Sky & Telescope - May 2021 - 39

G REGG DINDER M A N / S&T, SOURCE: J. G RE AV ES E T A L. / A R XIV 2011.0 8176

²⁰¹⁸

ExoMars Trace Gas
Orbiter (TGO) reports
absence of methane, with
upper limit of 0.05 ppbv

2013

2014

²⁰¹³

2015

Curiosity
measures
methane
spike of
6 ppbv

2016

2017

2018

²⁰¹⁹

2019

Curiosity measures
methane spike of
21 ppbv at night. Mars
Express detects none
at daytime, 5 hours later

Fraction emitted
Fraction absorbed

The third pillar of the biosignature framework is atmospheric
science, a broad and highly computational specialty that
looks at the physics, chemistry, and dynamics of planetary
atmospheres. Scientists build environmental simulations
starting with physical and chemical rules, then input observations and other facts about the world in question. Then
they " press go " and watch what happens to their toy planet.
Do the results align with the real world, or does everything
fall apart?
Through a process of trial and error, these simulations
help us to understand both local phenomena like weather and
even global phenomena, such as the lifetime of methane and
oxygen in Earth's atmosphere. Models that focus on chemistry allow scientists to make predictions about what might
be causing unexpected gas emissions. But even a detection of
methane and oxygen in thermodynamic disequilibrium on
a rocky world in a star's habitable zone wouldn't necessarily
" prove " anything. After all, there might be completely abiotic
ways to produce this that we are unaware of.
Sukrit Ranjan (now at Northwestern University) had the
task of ﬁguring out how well phosphine could survive in
Venus's hot and acidic atmosphere. First, he took everything
known about Venus's surface, subsurface, and atmospheric
composition (which is far from complete). He then introduced disruptive simulations of things like volcanic eruptions
and meteorite impacts to the model. In theory, these events
could create phosphine. But they didn't make enough.

4×10-5
2×10-5
0
-2×10-5
-4×10-5
-6×10-5

-8×10-5
-0.00010
266.1

266.161

266.2

Radio frequency (GHz)
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Cloudy with a Chance of Phosphine

Fraction absorbed

useful reference for the community.
" Our database is a tool for modeling planetary atmospheres, " Petkowski says. " Volatile gases react with other
atmospheric components. Some will not be detectable,
because they will be destroyed by atmospheric radicals and
UV. Our goal was to ﬁnd if there are groups of gases that are
better candidates to be remotely detected than others. The
creation of this database is just the ﬁrst step on that road. "

4 ×10-5
2 ×10-5

0
-2 ×10-5
-4 × 10-5
-6 ×10-5
-8 ×10-5
-0.00010

266.9

266.944

267.0

Radio frequency (GHz)
S A SIGN OF PHOSPHINE? These plots are two narrow slices of Venus's spectrum, taken by ALMA and fixed from an earlier version. Jane
Greaves and her team took targeted observations of Venus at multiple
frequencies, one centered where they would expect to see absorption
from phosphine (bottom), and another where they should see the more
familiar signal of the water isotope HDO (top). The observed bands
were too narrow to include many other molecules, but the absorption
features (dips at centers of plots) appeared right where expected for both
molecules. Finding HDO's absorption line where they expected it gives
the researchers confidence that they're correctly identifying PH3. The
PH3 line also matched the predicted phosphine spectrum (dotted line),
calculated from lab measurements and scaled to the conditions in Venus's atmosphere. The V-shape for both absorption features is spreading
caused by the molecules' motion in the planet's atmosphere.

" We tried to see if abiotic mechanisms could explain the
presence of phosphine at the inferred abundances, " Ranjan
says. " But we couldn't account for it. "
The phosphine detection itself is tentative, and a recent
ﬁ x of the primary data out of ALMA has reduced astronomers' conﬁdence in the observation. But even if future in
situ observations verify the sighting, this doesn't mean we've
discovered alien life.
The phosphine debate may have a similarly drawn-out fate
as methane on Mars. Methane is easily destroyed by sunlight
and should be fairly short-lived on the Red Planet. Astronos k y a n d t e l e s c o p e . o r g * M AY 2 0 2 1

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