Publication Date:
2014-05-23
Description:
13C/12C and 15 N/14 N isotopic ratios are pivotal for our understanding of the Martian carbon cycle, history of the Martian atmospheric escape and origin of the organic compounds on Mars. Here we demonstrate that the carbon and nitrogen isotopic composition of the surface rocks on Mars can be significantly altered by the continuous exposure of Martian surface to cosmic rays. Cosmic rays can effectively produce 13C and 15 N isotopes via spallation nuclear reactions on oxygen atoms in various Martian rocks. We calculate that in the top meter of the Martian rocks the rates of production of both 13C and 15 N due to galactic cosmic rays (GCRs) exposure can vary within 1.5-6 atoms/cm3/s depending on rocks’ depth and chemical composition. We also find that the average solar cosmic rays (SCRs) can produce carbon and nitrogen isotopes at a rate comparable to GCRs in the top 5–10 cm of the Martian rocks. We demonstrate that if the total carbon content in a surface Martian rock is 〈10 ppm then the “light”, potentially “biological” 13C/12C ratio would be effectively erased by cosmic rays over 3.5 billion years of exposure. We found that for the rocks with relatively short exposure ages (e.g. 100 million years), cosmogenic changes in 15 N/14 N ratio are still very significant. We also show that a short exposure to CRs of ALH 84001 while on Mars can explain its high-temperature heavy nitrogen isotopic composition (15 N/14 N). Applications to Martian meteorites and the current MSL mission are discussed.
Print ISSN:
0148-0227
Topics:
Geosciences
,
Physics
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