Feature

Martian Invasion

Probing lively puzzles on the Red Planet

By
6:14am, November 5, 2003
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Just 2 months ago, Mars loomed high in the sky, its ruddy countenance so close that anyone with a backyard telescope could make out the planet's white south-polar cap and a central smudge known as Syrtis Major. Not in 60,000 years had Mars and Earth been so close, and they won't be again for another 2 centuries. But even as the two planets now drift slowly apart, three envoys from Earth are racing to the Red Planet.

If all goes according to plan, the European Space Agency's Mars Express will begin orbiting Mars next month, using radar to search for hidden reservoirs of water. The craft will also jettison a suitcase-size stationary lander, Beagle-2, that will look for signs of life by examining soil at and just below the surface of a region called Isidis Planitia.

Then, in January, two NASA craft bearing identical rovers, named Spirit and Opportunity, will touch down in regions of the planet that may once have had water coursing through them and so could have hosted primitive life.

"Successful landings of all three spacecraft will more than double our experience with the . . . environments of Mars," says James B. Garvin, NASA's Mars-program scientist in Washington, D.C. "I am anticipating major breakthroughs in our understanding."

Planetary scientists studying Mars could use a breakthrough. Recent evidence has shaken what has been one of the most tantalizing core beliefs about the Red Planet–that ancient Mars was much wetter and warmer than the planet is today and even harbored a planetwide ocean.

On the one hand, the planet's now bone-dry surface is scarred by sinuous channels, apparent lake beds, deep canyons, and thousands of gullies. These all bear the marks of having been carved by liquid water. On the other hand, there's a troubling scarcity of minerals such as limestone and other carbonates, which commonly form in the presence of liquid water.

There is a "direct conflict" between the geological and mineralogical evidence for water on Mars, says Bruce M. Jakosky of the University of Colorado in Boulder.

Determining whether parts of Mars ever carried a substantial amount of liquid water and, if so, for how long would help answer the ultimate question about the Red Planet: Is it now or has it ever been a living world?

Missing minerals

The water conundrum intensified late last summer, when Philip R. Christensen of the Arizona State University in Tempe and his colleagues reported the results of a 6-year study with an infrared spectrometer aboard the orbiting Mars Global Surveyor observatory. The instrument scrutinized large swaths of the Martian surface and atmosphere for carbonates, minerals that are associated with water. On Earth, carbonates such as limestone form when carbon dioxide from the atmosphere dissolves in water, making carbonic acid. The acid eats away at rocks, and their remains precipitate out as carbonate deposits. A notable example is the White Cliffs of Dover.

Researchers had been looking for carbonates on Mars for more than a decade, and in the Aug. 22 Science, Christensen's team announced that it had finally found some. But there was little reason to rejoice. Carbonates were detected in only small amounts–up to 5 percent–in the planet's surface dust.

"We believe that the relatively small amounts that we see probably did not come from oceans, but from trace amounts of water vapor in the atmosphere interacting directly with dust," Christensen says.

This study, as well as other new evidence (see "Bone-dry Mars?" in this week's issue: Available to subscribers at Bone-dry Mars?), "really points to a cold, frozen, icy Mars that has probably always been that way, as opposed to a warm, humid, oceanic Mars some time in the past," Christensen adds. The extensive carbonate layers that would have formed early in Martian history if the climate had been warm and oceans plentiful "are simply not there," he says.

There may be geologic processes that could have hidden or transformed carbonates at the Martian surface to make them undetectable from orbit, acknowledges Chris Chyba of the SETI Institute in Mountain View, Calif. However, he says, "unless and until there's strong evidence of such a mechanism, I think we should take the data at face value."

Roving for water

The evidence that Chyba seeks can come only from spacecraft that land. On the surface, their instruments can search for carbonates and other water-derived minerals, along with water-carved features, on scales far finer than those at which an orbiting observatory can investigate. NASA's rovers will explore two strikingly disparate places. Both regions appear to have had an encounter with liquid water but have different tales to tell about their aquatic past, says Steve Squyres of Cornell University. His team built the twin rovers' array of instruments.

"We think these will be the most exciting landed missions of exploration since the Apollo program," declares NASA's Garvin. Each vehicle is about five times as large as its diminutive cousin, Sojourner, which on July 4, 1997, became the first rover on the Red Planet.

Compared with Sojourner's single scientific instrument, an X-ray spectrometer, each of the new rovers has nine cameras, three spectrometers, and a robotic arm. The spectrometers will analyze the chemical composition of the rocks, while a microscope imager at the end of the arm will act like a hand lens, revealing the texture and shape of minerals. Like the Beagle-2, each rover also has a scraper that can remove about half a millimeter of material–about the thickness of a nickel–from the surface of rocks covered with dust or other debris. The solar-powered, all-terrain vehicles can travel about 40 meters a day, compared with Sojourner's 1 meter. Researchers expect them to explore rocks for at least 3 months, about the same as Sojourner's life span.

On Jan. 3, Spirit, the first of the two $400-million rovers, will descend along with its mother ship into Gusev crater, a 160-kilometer-wide crater that appears to have a dried-up riverbed running into it. "We'll look for lake sediment or sea sediment–the rocks that may have been deposited by running water and that may have been entombed there," says Garvin. Spirit's close-up examination will seek sedimentary layers of rock that might be present and determine whether the rock was chemically altered by water that vanished long ago.

Water and hematite

Three weeks after the spacecraft carrying Spirit lands, the second NASA craft and its rover, dubbed Opportunity, will touch down at the edge of a smooth plain called Meridiani Planum.

