Image caption The Hellas basin is the largest impact structure on Mars

US scientists have found further evidence that huge seas existed long ago on Mars.

A geological mapping project found sedimentary deposits in a region called Hellas Planitia which suggest a large sea once stood there.

The 2,000 km-wide, 8km-deep Hellas basin is a giant impact crater - the largest such structure on Mars.

The researchers say their data support a lake between 4.5 and 3.5 billion years ago.

Some scientists believe that conditions on Mars were more favourable for the evolution of life at this time than they were on Earth.

"This mapping makes geologic interpretations consistent with previous studies, and constrains the timing of these putative lakes to the early-middle Noachian period on Mars," said Dr Leslie Bleamaster, research scientist at the Planetary Science Institute in Tucson.

The researchers say that fine-layered outcrops around the eastern rim of Hellas are likely to be sedimentary deposits.

They were formed through the erosion and transport of rock and soil from the Martian highlands into a standing body of water.

The results support earlier studies of the western part of the Hellas basin.

Further study of the region could provide clues about where this water went and to how the Martian climate changed over geological history.

The mapping effort used data from a number of instruments aboard Nasa spacecraft, including the Viking orbiter, Mars Global Surveyor and Mars Odyssey.