Selection of landing sites for the two Mars Exploration Rovers required more than two years of intensive study. More than 100 scientists and engineers participated in evaluating sites both on the basis of favorable criteria for safe landings and on the prospects for outstanding science opportunities after the rovers reach the ground.

To qualify for consideration, candidate sites had to be near Mars' equator, not too rugged, not too rocky, not too dusty, and low enough in elevation so the spacecraft would pass through enough atmosphere to slow down sufficiently. In all, 155 potential sites met the initial safety constraints. Detailed observations by two active orbital spacecraft, Mars Global Surveyor and Mars Odyssey, provided an unprecedented amount of information for evaluating finalist candidate sites.

The pair that made the final cut satisfied all the safety criteria; they also show powerful evidence of past liquid water, but in two very different ways.

Gusev Crater is seen here in its geological context from NASA Viking images. Credit: NASA/JPL

The first Mars Exploration Rover, Spirit, is flying to Gusev Crater, a bowl bigger than Connecticut that appears to have held a lake long ago. Scientists will use the robot's instruments to seek and analyze geological evidence about past environmental conditions in the crater. If sedimentary rocks lie on the surface, they may yield telltale clues to whether the crater ever did hold a wet environment that might have been suitable for sustaining life.

An asteroid or comet impact perhaps as much as 4 billion years ago dug Gusev Crater. Many smaller, younger impact craters pock Gusev's 150-kilometer-diameter (95-mile) floor. One of the largest branching valleys on Mars, likely carved by flowing water more than 2 billion years ago, leads directly into Gusev Crater through a breach in the crater's southern rim. Gusev sits at 15 degrees latitude south of Mars' equator at longitude 184.7 degrees west, in a transition zone between the ancient highlands on the southern part of the planet and smoother plains to the north. The valley, called Ma'adim Vallis, snakes northward Nile-like about 900 kilometers (550 miles) from the highlands to Gusev. In places, it gapes more than 25 kilometers (16 miles) wide and 2 kilometers (1.2 miles) deep.

Water flowing down the valley would have pooled in Gusev Crater, dropping sediments there before exiting through a gap in the crater's northern rim. Comparable crater lakes, such as Lake Bosumtwi in Ghana, exist on Earth. Gusev's lake, if indeed it did exist, is now gone. But the floor of Gusev Crater may hold water-laid sediments that preserve records of the lake environment, of the sediments' highlands origins and of the sediments' river trip.

As a potential complication, sedimentary layers may lie buried under later deposits from volcanic eruptions or wind-blown dust. If so, the best chances for finding sedimentary rocks may be in material thrown outward when younger craters were excavated by impacts that punched through the covering layers.

The targeted landing area for Spirit is an ellipse about 78 kilometers (48 miles) long and 10.4 kilometers (6.5 miles) wide near the center of Gusev Crater. Several small craters in and near the ellipse have likely stirred up rocks from underneath the top veneer of Gusev's flat floor. Whether they have dug deep enough to expose lake-related material if volcanic overburden is deep remains to be seen.

A Mars Exploration Rover is well equipped to pursue clues to Gusev's past environment. The panoramic camera and miniature thermal emission spectrometer will scan the scene for an initial survey of the surroundings after landing. Decisions about where to drive Spirit and how to use its other tools will depend on what that survey shows, such as whether any sedimentary rocks appear to be accessible. As the rover drives to new locations during its planned three months of Mars surface operations, a succession of further panoramic surveys will multiply the number of candidate rocks to consider for up-close examination.

If Spirit can find and approach sedimentary samples, several physical traits that the panoramic camera and the microscopic imager could reveal might testify about the long-ago environment. The rock abrasion tool could provide the cameras with fresh, unweathered surfaces to examine. The types of traits scientists may be checking for include:

• Grain size. Larger particles can settle out of water even when the water is moving. Smaller ones form sediments where water is still. The size of the particles that are consolidated into a sedimentary is a major clue about the conditions that existed when the sediments accumulated.
  
  
• Grain uniformity. A sedimentary rock with an assortment of grain sizes suggests jumbling by dynamic conditions such as a mudslide or a variable current. Uniformity of grain size suggests more stable conditions over time.
  
  
• Grain angularity. The shapes of grains in a sedimentary rock may be sharply angular or may be more rounded. Round grains tell a geologist that they may have worn off their edges by tumbling in a river for a long distance from where they started.
  
  
• Cross-bedding. Some sedimentary rocks have evenly stacked, horizontal layering; others have some layers at an angle to the stack. This second pattern, called cross-bedding, can result from an episode of migrating sand waves or ripples creating cyclical patterns of sediments that build up, then partially erode away, then rebuild.
  
  
• Fine layering. On Earth, some sedimentary rocks show annual layers that result from seasonal changes in the environment, like the growth rings of trees. Layers resulting from faster deposition in one season alternate with layers resulting from slower deposition the rest of the year. Scientists will be watching for anything similar in Mars rocks.

Spirit's miniature thermal emission spectrometer, alpha particle X-ray spectrometer and Mossbauer spectrometer could provide a different set of clues about Gusev Crater's past. These three instruments analyze the composition of rocks and soils. Scientists may use them to look for evidence such as:

• Weathering. Interaction with water can alter the chemical composition of rock-forming material. The water's temperature affects those changes. Information from the spectrometers could thus provide evidence about the wetness and temperature of the past environment, two key factors in whether that environment was hospitable to life.
  
  
• Evaporites. Some minerals are formed when dissolved salts get left behind as water evaporates. Finding and identifying any "evaporite" minerals at Gusev would suggest that the crater once held a salty, shallow lake.
  
  
• Carbonates. Carbonate minerals, such as limestone, can form from chemical reactions that pull carbon dioxide out of the atmosphere into bodies of water. If Spirit's spectrometers identify carbonate rocks, images from the rover's cameras could yield clues about how long the environment stayed wet and whether water was in the form of hot springs.

The geographical coordinates for the center of Spirit's landing ellipse target are 14.59 degrees south latitude and 175.3 degrees east longitude.

Gusev Crater was named in 1976 for Russian astronomer Matvei Gusev, who lived from 1826 to 1866. Ma'adim Vallis takes its name from the Hebrew word for Mars.