The atmospheric pressure of ancient Mars may not have been sufficient to warm the surface of the planet to above freezing at the time when rivers flowed, according to a study published online in Nature Geoscience. The finding challenges a common explanation for the presence of liquid water at the martian surface - that ancient Mars had a dense atmosphere that was rich with greenhouse gases.
Edwin Kite and colleagues identified small craters embedded within river deposits near Gale crater on Mars that date back to about 3.6 billion years ago. Whether a meteoroid of a given size will survive transit through the martian atmosphere and form a crater, without being broken apart, depends on the density of the air it must penetrate. Therefore, the size of the smallest craters in the river deposits yields information of the atmospheric pressure on Mars near the time the rivers flowed. The researchers compared the sizes of the craters to numerical simulations over a range of atmospheric pressures, and suggest that the atmospheric pressure of Mars 3.6 billion years ago would have been much greater than it is today.
However, the estimated atmospheric pressure is less than required to warm the surface above freezing. Instead, short-lived surface warming by volcanic eruptions, asteroid impacts or orbital changes of the planet may have temporarily permitted liquid water on the martian surface.
In an accompanying News and Views article, Sanjoy Som writes that “the longevity of stable liquid water on the ancient Martian surface may prove to be a key factor in considering whether life could have taken hold early in the planet’s history.”