What happened to early Mars’ atmosphere? New study eliminates one theory

Scientists may be closer to solving the mystery of how Mars
changed from a world with surface water billions of years ago to the arid Red
Planet of today.

A new analysis of the largest known deposit of carbonate
minerals on Mars suggests that the original Martian atmosphere may have already
lost most of its carbon dioxide by the era of valley network formation.

“The biggest carbonate deposit on Mars has, at most,
twice as much carbon in it as the current Mars atmosphere,” said Bethany
Ehlmann of the California Institute of Technology and NASA Jet Propulsion
Laboratory, both in Pasadena. “Even if you combined all known carbon
reservoirs together, it is still nowhere near enough to sequester the thick
atmosphere that has been proposed for the time when there were rivers flowing
on the Martian surface.”

Carbon dioxide makes up most of the Martian atmosphere. That
gas can be pulled out of the air and sequestered or pulled into the ground by
chemical reactions with rocks to form carbonate minerals. Years before the
series of successful Mars missions, many scientists expected to find large
Martian deposits of carbonates holding much of the carbon from the planet’s
original atmosphere. Instead, these missions have found low concentrations of
carbonate distributed widely, and only a few concentrated deposits. By far the
largest known carbonate-rich deposit on Mars covers an area at least the size
of Delaware, and maybe as large as Arizona, in a region called Nili Fossae.

Christopher Edwards, a former Caltech researcher now with
the U.S. Geological Survey in Flagstaff, Arizona, and Ehlmann reported the
findings and analysis in a paper posted online by the journal Geology. Their
estimate of how much carbon is locked into the Nili Fossae carbonate deposit
uses observations from numerous Mars missions, including the Thermal Emission
Spectrometer (TES) on NASA’s Mars Global Surveyor orbiter, the mineral-mapping
Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) and two telescopic
cameras on NASA’s Mars Reconnaissance Orbiter, and the Thermal Emission Imaging
System (THEMIS) on NASA’s Mars Odyssey orbiter.

Edwards and Ehlmann compare their tally of sequestered
carbon at Nili Fossae to what would be needed to account for an early Mars
atmosphere dense enough to sustain surface waters during the period when
flowing rivers left their mark by cutting extensive river-valley networks. By
their estimate, it would require more than 35 carbonate deposits the size of
the one examined at Nili Fossae. They deem it unlikely that so many large
deposits have been overlooked in numerous detailed orbiter surveys of the
planet. While deposits from an even earlier time in Mars history could be
deeper and better hidden, they don’t help solve the thin-atmosphere conundrum
at the time the river-cut valleys formed.

The modern Martian atmosphere is too tenuous for liquid
water to persist on the surface. A denser atmosphere on ancient Mars could have
kept water from immediately evaporating. It could also have allowed parts of
the planet to be warm enough to keep liquid water from freezing. But if the
atmosphere was once thicker, what happened to it? One possible explanation is
that Mars did have a much denser atmosphere during its flowing-rivers period,
and then lost most of it to outer space from the top of the atmosphere, rather
than by sequestration in minerals.

“Maybe the atmosphere wasn’t so thick by the time of
valley network formation,” Edwards said. “Instead of Mars that was
wet and warm, maybe it was cold and wet with an atmosphere that had already
thinned. How warm would it need to have been for the valleys to form? Not very.
In most locations, you could have had snow and ice instead of rain. You just
have to nudge above the freezing point to get water to thaw and flow
occasionally, and that doesn’t require very much atmosphere.”

NASA’s Curiosity Mars rover mission has found evidence of ancient
top-of-atmosphere loss, based on the modern Mars atmosphere’s ratio of heavier
carbon to lighter carbon. Uncertainty remains about how much of that loss
occurred before the period of valley formation; much may have happened earlier.
NASA’s MAVEN orbiter, examining the outer atmosphere of Mars since late 2014,
may help reduce that uncertainty.

Arizona State University, Tempe, provided the TES and THEMIS
instruments. The Johns Hopkins University Applied Physics Laboratory, Laurel,
Maryland., provided CRISM. JPL, a division of Caltech, manages the Mars
Reconnaissance Orbiter and Mars Odyssey project for NASA’s Science Mission
Directorate, Washington, and managed the Mars Global Surveyor project through
its nine years of orbiter operations at Mars. Lockheed Martin Space Systems in
Denver built the three orbiters.

For more information about the Mars Reconnaissance Orbiter
mission, visit:

For more information about the Mars Odyssey mission, visit:

Story Source:

The above post is reprinted from materials provided by
NASA/Jet Propulsion Laboratory. Note: Materials may be edited for content and

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Posted on September 9, 2015, in Useful Information. Bookmark the permalink. Leave a comment.

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