From Science, but you didn't hear it from me.... ;)

Richard A. Kerr

The three landers heading for Mars in search of signs of ancient life will also be testing geologic tales spun from decades of orbital data--less glamorous work but no less exciting for planetary scientists
For two generations, planetary scientists have been staring down at the surface of Mars from hundreds of kilometers above, picking out ever-finer details in a wider and wider range of the electromagnetic spectrum. Three times--twice in 1976 with the Viking landers and in 1997 with Mars Pathfinder--they've gotten infinitely closer to their subject through landers. But even then they learned mostly about martian dust and soils and precious little about rocks and the stories they might tell of how the planet has worked over its 4.5 billion years.
That could change with the coming round of landers. The U.S. rovers Opportunity and Spirit, which are scheduled to arrive on Mars in January, carry instruments capable of confirming or denying geologic stories of long-ago water told from orbital data alone. The two NASA landers are "all about testing hypotheses," says planetary geologist Ronald Greeley of Arizona State University (ASU) in Tempe. The European Space Agency's Beagle 2 lander, which is due to arrive on Christmas Eve, will not be roving, but it will conduct a broader search for life than the Viking landers ever did, while getting a close look at a weird and mysterious martian terrain. "Everybody's excited to be going," says planetary scientist Wendy Calvin of the University of Nevada, Reno.

But even if all goes according to plan, understanding the geology of just these few spots "may be hard to do," says planetary geologist Harry McSween of the University of Tennessee, Knoxville. Gusev Crater, where Spirit is headed, "has some complicated geology. Neither of these [NASA] sites is going to be really straightforward." And even the latest in rover technology has significant limitations (see p. 1834). However, adds McSween, "the complexity does make it fun."

Rust and water at Meridiani?

The hypothesis testing at Opportunity's landing site on Meridiani Planum--a flat, smooth (and possibly visually boring) plain on the martian equator--will be rooted in the study of color. Readings of the subtle shapes of spectra returned from orbit have generated a range of hypotheses for the origin of Meridiani's minerals. The most dramatic question scientists hope to answer is whether water--a necessity for life--was involved.

As a glance up in the evening sky suggests, the "Red Planet" is actually a yellowish brown. Spectroscopists have long interpreted that color as a sign of nanoscale bits of iron oxide or hematite in the pervasive martian dust. But in 1999, the Thermal Emission Spectrometer (TES) onboard the orbiting Mars Global Surveyor (MGS) returned spectra with such high resolution that researchers could recognize three restricted regions that contained the mineral called gray hematite. Its crystals are large enough to give a gray color rather than the yellow-brown of nanoscale hematite. The largest area by far of gray hematite is Meridiani Planum, where Opportunity is headed.

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CREDIT: KEES VEENENBOS
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If Opportunity lands safely on 24 January, it should confirm the existence of gray hematite. That's about all that researchers using orbital data can agree on; how the mineral got there is still up for grabs. On Earth, gray hematite "is almost always associated with water," says spectroscopist Melissa Lane of the Planetary Science Institute in Tucson, Arizona. By analogy with Earth, the hematite of Meridiani Planum in all likelihood formed with water, she says.

Most researchers would agree that water reacted with other minerals to produce the gray hematite, suggesting an environment conducive to life, at least in the distant past. But a few have suggested it could have formed in lavas without water. Planetary scientist Virgil Sharpton of the University of Alaska, Fairbanks, says that the best images from the camera on MGS show pervasive dunes across Meridiani Planum, suggesting that the hematite grains have been concentrated there by the wind. If so, they could have formed in any sort of environment, including volcanic lavas or ash or in the melted rock of impact sites.

Even if water were involved, as most believe, researchers disagree about what the wet environment was like. It might have been the floor of an ocean or lake, some have suggested, as was the case for Earth's ancient banded iron formations. But the favorite hypothesis is a subterranean, water-logged layer of possibly hot rock later exposed to view by some sort of erosion. Lane sees signs in the shape of spectra from TES that Meridiani's hematite formed as tiny plates aligned with each other. On Earth, such platy hematite crystals form as water alters minerals under the pressure of overlying rock. "It looks as if this whole area was buried in sediment and only recently exhumed" by erosion, says Lane.

