The atmospheric pressure of ancient Mars may not have been enough to warm the surface of the planet to above freezing at the time when rivers flowed, according to a new study.
The finding challenges a common explanation for the strong signs of liquid water on the martian surface; that ancient Mars had a dense atmosphere rich in greenhouse gases.
The study effectively rules out a long-lasting thick atmosphere allowing rivers to flow over hundreds of kilometres across the martian surface.
Dr Edwin Kite of Princeton University and colleagues identified small craters embedded within river deposits near Gale crater on Mars which date back to 3.6 billion years.
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 on the atmospheric pressure on Mars near the time the rivers flowed.
The researchers compared the sizes of the craters to simulations say the atmospheric pressure of Mars 3.6 billion years ago would have been much greater than it is today.
However, that 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 allowed liquid water on the martian surface.
The results of the new study are published in the journal Nature Geoscience.
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