A massive primordial atmosphere on early Mars

Mars finished forming while the solar nebula was still present, and acquired its primordial atmosphere from this reservoir. The absence of a detectable cometary xenon signature in the present-day Martian atmosphere suggests that the capture of solar nebular gas was significant enough to dilute later cometary contributions. By quantifying the mass of cometary material efficiently retained on Mars, we place a lower bound on the mass of the primordial Martian atmosphere. To test the robustness of our conclusions, we use cometary bombardment data from two independent studies conducted within a solar system evolutionary model consistent with its current structure. Our calculations show that, even under the most conservative scenario, the minimal mass of the primordial martian atmospheres would yield a surface pressure of no less than 2.9 bar. Such a massive nebular envelope is consistent with recent models in which atmospheric capture is strongly enhanced by the presence of heavier species on Mars - due to outgassing or redox buffering with a magma ocean. • Kr and Xe isotopic signatures in the Martian atmosphere have a solar origin. • Mars finished forming before the dissipation of the solar nebula. • Mars accreted its primordial atmosphere from the solar nebula, enabling dilution of later cometary xenon inputs. • By quantifying cometary delivery and retention on Mars, we establish a lower bound for its primordial atmospheric pressure.