|Aerial image of an ancient Martian riverbed taken with ESA's Mars Express orbiter.|
|Phase Diagram for water. Note that 1 bar corresponds to Earth's atmospheric pressure.|
One of the single most underrated aspects of Earth that allows us to exist at all is Earth's magnetic field. Our magnetic field is created by the rotation of Earth's liquid metal outer core (famously portrayed as shutting down and being nuked into motion again in The Core). Earth's magnetic field has the important job of shielding Earth from the stream of plasma known as the solar wind. The solar wind specifically consists of a stream of protons and electrons emitted by the upper layers of the Sun's atmosphere (specifically the solar corona). This matters because ionized particles are generally bad news for both Earth's atmosphere and living organisms.
|Diagram showing how a planet's magnetic field affects its interactions with the solar wind.|
To summarize the above, it appears that Mars did have an atmosphere in the past. It also briefly had a magnetic field that allowed it to keep that magnetic field for some time. There is also abundant evidence that Mars had liquid water on its surface in the past. As such, we have every reason to believe that Mars used to be habitable.
While this seems straightforward, there's actually a big complicating factor known as the "Faint Young Sun problem". The Faint Young Sun problem was first noted by the famous Carl Sagan and George Mullan in 1972. In short, the Sun used to be less luminous than it is today, which would move the habitable zone closer in to the Sun, which would make young Earth, which we know from geologic evidence to have been quite hot, cooler than it should be. This applies equally well to Mars, and possibly more so because Mars is on the edge of the habitable zone today, which could put it entirely out of the habitable zone in the early solar system. I actually alluded to this idea towards the end of my previous post, Discussion Questions: 9/5.
Well that's problematic. We know that Mars had habitable conditions, but suddenly it's a lot harder for us to re-create them in our models. The typical solution to this problem is that Mars likely had a much thicker atmosphere than we initially thought. It is not unfeasible that Mars once had a thick atmosphere. Whether or not it could have lost that atmosphere over the past 4 billion years is still an open question, though it is an active area of research.