Here, the hunt for evidence of past water will primarily rely on chemistry. The region is one of two places on Mars covered with a vast deposit of hematite, a gray, crystalline iron oxide that on Earth usually forms in the presence of water. (A ubiquitous, finer-grained version of the same oxide gives Mars its rusty-red hue.) Observations of Meridiani Planum from orbit suggest that the gray hematite was deposited by a watery source, perhaps an ancient and vast hot spring.

The rover's capacity to examine the texture and distribution of the hematite, even its microscopic grain structure, will be critical in determining how the material and related minerals got there, notes James F. Bell of Cornell.

Christensen has proposed several scenarios to explain the hematite; all but one require substantial amounts of water.

In one scenario, the hematite is but one layer of a band of iron-rich mineral deposits, like the iron-oxide bands seen in Lake Superior and other large standing bodies of water on Earth. The bands form when dissolved iron particles combine with oxygen and precipitate out as layers of hematite.

Each layer, says Christensen, demarcates a significant climate change, such as a dramatic difference in the temperature of the water or the concentration of oxygen in it. "If you get [to the planet], and you pick up a rock and it has some banded layers of hematite in it, that's a smoking gun for a lake deposit."

A second explanation for the hematite centers on massive amounts of rain or snow that may have dissolved minerals in the topmost layer of the region. Hematite might then have recrystallized in a deeper layer. Millions of years of wind erosion may have brought that once-buried layer to the surface. If that's the case, then the rover should detect iron-poor rocks, the vestiges of the topmost layer in which minerals were dissolved away, as well as iron-rich ones.

Or maybe water had nothing to do with hematite. The material could have arisen when iron-rich volcanic eruptions interacted with oxygen in the atmosphere. If so, Opportunity will find the hematite to be a fine-grained ash dispersed uniformly through the rocks the rover analyzes, Christensen says.

If Opportunity finds that hematite is merely a surface coating, like the rust on a junkyard car, it could mean that the rocks were exposed to water for only a short amount of time or simply interacted with trace amounts of water vapor in the atmosphere.

Magnified images that reveal the specific shapes of mineral will provide additional clues to the origin of the iron oxides at either landing site, notes Bell. For instance, large, rounded grains of hematite suggest transport by a flow of water, while fine, flatter grains point toward the rust-formation mechanism.

Christensen says his favorite explanation for the hematite at Meridiani Planum is that a warm or hot spring percolated through the rocks there. The water, he suggests, could have come from a frozen lake covering layers of iron-bearing sediment. If some underground heat source, such as an erupting volcano, melted the ice, the water would have percolated through the sediment. If this scenario holds true, Opportunity "would see a hematite cement" filling in the rock pores, Christensen says.

Even if the hematite's origin remains ambiguous, trace amounts of other minerals could serve as additional markers of past water. Consider goethite, the water-bearing iron mineral named for the German poet Johann Wolfgang von Goethe, who dabbled in mineralogy. Goethite formation requires water, and the mineral, when heated, can slowly convert to hematite. Small amounts of goethite would clinch a watery origin for the hematite at Meridiani Planum.

There is at least one way that liquid water could have been present on Mars without leaving behind carbonate fingerprints. Proposed by Christensen and other planetary scientists, this hypothesis could have important implications for future missions to Mars, especially in determining where to look for life.

If the Martian surface were somewhat warmer in the past–just above –20C instead of today's average of –60C–frozen water could liquefy as thin films at the boundary between layers of ice and dust, Jakosky notes. At that temperature, the chemical reactions that would lead to the formation of carbonates proceed so slowly that little carbonate would be made before the water refroze.

Supporting this idea, Christensen notes that many of the features on Mars that appear to have been sculpted by flowing water, including channels, don't require liquid water to last very long or to cover vast stretches of the planet. For example, the sudden melting of a reservoir of ice, creating a flash flood lasting for just a few weeks on a section of the Martian surface, would suffice to create a channel.

New synthesis, new missions

Brief interludes when water was liquid also seem to have occurred recently. A camera aboard the Mars Global Surveyor spacecraft spied thousands of gullies at high latitudes where water would typically be frozen but under some circumstances could liquefy for brief periods. Free of craters and other blemishes typically acquired by older surfaces, the gullies look remarkably young, suggesting that water flowed there as recently as a few million years ago.

Such episodic flows would permit dormant organisms "to revive and repair themselves every few million years," Jakosky and his colleagues argue in the summer 2003 Astrobiology. Studies of life in extreme environments on Earth suggest that, "even at very low temperatures that would allow metabolism but not necessarily growth, organisms could effectively reset any damaged systems, including DNA, and thus allow very long-term survival," the team says. "Mars today appears to be right at the edge of being habitable by microorganisms."

According to this hypothesis, some of the best places to look for life–or its remains–have vast reserves of ice. These promising regions include the north and south poles of the planet, according to data gathered by the Mars Odyssey spacecraft.

Because the tilt of Mars's polar axis dips as much as 60 every million years or so, some of this ice could have been liquid in the not-so-distant past. At such places, a lander or rover might discover primitive life or its vestiges in the topmost layers of ice, says Jakosky. Last summer, NASA announced plans to launch a lander to the north-polar regions of Mars in 2007.

"At the same time that evidence is accumulating that early Mars may globally not have been very warm, evidence is also accumulating that contemporary Mars still intermittently has water flowing on its surface in specific locations," says Chyba. "There are over 100 locations where water seems to have flowed recently," he notes. "It will be fascinating to see what the rover missions tell us."

Back at NASA, Garvin says he is looking forward to what could be the most nerve-wracking 6 minutes of his life. That's when the first rover and its mother ship will careen through the Martian atmosphere at 20,000 kilometers per hour with only a cocoon of airbags to cushion their fall. Says Garvin: "We'll all be on the edge of our seats."

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