The primordial atmosphere of Mars was massive

Mars is thought to have formed rapidly within the first 4 million years of the solar system's history (Dauphas & Pourmand, 2011), while the solar nebula was still present (Wang et al. 2017) . Isotopic analyses of heavy noble gases suggest that Mars acquired its early atmosphere directly from this solar reservoir (Ott et al. 1988; Conrad et al. 2016; Avice et al. 2018). In contrast, the krypton isotopic composition of the Martian mantle is consistent with a chondritic source (Peron et al. 2022). This is consistent with the scenario in which Mars' mantle rapidly formed from chondritic material, before accreting its atmosphere from the solar nebula.

After solar nebula dispersal, a dynamical instability among the giant planets scattered comets into the inner solar system (Gomes et al. 2005; Nesvony 2018). The lack of a detectable cometary xenon signature in the present-day Mars' atmosphere - in contrast to Earth's (Marty et al. 2017) - is therefore surprising, and suggests that the capture of solar nebular gas was significant enough to dilute later cometary contributions.

In this study, we combine geochemistry and solar system dynamics to quantify the mass of cometary material efficiently retained on Mars, thereby placing a lower bound on the mass of its primordial atmosphere. To test the robustness of our conclusions, we use cometary bombardment data from two independent studies (Joiret et al. 2024; Ribeiro et al. 2025). Our results indicate a minimal atmospheric pressure of a few bars at the Martian surface.  This estimate is compatible with recent findings showing that heavier molecular species from outgassing can substantially increase nebular gas retention (Pahlevan et al. 2025).