  The isotopic signature of atmospheric carbon offers a unique tracer for the
history of the Martian atmosphere and the origin of organic matter on Mars.
Photolysis of CO$_{2}$ is known to induce strong isotopic fractionation of
carbon between CO$_{2}$ and CO. However, its effect on the carbon isotopic
compositions in the Martian atmosphere remains uncertain. Here we develop a 1-D
photochemical model considering isotopic fractionation via photolysis of
CO$_{2}$ to estimate the vertical profiles of the carbon isotopic compositions
of CO and CO$_{2}$ in the Martian atmosphere. We find that CO is depleted in
$^{13}$C compared with CO$_{2}$ at each altitude due to the fractionation via
CO$_{2}$ photolysis: the minimum value of $\delta ^{13}$C in CO is about $-170$
per mil under the standard eddy diffusion setting. This result supports the
hypothesis that fractionated atmospheric CO is responsible for the production
of the $^{13}$C-depleted organic carbon in Martian sediments detected by
Curiosity Rover through the conversion of CO into organic materials and their
deposition on the surface. The photolysis and transport-induced fractionation
of CO we report here leads to a $\sim 15$ % decrease in the amount of inferred
atmospheric loss when combined with the present-day fractionation of the
atmosphere and previous studies of carbon escape to space. The fractionated
isotopic composition of CO in the Martian atmosphere may be observed by ExoMars
Trace Gas Orbiter (TGO) and ground-based telescopes, and escaping ion species
produced by the fractionated carbon-bearing species may be detected by Martian
Moons eXploration (MMX) in the future.
