The outcrops were seen in many regions of the planet by the Mars Global Surveyor spacecraft (MGS).
High-resolution pictures of Mars show evidence for sedimentary rocks laid down by ancient lakes and shallow seas. It is a discovery of huge importance, say scientists.
In 1996, some astronomers claimed to have evidence for microfossils in a Martian meteorite. Their case, although not proven, has not been totally discounted.
Certainly, if life ever existed on the Red Planet, the fossil evidence would most likely be found in the types of sedimentary rocks just imaged.
Vanished oceans
Astronomers, already excited by Martian discoveries of recent years, are ecstatic at finding rocks laid down by long vanished seas.
The outcrops were seen in many regions of the planet by the Mars Global
Surveyor spacecraft (MGS).
They look as though they are made of fine-grained materials deposited in horizontal layers - the hallmark of sedimentary rock.
"We see distinct, thick layers of rock within craters and other depressions for which a number of lines of evidence indicate that they may have formed in lakes or shallow seas," said Dr Michael Malin, principal investigator for the camera on MGS.
"We have never before had this type of irrefutable evidence that sedimentary rocks are widespread on Mars."
On Earth, such layered rock structures occur where there were once lakes.
"We caution that the Mars images tell us that the story is actually quite complicated and yet the implications are tremendous," said Dr Ken Edgett of Malin Space Science Systems, the company that operates the MGS.
"Mars has preserved for us, in its sedimentary rocks, a record of events unlike any other that occur on the planet today."
The scientists say that the sedimentary rocks are now the place to look for evidence of past life on Mars.
'Really shaken'
"I was really shaken when I first saw our first high-resolution pictures of Candor Chasma - the nearly identically thick layers would be almost impossible to create without water," Dr Malin added.
"This finding might just be the key to solving some of the biggest mysteries on Mars," said Dr Ed Weiler of the American space agency Nasa.
"It also tells us that our new Mars exploration programme needs the flexibility to follow up in a carefully thought-out manner."
Biologists say the confirmation of sedimentary rocks is a major step forward. Dr Ken Nealson, director of the Center for Life Detection at Nasa, said: "Perhaps the favourite sites for biologists to search for fossils or evidence of past life on Earth are layered lake or oceanic sediments such as these sites."
These findings add great weight to the view that in its early few hundred million years, Mars was wetter and warmer with a thicker atmosphere.
Many scientists believe that life could have arisen just as easily on Mars as it did on Earth. Later, Mars turned colder, although some speculate that life may hang on in isolated wetter regions or below the surface.
That evidence may be recorded in Mars' sedimentary rocks. Who knows, in just a few years from now, scientists may be able to land on those rocks and search for life.