption, ranging from 60 to 95 km.

Figure 8: The SOFIA observatory, which consists of a 2.5m telescope and
instruments mounted inside a 747 jumbo jet, and capable of flying above much
of the far-IR absorption in the Earth’s atmosphere. Credit: NASA/DLR

A third approach to confirm the detection of phosphine is to search for
absorption lines in the far-infrared, at frequencies around 534 and 1067 GHz
[52]. While the Earth’s atmosphere is completely opaque at these frequencies
at sea level and even on tall mountains like Mauna Kea, the SOFIA observatory
(Stratospheric Observatory For Infrared Astronomy) - essentially a 747 Jumbo
Jet with a hole cut in the fuselage with a 2.5m telescope pointing out (see
Figure 8) - can perform these observations since it flies at an altitude of
about 13 km, above much of the water vapour that absorbs far-IR radiation in
the Earth’s atmosphere555Sadly such observations can no longer be performed
since the SOFIA observatory was decommissioned and retired at the end of
September 2022..

SOFIA observations of Venus in search of phosphine were carried out in
November 2021 [52] using the GREAT (German REceiver At Terraherz frequencies)
instrument, a receiver similar to the JCMT receivers but operating at much
higher frequencies. Data reduction and analysis by the original authors failed
to find any sign of phosphine, setting an upper limit of 0.8 ppb from the
J=4-3 line and $\sim$ 2 ppb for the J=2-1 line. However, subsequent reanalysis
of the SOFIA data found that the calibration stage that sets an absolute flux
scale adds noise and artefacts to the resulting spectra. This calibration
stage is not needed if we are only interested in the line-to-continuum ratio,
as is the case when measuring an absorption line. By purely analysing the
line-to-continuum ratios [53], phosphine at a level of $\sim$1-2 ppb is found,
averaged over altitudes from 75-110 km, with 6.5$\sigma$ significance.

These other observations in search of phosphine absorption using different
approaches, whether from the ground or from Venus Express, have produced a
number of conflicting results. They need to be carefully interpreted since the
different wavelengths and observational approaches are in fact probing the
presence of phosphine at different altitudes and times, as we shall see below.
None has yet definitively disproved the original JCMT and ALMA results of [6],
and the SOFIA observations may in fact have provided some level of
confirmation, depending on which analysis approach is used.

####  6.3.5 In Situ Confirmation

The ideal way to determine the presence and amount of phosphine in the
atmosphere of Venus would be to send a space probe directly into the
atmosphere equipped with instrumentation that can detect and measure the
presence of the gas in situ. This would avoid all the difficulties of
observing phosphine remotely, as well as all issues of interpretation. At this
point, as we will see below, we are some years away from any such future
mission. However, past missions to Venus did send probes into the planet’s
atmosphere. Principal among these, for our current purposes, is the Pioneer
Venus Multiprobe (also known as Pioneer Venus 2 or Pioneer 13) [54] (see
Figure 9) which, among many other instruments, sent a mass spectrometer into
the atmosphere of Venus on its largest entry probe.

Figure 9: The Pioneer Venus Probe Spacecraft. The mission consisted of an
orbiter and four probes that were sent into the atmosphere of Venus, seen here
in artists conception. Credit: NASA

Data from the Pioneer Venus Large Probe’s Neutral Mass Spectrometer (LNMS)
were reanalysed in 2021 [40] subsequent to the announcement of the discovery
of phosphine by [6]. This reanalysis of data taken during its descent into the
atmosphere of Venus on 9 December 1978, was the first to look for trace or
minor constituents of the atmosphere beyond methane and water. The LNMS takes
gas in from the atmosphere through inlet tubes. Molecules in the gas are then
ionised by an electron source, accelerated by an electric field and then
passed through a magnetic field which deflects the ions by an amount that
depends on their mass. These ions are subsequently detected, allowing their
mass and abundance to be determined. For more information see [55].

The data reanalysed in search of phosphine came from within the clouds, at an
altitude of 51.3 km above the surface, part of the atmosphere that is largely
inaccessible to ground or space based observations, but which is of critical
importance to searches for possible life in the clouds, as this is where that
life might actually live. The detailed analysis found evidence for phosphine
at 0.1 to 2 parts per million (ppm) levels in the clouds themselves, a much
higher abundance than is seen in the JCMT or ALMA observations. It also found
evidence for other species such as nitrite, nitrate, nitrogen and possibly
ammonia. Taken together these molecules indicate that unexpected chemical
processes are underway in the clouds and suggest chemical disequilibrium.
Whether this disequilibrium is due to biological or some other as-yet unknown
chemical process is yet to be determined.

