  Mars has a thin (6 mbar) CO2 atmosphere currently. There is strong evidence
for paleolakes and rivers formed by warm climates on Mars, including after 3.5
billion years (Ga) ago, which indicates that a CO2 atmosphere thick enough to
permit a warm climate was present at these times. Since Mars no longer has a
thick CO2 atmosphere, it must have been lost. One possibility is that Martian
CO2 was lost to space. Oxygen escape rates from Mars are high enough to account
for loss of a thick CO2 atmosphere, if CO2 was the main source of escaping O.
But here, using H isotope ratios, O escape calculations, and quantification of
the surface O sinks on Mars, we show for the first time that O escape from Mars
after 3.5 Ga must have been predominantly associated with the loss of H2O, not
CO2, and therefore it is unlikely that >250 mbar Martian CO2 has been lost to
space in the last 3.5 Ga, because such results require highly unfavored O loss
scenarios. It is possible that the presence of young rivers and lakes on Mars
could be reconciled with limited CO2 loss to space if crater chronologies on
Mars are sufficiently incorrect that all apparently young rivers and lakes are
actually older than 3.5 Ga, or if climate solutions exist for sustained runoff
on Mars with atmospheric CO2 pressure <250 mbar. However, our preferred
solution to reconcile the presence of <3.5 Gya rivers and lakes on Mars with
the limited potential for CO2 loss to space is a large, as yet undiscovered,
geological C sink on Mars.