Raymond Arvidson of Washington University in St. Louis, Missouri, who will help supervise scientific operations of the landers, can spin a geologic story from all this. In his best-guess scenario, volcanic eruptions laid down 300 meters of debris on top of the 4-billion-year-old cratered Meridiani crust, followed by a heavy layer of wind-blown dust. Water heated by rising magma then flowed through the topmost 10 meters of buried volcanic ash layer, altering some of its minerals to hematite. That would be the prime time for mineral-eating microbes, if any existed, to have taken up residence. Then the wind removed the dust to expose the hematite-rich layer with the occasional crater rim protruding. That's as close to a consensus as anyone comes, but, says Arvidson, "I could be all wrong."

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Single-minded. The Beagle 2 lander will search for signs of past life by analyzing isotopes of any carbon it can find.
CREDIT: BEAGLE 2
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To test that scenario, "we're really focusing on the rocks and reading the rock record," Arvidson says. "The rock provides the detailed mineral history. Viking didn't have much in the way of mineralogy [instruments]. Mars Pathfinder had mobility but didn't have very good instruments. Elemental composition [analysis], which Pathfinder had, is good, but mineralogy is even better. We'll have both."

Each rover carries the basic tools of a field geologist. One is the rock abrasion tool, the equivalent of a hammer for exposing fresh rock. The other two field tools are a panoramic camera for the big picture and a microscopic imager for the fine details. Combined, the two should determine whether the hematite comes in Lane's platy form and was formed by the action of subterranean water. They could even see any mineralogical layering indicative of lakebed formation. But if it's all loose sand- and silt-sized grains, Sharpton could be right about the hematite being the winnowed remains of volcanic debris.

Opportunity and Spirit also carry instruments found in a mineralogy laboratory. An alpha particle x-ray spectrometer determines the amount of most elements, a Mössbauer spectrometer identifies and measures the abundance of iron minerals, and a mini-thermal emission spectrometer determines mineral composition spectrally as TES does from orbit. At Meridiani, "it's the minerals in association with the hematite that will be important," says Arvidson. Minerals such as carbonates, olivine, and iron silicates could tell what minerals water weathered to form the hematite. They could also help tell just what conditions were like during weathering and how conducive to life they might have been.

So far, spectroscopic studies from orbit have failed to find any sign of such accessory minerals on Meridiani Planum. Indeed, Mars has proved to be spectrally bland at the wavelengths examined in varying detail from orbit. "Perhaps Mars is mineralogically impoverished," says planetary scientist John Mustard of Brown University in Providence, Rhode Island. Even close up, Pathfinder found little spectral diversity, he notes. Opportunity "will allow us to validate the orbital data," says Mustard, sorting out what part of the martian mineralogical story may have been missed from orbit.

A watery mirage?
At Gusev Crater, Spirit's landing site, the geologists' tales of ancient water will be tested not against spectroscopy and mineralogy but primarily by geological deduction from images. At first glance, images from orbit tell a story of bountiful water flowing into Gusev, ponding, and flowing away. The channel of 900-kilometer-long Ma'adim Vallis cuts through the southern rim of 150-kilometer-wide Gusev, fans of sediment spread across the western half of the crater floor, and a lower cut in the north rim looks like an outlet. Nathalie Cabrol and Edmond Grin of NASA's Ames Research Center in Mountain View, California, have argued that water flowed in, formed a lake, and flowed out the other side.

Gusev as paleolake--a great spot for "following the water" and maybe even stumbling on signs of past life--was the prime justification for selecting the crater as a second landing site. But some are questioning its seemingly straightforward crater-lake history. "Gusev is telling us an interesting story," says planetary geologist James Rice of ASU; "I just don't think it's the lake story."