###  6.4 Where and When is the Phosphine Seen?

The forgoing sections, describing the various observations in search of
phosphine in the atmosphere of Venus, can seem confusing and mutually
contradictory. This is at least partly because observational constraints mean
that they sample the atmosphere of Venus at different altitudes and, because
they encompass datasets that span over 40 years, at different times. We
already know that some species in Venus’ atmosphere are highly variable - SO2
levels, for example, can vary by large factors on timescales of both years and
days at various altitudes [56] \- so this may also apply to phosphine. Spatial
variations across the disk of the planet are also possible, but this is
difficult to assess for phosphine since many of the observations to date have
been of the average phosphine level across the planetary disk.

Of particular importance is the amount of phosphine in the atmosphere as a
function of altitude. While the in situ observations of the LNMS and Venus
Express have a clearly determined altitude, this is harder to extract for the
observations from Earth. In principle, the effect of pressure broadening on
the absorption lines can be used to determine the vertical abundance profile
of an absorbing molecule. Pressure broadening of an absorption line occurs
when the absorbing molecules interact collisionally with other molecules in
the atmosphere. These interactions shorten the characteristic time of the
absorption process, in accordance with Heisenberg’s uncertainty principle,
increasing the uncertainty of the absorption frequency and thus broadening the
line (see eg. [57]). The overall effect is to make the line shape a Lorentzian
function, which has much broader wings than the usually assumed Gaussian
shape. The exact width of the Lorentzian line depends on the pressure,
temperature and nature of the molecules that are interacting. The higher the
pressure, the broader the wings, so a full analysis of the shape of the
phosphine absorption line can reveal its vertical abundance profile in the
atmosphere of Venus.

There are, however, a number of problems with a full pressure broadening
analysis of the phosphine line seen in the atmosphere of Venus. Firstly, the
pressure broadening coefficient for phosphine in CO2, the dominant constituent
of Venus’ atmosphere, is not currently known. Analyses have so far used either
a modification of the phosphine broadening coefficient in air [50] or have
used the CO2 pressure broadening coefficient for NH3 as an analog to that of
phosphine [6]. Secondly, and more significantly for the immediate
understanding of phosphine in Venus, the data reduction techniques used to
date to extract the absorption line remove any broad line wings as part of the
process that removes baseline ripples. This just leaves narrow line cores,
meaning that the observations are insensitive to any significantly broadened
lines, and thus are only sensitive to phosphine at altitudes of 75 to 80 km.
Most recently, an experimental data processing approach applied to new
observations of Venus from the JCMT-Venus project (PI: D.L. Clements) seems to
be able to recover the broad line wings of the J=1-0 phosphine line,
suggesting an abundance of phosphine at the ppm level inside the clouds at an
altitude of about 60 km, consistent with the high levels seen by the LNMS.

The other factor to consider is the timing of the observations. While we do
not yet have enough observations to allow us to monitor any changes in the
abundance of phosphine with time or in relation to other species such as HDO
or SO2, we can see if there are any correlations between the amounts of
phosphine seen and the timing of the observations relative to the illumination
of Venus’ atmosphere by the Sun. This may well be an important factor since
photolysis by sunlight is a significant destruction route for phosphine in the
Earth’s atmosphere [22]. If we combine all the phosphine observations -
detections and non-detections - together with information about whether the
Sun is rising or setting on the atmosphere at the time of observation we
perhaps begin to see a pattern (see Figure 10).

Figure 10: The trend of phosphine abundance by altitude from the currently
available data. Shading indicates cloud (orange, centred at $\sim$ 60 km) and
haze (grey, centred at $\sim$ 80 km and 40 km) layers of Venus’ atmosphere.
Superposed symbols indicate candidate detections plus upper limits for
phosphine abundance. Rising arrows indicate observations made where the
atmosphere was rising into sunlight and falling arrows indicate observations
made when the atmosphere was descending towards the nightside. Abundances are,
from top: 20, 25 ppb from J=1-0 data [6]; $\sim$1 ppb or $<$ 0.8 ppb from
J=4-3 data [52, 53]; $<7$ ppb at 62 km from 4 $\mu$m spectra [51]; $<$ 5ppb at
60 km from 10 $\mu$m spectra [50]; $\sim$2 ppm at 60 km from initial JCMT-
Venus processing; high ppb to 2 ppm at 51 km from Pioneer-Venus in situ
sampling [40]. As can be seen all the significant detections of phosphine take
place as the atmosphere is moving out of night and into sunlight, while the
non-detections take place as the atmosphere is moving from sunlight into
night. If sunlight destroys phosphine at high altitudes during daylight, as is
the case on Earth, this would explain the apparent contradictions between some
of the observations. From: Greaves et al. in prep, by permission.

