Mars is cold and dry, with a thin atmosphere not even 1% as dense as ours.
Now, that is. The question is, what was it like around 3 billion years ago? Evidence abounds on the surface that plenty of water existed there back then, which also means it had a thicker atmosphere and was warmer, too.
However, a new paper has come out questioning that interpretation of the evidence. Instead of it being warm and wet, the scientists argue, instead it was cold and, well, not dry exactly, but frozen. This is hardly the semi-tropical Earth-like planet that for the past decade or two has been the growing consensus.
In this new work, they looked at the valley networks, systems of interconnected tributaries that have been carved into the surface of Mars, mostly in the southern hemisphere and equatorial regions. On first glance, they look very much like river valleys on Earth, carved by flowing water that’s fed by either rain or a process called groundwater sapping, erosion from water that originally bubbles up from underground like in a spring or seeps out of rocks and sediment.
That conclusion would support a warm, wet early Mars with a thicker atmosphere. ...