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What role did liquid water play in shaping the surface of Mars? The Mars Global Surveyor (MGS) currently in orbit around the Red Planet has provided strong visual evidence that water once flowed on Mars, cutting deep channels, pooling in lakes, perhaps even filling the Northern third of Mars with a vast ocean. But probing the question more deeply requires landing and taking a look around.

That is precisely what NASA hopes to do in 2003. The space agency recently announced plans to launch two rovers, one in May and the other in June of 2003. The two will land in January 2004 at different locations.

According to Steve Squyres, the principal investigator for the 2003 mission, its primary scientific goal “is to go to two sites on the Martian surface where we believe, based on orbital data, that liquid water has played an important role, and to do the best that we can to read the geologic record and to understand the history of water and climate at that location.”

The Mars Exploration Rovers (MERs) will not actually look for present-day water. Rather they will search for the effects that liquid water in Mars’s past may have had on rocks and soil.

The MERs will are the next-generation descendants of the Sojourner rover that roamed the Ares Vallis region of Mars in July 1997. They will use the same parachute-and-airbag landing system that was used successfully in that mission. The 2003 rovers are larger and contain more advanced scientific payloads, however. Squyres, who teaches astronomy at Cornell, refers to MER as a “robotic field geologist.”

Two of MER’s instruments are designed for panoramic surveys of the landing site. The Panoramic Camera (Pancam), which serves a similar function to that of the Pathfinder IMP camera, will take 3D images of the terrain surrounding the site.

The Miniature Thermal Emission Spectrometer (Mini-TES) will also survey the surroundings, scanning for spectral variations in the variations in the infrared radiation given off by different rocks. By analyzing these spectra – which frequencies of infrared are absorbed and which are reflected – will be able to determine the mineral composition of the rocks.

Scientists will combine the Pancam visual images with the mineralogical information from the Mini-TES to identify rocks for the rover to approach and investigate further.

To examine a rock more closely, MER will use a microscope and two additional spectrometers. Its Microscopic Imager will provide scientists on Earth with the first high-magnification look at the composition of rocks and soil on the Martian surface. The size and structure of the grains within a rock can help determine what role, if any, water may have played in its formation.

MER’s Alpha Proton X-Ray Spectrometer (APXS) will provide data on the elemental chemical composition of rocks and soils. The MER APXS is an improved version of a similar instrument that was aboard the Sojourner rover. MER’s Moessbauer Spectrometer will study in detail a rock’s iron-containing minerals.

In addition, MER will contain a RAT (rock abrasion tool), to grind away the surface of rocks, exposing their internal features. The lack of such a tool on the Sojourner rover made it difficult to tell whether its instruments were analyzing the composition of Mars rocks or of the pervasive dust that coated them.

By combining the information from all of MER’s instruments, scientists hope to be able to reconstruct a detailed history of how natural forces – water in particular – shaped the area around the landing site.

Although the long-term goal of Mars exploration is to determine whether life ever took hold – or still exists – on the planet, MER will not include any experiments designed to search directly for life or for the organic molecules that are the building blocks of life.

Nevertheless, Matt Golombek, who was the project scientist for the Pathfinder/Sojourner mission, believes that “the MER is exactly the next step beyond what Pathfinder did.” Because it will enable a detailed study of the mineral composition of Martian rocks and soil, it will tell scientists far more than was previously known about the history of water on Mars.

For the 2003 missions, Golombek is responsible for heading up the landing-site-selection process. That process is just now beginning and could take as long as two years.

Because of the constraints placed on site selection by safety concerns, some locations that scientists would most like to explore are simply out of the question for 2003.

According to Golombek, the most important characteristic of a good landing site is the ability to land safely. “The science desires are really rather secondary. Although we’d like to think otherwise, given our capabilities for landing spacecraft right now, we’re just not in that realm yet.”

Golombek’s cautions notwithstanding, finding sites of astrobiological importance does rank high on NASA’s list of site criteria. The NASA Astrobiology Institute’s (NAI) Mars Focus Group, headed by Jack Farmer (Arizona State University) provides input to NASA on landing-site selection. Scott Hubbard, interim manager of the NAI when it was first established, is now the Mars program director at NASA’s Washington, DC, headquarters.

What Next?

In October 2000, NASA announced the outlines of a long-term plan to explore the Red Planet. In addition to the previously announced 2001 Mars Odyssey orbiter and the 2003 MER missions, NASA’s plans include: a Mars Reconnaissance Orbiter (2005) that will take super-high-resolution images of Martian landscapes; a mobile science laboratory; and sample-return missions in 2014 and 2016, possibly as early as 2011.