##  7 The (Im)Possible Origins of Phosphine on Venus

The presence of phosphine in the atmosphere of Venus is a surprise since a
compound of phosphorous with hydrogen should not naturally appear in the
atmosphere of a planet, such as Venus, which has an oxidised atmosphere. On
Earth, phosphine does not occur through normal chemical processes and is
produced only by anaerobic life or through human industrial activity. While
this is the expectation, Venus is a complex environment with a wide range of
chemical and physical processes underway from the surface to the top of the
atmosphere. A detailed analysis is thus necessary to see if there are any
possible routes through which the levels of phosphine seen might occur through
normal chemical processes. Such an analysis was conducted in [58] where a wide
range of chemical processes were examined to see if there is any potential
source of phosphine in sufficient abundance to explain the observations with
processes that we know are underway on the planet. The processes examined
included gas reactions, geochemical reactions, photochemistry, volcanism (see
also [59]), lightning and impactors. An example of the kind of chemical
reaction network considered is shown in Figure 11, where the reaction rate and
the destruction rates are compared. Only segments of this reaction network
where the ratio of the production rate over the destruction rate is $\geq 1$
can produce an accumulation of the relevant chemical. For phosphine to be
produced in significant amounts the whole reaction network must have this
ratio $\geq 1$ but, as can be seen, critical segments of the network have
ratios orders of magnitudes less than this. More generally, it was found that
the lifetime of free phosphine at various altitudes in the atmosphere of Venus
ranged from $<$ 1 second to perhaps a century [59], making it highly unlikely
that a significant amount of phosphine can accumulate from any hypothetical
source.

Figure 11: Potential chemical pathways for the synthesis of phosphine in the
atmosphere of Venus, and their derived production vs. destruction rate. There
are stages where, for all possible pathways, the rate of destruction of
phosphine exceeds its formation by many orders of magnitude, as shown in
red/purple. As can be seen, there is no route to produce phosphine by these
processes that can account for the amounts observed. From [58] where more
details can be found.

The most obvious conclusion that can be drawn from this analysis is that we do
not know how phosphine came to be in the atmosphere of Venus. There may be
geochemical or photochemical processes that can produce it in sufficient
amounts, but these are currently not known to us. The alternative, that, by
analogy with Earth, phosphine is being produced by anaerobic biological
processes, is another potential explanation. However, before we can make this
particular leap, and claim that we have found evidence for life in the clouds
of Venus, we must first exclude all other possible origins, and also explain
how life is able to survive in the extremely acidic environment of Venusian
cloud droplets. One possible solution to the latter problem is that ammonia,
if present, is able to buffer the sulphuric acid in these droplets to some
extent [34]. The possible detection of ammonia in the clouds of Venus by the
LNMS [40] and in preliminary analysis of data from the Green Bank Telescope
(Greaves et al., private communication), is thus rather interesting.

##  8 The Next Steps

As has become clear in the previous section, studies of phosphine, and the
search for life on Venus, are very much works in progress. While the current
results are intriguing, there are no solid conclusions that can yet be
determined. Much more work needs to be done, and it will be the work of many
years before we can have a definitive answer to the question of whether there
is life in the clouds of Venus. This will require not only observations from
Earth, but also in situ probes and, ideally, missions that can return samples
from Venus to Earth. In this section we look at some of the projects that are
planned or already underway to improve our knowledge of the clouds of Venus.

###  8.1 Earth-based Studies

Observations from Earth were responsible for the first detections of
phosphine, and these are continuing to both monitor phosphine and to search
for other molecules that may have a bearing on the chemistry, or biochemistry,
underway in the clouds of Venus.

