Examples of layering have been known since the Mariner 9 mission in the early 1970s, said lead researchers Michael Malin and Kenneth Edgett of Malin Space Science Systems in San Diego, California. But the layering had not been seen in such detail, nor did anyone know it was so widespread. Other recent findings have shown evidence of flowing water and water dribbling from canyon walls on the Red Planet.
"But this would be the first evidence for widespread standing water," said Wes Huntress, NASA's former space science chief and now director of the Geophysical Laboratory, Carnegie Institution of Washington. "If in fact this holds up under scrutiny from the rest of the community, there's no doubt it's a fantastic discovery."
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Huntress was not involved in the research, but he oversaw the development of the Mars Global Surveyor, which used its Mars Observer Camera (MOC) to produce the images. He told SPACE.com that like all new discoveries, this one "is going to be debated heavily."
The debate began before the images were even released.
Robert Craddock, a geologist with the Smithsonian Institutions National Air and Space Museum, said he reviewed the paper for the journal Science and recommended it be rejected.
"In reality, they havent seen anything we havent seen before," Craddock said.
"Weve known for three decades about these layered deposits across the planet," he added. "[The paper] fails to place these new observations into context and you end up with the impression theyre making new discoveries, when all theyre doing is looking at it with new data."
Others, however, were encouraged that the new images do represent new information.
"This provides much more compelling evidence for the layering, although we can still argue about what the mechanism is," said Bruce Jakosky, a professor of geological sciences at the University of Colorado.
"What were seeing is a couple of things: One, [the layering is] much more widespread than people had thought. The second is that the evidence is incontrovertible," Jakosky said.
About the outcrops
The Martian outcrops, in some cases a few miles (kilometers) thick, appear to be made of fine-grained materials deposited in horizontal layers, the hallmark of sedimentary rock. These outcrops are found inside and between craters, and within chasms, said Malin and Edgett.
A lake is not required to make sedimentary deposits. Wind, volcanic activity and even an asteroid impact could have similar effects. But the prevalence of the Martian sedimentary outcrops within basin-like features suggests that they were deposited by water, perhaps in lakes that formed within the craters and chasms, the researchers said.
Under this scenario, sediments may have been transported into the lakes in regular, swift pulses, building up thin layers. Larger blocks of sediment may have been deposited when a lake became stagnant or deep enough to cause sediments to sift down through the water over longer intervals.
"Some of the MOC images of these outcrops show hundreds and hundreds of identically thick layers, which is almost impossible to have without water," Malin said.
If researchers can prove that Mars was once wet, it could be a significant boost for future Mars missions, with the ultimate goal being the search for signs of life, past or present.
"I think most of us have been convinced that [there was once water on Mars] just from the photographs that show erosion that could only have been done by water," said Morris Aizenman, senior science associate in the National Science Foundation's Mathematical and Physical Science Directorate. "This just adds another point to that very convincing argument."
And Aizenman points out that water is widely viewed as a necessary ingredient for life as we know it. "With the existence of water is the possibility that there was once life on Mars," he said.
Next page: Mysteries in the images
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