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Monday, September 27, 2010

Sunday, September 26, 2010

Where Will Next Mars Rover Land?

The next Mars rover is only about a year away from taking off, and it's already stretching its arms and spinning its wheels in a lab in California. But scientists are still debating exactly where to drop it.

Curiosity (or more formally, the Mars Science Laboratory) is slated to launch in late 2011, and its chief objective is looking for life. That means landing in a spot where the soils formed in water, and where rocks could have preserved chemical traces of living organisms.

Now, after four years of deliberation, the rover crew has narrowed the choice down to their four favorites: a rugged valley full of water-bearing clays; and three craters that may once have been basins, lakes or river deltas. Hundreds of planetary scientists will descend on Monrovia, California, next week to continue the debate.

Mawrth Vallis

One of the top contenders is Mawrth Vallis, one of the oldest valleys on Mars. Mawrth's appeal comes from its mineral composition: It's chock-full of clay-like minerals called phyllosilicates that form only in the presence of water.

"The reason people are really excited about clay minerals is that they're good at trapping organic molecules, and they only form in relatively benign watery situations," said Cornell University graduate student Ryan Anderson. If Mars ever had life, clays are a good place to look for the evidence.

Scientists know what's down there by checking out the spectra of the rocks. The CRISM, or Compact Reconnaissance Imaging Spectrometer for Mars, instrument on the Mars Reconnaissance Orbiter and similar instruments on other satellites take in sunlight reflecting off Mars' surface and break it up into a spectrum, like a prism spreading white light into a rainbow. Different rocks cast distinctive rainbows based on their compositions.

As seen from orbit, Mawrth is "this glowing beacon of clay minerals," Anderson said. Observations show that the rocks at Mawrth could be made of up to 60 percent phyllosilicates, a huge fraction. And they come in different flavors, too. The distribution of aluminum-rich clays and iron-rich clays found at Mawrth is also found on Earth -- in tropical rainforests.

Mawrth is also the only site where the rover would land right on top of the interesting stuff.

"That's an advantage it has going for it: The rover won't have to drive far to find the most interesting mineral deposits," said planetary scientist Jim Bell of Cornell, who manages the panoramic cameras on rovers Spirit and Opportunity.

But the rocks at Mawrth are a mess. "It looks like a nasty, battered, cratered, rocky place," Anderson said. There are no obvious geological clues that tell astronomers how all those tempting clays formed. They could have formed in pools of standing water, but they could equally as well have been a pile of ash that was altered.

"There's a lot of confusion about the underlying story at that site," Anderson said. "But you could make the argument that if we landed there, we would know the story."

Eberswalde Crater

Rover drivers are also tempted by Eberswalde Crater, just south of the Martian equator. Eberswalde is considered some of the best evidence on all of Mars that the planet had persistent water. Just to the left of the 40-mile-wide basin lies a wide, meandering delta.

"That's delta as in the Mississippi Delta," Anderson said. "It looks for all the world like a bunch of water was flowing into a lake. That's really exciting."

The delta also shows evidence of containing clay-like minerals. When clays are deposited in a gentle, watery place like a delta, the conditions are perfect for preserving fossil traces of life.

No one expects fossils on Mars to look like the calcified bones and shell imprints found on Earth. Instead, Curiosity will look for "molecular fossils" -- organic molecules that have a certain pattern to their structure that is only formed by life.

For example, the building blocks of proteins can come in "left-handed" and "right-handed" versions that are mirror images of each other, but otherwise identical. For some reason, all earthly life uses left-handed molecules.

That's the kind of shapely signature that Curiosity's instruments will be able to detect. But finding organics on Mars at all will be a huge discovery. The Viking Mission in the 1970s found even less organic material on Mars than there ought to be on the moon -- possibly because the Viking lander itself destroyed them.

Holden Crater

Just to the south of Eberswalde lies Holden Crater, another possible ancient lake that may have lived through a catastrophic flood.

"The cool thing about Holden is we know that the crater was filled with water," Anderson said. A river channel snakes across the Martian landscape to a breach in the 87-mile-wide crater's southern wall, suggesting that water built up at the crater's edge and then suddenly burst through.

