NASA's Mars rover Opportunity found evidence for a lake or sea on Mars, and new research suggests the body of water was deep, large and long-lasting.

Opportunity studied a rock outcropping rich in a mineral called hematite, which combined with a certain type of sulfate revealed there must once have been water at the landing site. NASA announced the discovery in March. The new study, which includes observations from the orbiting Mars Global Surveyor and Mars Odyssey spacecraft, finds that similar outcrops are widespread.

Brian Hynek of the University of Colorado at Boulder thinks the broad distribution of similar rock outcrops -- though they have not been studied up close -- adds up to an expanse of water about the size of the Baltic Sea.

The lake or sea probably covered about 127,000 square miles (330,000 square kilometers), Hynek said today. The study is detailed in the Sept. 9 issue of the journal Nature.

"If the outcrops are a result of sea deposition, the amount of water once present must have been comparable to the Baltic Sea or all of the Great Lakes combined," Hynek said, speculating also that future studies may show that the ancient sea was even larger. He also speculates that the water may have been deep and long lasting, allowing sediments to build up to a depth of a third of a mile.

"For this to occur, the ancient global climate of Mars must have been different from its present climate and have lasted for an extended period," Hynek wrote in the Nature paper.

NASA's Odyssey orbiter uses a thermal emission imaging system to infer the particle size of rocks near or on the surface of Mars. Larger rocks heat up more slowly in daylight and cool more slowly in evenings. The region shows a preponderance of large rocks, the data show, which are presumed to be outcroppings of bedrock similar to what Opportunity studied.

Scientists are interested in the water history of Mars because water is a key element needed for life as we know it. The presence of water does not make life inevitable, but possible. Nobody knows, however, how long on Earth it took for life to emerge.

"It is important to understand how extensive these water-rich environments were and how long they persisted, because life required at least some degree of environmental stability in order to begin and to evolve," said David Des Marais, an astrobiologist at NASA's Ames Research Center and a member of the rover science team.