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  Vast ocean once covered Mars, say scientists

Researchers believe their study of apparent marine deltas and valley
networks suggest a third of Mars was once under water

Artist's impression of a Martian ocean shoreline seen from the air 3.5bn
years ago Artist's impression of a Martian ocean shoreline seen from the
air 3.5bn years ago, based on satellite images. Photograph: G Di
Achille/Press Association

Scientists have revived arguments over whether there was once an ocean
on the surface of Mars <http://www.guardian.co.uk/science/mars> by
claiming that their analysis of existing data supports the hypothesis
that water covered much of the red planet's northern hemisphere 3.5bn
years ago.

They believe their study of apparent marine deltas and valley networks
in the journal Nature Geoscience
<http://www.nature.com/ngeo/journal/vaop/ncurrent/abs/ngeo891.html>
bolsters the possibility that up to a third of Mars was under about 30
million cubic miles of water.

Previous spacecraft investigations have pointed to the possible presence
of an ancient ocean, with supporters for the idea that there is still a
substantial amount of water under the surface as liquid or ice. Climate
change over millions of years might have led to the disappearance of the
atmosphere, which would mean that any water on the surface would boil away.

Volcanic activity is among other explanations for apparent gullies,
<http://science.nasa.gov/science-news/science-at-nasa/2000/ast22jun_2/>river
valleys, flood plains, lakes, seas and other signs of water that have
vanished.

Gaetano Di Achille and Bryan Hynek, of the University of Colorado, led a
team that analysed the distribution of supposed ancient delta deposits
and river-valley networks on Mars. They found many of the deltas were at
a similar elevation and suggested these might ring an ancient shoreline,
providing strong support for a vast ocean once covering the northern
plains of the planet. While admitting that the idea remained one of the
greatest uncertainties in Mars research, they added that: "Our findings
lend credence to the hypothesis that an ocean formed on early Mars as
part of a global and active hydrosphere."

/This article was corrected on 14 June 2010. It originally stated that
up to a third of Mars could have been under 30 cubic miles of water.
This has been corrected./