Their assessment is based on pictures of the planet's near-equatorial Elysium region that show plated and rutted features across an area 800 by 900km.
The team think a catastrophic event flooded the landscape five million years ago and then froze out.
They tell a forthcoming edition of Nature magazine that sediments covered the ice, locking it in place.
Large reserves of water-ice are known to be held at the poles on Mars but if this discovery is confirmed by follow-up observations, it would be a first for a region at such a low latitude.
Dust covering
"It's been predicted for a long time that you should find water close to the surface of Mars near the equator," Jan-Peter Muller, from University College London, UK, said.
"This is an area where there are a lot of river features but no-one has ever seen a sea before, and certainly no-one has ever seen pack ice before," he told the BBC News website.
The interpretation is based on images taken by the High Resolution Stereo Camera on Europe's Mars Express spacecraft. These show extensive fields of large, platy features - reminiscent of the fractured ice floes found in polar regions on Earth.
Finding exposed ice at the equator would be unlikely. Very low pressures on the planet would lead to sublimation - the ice would erode over time straight to water vapour.
But the research group, led by John Murray, from the Open University, UK, tells Nature that a crust of dust and volcanic ash, perhaps just a few centimetres thick, has prevented this happening.
"The story runs that water flowed in some kind of massive catastrophic event; pack ice formed on top of that water and broke up, and then the whole thing froze rigid," explains Professor Muller.
"Large amounts of dust then fell over that area. The dust fell through the water and on top of the pack ice, which explains why the pack ice is a different hue to the area around it."
Source:
http://news.bbc.co.uk/2/hi/science/nature/4285119.stm
Mars pictures reveal frozen sea
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