In findings that are as scientifically significant as
they are crushing to the popular imagination, NASA
reported Thursday that its Mars rover, Curiosity, has
deflated hopes that life could be thriving on Mars
today.
The conclusion, published in the journal Science,
comes from the fact that Curiosity has been looking
for methane, a gas that is considered a possible
calling card of microbes, and has so far found none
of it. While the absence of methane does not rule out
the possibility of present-day life on Mars — there
are plenty of microbes, on Earth at least, that do not
produce methane — it does return the idea to the
realm of pure speculation without any hopeful data
to back it up.
The history of human fascination with the possibility
of life on Mars is rich, encompassing myriad works of
science fiction, Percival Lowell’s quixotic efforts to
map what turned out to be imaginary canals, Orson
Welles’s panic-inducing 1938 “War of the Worlds”
radio play, and of course Bugs Bunny’s nemesis,
Marvin the Martian.
But Marvin apparently did not emit enough methane
for Curiosity’s sensitive instruments to find him.
A view of Gale Crater near Mars’s equator. The
panorama comprises nearly 900 images taken by
Curiosity.
/ NASA, VIA ASSOCIATED PRESS
“You don’t have direct evidence that there is
microbial process going on,” said Sushil K. Atreya, a
professor of atmospheric and space science at the
University of Michigan and a member of the science
team.
But NASA scientists are going strictly by their data,
and they are leery about drawing broader
implications to the question once posed by David
Bowie, “Is there life on Mars?” John P. Grotzinger,
the project scientist for the Curiosity mission, would
go only so far as to say that the lack of this gas “does
diminish” the possibility of methane-exhaling
creatures going about their business on Mars.
“It would have been great if we got methane,” Dr.
Atreya said. “It just isn’t there.”
Curiosity, which has been trundling across the
planet for a little over a year, made measurements
from Martian spring to late summer, coming up
empty for methane.
The Darwin site, with rocks of particular interest.
NASA / JPL-CALTECH / MALIN SPACE / EUROPEAN
PRESSPHOTO AGENCY
Scientists have long thought that Mars, warm and
wet in its early years, could have been hospitable for
life, and the new findings do not mean that it was
not. But that was about three and a half billion years
ago. Methane molecules break apart over a few
centuries — victims of the Sun’s ultraviolet light and
of chemical reactions in the atmosphere — so any
methane in the air from primordial times would have
disappeared long ago.
That is why reports of huge plumes of methane
rising over Mars in 2003 fueled fresh hopes for
Martian microbes. Those findings, based on data
from telescopes on Earth and a spacecraft orbiting
Mars, set off a surge of speculation and scientific
interest.
On Earth, most of the methane comes from micro-
organisms known as methanogens, but the gas is
also produced without living organisms, in
hydrothermal vents. Either possibility would be a
surprising result for Mars.
After the 2003 methane readings, “a lot people got
excited and started working on it,” said Christopher
R. Webster of NASA’s Jet Propulsion Laboratory in
Pasadena, Calif., and the lead author of the paper in
Science. “It was a very important result, because of
the magnitude of methane.” The fresh data from
Curiosity brings the earlier claims into question.
Not everyone is daunted. Robert Zubrin, president of
the Mars Society, a nonprofit group dedicated to the
planet’s exploration and settlement, said he was still
convinced that Martian life was waiting to be
discovered in underground aquifers.
“If it had found methane, that would have been
killer,” Dr. Zubrin said, referring to Curiosity. “Yes,
it’s disappointing in that we didn’t get a pony for
Christmas. But it doesn’t mean there aren’t ponies
out there.”
One of the scientists who found the methane plumes
in 2003, Michael J. Mumma, of NASA’s Goddard
Space Flight Center in Greenbelt, Md., said in an
interview this week he was certain that his earlier
measurements were still valid. He said he now
believed that methane on Mars was episodic —
released in large plumes and then quickly destroyed.
He suggested, half-jokingly, that there could be
huge colonies of methane-eating microbes on Mars
that eliminated the gas from the air.
Dr. Mumma acknowledged that he could not identify
any phenomena that would explain why methane
plumes spurted out that year but not more recently,
or how methane could be destroyed much more
quickly on Mars than on Earth.
“Mars may not be operating the same way,” he said.
“It’s a puzzle.”
Dr. Atreya of the Curiosity team said he originally
thought that highly reactive chemicals on the
Martian surface could be destroying methane, as Dr.
Mumma envisioned. But “that’s not panning out,”
Dr. Atreya said.
A simpler explanation would be that there was never
much in the way of methane — or microbes — on
Mars.
Kim Stanley Robinson, a science fiction author,
wrote three novels in the 1990s about the
colonization of Mars by people from Earth, which in
his version of things begins in 2026. Except for one
conversation between two scientists, he completely
leaves out the possibility of indigenous Martian
microbes.
“In my Mars trilogy, I assumed what everyone
assumed back then, which was that it was a dead
rock,” Mr. Robinson said by e-mail on Thursday.
“Actually, it would be very problematic to write that
book today.”
These days there are plans, on paper, to send
humans to Mars in roughly Mr. Robinson’s time
frame. One of them, a private effort called Mars One,
which has yet to prove it has the technology to
achieve its goals, has nevertheless attracted
hundreds of thousands of people to apply for a one-
way trip, which theoretically would arrive in 2023.
Mars is smaller than Earth and would have cooled off
sooner after the formation of the solar system. Some
scientists have even suggested that all life on Earth
could be descended from Martian microbes that
were carried here embedded within meteorites. As
the surface of Mars turned cold and dry and most of
the air dispersed to space, microbes could have
migrated underground and persisted, the thinking
goes.
To pursue the methane mystery, Curiosity was
outfitted with an instrument that can measure
minute quantities of methane
and other gases. The first measurements by
Curiosity last fall showed a definite signal from
methane. “When we saw it for it for the first time, we
went ‘Oh, my gosh,’ ” Dr. Webster said.
But that turned out to be from residual air from
Earth carried all the way to Mars. Once the Earth air
was pumped away, the methane readings
disappeared, too. Last November, the scientists
reported an upper limit of 6 parts per billion. Now
they have pushed that down to 1.3 parts per billion
and expect to improve their precision by at least
another factor of 10 in the coming months.
As exciting as it is to see the beautiful full-color
pictures of the Martian landscape that Curiosity
sends back, it is the tantalizing prospect of creatures
living on a neighboring planet that fuels public
interest the most, space enthusiasts say.
“That’s the mythology,” said Seth Shostak, an
astronomer with the Seti Institute in Mountain View,
Calif., which searches for intelligent life in the
universe. “Mars is about life, not geology, as
interesting as that is. That’s the triumph of hope
over measurement, and maybe it is.”
In a month, India is to launch a Mars orbiter that
also has a methane-measuring instrument to look for
the gas from orbit. “They may be disappointed when
they try to create maps of methane,” Dr. Webster
said.
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