The black-and-white panoramic image provides a more-detailed view of the light-colored bedrock outcrop directly in front of the rover's landing platform. Clearly visible in the image are distinct layers, some as thin as one centimeter (about one-third of an inch).

The images do not yet provide enough information to enable scientists to determine how the rocks formed. But the thinness of the layers has already led the science team to rule out one possibility.

"These aren't lava flows," Squyres said. Flowing lava creates layered terrain, but the layers it produces are invariably much thicker.

While lava flows are ruled out, volcanoes could still have played a role in the outcrop's formation. Of the remaining theories about how they formed, one involves wind-borne volcanic ash deposits. The other possibility is that the layers are sedimentary - possibly deposited by water.

Geologists on the science team can't yet tell whether volcanic or sedimentary processes were responsible. The images received to date don't show enough detail to distinguish between the contending theories.

On Earth, examples of both types of layering can be found. According to Harvard geologist and science team member Andy Knoll, similar-looking rocks, if found in Kentucky, would be sedimentary. But if one saw them in Hawaii, they would be composed of volcanic ash.

More-detailed Pancam images, which scientists expect Opportunity to send to Earth in the next few days, may reveal the rocks' origin. Knoll said that if the outcrop is composed of volcanic ash, all of the layers will appear parallel to each other. If, however, the layers exhibit cross-bedding - adjacent layers deposited at different angles - that would be evidence for a sedimentary origin.

But a definitive answer may have to wait until the rover can place its robotic arm up against the rocks and examine them with its Microscopic Imager and Mini-TES instruments.

The microscopic imager will show rock textures and individual grains of the material in detail. The presence of more rounded grains will imply that the rocks have a sedimentary origin.

Mini-TES will gather information about the mineral composition of the outcrop. The presence of minerals that have undergone chemical alteration will indicate that water was involved in their formation.

If Opportunity determines that the rocks are volcanic, scientists will be left scratching their heads. There is no known volcano nearby that the ash could have come from.

But, said Knoll, "I'm not particularly bothered by the idea that there's no volcanic cone 10 kilometers away." Quoting a colleague, he added, "Anything that did happen can happen." If the deposit turns out to be volcanic ash, there must have been a volcano somewhere that produced it.

If the rocks turn out to be sedimentary, scientists will face a similar problem: Where did the water come from? Unlike Gusev Crater, Spirit's landing site, Meridiani Planum is not a large catchment basin. If it were to rain there today, the water would simply run off downhill. Nor is there any obvious channel that could have fed water into Meridiani.

Nevertheless, the discovery of sedimentary rocks would undoubtedly raise another question: Was Meridiani Planum once a potential habitat for life? That, says Knoll, depends on how long the water was around.

"The time scale for accumulating that thickness of [sedimentary] rocks would be, on the short end, a single event - a Thursday. A long time scale would be perhaps thousands of years." Only water's presence over a long time could have created a habitat suitable for life as we know it.

Whatever scientists learn about the local rock forms will be applicable to a much wider area. The outcrop sitting directly in front of the Opportunity lander is quite small - 1 to 2 meters (several feet) high and perhaps 5 to 10 meters (15 to 30 feet) wide. But Meridiani Planum is vast, covering an area the size of Oklahoma. And in images taken from orbit, material can be seen scattered throughout the entire region that is similar in color to the nearby bedrock.

No one can say for certain what Opportunity will find when it begins to explore the martian crater that has become its new home. But it's bound to be an exciting adventure.

As Steve Squyres put it: "We are about to embark on what is arguably the coolest geological field trip in history."

Article is courtesy of NASA's Astrobiology Magazine team at Ames Research Center. This article is public domain and available for reprint with appropriate credit.

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