Sky & Telescope - May 2021 - 38

Looking for Life

It's All About the Gas
The All Small Molecules project (ASM) at MIT is a database
of more than 16,000 molecules meant to guide research on
biosignature gases. Sara Seager started it in the early 2010s
with William Bains and Janusz Petkowski, and they published
the list in 2015. The project was a tacit admission that until
we have full workups on all the small molecules produced by
life, we do not have even the most basic foundational knowledge with which to gauge whether a particular detection is a
biosignature or not. So they set out to organize what is known

2001

2002

¹⁹⁹⁹-²⁰⁰³

2003

Ground-based observations
indicate 10 ppbv; later work
suggests values 0-50 ppbv
(Note: the 1999 observations
were reported in 2004.)

M AY 2 0 2 1 * S K Y & T E L E S C O P E

into a kind of Webster's Dictionary for astrobiologists.
" Life on Earth produces thousands upon thousands of
different volatile gases, " Petkowski says. " So, what we tried to
do with ASM is to come up with an exhaustive list of all these
gases, which then could be assessed in terms of their biosignature potential. "
Unlike ExoMol, which produces spectral information that
is generally useful to astronomy, the ASM database was built
explicitly to aid in the search for extraterrestrial life. Most of
the information it contains was not created by the team but
collected by trawling existing literature. Certain large gaps in
our knowledge have emerged, such as microbial byproducts.
It turns out that biologists don't do a lot of fundamental
research into trace gases produced by microbes unless they
pose a threat to human life. This is how we know about the
bacteria that produce phosphine: The toxic gas was popping
up in water treatment and sewage plants, and people wanted
to know why.
An offshoot project called RASCALL (Rapid Approximate
Spectral Calculations for All) aims to take this work a step
further and obtain spectra for all of the ASM molecules. This
contribution will still not be as good as a laboratory measurement, like what Stanford's Gasdynamics Lab does, or a full ab
initio quantum chemical calculation, like what ExoMol does.
But that's okay. RASCALL and ASM are not trying to create
knowledge, but rather to collect what is already known into a

2004

2005

2006

Created
eated by Oleksandr
Oleksandr Panasovskyi
Panasovskyi
om the Noun
Noun Pr
Project
oject
rom

²⁰¹²-²⁰¹⁴

Mars Express
preliminary
measurements report
variations of 0-30 ppbv

A summary of key methane
measurements at Mars

2000

Organisms that don't need oxygen or would
die in its presence might sound alien, but these
anaerobic organisms are more common on
Earth than you might think. Examples include
E. coli and Vibrio cholerae (which causes
cholera). They're also widespread in your
mouth and lower gastrointestinal tract.

²⁰⁰⁴

Mars Methane Mystery

1999

ANAEROBES

A N A EROBES: OLEKSA NDR PA N ASOVSK YI / THE NOUN PROJECT; TIMELINE:
TERRI DUBÉ / S&T, SOURCE: ESA; M A RS L A NDSCA PE: N ASA / JPL / COR NELL

ide, and methane. They have the longest, most comprehensive
experimental record, and their biosignature potentials are
relatively well understood.
When it comes to expanding beyond these molecules,
most astrobiology research leans on extremophiles as aliensby-proxy. Extremophiles are microorganisms that live in conditions deadly to " normal " lifeforms - environments with
extreme temperatures, chemical concentrations, or pH levels.
One example is anaerobes, which either don't need oxygen
or would die if it's present. Venus's atmosphere has very little
free oxygen, so any life there would be anaerobic. Phosphine
is a byproduct of anaerobic bacteria here on Earth. And as scientists know of no abiotic production method for phosphine
on rocky worlds, it is potentially a very good biosignature.
Clara Sousa-Silva (now Center for Astrophysics, Harvard &
Smithsonian), one of the researchers on the phosphine study,
completed her PhD at UCL. While working for ExoMol, she
built a template of more than 16 billion different spectral
permutations for phosphine, each of which could be a signal
a telescope might pick up, depending on the planet's conditions. The uncertainties about our neighbor's atmosphere
are huge, however. If you shift the gas content, mixing, and
altitude of the models just a tiny bit, one chemical ﬁngerprint starts to look very much like another. This is one of the
reasons other researchers suspect the group actually detected
something else in Venus's clouds, perhaps sulfur dioxide.

Mars Express ﬁnds no
methane, except for one
15 ppbv spike one day
after Curiosity detection

2007

2008

2009

2010

2011

²⁰¹²-²⁰¹⁸

Curiosity's
ﬁrst years of data
suggest seasonal
background variation
of 0.2-0.7 ppbv

2012

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