The minerals on the crater floor support the flood story. These minerals include light-toned clays and shocked, broken rock called breccia. Breccia typically forms when some catastrophic event -- an impact, a landslide or a flash flood -- cements broken, tumbled rock fragments together. The rocks at Holden are so big -- about the size of a large house -- that astronomers call them megabreccia.

The flood story is compelling, Bell says, but it could be bad for signs of life.

"That concerns some of the biologists," he said "Maybe that would be not a good environment for preserving potential biological sediments, because things would be broken down physically."

Gale Crater

The final contender is Gale Crater, a hole 96 miles across with a mountain nearly 4 miles high stretching up from its center. Several gorges flow into the crater, but nothing flows out.

"I've heard people say, if there was ever a lake on Mars, then it should have been in Gale," Anderson said. "If we don't find evidence of a lake if we go to Gale, that's a big deal."

Gale Crater contains both phyllosilicates and sulfate minerals, which tend to form in the presence of salty waters. But the main attraction is the enormous central mound. From orbit, the mound looks like a stack of layers of several different kinds of material. Because the crater is so old -- about 4 billion years -- the layers trace the history of geology in this part of Mars.

"It's like the Grand Canyon, where you have most of Earth's history laid out before you," Anderson said. "In Gale, you have so much stratigraphy that you can learn a lot about the planet's history just by going uphill."

Unfortunately, Curiosity would have to land 6 to 10 miles away from the mound. Despite being twice their size, Curiosity can't drive much faster than Spirit and Opportunity -- only a few hundred feet a day. It could take several days of hard driving just to set the rover's wheels on the base of the mountain.

There are still questions about what exactly Gale is made of, though. Gale is the dustiest potential landing site, which means it's harder to tell from orbit what minerals it contains. And the layers could have been laid down in several ways.

"The big question is what the layers are. Sediments from a lake? Volcanic ash? Sand dunes?" Anderson said. "It's hard to tell unless we go there."

Viking Landers

Curiosity is the latest in a long line of robots on Mars. The Viking 1 lander (below) became the first spacecraft to successfully land on Mars when it touched down in western Chryse Planitia on July 20, 1976. The site showed strong evidence for running water on Mars' surface.

Viking 2 (above) landed in Utopia Planitia on Sept. 3, 1976, where patches of frost formed around dark rocks. Neither lander found any evidence of life.

Mars Pathfinder

Mars Pathfinder landed in an ancient flood plain in Mars' northern hemisphere called Ares Vallis on July 4, 1997. The Sojourner rover, the first independent moving robot on Mars, drove up to 1,600 feet from the lander and investigated rocks named Barnacle Bill, Yogi and Scooby Doo.


Phoenix landed near Mars' north pole in an area called the Green Valley on May 25, 2008. Among other analyses of Martian soil and weather, the lander confirmed the presence of subsurface water ice on Mars.


Curiosity will, however, have a few advantages over its predecessor rovers. Its bulked-up size will let it roll right over obstacles that Spirit and Opportunity would have to drive around. And it's powered by plutonium, meaning it won't have to worry about the angle of the sun, or its solar panels getting dusty.

The final decision about where to land Curiosity will probably come down to engineering constraints. Safely landing Spirit and Opportunity in 2004 was tricky business: The twin rovers bounced and rolled across the Martian surface in a bubble of protective airbags (Spirit's landing site, Gusev Crater, is shown above). This slapdash landing gear meant that deep craters and gullies were out of bounds -- the rover could roll into one and not have room to climb out of its airbags.

Curiosity will take a more precise, if death-defying, route down to Mars. After detaching from its heat shield, the rover will be deftly dropped on the surface by a hovering hoist called a Sky Crane.

"Wherever we go," Anderson said, "we're going to see a lot more spectacular stuff than we've seen at other landing sites, just because we can land near the tall, interesting stuff."


The two earlier rovers got lucky -- especially Opportunity, which landed in a tiny crater on Meridiani Planum that happened to be full of the water-formed mineral hematite.

Astronomers already know more about the final four Curiosity sites than they did about Spirit and Opportunity's sites before landing.

"If we only had orbital data, we would still not know that Gusev and Meridiani are as interesting as they are," Bell said. "But having the orbital data beforehand helps us be confident that we'll find at least as interesting stuff -- but hopefully a lot more -- once we get down on the ground."

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