  Lithium is overabundant in cosmic rays because protons impact on carbon and
oxygen nuclei and fission them. Among the products of this fission is lithium.
Given this preference for carbon and oxygen atoms, in this work I propose that
in an atmosphere of almost pure CO2, such as Mars and Venus atmospheres,
lithium nuclei are produced by interaction with cosmic rays. I calculated the
production rate of lithium and came to the conclusion that, for pressures of
two bars or greater, are produced between 21 and 81 lithium nuclei for each
primary cosmic rays proton. For lower pressures, the production is less and
almost nil with the current pressure of Mars or Earth (pressure of CO2).
Assuming a rate of cosmic ray arrival at Mars equal to that of Earth, and a
pressure greater than two bars throughout the history of Mars, the amount of
lithium that would occur would be between 162 and 642 million metric tons (in
the Earth lithium estimated reserves are 30 million metric tons). These values
are an upper limit; the actual amount of lithium on Mars will depend on the
time in which the planet had a dense atmosphere (> 2 bars). That is, the amount
of lithium produced by cosmic rays, serves to estimate the time that Mars had a
thick atmosphere and therefore the capacity for have liquid water on surface.

Earth and Planetary Astrophysics