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Geologic Column Where to land?
Lisa Rossbacher
Everyone has a favorite spot ... on Mars. At least, geoscientists who have spent more than two minutes looking at a map or photograph of Mars have a location they would like to visit. "If I could just look right here," any given planetary geoscientist might think, "I would understand what's going on." Planetary scientists are currently involved in a three-year planning process to identify the next site for fieldwork on Mars. In 2010, NASA's Mars Science Laboratory (MSL) will land on the red planet, and scientists on Earth need to decide where the martian "geologist on wheels" will work (see Geotimes, July 2006). The first of four meetings of NASA's Landing Site Steering Committee was held in June, and it drew more than 100 scientists  -  nearly all with an opinion about the best place to send the MSL.
The optimal landing site has two key requirements: safety and scientific interest. For the MSL, the science will focus on understanding the climate and geology of Mars, finding evidence of whether life ever existed there, and preparing for future human exploration. But safety is important too. If a spacecraft doesn't survive the landing, then no data are collected. On the other hand, landing sites that are broad, flat and featureless  -  and therefore relatively safe  -  are not likely to reveal much of the planet's history.
The hazards of Mars exploration are many. The complex technologies of launch, space travel and landing present one set of challenges. But even when the technology works, the surface of Mars is a hazardous environment, with rugged boulders, steep slopes, strong winds, frigid temperatures and high radiation levels.
NASA's Viking missions' landings in 1976 were just plain lucky. The resolution of the images used in selecting the landing sites was 150 to 300 meters  -  not high enough to spot boulders the size of a house, or possibly to identify even a football field. Both Viking landers arrived safely, however, and both sites proved to be scientifically interesting, with rocks, soil and vistas to provide our first human-scale view of Mars.
Yet not all missions to Mars have been so fortunate. NASA's 10 successful missions have been offset by six failures between 1964 and 1999. The USSR space program had three successes  -  and 16 failures. Japan's single Mars mission failed, and the European Space Agency's (ESA) Beagle 2 Lander was lost in 2003, although the companion Mars Express Orbiter is still collecting data. No one should take success for granted. Despite a history of problems, however, five missions are currently returning data from Mars: four operated by NASA and one by ESA. And the hunt is now on for a new landing spot.
The terrestrial principles of selecting a field area also apply on Mars, and the scientists have to ask the same questions: What's the most important question? For Mars, the relative importance of looking for evidence of surface water, detecting past life or analyzing soil composition each points to a different location.
Should we explore a previously visited area in more depth or try to look at someplace new? The MSL will be able to travel up to 200 meters per day, and up to 20 kilometers total, conquering 30-degree slopes and 0.75-meter-tall rocks, so it will have much better capability to study a particular area than the current Mars Exploration Rovers, which are challenged by 25-centimeter-tall obstacles.
What is our tolerance for risk? One hundred percent safe is likely to be 100 percent boring. Yet, 0 percent safe means 0 percent chance of collecting data. Even the most remote field areas on Earth are relatively accessible compared with Mars. Fieldwork on other planets is robotic, planned years in advance, full of compromises and has little room for error.
An interesting comparison is to think about how a martian geologist might explore Earth, using a single lander or rover with a 20-kilometer exploration radius. (Mars has about the same land area as Earth, so the parallel may be better than it first seems.) The Himalayan Mountains would provide a wealth of data ... but the odds of landing safely are about zilch. Parts of the Red River Valley might look safe, but the prospects for learning much about Earth's geology and climate within a 20-kilometer radius of a landing site are probably limited. (This assumes, of course, that the martian has figured out that the relatively smooth, dark "maria" that cover 75 percent of our planet fall into the "unsafe" category for landing purposes.)
The researchers at the first landing site workshop for the $1.5 billion MSL proposed 33 locations, and, by the end of the meeting, all the sites were ranked. The prioritized landing sites include areas that appear to have layered deposits, craters, deltas, canyons, plains and possibly glaciers. Some present prime possibilities for finding clays, sulfates, water-bearing minerals, lake sediments, volcanic ash or windblown dust. All the sites are within a latitude band between 23 degrees north and 57 degrees south. The top prospects, as of now, have study targets of phyllosilicates, layered deposits, a delta, sulfates, sedimentary layers and a paleolake. Sites that were ranked lower  -  but which remain under consideration  -  focus on hematite deposits, chert, crater lake sediments, volcanic and fluvial depositions, and glacial features. Over the next three years and three subsequent conferences, the science community will narrow this list of possible landing sites to a single location. NASA is committed to ensuring that the site selection process is open. The status of the debate is available on the Web at . The Web site also provides status reports for all the active missions currently collecting data about Mars. By October 2008, one year before the launch, the single site will be identified. Meanwhile, I'd like to cast a vote for a close look at the inverted channels in the Erythraeum region, around 24.3 degrees south, 33.5 degrees west. The location is currently #2 on the priority list. The campaign has started.
Rossbacher, a geologist, is president of the Southern Polytechnic State University in Marietta, Ga. Links:
"Revolutionary Robotic Explorers," Geotimes, July 2006
Mars Exploration Program Landing Sites