In the first patch of soil examined in detail by instruments aboard the Mars Spirit rover, scientists were surprised to find olivine, a silicate mineral that typically forms in igneous rocks of volcanic origin. It also weathers rapidly in the presence of water, posing a mystery of sorts for the rover science team.

The yellow box in this high-resolution image from the panoramic camera on Spirit outlines the patch of soil scientists examined at Gusev Crater. Credit: NASA/JPL/Cornell Download a larger version here

Spirit's landing site - Gusev Crater - is believed to have harbored a vast lake in the distant past and the major goal of the rover mission is to find out if water might have existed on the martian surface long enough for life to have evolved.

Olivine was discovered earlier, elsewhere on Mars, by the Mars Global Surveyor spacecraft currently in orbit around the red planet. The mineral quickly weathers to form clays and iron oxides and its presence could imply an absence of long-standing water in Gusev Crater.

Spirit principal investigator Steve Squyres said he has no doubt the 100-mile-wide crater held a lake at some point in the distant past. But the tortured history of the red planet has likely turned the crater floor into a complex mixture of rock and soil types that will take time to sort through.

"Mars is not going to give up secrets easily in this place," he told reporters today. "It's going to take a long time to puzzle through this. But the key is, we've got the tools to do it."

The discovery of olivine, he said, did two things: It provided an intriguing puzzle for geologists to ponder and served as welcome proof that Spirit's full complement of scientific instruments, including a sophisticated Mossbauer spectrometer supplied by Germany, is working properly.

"What is olivine? It's a mineral, it's a silicate mineral so it has silicon, oxygen, it has iron in it, it has magnesium in it," Squyres said. "It is the kind of mineral that one finds in igneous rocks, volcanic rocks, lava, basalt.

"Now it forms in a number of different kinds of rocks, but it is a primary igneous mineral for the most part, it's not something that you form as a result of lots of chemical weathering.

"Now there are a couple of different ways to interpret this," he said. "One possibility is that this martian soil, rather than being the result of a chemical weathering process, is simply very finely ground lava, very finely ground rock. That's one possibility, that would be a surprise to me.

"Some people on my team are so surprised to see the olivine in this measurement that they don't think we're looking at the soil. It's entirely possible that a millimeter down, two millimeters down beneath those grains is solid rock. Some people on my team believe we're seeing through that fluffy stuff ... so there may be rock solid rock beneath this stuff."

It is possible volcanic activity after the hypothesized lake vanished covered over earlier lakebed deposits. Even so, Squyres is optimistic about eventually finding such rocks.

"I believe it is unavoidable that somewhere beneath our wheels are lake sediments," he said. "How far down do we have to go to get to them? I don't know. There are a couple of possibilities. One of them is this stuff has been churned up so completely by impacts that whatever was present in the way of sedimentary layering is long gone and what we're seeing at the surface is some mixture of maybe volcanic materials, windblown materials, stuff that's been churned up by the impact process. So that's one possibility.

"Another possibility is that what we're sitting on is just stuff that's been blown in by the wind, and volcanic (activity) and so forth and that the lake sediments are, in fact, buried. As I said, it's beneath our wheels somewhere, but I don't know how far down."

Luckily, a 650-foot-wide crater is within reach of Spirit, a mere 820 feet or so away from its current position. The impact that created a crater that wide could have blasted out material from as much as 150 feet below the current surface.

"As we get close to that crater and we start to see the ejecta, the stuff that was thrown out of it, we're going to be getting deeper and deeper into materials that came from far beneath our wheels," Squyres said. "I don't think there's any question, as I said, that there was once a lake at Gusev Crater. But I also said Mars is not going to give up her secrets very easily. And finding those materials is going to take the full capabilities this vehicle has to offer."

Spirit rolled off its lander and onto the floor of Gusev Crater late last week. Its first scientific target was a small patch of fine-grain soil just a few feet from the lander. The rover's instrument deployment device, or IDD - a robot arm by any other name - pointed a microscope at the soil and then two spectrometers, the Mossbauer and the Alpha Particle X-ray Spectrometer, also provided by Germany. The former is designed to detect iron-bearing minerals while the latter sniffs out rock-forming elements.

The Mossbauer detected olivine and two different forms of iron while the APXS detected high concentrations of silicon and iron, along with calcium, sulfur, chlorine and nickel.

On Sunday, Spirit was ordered to drive about 9.4 feet to a football-size rock nicknamed "Adirondack" and that's where the rover remained today. The rock has been photographed by the rover's microscope and spectrometer runs will be carried out overnight.

This true color image taken by the panoramic camera onboard the Mars Exploration Rover Spirit shows "Adirondack," the rover's first target rock. Scientists named the angular rock after the Adirondack mountain range in New York. The word Adirondack is Native American and is interpreted by some to mean "They of the great rocks." Credit: NASA/JPL/Cornell Download a larger version here

The only instrument, or tool, aboard Spirit that has not yet been put to work is its rock abrasion tool, or "RAT," which uses a spinning rotor to bore into selected rocks to reach pristine, non-weathered material. Scientists are debating whether to use the RAT on Adirondack overnight Tuesday.

"We're starting to put together a picture of what the soil at this particular place in Gusev Crater is like," Squyres said. "There are some puzzles, there are some surprises, we have much that we still have to learn, but we're starting to put together an interesting story."

Video coverage for subscribers only:
   VIDEO: MOSSBAUER SPECTROMETER RESULTS FROM SOIL STUDY QT
   VIDEO: ALPHA PARTICLE X-RAY SPECTROMETER DATA RELEASED QT
   VIDEO: TUESDAY'S SCIENCE AND MISSION UPDATE BRIEFING QT

   VIDEO: MOVIE FROM ONBOARD IMAGES OF DRIVE TO ROCK QT
   VIDEO: ACTUAL ANIMATION OF DRIVE WITH NARRATION QT
   VIDEO: 3D VIEW OF ROCK SPIRIT WILL STUDY W/NARRATION QT
   VIDEO: TODAY'S BRIEFING ON SCIENCE AND LATEST DRIVE QT
   MORE: PREVIOUS MISSION COVERAGE!
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