Four potential exploration sites for future Mars probe

The latest Mars robot may be dead, but NASA scientists have plenty to keep them busy as they scout out four potential stomping grounds for an ambitious new rover pegged to be the next red planet explorer.

NASA declared the Phoenix Mars lander – its youngest Mars probe – officially dead in late May after photos taken of it from orbit revealed substantial damage from its environment in the Martian arctic. Those photos came from the same powerful orbiter that has been searching for the ultimate drop zone for NASA's new Mars Science Laboratory (MSL) which is currently set for a November 2011 launch.

The new roving robot lab, known as Curiosity, is expected to determine whether Mars is or was ever habitable to microbial life. The rover's combination of technical improvements should make any potential landing sites more scientifically rich than anywhere Mars landers have gone before.

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"We will either land in Disneyland or in the parking lot next to Disneyland," said Ashwin Vasavada, Curiosity deputy project scientist at NASA's Jet Propulsion Laboratory (JPL) in Pasadena, Calif., echoing the project's mission manager. [Best and worst Mars landings.]

Stomping grounds on Mars

NASA is closing in on the end of a long process – which began three years ago – to poll the Mars science community for potential landing sites, then weigh the pros and cons of each.

Now, out of some 60 possible sites considered at different stages of the process, the list has been whittled down to four. They are regions of Mars known as Mawrth Vallis, Gale crater, Holden crater and Eberswalde crater.

"These are the best places you could possibly imagine you would want to go, and for the first time, you can actually land near them and get to them," said Matthew Golombek of JPL, co-chair of the Curiosity rover landing site steering committee.

Mars rover bull's eye

The Curiosity rover is the first Mars mission ever built to use a guided entry, meaning it will steer itself through the Martian atmosphere like a guided missile instead of flying passively like a shuttlecock. Because of this, the spacecraft can hit a much smaller landing target than ever before.

NASA's Viking missions required landing zones 190 miles (300 km) long to account for possible drift as the craft descended through the atmosphere. Having to find a safe area of that extent – free of craters, cliffs and rocks – drastically limited the potential landing sites. Later, the Mars Pathfinder mission and Spirit and Opportunity rover landings worked with target ellipses 60 miles (100 km) long.

The Curiosity rover, however, is designed to hit a target just over 15 miles (25 km) long and 12 miles (20 km) wide.

"It opens up a lot more possibilities of squeezing the ellipse within the terrain and closer to features of interest," said Vasavada. After landing, Curiosity will also be able to drive up to 20 kilometers to reach targets, or "go-to" sites.

The new rover is also built to decelerate faster than previous missions, meaning it can land on more elevated terrain, which opens up even more of the red planet for exploration. And because Curiosity lacks solar panels and is designed to withstand severe cold temperatures, mission planners can target anywhere from thirty degrees North to thirty degrees South latitude.

Where to land on Mars?

These broader landing possibilities give researchers roughly half the planet to choose from, or triple the area open to Spirit and Opportunity rovers. To zero in on the most interesting landing sites, Mars scientists made use of data collected from orbit.

In 2005, the OMEGA spectrometer on board ESA's Mars Express orbiter picked up the presence of phyllosilicates – clay minerals that formed in the presence of water – on the Martian surface. For the past few years, Mars Reconnaissance Orbiter (MRO) has been mapping those phyllosilicates at high resolution.

All four of the landing sites under consideration contain layered phyllosilicates, which must have been deposited over a period of time. Vasavada said the phyllosilicates are all found in terrain that dates back to roughly the first billion years of Mars history, known as the Noachian era. Curiosity will be the first rover to land on such ancient terrain.

Ideally, Mars scientists would like a landing site in which they know going in how the phyllosilicates were formed, to be sure they can construct a complete story of the features Curiosity will be examining.

The most comprehensive of possible landing sites for the Curiosity rover are Holden crater and Eberswalde crater, adjacent to each other in the Southern highlands of Mars.

Based on MRO observations, Holden crater was formed from an impact that disrupted an existing river system. The river eventually broke through the crater and flooded it, leaving behind phyllosilicate minerals. Holden may have also contained lakes prior to the flooding, which would have left their own traces.

Eberswalde predates Holden and contains something very exciting to researchers – the sedimentary remains of a river delta on the western side of the crater.

"Here the geologic story is really tight," Vasavada said. "It's basically a pile of sediment that was deposited by a river."

Curiosity could be used to figure out how long the river system was running, what kind of sediment was deposited and where it came from. River deltas trap sediment along with organisms and organic molecules, which are all the things mission scientists want to study.

"The downside," according to Vasavada, "would be that the delta may have only been active for very short time in Mars history."

Riskier Mars targets

Riskier from the perspective of geological understanding but potentially more rewarding in terms of mineral remains are Mawrth Vallis and Gale crater.

Mawrth Vallis is one of the oldest valleys on Mars, formed approximately 3.7 billion years ago. It also contains the best-exposed phyllosilicates on Mars.

In images from MRO's CRISM instrument, scientists have identified multiple layers of phyllosilicates made of different materials. Some are rich in iron and magnesium; others are rich in aluminum.

"It suggests a change in environment, a change in chemistry" when Mars was warmer and wetter, said Golombek. "All of that is incredibly interesting to go look at."

What Mawrth Vallis lacks is an understanding of how the minerals got there, Vasavada explained. "That's something we could discover by putting a rover there and driving around, [but] we would like to know that before."

A similar situation holds for Gale, a 90-mile (150-km) wide crater near the bottom of Elysium Planitia.

Some 3.5 to 3.8 billion years old, Gale contains a mound of phyllosilicates in the middle that towers 3 miles (5 km) high – higher even than the crater's rim. At the top of the mound are younger sulfate minerals, which would have formed in more acidic water than the phyllosilicates below them.

If Curiosity landed at Gale, it might be able to survey a billion years of Mars history, Vasavada said. The only problem, he added, is that researchers can't be sure whether the minerals were deposited by water or air.

"Everybody's wishing we had the perfect site where we could sort of do it all," said Vasavada. "Really there are strong supporters behind each of these four sites for different reasons."

More debate ahead

At the next landing site workshop in September, researchers will continue debating the pros and cons of the four sites, including how long it will take Curiosity to reach the phyllosilicates in each case and the potential pitfalls along the way such as cliffs and rocks. "If it takes a year to drive out of the ellipse, that's a factor you have to take into account," said Golombek.

The final recommendation won't come until next May or June, after MSL scientists construct simulations of the surface conditions following a fifth workshop next March or April. After that, it will be up to NASA's Associate Administrator for Space Science to make the final decision.

It's too soon to make an informed choice, said Golombek. "The science is only half of the issue. I don't think we fully understand all the potential hazards yet."

• Gallery - Latest Mars Photos From Spirit and Opportunity
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