Mars once had an atmosphere that was thicker than Earth’s today

The current thin Martian atmosphere may not reflect its past, when it was likely many times denser, offering a blanket against frequent asteroid impacts.

During the early formation of the solar system roughly 4 billion years ago, Mars had already developed significantly in a chaotic environment filled with gas and dust swirling around the young sun, termed the solar nebula. It was previously believed that Mars lost its gathered gases quickly after the solar nebula dissipated, resulting in a thin atmosphere today.

However, a recent study led by Sarah Joiret from Collège de France posits that Mars may have retained a substantial amount of this gas, yielding a rich, primordial atmosphere instead. Shortly after the solar nebula receded, it’s theorized that the shifting orbits of massive planets such as Jupiter and Saturn disrupted the paths of comets and asteroids, sending them toward the inner solar system where they violently interacted with rocky planets like Mars.

Evidence of this event remains abundant on Earth and other bodies, yet volcanic activity on Mars has obscured the historical record there. Joiret noted at the Europlanet Science Congress held recently in Helsinki, Finland, that “No terrestrial planet was spared bombardment by comets during this period, and Mars must have been affected too, leaving some indication of these impacts.”

The research team theorizes that a thick, hydrogen-laden atmosphere would have acted to dilute any comet-derived materials that reached Mars. By using simulations of early solar dynamics to gauge how much of this comet material should have arrived, they deduced the mass of Mars’ primordial atmosphere, estimating it could have had a pressure of 2.9 bar—almost three times that of Earth today.

Nonetheless, this dense atmosphere would have been transient, existing only for about a million years, dissipating before the emergence of liquid water, stated Raymond Pierrehumbert from the University of Oxford, who was not directly involved in the study. The conditions necessary for liquid water to exist on Mars likely required a different atmospheric makeup that was not sustainable in the thick primordial environment.