Neutron Capture Isotopes in the Martian Regolith and Implications for Martian Atmospheric Noble Gases

Abstract

Impact-produced glasses in some martian meteorites have trapped significant amounts of the recent martian atmosphere. From literature data, we estimate that ∼9% of the trapped 80Kr in these meteorites was produced from neutron capture on 79Br. Estimates of neutron fluences made from 80Kr and 149Sm for bulk samples of meteorite EET79001 indicate that 80Kr excesses in the impact glass were not produced in situ. Theoretical calculations independently predict production of a large neutron-capture component of 80Kr and 36Ar in the martian regolith, and part of this component presumably escaped into the martian atmosphere. These calculations were made by using the Los Alamos High-Energy Transport Code to calculate the fluxes of galactic cosmic ray (GCR)-produced thermal neutrons as a function of depth in the uppermost 500 g cm −2 of the martian surface, and by adopting average Cl, Br, and I concentrations of the upper martian surface of ∼0.3%, ∼20 ppm, and ∼0.5 ppm, respectively. Combining these data with the appropriate neutron-capture cross sections, we calculate Mars global production rates of 80Kr n=2.4×10 16atoms sec −1, 36Ar n=5.5×10 18 atoms sec −1, and 128Xe n=3×10 13 atoms sec −1. Calculated global production rates of spallogenic 80Kr sp, and 36Ar sp, are smaller by factors of ∼770 and ∼29, respectively. It would require ∼330 Myr to produce an amount of 80Kr n equivalent to the amount inferred to be present today in the martian atmosphere (∼2.5×10 32 atoms). Production of these neutron-capture components probably has occurred over the past ∼4 Gyr, as only an atmospheric pressure substantially higher than today's would appreciably decrease the neutron flux in the regolith. Thus, most of the neutron-capture noble gases produced over time probably remain in the martian regolith and would make sensitive indicators of the time period a sample has resided near the martian surface. Assuming mixing of the martian surface to an average depth of 100 m, the predicted average regolith concentrations of 80Kr n, 36Ar n, and 128Xe n are ∼4×10 −9 cm −3 g −1, ∼1×10 −6 cm 3 g −1, and ∼5×10 −12 cm 3 g −1, respectively. If similar fractions of these neutron-capture isotopes have escaped into the atmosphere, they would comprise ∼3% and ∼0.2% of the present atmospheric inventories of 36Ar and 128Xe, respectively. The fractional excess of 80Kr n in ancient martian meteorite ALH84001 appears similar to that in shock-glass phases of young shergottite meteorites. If ALH84001 acquired its atmospheric gases ∼4 Gyr ago, this implies that, prior to that time, halogens were greatly concentrated at the martian surface by crustal formational and weathering processes, impacts efficiently degassed the regolith, and Mars did not have a significant atmosphere to shield the surface.