In puffs of gas from rocks more than 3 billion years
old dug up by one of NASA's robotic explorers on Mars, scientists have
identified several complex organic molecules — possible building blocks
for ancient life.
It's not aliens. (It's never aliens.)
But it is “consistent with the past presence of biology,” said Ken Williford,
an astrobiologist at NASA's Jet Propulsion Laboratory. “And it makes us
more confident that if biomarkers” — or direct evidence of biologic
activity — “are there, we might find them.”
In two studies published today in the journal Science, this new finding from NASA's Curiosity rover is paired with another discovery:
The planet's methane — another organic molecule usually (but not
always) produced by living beings — varies with the seasons. In the
past, scientists have seen plumes and patches of this intriguing
substance, but this is the first time they've been able to discern a
pattern in its presence. The result could pave the way for future
missions to pin down the methane's source.
“The closer we look, the more we see that Mars is a
complex, dynamic planet that — particularly early in its history — was
more conducive to life than we might have previously imagined,” said
Williford, who was not involved in either study.
A
reminder: Organic molecules aren't necessarily produced by organisms;
they're just chemical compounds that contain carbon. But they're of
interest to astrobiologists because they are the essential ingredients
in all the chemistry that drives life on Earth.
Mars's
Gale Crater, where Curiosity has been trolling around for the past six
years, is a particularly interesting place to look for those
molecules. About 3.5 billion years ago, research suggests, this pockmark
on the Martian surface was brimming with water.
But
the water vanished when most of the Martian atmosphere was stripped
away by brutal solar winds. And, given the intensity of the radiation
bombarding the planet's surface, it wasn't clear whether any relics from
that warm, wet period could still be preserved in mudstones on the
lake's dried-up floor.
Using Curiosity's Sample Analysis at Mars instrument — which heats soil
and rock samples to examine their contents — astrobiologist Jennifer Eigenbrode
and her colleagues were able to identify an array of interesting
organic molecules: ring structures known as aromatics, sulfur compounds
and long carbon chains. Even more compelling was the fact that these
compounds seemed to have broken off even bigger, more complex
“macromolecules” — substances found on Earth in coal, black shale and
other ancient organic remains.
“What we have detected is what we would expect from
a sample from an ancient lake environment on Earth,” said Eigenbrode,
of NASA's Goddard Space Flight Center.
There
are some non-biological explanations for the detection — this
combination of compounds has also been found in meteorites. But that
explanation, too, suggests a provocative possibility; even if the
organic molecules didn't come from life, they are exactly what life
likes to eat. Perhaps the meteorite-delivered molecules provided fuel
for ancient alien organisms.
Regardless,
the detection is a technical achievement, said Williford, because it
demonstrates that organic molecules can persist near Mars's surface for
billions of years. If scientists keep drilling deeper and more widely,
as they plan to do with the European and Russian space agencies' ExoMars
rover and NASA's Mars 2020 mission, who knows what they might find?
(Williford is deputy project scientist for Mars 2020.)
The methane study, spearheaded by JPL atmospheric scientist Chris Webster,
is also intriguing for astrobiologists. On Earth, 1,800 out of every
billion molecules in the atmosphere is methane, and 95 percent of it
comes for biological sources: burning fossil fuels, decomposing debris, burping cows.
Some of our planet's earliest organisms may have been methanogens —
microbes that eat organic molecules and exhale methane gas.
Several spacecraft including Curiosity have detected whiffs of this gas
that “defied explanation,” Webster said. Methane is quickly broken down
by radiation, so it must be replenished by some source on the planet.
One explanation “that no one talks about but is in the back of
everyone's mind,” as Goddard planetary scientist Mike Mumma put it to Science last winter, is that methanogens beneath the Martian surface were breathing it out.
“You'd expect life to be seasonal,” Mumma noted. But it was also
possible that puffs of methane were delivered to the desert world by
crashing meteorites or other less thrilling sources.
By examining data spanning nearly three Martian
years (six Earth years), Webster and his colleagues discerned the first
repeating pattern in Martian methane. During the summer months, levels
of the gas detected by Curiosity rose to about 0.7 parts per billion; in
winter, they fell to roughly half that. They suggest that warmer
conditions might release the gas from reservoirs beneath the surface.
The
results don't explain shorter-lived spikes in methane levels — as high
as 45 parts per billion — that have been detected. And even if the
reservoir explanation is correct, it remains to be seen what's feeding
them.
To determine whether the methane is
biological, Webster said, scientists can weigh the kinds of carbon atoms
it contains (life prefers the lighter versions). Future missions might
also seek places where there's “significant seepage” and attempt to
figure out its source.
In a commentary for Science, astrobiologist Inge Loes ten Kate of the Utrecht University in the Netherlands, explained what makes these two studies so compelling:
“Curiosity
has shown that Gale crater was habitable around 3.5 billion years ago,
with conditions comparable to those on the early Earth, where life
evolved around that time,” she wrote. “The question of whether life
might have originated or existed on Mars is a lot more opportune now
that we know that organic molecules were present on its surface at that
time.”
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