Photochemistry of Methane and Ethane in the Martian Atmosphere

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 (CH 4 ) and ethane (C 2 H 6 ),a longer‐chain hydrocarbon that can be used to differentiate between abiotic and biotic surface releases of CH 4 . We find that CH 4 is most volatile between 20 and 50 km during Mars' northern summer, where the local atmospheric CH 4 lifetime lowers to 25–60 years. We study atmospheric formaldehyde (HCHO) and formic acid (HCOOH), as the two common oxidation products of CH 4 and C 2 H 6 , and acetaldehyde (CH 3 CHO) and acetic acid (CH 3 COOH) as unique products of C 2 H 6 . 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 CH 4 and C 2 H 6 . From this initial condition, CH 4 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 C 2 H 6 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 C 2 H 6 and CH 4 . We also find that oxidation of 50 pptv of C 2 H 6 results in 10 −3  pptv of CH 3 CHO and 10 −4  pptv of CH 3 COOH. Subsequent UV photolysis of this CH 3 CHO results in 10 −4 pptv of atmospheric CH 4 , potentially representing a new atmospheric source of Martian CH 4 .