In the catalogue of cosmic discovery, the outcropping of Martian rocks just a few yards from where NASA's Opportunity rover came to rest two weeks ago might not appear all that enthralling. Rocks, after all, are not something that Mars seems to lack. Already, Martian missions have given the world Yogi and Sushi, Adirondack and Barnacle Bill.
But as Opportunity today stands beside that chain of rocks, nestled just beneath the lip of shallow crater on the smudge-gray plains of Meridiani, it stands on the edge of an untold story. Unlike any rocks ever before seen on Mars, these are the very bones of the planet - the bedrock shaped by ancient forces long spent. And they are, in many ways, what Opportunity came to Mars to find.
To geologists, bedrock is nothing less than an open history book, meaning that the coming days will mark the most important period of science since the mission began. Poring over the tiniest traces and textures in the rock, scientists will seek to fulfill the mission's primary purpose: to discover if water ever played a role in Mars' mysterious past, and if it created the conditions necessary to support life.
"They've struck gold," says Bruce Betts of the Planetary Society in Pasadena, Calif.
Indeed, scientists had expected to send their rovers great distances in search of bedrock. They counted themselves fortunate when they discovered that Spirit, Opportunity's twin rover on the other side of the planet, had landed 800 feet from a crater on Jan. 4. The hope was that the crater was deep enough to expose bedrock around the rim.
Later that month, however, when Opportunity landed and beamed back its first photos, scientists realized that the rover had bounced directly into just such a crater. One team member dubbed it "a 300 million-mile interplanetary hole in one." So while Spirit, now recovered from its technical malfunctions, is only beginning its trek toward a crater that might or might not have bedrock, Opportunity is already on the job.
What makes bedrock worth so much bother is the fact that it is the roots of the Martian landscape, and has been there for billions of years. In contrast, the rocks at Spirit's site - like the rocks nosed by Sojourner more than six years ago - could have come from almost anywhere, hurled by massive eruptions or floods. While they tell some of the same interesting stories as bedrock, they are like history books with the place names omitted.
Large bedrock outcrops like the one in Opportunity's crater also hold other treats for geologists. They lock huge amounts of the geologic record in one place. The texture and composition of the rock can show how it was shaped - by wind, lava, or water. Patterns, meanwhile, can hint at how different periods of geologic history developed and interacted.
"[Bedrock] will tell you how that material was laid down," says Michael Carr of the United States Geological Survey in Menlo Park, Calif. "Was it a continuous process? Was it interrupted? Because we have not encountered an outcrop like this before, this will be the first time we get that kind of information."
The necklace of pale rocks around the crater rim has already begun to reveal some of its secrets - and add more. About as long as a basketball court is wide, it's set on a slope roughly twice as steep as an average wheelchair ramp. In most places the bedrock rises only four to eight inches above the dark soil surrounding it.
While sensors have suggested that the crater soil is rich in hematite - a material that usually forms with water - none seems to be in the bedrock. On the other hand, pictures show signs of layering in the bedrock - a process often associated with water.
"The science team has developed a number of theories they want to test," says Joy Crisp, project scientist for the rover missions at the Jet Propulsion Laboratory (JPL) in Pasadena, Calif.
That testing has already begun. This weekend, the rover arrived at the outcrop and started to analyze it with an array of instruments on its robotic arm, including sensors and a microscope. For most of the rest of the week, Opportunity will drive the length of the 50-foot outcrop, snapping pictures along the way in what scientists call the "shoot and scoot."
"One of the first things to do is reconnaissance to figure out what is there," says Jim Rice, an astrogeologist working on both the Opportunity and Spirit missions at JPL. "Then you go back and look at it in more detail."
Depending on what it finds, Opportunity could spend as much as a month at the outcrop before heading off onto the pan-flat plains of Meridiani. Dr. Rice is in no rush to leave. "This is exactly what you want to find," he says. "To get to Martian bedrock and be able to analyze it with this suite of instruments - it's a gift."