There are multiple lines of evidence suggesting that Mars was not always desiccated.
The red planet used to be so wet that a megatsunami crashed across the landscape. Why did this happen? The one that killed the dinosaurs 66 million years ago is said to be a giant asteroid impact.
The most likely origin of the mystery wave is an enormous impact crater that has been found by a group of researchers.
It was located within an area scoured with catastrophic flood erosion which was first identified in the 1970s.
There was a boulder-strewn plain when NASA's Viking 1 probe landed on Mars in 1976.
A team of scientists led by Rodriguez decided in a 2016 paper that the cause of this was the result of waves from the sea.
They thought that there were two different impact events that triggered the waves. Scientists used numerical simulations to find the source of the later wave.
The source of the previous wave remained a mystery. The northern plains of Mars are very cratered and difficult to interpret. Rodriguez and his team combed maps of the Mars surface, looking for impact craters that could be linked to big waves.
They hit upon Pohl, located some 900 kilometers (560 miles) northeast of the Viking 1 landing site, a crater that scientists think would have been sea level.
The researchers thought that Pohl may have formed at this time because of the rocks around the crater. The crater may have formed during a marine impact.
The researchers did impact simulations to confirm their suspicions. Two scenarios fit the site.
An asteroid 9 kilometers across encountered strong ground resistance and exploded. The asteroid 3 kilometers across encountered weak ground resistance and released half a million mega tons of energy.
In the simulations, both of the scenarios resulted in a crater that was over 100 kilometers wide, and a megatsunami that traveled as far as 1,500 kilometers from the impact site.
The researchers say their simulations match the older megatsunami deposit's margins and predict fronts to reach the Viking 1 landing site.
The site's location is aligned to erosional grooves.
The site is similar to the Chicxulub impact.
The first occurred in a shallow marine environment, the second in the ground, and the final one in the ocean.
"Our findings allow that rocks and soil salts at the landing site are of marine origin, inviting the scientific consideration of information gathered from the first in-situ measurements on Mars," they wrote.
The research was published in a scientific journal.
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