by Johnny Bontemps
A careful study of images taken by the NASA rover Curiosity
has revealed intriguing similarities between ancient sedimentary rocks
on Mars and structures shaped by microbes on Earth. The findings
suggest, but do not prove, that life may have existed earlier on the Red
Planet.
The photos were taken as the Mars rover Curiosity
drove through the Gillespie Lake outcrop in Yellowknife Bay, a dry
lakebed that underwent seasonal flooding billions of years ago. Mars and
Earth shared a similar early history. The Red Planet was a much warmer
and
wetter world back then.
On Earth, carpet-like colonies of microbes trap and rearrange sediments
in shallow bodies of water such as lakes and coastal areas, forming
distinctive features that fossilize over time. These structures, known
as
microbially-induced sedimentary structures (or MISS), are found in shallow water settings all over the world and in ancient rocks spanning Earth's history.
Nora Noffke, a geobiologist at Old Dominion University in Virginia, has
spent the past 20 years studying these microbial structures. Last year,
she reported the discovery of MISS that are 3.48 billion years old in
the Western Australia's Dresser Formation, making them potentially the
oldest signs of life on Earth.
In a
paper published online last month in the journal
Astrobiology
(the print version comes out this week), Noffke details the striking
morphological similarities between Martian sedimentary structures in the
Gillespie Lake outcrop (which is at most 3.7 billion years old) and
microbial structures on Earth.
The distinctive shapes include
erosional remnants, pockets, domes, roll-ups, pits, chips and cracks,
which on Earth can extend from a few centimeters to many kilometers.
Although Noffke makes a tantalizing case for possible signs of
ancient life on Mars,
her report is not a definitive proof that these structures were shaped
by biology. Getting such confirmation would involve returning rock
samples to Earth and conducting additional microscopic analyses, a
mission that isn’t scheduled anytime in the near future.
"All I
can say is, here's my hypothesis and here's all the evidence that I
have," Noffke says, "although I do think that this evidence is a lot."
"The fact that she pointed out these structures is a great contribution to the field," says
Penelope Boston, a geomicrobiologist at the New Mexico Institute of Mining and Technology. "Along with the recent reports of methane and organics on Mars, her findings add an intriguing piece to the puzzle of a possible history for life on our neighboring planet."
A careful analysis
"I've seen many papers that say 'Look, here's a pile of dirt on Mars, and here's a pile of dirt on Earth,'" says
Chris McKay,
a planetary scientist at NASA's Ames Research Center and an associate
editor of the journal Astrobiology. "And because they look the same, the
same mechanism must have made each pile on the two planets.'"
McKay adds: "That's an easy argument to make, and it's typically not
very convincing. However, Noffke's paper is the most carefully done
analysis of the sort that I’ve seen, which is why it's the first of its
kind published in Astrobiology."
The images on which Noffke drew are publicly available on the
Mars Science Laboratory page on NASA’s website.
"In one image, I saw something that looked very familiar," Noffke
recalls. "So I took a closer look, meaning I spent several weeks
investigating certain images centimeter by centimeter, drawing sketches,
and comparing them to data from terrestrial structures. And I've worked
on these for 20 years, so I knew what to look for."
Noffke compared the rover pictures to images taken at several sites on
Earth, including modern sediment surfaces in Mellum Island, Germany;
Portsmouth Island, USA; and Carbla Point, Western Australia; as well as
older fossils of microbial mats in Bahar Alouane, Tunisia; the Pongola
Supergroup in Africa; and the Dresser Formation in Western Australia.
The photos showed striking morphological similarities between the terrestrial and Martian sedimentary structures.
The distribution patterns of the microbial structures on Earth vary
depending on where they are found. Different types of structures are
found together in different types of environments. For instance,
microbial mats that grow in rivers will create a different set of
associations than those that grow in seasonally flooded environments.
The patterns found in the Gillespie Lake outcrop are
consistent with the microbial structures found in similar environments
on Earth.
What’s more, the terrestrial structures change in a
specific way over time. As the microbial mats form, grow, dry up, crack
and re-grow, specific structures become associated with them. Here
again, Noffke found that the distribution pattern in Martian rocks
correspond with microbial structures on Earth that have changed over
time. Taken together, these clues strengthen her argument beyond simply
pointing out the similarities in shape.
In her paper, she also
describes alternative processes through which these could have formed.
For instance, the chips, pits and cracks could be the product of erosion
by salt, water, or wind.
"But if the Martian structures aren’t
of biological origin," Noffke says, "then the similarities in
morphology, but also in distribution patterns with regards to MISS on
Earth would be an extraordinary coincidence."
"At this point, all I’d like to do is point out these similarities,"
she adds. "Further evidence must be provided to verify this hypothesis."
Confirmation pending
At the end of her report, Noffke outlines a detailed strategy for
confirming the potential biological nature of the Martian structures.
Unfortunately, one important step — returning samples to Earth for
further analyses — is just not feasible yet.
Noffke also lists a series of measurements Curiosity could potentially
do to strengthen the case if it came across such structures again,
including looking for organic or chemical signatures using its
Sample Analysis at Mars (SAM) instrument.
But McKay says this likely would not work. "In principle, that
instrument could tell us something about the nature of these materials
biologically, if there were still large amounts of biological organics
in the samples," he explains. "But these are just ancient sedimentary
structures, and biology has long since left."
"What's more, in practice this instrument is restricted," he adds.
"There was a contamination spill in the instrument presumably during
landing. So it has a very high background contamination level."
On Earth, scientists typically confirm the biological nature of
microbial sediment structures by searching for specific microscopic
textures, which involves cutting rocks into thin slices and studying
them under a microscope.
On
Mars, this would be very difficult do from an engineering perspective,
although McKay doesn’t rule out the possibility for future missions. "I
don't know if it can be done, but engineers are pretty smart," he says.
"If you give them a challenge, they usually find a solution."
He adds: "A sample-return mission would be the gold standard. But that's just unlikely to happen anytime soon."
