After analyzing powdered rock samples collected from the surface of Mars by NASA’s Curiosity rover, scientists today announced that several of the samples are rich in a type of carbon that on Earth is associated with biological processes.
While the finding
is intriguing, it doesn’t necessarily point to ancient life on Mars, as
scientists have not yet found conclusive supporting evidence of ancient or
current biology there, such as sedimentary rock formations produced by ancient
bacteria, or a diversity of complex organic molecules formed by life.
“We’re finding
things on Mars that are tantalizingly interesting, but we would really need
more evidence to say we’ve identified life,” said Paul Mahaffy, who served as the principal
investigator of the Sample Analysis at Mars (SAM) chemistry
lab aboard Curiosity until retiring from NASA’s Goddard Space Flight Center in
Greenbelt, Maryland, in December 2021. “So we’re looking at what else could
have caused the carbon signature we’re seeing, if not life.”
This image shows the Highfield drill hole made by NASA’s Curiosity rover as it was collecting a sample on Vera Rubin Ridge in Gale crater on Mars. Drill powder from this hole was enriched in carbon 12. The image was taken by the Mars Hand Lens Imager on the 2,247th Martian day, or sol, of the mission. Credits: NASA/Caltech-JPL/MSSS.
In a report of their findings to be
published in the Proceedings of the National Academy of Sciences journal on
January 18, Curiosity scientists offer several explanations for the unusual
carbon signals they detected. Their hypotheses are drawn partly from carbon
signatures on Earth, but scientists warn the two planets are so different they
can’t make definitive conclusions based on Earth examples.
“The hardest thing is letting go of Earth and letting
go of that bias that we have and really trying to get into the fundamentals of
the chemistry, physics and environmental processes on Mars,” said Goddard
astrobiologist Jennifer L. Eigenbrode,
who participated in the carbon study. Previously, Eigenbrode led an
international team of Curiosity scientists in the detection of myriad organic molecules —
ones that contain carbon — on the Martian surface.
“We need to open our minds and think
outside the box,” Eigenbrode said, “and that’s what this paper does.”
The biological explanation Curiosity scientists
present in their paper is inspired by Earth life. It involves ancient bacteria
in the surface that would have produced a unique carbon signature as they
released methane into the atmosphere where ultraviolet light would have
converted that gas into larger, more complex molecules. These new molecules
would have rained down to the surface and now could be preserved with their
distinct carbon signature in Martian rocks.
Two other hypotheses offer nonbiological explanations.
One suggests the carbon signature could have resulted from the interaction of
ultraviolet light with carbon dioxide gas in the Martian atmosphere, producing
new carbon-containing molecules that would have settled to the surface. And the
other speculates that the carbon could have been left behind from a rare event
hundreds of millions of years ago when the solar system passed through a giant
molecular cloud rich in the type of carbon detected.
“All three explanations fit the data,” said Christopher
House, a Curiosity scientist based at Penn
State who led the carbon study. “We simply need more data to rule them in or
out.”
To analyze carbon in the Martian surface,
House’s team used the Tunable Laser
Spectrometer (TLS) instrument inside the SAM lab.
SAM heated 24 samples from geologically diverse locations in the planet’s Gale
crater to about 1,500 degrees Fahrenheit, or 850 degrees Celsius, to release
the gases inside. Then the TLS measured the isotopes from some of the reduced
carbon that was set free in the heating process. Isotopes are atoms of an
element with different masses due to their distinct number of neutrons, and
they are instrumental in understanding the chemical and biological evolution of
planets.
Carbon is particularly important since this element is
found in all life on Earth; it flows continuously through the air, water, and
ground in a cycle that’s well understood thanks to isotope measurements.
For instance, living creatures on Earth use the
smaller, lighter carbon 12 atom to metabolize food or for photosynthesis versus
the heavier carbon 13 atom. Thus, significantly more carbon 12 than carbon 13
in ancient rocks, along with other evidence, suggests to scientists they’re
looking at signatures of life-related chemistry. Looking at the ratio of these
two carbon isotopes helps Earth scientists tell what type of life they’re
looking at and the environment it lived in.
On Mars, Curiosity researchers found that nearly half
of their samples had surprisingly large amounts of carbon 12 compared to what
scientists have measured in the Martian atmosphere and meteorites. These
samples came from five distinct locations in Gale crater, the researchers
report, which may be related in that all the locations have well-preserved,
ancient surfaces.
“On Earth, processes that would produce the carbon
signal we’re detecting on Mars are biological,” House said. “We have to
understand whether the same explanation works for Mars, or if there are other
explanations, because Mars is very different.”
Mars is unique because it may have started off with a
different mix of carbon isotopes than Earth 4.5 billion years ago. Mars is
smaller, cooler, has weaker gravity, and different gases in its atmosphere.
Additionally, the carbon on Mars could be cycling without any life involved.
“There’s a huge chunk of the carbon cycle on Earth
that involves life, and because of life, there is a chunk of the carbon cycle
on Earth we can’t understand, because everywhere we look there is life,” said Andrew Steele, a Curiosity scientist based at the Carnegie Institution for Science in
Washington, D.C.
This mosaic was made from images taken by the Mast Camera aboard NASA’s Curiosity rover on the 2,729th Martian day, or sol, of the mission. It shows the landscape of the Stimson sandstone formation in Gale crater. In this general location, Curiosity drilled the Edinburgh drill hole, a sample from which was enriched in carbon 12. Credits: NASA/Caltech-JPL/MSSS
Steele noted that scientists are in the early stages of understanding how
carbon cycles on Mars and, thus, how to interpret isotopic ratios and the
nonbiological activities that could lead to those ratios. Curiosity, which
arrived on the Red Planet in 2012, is the first rover with tools to study
carbon isotopes in the surface. Other missions have collected information about
isotopic signatures in the atmosphere, and scientists have measured ratios of
Martian meteorites that have been collected on Earth.
“Defining the carbon cycle on Mars is absolutely key to trying to
understand how life could fit into that cycle,” Steele said. “We have done that
really successfully on Earth, but we are just beginning to define that cycle
for Mars.”
Curiosity scientists will continue to measure carbon isotopes to see if
they get a similar signature when the rover visits other sites suspected to
have well-preserved ancient surfaces. To further test the biological hypothesis
involving methane-producing microorganisms, the Curiosity team would like to
analyze the carbon content of a methane plume released from the surface. The
rover unexpectedly encountered such a plume in 2019 but there’s no way to
predict whether that will happen again. Otherwise, researchers point out that this
study provides guidance to the team behind NASA’s Perseverance rover on the best types
of samples to collect to confirm the carbon signature and determine
definitively whether it’s coming from life or not. Perseverance is collecting
samples from the Martian surface for possible
future return to Earth.
Curiosity’s mission is led by NASA’s Jet Propulsion Laboratory in Southern
California; JPL is managed by Caltech.
Banner image: NASA’s
Curiosity rover captured these clouds just after sunset on March 19, 2021, the
3,063rd Martian day, or sol, of the rover’s mission. The image is made up of 21
individual images stitched together and color-corrected so that the scene appears
as it would to the human eye. Credits: NASA/JPL-Caltech/MSSS
By Lonnie Shekhtman
NASA’s Goddard Space Flight Center, Greenbelt, Md.
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