This view combines information from two instruments on a NASA Mars orbiter to map color-coded composition over the shape of the ground within the Nili Fossae plains region of Mars. Credit: NASA/JPL-Caltech/JHUAPL/Univ. of Arizona
How did Mars change from a world with 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 CO2 by the era of valley network formation.
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 in Nili Fossae.
The estimate of how much carbon is locked into the Nili Fossae uses Thermal Emission Spectrometer (TES) on NASA’s Mars Global Surveyor orbiter, mineral-mapping Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) and 2 telescopic cameras on NASA’s Mars Reconnaissance Orbiter, and Thermal Emission Imaging System (THEMIS) on NASA’s Mars Odyssey orbiter.
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. It would take >35 carbonate deposits the size of the one examined at Nili Fossae. It is unlikely that so many large deposits have been overlooked in numerous detailed orbiter surveys of the planet. 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.
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 has found evidence of ancient top-of-atmosphere loss, based on the modern Mars atmosphere’s ratio of heavier to lighter C. 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.
http://www.jpl.nasa.gov/news/news.php?feature=4708
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