A Thick Blanket for Early Mars: Evidence of a Massive Primordial Atmosphere
Sarah Joiret and collaborators set out to investigate how thick Mars’s atmosphere may have been when the planet first formed. By looking at noble gases, especially xenon (Xe), and combining them with models of cometary impacts, the authors provide new evidence that Mars likely held onto a surprisingly massive primordial atmosphere, far denser than previously expected.
Background: Mars’s Atmosphere Origins
Mars finished forming very early, while the Sun was still surrounded by the solar nebula, a disk of gas and dust. Evidence from noble gases suggests that Mars captured much of its original atmosphere directly from this nebular gas. In contrast to Earth, which later lost its nebular signature due to giant impacts, Mars retained noble gas ratios consistent with the solar nebula. However, Mars also experienced a heavy bombardment of comets, which should have added their own chemical “fingerprints.” The puzzle is that Mars’s atmosphere today shows almost no cometary xenon signature, unlike Earth’s.
Calculating the Cometary Contribution
To solve this puzzle, Joiret and her team estimated how much cometary material Mars could realistically have captured. They used two sets of simulations: one with a violent planetary instability leading to fast, high-energy comet impacts, and another with a gentler instability producing fewer, slower collisions. They then ran hydrodynamic impact models to determine how much comet material, especially volatiles like Xe, could be retained after these impacts. Even when assuming the most conservative case, many high-speed collisions with low retention, Mars would still have trapped a measurable amount of cometary xenon.
Setting a Lower Bound on the Atmosphere
The key step was comparing this expected cometary xenon with the fact that Mars’s actual xenon today is overwhelmingly solar in origin. The authors concluded that the only way for Mars’s atmosphere to “drown out” the cometary contribution is if it had been much thicker to begin with. Their calculations suggest a lower bound of about 2.9 bar of surface pressure, several times Earth’s current atmospheric pressure, rising to as much as 14.5 bar depending on the assumptions.
Implications for Early Mars
These results imply that Mars initially accreted a large envelope of nebular gas, consistent with scenarios where the presence of heavier species like CO₂ or water vapor boosted atmospheric retention. Such a massive hydrogen-rich atmosphere would also have acted as a strong greenhouse gas, raising the possibility that early Mars was warmer than often assumed, at least during the short-lived pre-Noachian era. If part of this atmosphere persisted, it could help explain isotopic signatures found today and may even hint at more hospitable surface conditions.
Conclusions
Joiret and her colleagues argue that Mars must have started with a substantial primordial atmosphere, likely exceeding several bars of pressure. Whether most of this atmosphere was quickly lost to space or partially lingered is still debated, but either way, the planet’s early climate and geochemistry were profoundly shaped by its thick atmospheric beginnings. This study highlights how comet impacts and noble gas measurements together can reveal the hidden history of planetary atmospheres.
Source: Joiret
