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  1. Return to the Red Planet

    The Mars Global Surveyor (MGS) currently orbiting Mars has sent back a wealth of new information. It has revealed an enticing sample of the martian surface at a level of detail never previously achieved from orbit. It has shown us layered sedimentary deposits in crater basins. It has discovered what appear to be seepage gullies caused by recently running water, in regions of Mars believed to be far too cold for liquid water to flow.

    As is often the case, these discoveries have led to new questions. For example, if there are so many places where the martian landscape appears to have been altered by running water, why hasn’t the TES (Thermal Emission Spectrometer) instrument aboard MGS found any evidence of carbonates on Mars? (Carbonates are typically formed on Earth when water evaporates from ponds or lakes.)

    Another vexing question is: Where has all the water gone that many scientists believe once flowed on Mars? Some of it can be seen in the form of ice deposits of the northern polar cap. But the rest of it seems to have vanished. Did it evaporate into space? Did it somehow go underground? No one knows for certain.

    On April 7, NASA will launch a new orbiter, the 2001 Mars Odyssey, on a journey to Mars in hopes of finding some answers. There will be three scientific instruments on board: THEMIS (Thermal Emission Imaging System), GRS (Gamma Ray Spectrometer), and MARIE (Martian Radiation Environment Experiment).

    THEMIS is a successor to the TES instrument aboard the Mars Global Surveyor. THEMIS has two jobs. When it is over the daylight side of Mars, it will collect information about the mineral composition of the martian surface. It will do this by examining the infrared light radiated into space by the sunlit regions of Mars.

    Infrared radiation is not visible to the human eye. But, just like visible light, it covers a wide spectrum of wavelengths, each of which represents a different “color.” When illuminated by sunlight, different minerals radiate different combinations of infrared wavelengths. The intensity of the radiation at each wavelength within the range of the instrument forms what is called a spectrum. By looking at the spectral signatures of specific locations on Mars, the THEMIS team should be able to determine and map their mineral compositions.

    THEMIS is more precise than TES. It can examine much smaller spots on the Martian surface than its predecessor. According to Steve Saunders of the Jet Propulsion Laboratory in Pasadena, CA, who is the Project Scientist for the Mars Odyssey mission, THEMIS will have a resolution of 100 meters (328 feet) per pixel. “We’ll be able to do mineral maps at a much higher resolution” than TES was able to do.

    One theory about why TES didn’t find much evidence of carbonates is that they are present on Mars only in small patches. These patches are too small to be detected by TES, which has a resolution of about three or four kilometers (roughly two to two-and-a-half miles) per pixel. If this theory holds true, THEMIS should be more successful. THEMIS, says Saunders, is “pretty good at finding carbonates. At least it was somewhat optimized for that.”

    When THEMIS is in the Martian night sky, it will perform a different task. Mars, like Earth, is warmed by the Sun during the day. At night, much of that warmth is radiated back by the planet into the atmosphere and from there into space. This type of radiation, known as thermal infrared radiation, can also be detected by TES.

    Mars should emit nighttime thermal infrared radiation more-or-less evenly. But if there are hot spots below the surface of Mars — underground reservoirs of heated magma, for example — these spots will give off greater quantities of thermal infrared radiation than the norm. THEMIS will be able to detect these differences. Says Saunders, “We ought to be able to determine the actual temperature of the surface to within about a tenth of a degree.”

    Underground hot spots are important because they could indicate the presence of underground liquid water, hydrothermal vent-like systems and even — just possibly — life. These would be prime candidate landing sites for future Mars missions.

    THEMIS will also be able to take visible-light images of Mars, at a resolution of about 18 meters (59 feet) per pixel. This is mid-way between the resolution of Viking images (100 meters, or about 300 feet, per pixel) and that of images taken by the Mars Orbiter Camera (MOC) aboard MGS (3 meters, or about 10 feet, per pixel).

    “Features that you can see very nicely in Viking, like a volcanic flow, when you look at it with MOC, it doesn’t look like that, “ says Saunders. “What you’re seeing is something else, you’re seeing the effects of processes that are operating on another scale. This [THEMIS resolution] falls very nicely in between, so I think we’ll be able to bridge that gap.”

    Odyssey’s Gamma Ray Spectrometer (GRS) will detect a different type of radiation coming from Mars’ surface. Infrared radiation has very long wavelengths; gamma rays have extremely short wavelengths.

    All chemical elements emit gamma rays when they are exposed to cosmic radiation. Different elements emit gamma radiation at different wavelengths. By studying the gamma rays coming from the surface of Mars, then, Odyssey’s GRS will be able to determine its elemental composition. This will help scientists learn about the planetary processes, such as volcanoes, that shaped and continue to shape Mars.

    The GRS also includes two neutron detectors. These will be used to look for the presence of hydrogen in the upper meter of the Martian crust. The presence of hydrogen would imply the presence of water. “I think water is the best bet if we see a lot of hydrogen,” says Saunders. “For Mars there aren’t too many other alternatives.”

    The GRS instrument is a repeat of an instrument originally flown aboard the Mars Observer. Launched in 1992, the Mars Observer was lost as it approached the red planet.

    The third Odyssey instrument is MARIE. Every planet — Mars is no exception — is subject to ionizing radiation from space. This radiation, which breaks apart the carbon bonds in biological molecules, is harmful, potentially deadly, to all living things. Earth is to a large extent protected from this radiation by its magnetic field. But Mars has no magnetic field. MARIE’s goal is to determine the strength of the radiation that Mars is exposed to. This information will help future human Mars explorers plan to protect themselves properly against damaging radiation effects.

    NASA has a lot riding on this mission. Its spectacular success in 1997 with the Pathfinder/Sojourner lander-rover mission was followed by two successive failures: the Mars Climate Orbiter in September 1999, followed just two months later by the Mars Polar Lander. After exhaustive reviews of what went wrong with these two missions, NASA has taken steps to ensure the success of Mars Odyssey.

    What Next?

    After Odyssey, NASA’s next scheduled Mars mission will be a pair of Mars Exploration Rovers (MERs), similar to Sojourner. These will be launched in 2003 and arrive at Mars in early 2004. The MERs will study minerals on the Martian surface up-close, searching in particular for further evidence of water-altered minerals.

    The European Space Agency also plans a mission to Mars in 2003, a combined orbiter and lander. The lander, Beagle 2, will conduct biochemical experiments designed explicitly to detect evidence of past or present life on Mars. These will be the first direct Martian life-detection experiments conducted in nearly 30 years. The only previous biological experiments were performed by instruments aboard the two Viking landers that touched down on Mars in 1975.