[meteorite-list] What Happened to Early Mars' Atmosphere? New Study Eliminates One Theory

[Ron Baalke]

http://www.jpl.nasa.gov/news/news.php?feature=4708

What Happened to Early Mars' Atmosphere? New Study Eliminates One Theory
Jet Propulsion Laboratory
September 2, 2015

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:

http://mars.nasa.gov/mro

For more information about the Mars Odyssey mission, visit:

http://mars.nasa.gov/odyssey


Media Contact

Guy Webster
Jet Propulsion Laboratory, Pasadena, Calif.
818-354-6278
guy.webster at jpl.nasa.gov 

Jennifer LaVista
U.S. Geological Survey, Denver
303-202-4764
jlavista at usgs.gov 

2015-288