The largest of the projects currently underway is JCMT-Venus (PI: D.L.
Clements). This uses the ’Ū’ū receiver, the replacement for RxA3, together
with the ACSIS system to obtain whole disk spectra for Venus. The new receiver
has a wider bandwidth than RxA3 so we can simultaneously observe phosphine, HD
and SO2, and search for other molecules such as SO and PO2 which have spectral
features in the band covered by ’Ū’ū. By simultaneously monitoring phosphine,
HDO and SO2 we can see how these different species vary in relation to each
other. This should provide indications as to the chemical processes behind the
presence of phosphine. If, for example, phosphine is produced by reducing
processes in the upper atmosphere, the proportions of reduced compounds, like
phosphine, and oxidized compounds, like HDO and SO2, will be anticorrelated.
The JCMT-Venus project is a long term programme at the JCMT and has been
awarded 200 hours of time over a period of three years. The visibility of
Venus means that observations will be possible in three tranches, including
Feb 2022, July 2023 and September 2023. The first of these observing campaigns
has already taken place, with Venus observed over a period of 20 consecutive
mornings. The data obtained already contains 140 times as much information as
in the original JCMT observations, so is taking some time to process and
analyse, especially since ’Ū’ū has its own difficulties dealing with the
brightness of Venus and thus an interesting new set of baseline drifts and
ripples to be removed. Nevertheless the analysis is well underway and initial
results, some of which have been briefly discussed above, have already
emerged, including further confirmation of the presence of phosphine. When
complete, JCMT-Venus will provide a major new database of observations of
Venus in the mm band, including phosphine and other important molecules which
will provide significant new insights into the origin of phosphine.

Further ALMA observations have yet to be approved, but these hold the promise
of providing further information about the distribution of phosphine across
the face of the planet. The original ALMA observations provided some hints
that the distribution is not uniform, but a full map of the abundance of
phosphine across the planetary disk could not be made because of excess
ripples affecting the signal over significant portions of the disk. Additional
observations in the mid-IR from the IRTF and elsewhere will also be helpful.
Sadly, further observations with SOFIA are not possible since the observatory
has been decommissioned.

Studies related to the search for potential signs of life on Venus are also
underway that do not directly target phosphine. These include observations
with the Green Bank Telescope (GBT) at radio wavelengths to look for ammonia
(NH3) in absorption. This is important since detection of ammonia would
indicate the presence of another reduced molecule that should not be expected
in the oxidised atmosphere of Venus. Ammonia is also important because of its
buffering effect against the high acidity in the liquid droplets in Venus’
clouds [34]. Analysis of archival data from the 1970s as well as an initial
set of observations with the GBT suggest the presence of ammonia (Greaves et
al., private communication), as do the in situ measurements of the LNMS, but
more data is needed to confirm this.

Laboratory studies also have a role to play since they can validate and test
the various assumptions that went into the analysis of [34], and allow more
accurate predictions for the formation and destruction of phosphine and other
hydrogen-rich compounds in Venus-like conditions. Such studies are already
being planned.

###  8.2 Space-based studies

Venus is also being studied from much closer quarters by space probes. These
missions take many years to prepare and so have largely not been designed to
examine the possibilities of unusual chemistry, or even life, in the clouds of
Venus. Nevertheless, existing missions do have useful capabilities for these
purposes and future missions are being planned that can respond to recent
discoveries.

The Japanese mission AKATSUKI (see eg. Figure 1) is currently operating in
orbit around Venus. While it does not have any instruments that are directly
relevant to the search for phosphine, its UV imaging instruments are
monitoring the unidentified UV absorber, the origin of which is one of the
outstanding mysteries of the Venusian atmosphere. Comparing AKATSUKI’s results
with future data from the ground, especially any future imaging observations
with ALMA, may be able to see if there is a link between the presence of
phosphine and the presence of the UV absorber.

The next potentially important space mission to go to Venus, from the point of
view of phosphine observations, is not in fact a specific mission to Venus,
but the JUICE mission to the moons of Jupiter [5]. The JUICE spacecraft is
scheduled to be launched in the second quarter of 2023. It will then perform a
series of flybys of planets as gravity assists on its journey to Jupiter. Of
particular interest here is the flyby of Venus in August 2025 where an
observational campaign is possible. Of particular importance in the context of
phosphine on Venus is the Submillimetre Wave Instrument (SWI) which will be
able to observe higher J transitions of phosphine, including those observed
from the Earth by SOFIA. Whether JUICE will be able to undertake an observing
campaign at Venus will be up to the JUICE mission directors, and no decision
will be made until after launch.

