Mars Exploration Rovers - Landing Sites | Public Outreach | Mars Institute - To further the scientific study, exploration, and public understanding of Mars

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 evalu-ating 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.

Spirit to Gusev

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 condi-tions 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 sedi-mentary rocks may be in material thrown outward when younger craters were excavat-ed 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-relat-ed material if volcanic overburden is deep remains to be seen.

A Mars Exploration Rover is well equipped to pursue clues to Gusev's past environ-ment. 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 succes-sion of further panoramic surveys will multiply the number of candidate rocks to consid-er 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 parti-cles that are consolidated into a sedimentary is a major clue about the condi-tions that existed when the sediments accumulated.
Grain uniformity. A sedimentary rock with an assortment of grain sizes sug-gests 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 rip-ples 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 Mössbauer 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 wet-ness 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 chemi-cal 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 cam-eras could yield clues about how long the environment stayed wet and whether water was in the form of hot springs.

Spirit might not find any water-related rocks at all as it explores the landing-site region. Even if a lake once covered the Gusev floor, later deposits, such as ash from a vol-canic area north of Gusev, could have thoroughly buried sedimentary evidence of the lake. Spirit's examination of the surface geology might still provide new insights about Mars' history, such as the nature of ancient volcanic activity.

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.

Opportunity to Meridiani

The second Mars Exploration Rover, Opportunity, is targeted for Meridiani Planum, a smooth plain near the equator halfway around the planet from Gusev Crater. Intense scientific interest in the site results not from the shape of the terrain, as at Gusev, but from an unusual mineral deposit found by a Mars-orbiting spacecraft.

Scientists using an instrument called the thermal emission spectrometer on NASA's Mars Global Surveyor have discovered that Meridiani Planum is rich in gray hematite, a type of iron oxide mineral. On Earth, gray hematite usually -- but not always -- forms in association with liquid water. Some environmental conditions that can produce gray hematite, such as a lake or hot springs, could be quite hospitable to life. Others, such as hot lava, would not.

The gray hematite covers an estimated 15 to 20 percent of the surface in the vicinity of the planned landing site. It appears as a dark cap layer atop a brighter layer that is exposed at many places within the ellipse-shaped landing target. With the tools on Opportunity, scientists hope to determine which type of hematite-forming environment existed at Meridiani. Each of several possible past environments might leave geological clues to distinguish it from the others. For example:

Gray hematite can form in oxygenated water in an iron-rich lake or ocean. If Opportunity finds evidence of sedimentary layering in rocks associated with a Meridiani hematite outcropping, that would support such a scenario possibly hospitable to life.
As it percolates through the ground, iron-rich water heated by underground volcanism can deposit veins of gray hematite. This type of "hydrothermal" envi-ronment could offer microbes a favorable habitat. It would likely leave behind other types of telltale minerals that Opportunity's instruments could identify, such as carbonates.
Weathering in the presence of very small amounts of liquid water can create a veneer of gray hematite on rocks bearing other types of iron oxide. Scientists using the rock abrasion tool and two spectrometers on the rover's arm may determine whether the hematite at Meridiani fits this pattern.
Gray hematite can result from direct oxidation of hot, iron-rich lava. This process requires no liquid water and would not indicate a past environment hos-pitable to life. If Opportunity finds only volcanic rocks at Meridiani, that would support this scenario.

The geographical coordinates for the center of Opportunity's landing target are 1.98 degrees south latitude and 5.94 degrees west longitude. The targeted landing area is an ellipse about 85 kilometers (53 miles) long and 11 kilometers (6.8 miles) wide.

The site is within a large region that has been known as Meridiani since the earliest days of telescopic study of Mars because it lies near the planet's arbitrarily designated prime meridian, or line of zero longitude. "Planum" means plains, and the name fits: Meridiani Planum is one of the smoothest, flattest places on Mars.

Gusev Crater and Meridian Planum do have something in common. At both sites, sci-entists expect surprises. Mars has held surprises for every successful mission sent there so far. The types of observations and target rocks described here may well bear little resemblance to what end up being the Mars Exploration Rover mission's chief discoveries.