Astronomers searching the atmospheres of alien worlds for gases that
might be produced by life can't rely on
the detection of just one type,
such as oxygen, ozone, or methane, because in some cases these gases can
be produced non-biologically, according to extensive simulations by
researchers in the NASA Astrobiology Institute’s Virtual Planetary
Laboratory.
The researchers carefully simulated the atmospheric chemistry of
alien worlds devoid of life thousands of times over a period of more
than four years, varying the atmospheric compositions and star types.
"When we ran these calculations, we found that in some cases, there was a
significant amount of ozone that built up in the atmosphere, despite
there not being any oxygen flowing into the atmosphere," said Shawn
Domagal-Goldman of NASA's Goddard Space Flight Center in Greenbelt,
Maryland. "This has important implications for our future plans to look
for life beyond Earth."
Methane is a carbon atom bound to four hydrogen atoms. On Earth, much
of it is produced biologically (flatulent cows are a classic example),
but it can also be made inorganically; for example, volcanoes at the
bottom of the ocean can release the gas after it is produced by
reactions of rocks with seawater.
Ozone and oxygen were previously thought to be stronger biosignatures
on their own. Ozone is three atoms of oxygen bound together. On Earth,
it is produced when molecular oxygen (two oxygen atoms) and atomic
oxygen (a single oxygen atom) combine, after the atomic oxygen is
created by other reactions powered by sunlight or lightning. Life is the
dominant source of the molecular oxygen on our planet, as the gas is
produced by photosynthesis in plants and microscopic, single-cell
organisms. Because life dominates the production of oxygen, and oxygen
is needed for ozone, both gases were thought to be relatively strong
biosignatures. But this study demonstrated that both molecular oxygen
and ozone can be made without life when ultraviolet light breaks apart
carbon dioxide (a carbon atom bound to two oxygen atoms). Their research
suggests this non-biological process could create enough ozone for it
to be detectable across space, so the detection of ozone by itself would
not be a definitive sign of life.
"However, our research strengthens the argument that methane and
oxygen together, or methane and ozone together, are still strong
signatures of life," said Domagal-Goldman. "We tried really, really hard
to make false-positive signals for life, and we did find some, but only
for oxygen, ozone, or methane by themselves." Domagal-Goldman and
Antígona Segura from the Universidad Nacional Autónoma de México in
Mexico City are lead authors of a paper about this research, along with
astronomer Victoria Meadows, geologist Mark Claire, and Tyler Robison,
an expert on what Earth would look like as an extrasolar planet. The
paper appeared in the Astrophysical Journal Sept. 10, and is available
online.
Methane and oxygen molecules together are a reliable sign of
biological activity because methane doesn't last long in an atmosphere
containing oxygen-bearing molecules. "It's like college students and
pizza," says Domagal-Goldman. "If you see pizza in a room, and there are
also college students in that room, chances are the pizza was freshly
delivered, because the students will quickly eat the pizza. The same
goes for methane and oxygen. If both are seen together in an atmosphere,
the methane was freshly delivered because the oxygen will be part of a
network of reactions that will consume the methane. You know the methane
is being replenished. The best way to replenish methane in the presence
of oxygen is with life. The opposite is true, as well. In order to keep
the oxygen around in an atmosphere that has a lot of methane, you have
to replenish the oxygen, and the best way to do that is with life."
Scientists have used computer models to simulate the atmospheric
chemistry on planets beyond our solar system (exoplanets) before, and
the team used a similar model in its research. However, the researchers
also developed a program to automatically compute the calculations
thousands of times, so they could see the results with a wider range of
atmospheric compositions and star types.
In doing these simulations, the team made sure they balanced the
reactions that could put oxygen molecules in the atmosphere with the
reactions that might remove them from the atmosphere. For example,
oxygen can react with iron on the surface of a planet to make iron
oxides; this is what gives most red rocks their color. A similar process
has colored the dust on Mars, giving the Red Planet its distinctive
hue. Calculating the appearance of a balanced atmosphere is important
because this balance would allow the atmosphere to persist for
geological time scales. Given that planetary lifetimes are measured in
billions of years, it's unlikely astronomers will happen by chance to be
observing a planet during a temporary surge of oxygen or methane
lasting just thousands or even millions of years.
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