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Born to wander. The Spirit rover will look for geologic signs of an ancient lake in 150-kilometer Gusev crater.
CREDITS: JPL/NASA; KEES VEENENBOS
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Other craters in the region have no channels running into them but are filled with sediment anyway, Rice notes. Some of it has partially eroded away to expose enticingly regular layering that could be water-lain, windblown, or volcanic sediments. There hasn't been erosion to expose the sediments in the landing area, but Rice suspects that the bulk of Gusev fill got there without a lake forming. What water there might have been running through Ma'adim Vallis could have flowed in and out of Gusev without forming a lake, he says.

Most researchers are not as pessimistic as Rice about finding lakebed sediments, but no one is saying that testing the lake story using a rover is going to be easy. Gusev crater has been sitting there for most of the past 4 billion years collecting whatever has come its way. "I was bowled over" by images from the Thermal Emission Imaging System (THEMIS) on Mars Odyssey, says McSween. They highlight differences between the physical properties of rock, soil, and dust. "Gusev is surprisingly complex."

Sediment may well have settled out of a lake in nice neat layers from which a history of martian climate could be read. But Rice as well as McSween see in THEMIS images where a slurry of sediment flowed out of Ma'adim Vallis and across the western half of the landing area. If so, there would be no layering there. And unknown amounts of other materials have no doubt collected in Gusev as well: ash from nearby volcano Apollinaris Patera, ejecta from impact craters outside and inside Gusev, dust blown in on the wind, and even lava flows. All that could be on top of any lakebed. "We're going to see evidence of more than one or two processes," says Greeley. "The results are likely to be complex. Untangling that is going to take time."

Rover operators will be relying on the panoramic camera and microscopic imager to ferret out the shapes and patterns that can tell a geologic tale. And lacking the martian equivalent of riverbanks and road cuts, they will be searching for some of those impact ejecta blocks for a close look at now-buried deposits. Is there regular layering in the rock, as if sediment fell through a seasonally varying lake, or crosscutting beds typical of flowing water? Are the grains rounded, as water-borne particles would be, or angular, as fresh volcanic ash would be? Are grains sorted by size, as if they fell through standing water? A close look, if Spirit can get one, should tell a tale or two.

A shot in the dark

Beagle 2's primary goal is to test a single, simple question--"whether life began on Mars"--explains consortium leader Colin Pillinger of the Open University in Milton Keynes, U.K. But the compact, 33-kilogram lander built and operated by the British-led consortium has all the tools and instruments for mineralogical study that Opportunity and Spirit carry, with the exception of a thermal emission spectrometer. So it will take the opportunity to test any number of hypotheses prompted by the rocks in the landing area centered in the great Isidis impact basin.

The floor of the impact basin is most often called a lava plain, but scientists have described it as everything from an old sea floor to debris swept off the basin rim by an outburst of carbon dioxide. Moving inward from the rim, "the terrain becomes totally weird," says planetary geologist Larry S. Crumpler of the New Mexico Museum of Natural History and Science in Albuquerque. "Who knows what that material is?"

Given the general geologic uncertainty in Isidis, the main attraction is a hint of water. Odd strings of cones crisscrossing the floor are up to 50 meters high and are topped by pits. They were possibly formed when volcanic heat boiled subterranean water to steam that drove volcanic explosions, says Beagle 2 team member John Bridges of the Open University. Where there's been water, there may have been life, and life may have left traces of organic matter.

So Beagle 2 is going after the organic matter. "The experiments on the Viking landers were very specifically designed to detect [ongoing] biological activity," says Pillinger. "Our prime aim is looking for past life." For that, a "mole" will burrow beneath the surface, even under rocks, to retrieve samples whose organic matter may have been preserved from oxidation on the surface. Any organic as well as inorganic carbon will then be baked off and separately analyzed isotopically for signs of life having shifted the isotopic signature of the organic carbon. The British "have a more imaginative, a more daring payload," says planetary geologist Michael Carr of the U.S. Geological Survey in Menlo Park, California.

Mars researchers are tickled by the chance to do any kind of science on Mars, daring or otherwise. "Landing anywhere on Mars, you're going to learn something new and interesting," says Kenneth Edgett of Malin Space Science Systems Inc. in San Diego, California. "Who knows what we're going to find?" Adds James Head of Brown University: "That's one of the beauties of exploration."