Mars once had a thicker atmosphere than Earth does today.

While Mars today is just a thin gaseous remnant, it may once have had an atmosphere hundreds of times thicker with three times the pressure of Earth.

Mars' atmosphere may have once been hundreds of times thicker than it is today, acting like a blanket, protecting the planet from the asteroids that regularly ravage other planets.

While the Sun and most of the planets were still forming, about 4 million years after the Solar System began, Mars was already nearly complete. At this time, the planets existed in a large ball of hot gas and dust swirling around the young Sun, called the solar nebula, which some planets were able to temporarily absorb into their atmospheres. However, as the solar nebula retreated, it was thought that the planets would quickly lose this gas, reducing the density of their atmospheres.

Now, Sarah Joiret at the Collège de France in Paris and colleagues suggest that Mars may have held onto that gas for longer, forming a 'viscous' primordial atmosphere that persisted.

Soon after the nebula retreated, astronomers think the orbits of the giant planets Jupiter and Saturn shifted, and this disturbed the paths of comets and asteroids, sending them hurtling toward the solar system where they struck the rocky planets. We can find evidence of these bombardments in the chemical signatures of rocks on Earth and in our atmosphere, but the evidence for Mars is weaker.

' All the terrestrial planets were hit by comets during this period, and Mars could not have avoided it, so we should have seen traces of cometary strikes on Mars ,' Joiret said at the Europlanet Science Congress in Helsinki, Finland, on September 11.

Joiret and her colleagues suggest that a dense hydrogen-rich atmosphere at the time could have diluted any cometary material absorbed by the planet. By estimating the amount of cometary material that would have reached Mars using simulations of the early solar system, and comparing it to the amount that actually arrived, they calculated the mass of Mars's primordial atmosphere, and found it was equivalent to a pressure of 2.9 bars, about three times the atmospheric pressure at Earth's surface today.

However, this atmosphere would have been lost relatively quickly, within about a million years, and would have been largely gone by the time liquid water appeared on the surface of Mars, said Raymond Pierrehumbert of the University of Oxford at the conference. He was not involved in the study. The appearance of liquid water on Mars would have required different atmospheric conditions, including an abundance of carbon dioxide, that probably did not exist in that thick primordial atmosphere.

Micah Soto

Update 16 September 2025