In the 2030s three missions directly targeted at Venus are due to be launched.
These include the European Space Agency’s EnVISION mission [60], and NASA’s
VERITAS (Venus Emissivity, Radio Science, InSAR, Topography, and Spectroscopy)
[61] and DAVINCI (Deep Atmosphere Venus Investigation of Noble gases,
Chemistry, and Imaging) [62] missions. VERITAS and EnVISION are primarily
concerned with the surface and interior of Venus, studying the history and
role of volcanism on the planet. While they will doubtless reveal much that is
of interest, they are unlikely to have much to say about phosphine and the
processes underway in the clouds of Venus unless they uncover volcanic
activity vastly in excess of our current understanding [59]. DAVINCI, however,
is a much more interesting prospect.

The goal of DAVINCI is to study the atmosphere of Venus. To do this its
primary set of instruments are on board a descent stage that will fly through
the clouds of Venus, sampling the atmosphere as it goes. It will be the first
NASA mission to enter the atmosphere of Venus since Pioneer Venus Probe in
1978. Among the instruments on the descent stage is a mass spectrometer that
will be able to significantly improve on the results of the LNMS. This will be
able to detect phosphine and other trace gas species and see how their
abundance changes with altitude and other conditions. Other instruments
include a tuneable laser spectrometer which is able to measure even small
amounts of specific gases. Altogether, the four instruments on the DAVINCI
probe, combined with imagers on the orbiting mothership, will provide a vast
improvement in our in situ knowledge of Venus’ atmosphere. It will provide the
ground truth against which observations of the planet from Earth can be
compared.

National and international space agencies are not the only organisations
looking to send probes to Venus. Private companies now have the capability to
send missions to other planets independently of governments, and they are also
interested in the possibility of life on Venus. One company in particular,
Rocket Lab, is taking special interest in Venus and has set up a team to
develop a series of Venus Life Finder (VLF) missions [63]. The first of these
missions, which may launch as soon as mid-2023, is intended to look for
organic molecules using an ultraviolet autofluorescence technique. Further
missions are planned including a balloon borne laboratory that will be able to
float in the clouds for an extended period. Amongst the planned instruments
for this payload are not only mass spectrometers but also a microscope that
will search cloud droplets for evidence of biological cells.

Perhaps the most ambitious mission planned by the VLF team is a sample return
mission that will use a balloon to collect samples of cloud droplets and gas,
and return these to Earth for detailed laboratory study. If there is in fact
life in the clouds of Venus, a mission like this will be necessary to answer
fundamental questions about its origin and how it operates. It is perhaps the
dream mission in the search for evidence of life on Venus.

##  9 Conclusions

The discovery of phosphine in the atmosphere of Venus has caused some
controversy and has renewed discussions about the possibility of life in the
planet’s clouds. The observational evidence for phosphine has been challenged
and examined in detail. The JCMT and ALMA results have so far survived these
challenges, and there has been independent in situ confirmation of the
presence of phosphine from the Pioneer Venus LNMS instrument. Observations
from other telescopes in search of phosphine have produced rather more mixed
results, with several upper limits and one possible detection. However, the
apparent disagreement between these different sets of observations may soon be
understood in the context of day-night variations in the amount of phosphine
above the clouds thanks to photolysis by sunlight.

While the presence of phosphine in the atmosphere of Venus is becoming more
secure with the arrival of new and improved datasets such as JCMT-Venus, an
understanding of its origin still eludes us. It is clear that no conventional
chemical process can produce phosphine in the amounts observed, but it is
still far from clear whether biological processes are involved, or if there is
some as-yet unknown non-biological source.

More data is clearly necessary for us to understand what is really going on in
the atmosphere of Venus, and this is being sought by a number of different
ground and space-based approaches. Over the next several years our
understanding of the origin of phosphine on Venus will certainly improve, and
we will hopefully reach a point at which the question of life in the clouds of
Venus moves from being something that we can only speculate about, to
something about which we have clear and decisive knowledge. Whatever
conclusion we finally reach, we will have learnt a lot more about our nearest
neighbour planet, and this knowledge will help guide our search for possible
biospheres on planets orbiting other stars. Confirmation that there is in fact
life in the clouds of Venus would be a truly epoch making discovery, but we
are still a very long way from drawing that conclusion.

Acknowledgements It is a pleasure to thank Jane Greaves, Janusz Petkowski,
Anita Richards, and Wei Tang for many useful comments. It is also a pleasure
to thank all members of the phosphine team for their enthusiasm and expertise
in what has already been quite an exciting, and unexpected, adventure, which,
for me, started in a bar in Hilo.

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