  The climate of Mars likely evolved from a warmer, wetter early state to the
cold, arid current state. However, no solutions for this evolution have
previously been found to satisfy the observed geological features and isotopic
measurements of the atmosphere. Here we show that a family of solutions exist,
invoking no missing reservoirs or loss processes. Escape of carbon via CO
photodissociation and sputtering enriches heavy carbon (13C) in the Martian
atmosphere, partially compensated by moderate carbonate precipitation. The
current atmospheric 13C/12C and rock and soil carbonate measurements indicate
an early atmosphere with a surface pressure <1 bar. Only scenarios with large
amounts of carbonate formation in open lakes permit higher values up to 1.8
bar. The evolutionary scenarios are fully testable with data from the MAVEN
mission and further studies of the isotopic composition of carbonate in the
Martian rock record through time.
