We have known for a few years that Mars lost its atmosphere due to the relentless effects of solar wind, and now the same team has learned a bit more about the history of the Martian atmosphere.

The new study, published in Science, suggests that when Mars was a water-rich world over 3 billion years ago, it might have had an atmosphere with a similar pressure to our own, but composed mainly of carbon dioxide.

This idea comes from a detailed analysis of the data collected by NASA’s MAVEN (Mars Atmosphere and Volatile Evolution) spacecraft. The orbiter has been studying the Red Planet since late 2014 and has provided researchers with a deluge of information regarding the gasses and isotopes in the Martian atmosphere.

The team looked at the isotopic composition of argon at different heights in the atmosphere. Isotopes are elements with the same chemical properties but different physical properties, as some atoms are heavier than others. In this case, MAVEN measured Argon-36 (made of 18 protons and 18 neutrons) and Argon-38 (18 protons and 20 neutrons), which are two rare isotopes of the most common Argon-40.

Argon-36 is lighter than Argon-38 and is more abundant at higher atmospheres, which makes it easier for solar wind, solar storms, and ultraviolet light to scoop it up and take it away from Mars. According to the research, 65 percent of the Red Planet’s argon has been blown away since the planet formed 4.6 billion years ago.

The study has clear implications for the habitability of Mars, both past and present. When the water was present and abundant on the surface of the planet, simple life forms might have developed. This study suggests that the atmosphere was dense and rich in carbon dioxide – a possibility astronomers will need to consider when thinking about what life might have been like.

“We determined that the majority of the planet’s CO2 also has been lost to space by sputtering,” lead author Bruce Jakosky, principal investigator of MAVEN, said in a statement. “There are other processes that can remove CO2, so this gives the minimum amount of CO2 that’s been lost to space.”

As the atmosphere disappeared, water either evaporated or retreated underground. Some of the potential life forms might have migrated there as well and adapted to the more inhospitable climate. MAVEN is helping to clear up the picture of what the climate was like, but we are yet to find these life forms.

^{Animation of Mars losing its atmosphere and water. Lunar And Planetary Institute/Maven Mission/NASA}