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Mars was once covered in water
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Early Mars was covered in water just like the Earth today, scientists believe.
Researchers studied both evidence from ancient volcanic explosions and surface observations on the Red Planet by Mars rover Spirit to draw their conclusions.
One of the reasons water cannot exist on Mars is because its atmosphere is less than one per cent of the density of the Earth's.
But Professor Josef Dufek, of Georgia Tech university, looked back billions of years into the past to calculate whether this was always the case - and concluded its atmosphere was once 20 times denser.
He began his studies with a rock fragment propelled into the Martian atmosphere during a volcanic eruption roughly 3.5 billion years ago, reports journal Geophysical Research Letters.
The Mars rover Spirit landed at the site in 2007 and took a close look at the remaining fragment, enabling prof Dufek and colleagues to calculate the size, depth and shape of the divot.
The team then created bomb sags of their own, firing particles into grains the same size as those found by Spirit.
They calculated that the speed the particles must have been moving to recreate the divot would have required an atmospheric pressure 20 times greater than that found today.
This suggests Mars once had a thicker atmosphere, said prof Dufek.
He said: "Atmospheric pressure has likely played a role in developing almost all Mars' surface features.
"The planet's climate, the physical state of water on its surface and the potential for life are all influenced by atmospheric conditions.
"Our study is consistent with growing research that early Mars was at least a transiently watery world with a much denser atmosphere than we see today.
"We were only able to study one bomb sag at one location on the Red Planet. We hope to do future tests on other samples based on observations by the next rover, Curiosity."
An earlier study found gypsum on Mars - indicating there was once water on Mars and the possibility of life.
Journal Science reports that the gypsum could only have been formed in water below 60 c and writes: "That means that conditions conducive to life once existed on the edge of the crater."
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