Detecting the presence of atmospheric methane (CH$_4$) on
Mars is an ongoing scientific debate, with multiple observations
reporting elevated CH$_4$ amounts that are difficult to reconcile
with photochemistry models that describe Martian
atmospheric chemistry. We develop an existing 1-D photochemistry model to include a more comprehensive description of organic chemistry, including the
oxidation products of CH$_4$ and ethane (C$_2$H$_6$), a longer-chain
hydrocarbon that often accompanies abiotic releases of CH$_4$ on
Earth. We report the atmospheric lifetime of CH$_4$ as a function
of altitude along its solar orbit, highlighting regions above the water
vapour saturation point where the abundance of O($^1$D) reduces the
lifetime to 25–60~years, and a region between 50 and 70~km where
loss rates are at a minimum that result in a lifetime in excess of
1000~years. We find the two largest photochemical products of
CH$_4$ and C$_2$H$_6$ are formaldehyde (HCHO) and formic acid
(HCOOH). We show that a 14~ppb uniform profile of CH$_4$
photochemically results in a latitude-independent layered structure
of HCHO at 20–40~km during the Martian northern summer with
magnitudes peaking at 0.2~ppt, and oxidation of C$_2$H$_6$ produces HCHO at rates an order of magnitude larger than for CH$_4$. Formic acid is found to have atmospheric lifetimes spanning 10–200 sols below 10~km that show little temporal or zonal variability, and is produced in greater abundances by the oxidation of C$_2$H$_6$ than of CH$_4$. The
photochemistry of C$_2$H$_6$ has allowed us to identify an
atmospheric source of CH$_4$ from the UV photolysis of acetaldehyde.