UV production of methane from surface and sedimenting IDPs on Mars in light of REMS data and with insights for TGO

TL;DR

This study refines models of methane production on Mars via UV irradiation of interplanetary dust particles, incorporating recent UV environment measurements, and assesses implications for observed methane levels and organic carbon cycling.

Contribution

It updates methane production estimates by integrating new UV data from REMS, showing reduced production rates and implications for methane sources on Mars.

Findings

UV energies are 35% lower than previous estimates

Methane production from IDPs is insufficient to explain observed spikes

Less than 0.32% of organic carbon from meteor streams deposits in the atmosphere

Abstract

This paper refines model predictions for the production of methane from UV-irradiated interplanetary dust particles (IDPs) now that the Rover Environmental Monitoring Station (REMS) instrument onboard the Mars Science Laboratory (MSL) Rover has made the first measurements of the UV environment on the surface of Mars, at Gale Crater. Once these measurements are included in a UV radiative transfer model, we find that modelled UV sol-integrated energies across the planet are lower than pre-measurement estimates by 35% on average, considering all latitudes and seasons. This reduction, in turn, reduces the predicted production of methane from individual accreting IDPs, extending their lifetimes and increasing the surface concentration of organics that must accumulate in order to emit sufficient methane to balance the accretion of organic compounds to Mars. Emission from reasonable accumulations of IDPs could range up to ~7.9x10^-4 ppbv sol^-1. Richer deposits of organic carbon at the surface may emit methane at no more than 3.9 ppbv sol^-1. An examination of IDP-derived methane production during atmospheric settling indicates that no more than 0.32% of organic carbon from meteor streams may be deposited in the atmosphere. Thus, such a process cannot explain either the spikes observed in methane nor the low equilibrium values observed by MSL. Instead, this discrepancy may be explained if < 80 tons per year of organic carbon survives to the surface, the atmospheric lifetime of methane is < 110 years or the efficiency of the UV-CH4 process is < 7%. Under the assumption of reduced carbon input cycling in the Martian system from these processes, both soil concentrations of organic carbon and atmospheric measurements of methane observed by MSL are consistent with the UV-CH4 process. This refinement of methane production from IDPs and its geographical and vertical... <abstract continues in paper>.