Photochemistry of Methane and Ethane in the Martian Atmosphere

Abstract

We develop an existing 1-D photochemistry model to include a comprehensive description of organic chemistry on Mars that includes the oxidation products of methane (CH4) and ethane (C2H6),a longer-chain hydrocarbon that can be used to differentiate between abiotic and biotic surface releases of CH4. We find that CH4 is most volatile between 20 and 50 km during Mars' northern summer, where the local atmospheric CH4 lifetime lowers to 25–60 years. We study atmospheric formaldehyde (HCHO) and formic acid (HCOOH), as the two common oxidation products of CH4 and C2H6, and acetaldehyde (CH3CHO) and acetic acid (CH3COOH) as unique products of C2H6. We focus our analysis of these gases at Mars' aphelion and perihelion at latitudes between -30° and 30°, altitudes from the surface to 70 km, and from a homogeonous initial condition of 50 pptv of CH4 and C2H6. From this initial condition, CH4 produces HCHO in a latitude-independent layered structure centered at 20–30 km at aphelion with column-averaged mixing ratios of 10−4 pptv, and oxidation of C2H6 produces HCHO at 10−2 pptv. Formic acid has an atmospheric lifetime spanning 1–10 sols below 10 km that shows little temporal or zonal variability and is produced in comparable abundances (10−5 pptv) by the oxidation of C2H6 and CH4. We also find that oxidation of 50 pptv of C2H6 results in 10−3 pptv of CH3CHO and 10−4 pptv of CH3COOH. Subsequent UV photolysis of this CH3CHO results in 10−4 pptv of atmospheric CH4, potentially representing a new atmospheric source of Martian CH4.