Isotopic Constraints on the Origin and Evolution of Martian Volatiles
NASA’s Mars 2020 “Perseverance” rover, China’s first Mars mission “Tianwen-1”, and the United Arab Emirates’ “Hope” Mars orbiter have all recently successfully launched to eventually orbit or land on the red planet. Their scheduled arrivals in seven months will undoubtedly advance our understanding of Mars. From the first fly-by space probes to more recent Mars rovers, “follow the water” is one of the key themes of martian exploration, as water, and volatiles more broadly, are essential for discovering extraterrestrial life and examining habitability.
To characterize the volatiles of bulk Mars and compare to those of other terrestrial planetary bodies, several elemental and isotopic ratios have been developed over the past few decades, such as D/H, (Na,Ga,Br)/Al, K/(La,Th,U), Mn/Na, and Rb/Sr, which can be acquired through either direct analysis on martian meteorites or remote sensing of the martian surface (e.g., Odyssey GRS). According to these indexes, Mars has a higher inventory of volatiles than Earth. Therefore, Mars can be viewed as a volatile-rich (hence water-rich) planet. Although this conclusion is consistent with Mars’ greater heliodistance and has since become the current paradigm, it is sharply contradictory to the present-day conditions of Mars vs. Earth. One could explain this discrepancy with solar wind stripping mechanism as recently observed by MAVEN. However, how much volatiles have been removed over geological history through this mechanism remains uncertain, and so it is unclear whether this alone can justify the difference. Nevertheless, inherent difficulty in determining volatile/water inventories makes it challenging to directly compare the volatile budget of Mars to that of Earth.
Here I will reassess several key geochemical and isotopic constraints for the volatile budgets of Mars and Earth. Then I will introduce K isotopes, a newly proposed tracer for comparing planetary volatiles. New data suggest that Mars was in fact more depleted in volatiles than Earth, which implies much less water was initially accreted to Mars than previously thought. Most of the limited initially accreted water having been liberated during the early warmer climate of Mars. As observed by previous’ rovers and orbiters, this has left only morphological and mineralogical evidence of liquid water on Mars’ surface. Our study sets a baseline estimate of Mars’ volatile budget and provides a testable hypothesis for the future Mars 2020 “Perseverance” rover.
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