On 7 April, a probe will set out for Mars that could be pivotal in the search for water--and life--on the red planet. The 2001 Mars Odyssey orbiter will also be attempting to erase memories of the failure of the last two NASA missions to that destination.
The Odyssey's mission is to map the planet's geology, paying particular attention to the role of water, both past and present. Recent results from the Mars Global Surveyor (MGS), in orbit since 1997, have brought interest in water there to fever pitch. Last June, pictures from the MGS indicated that brief flows of liquid water could have been present on the planet as recently as any time between yesterday and two million years ago. In December, the discovery of layered deposits of rock at multiple locations on the planet suggested sedimentation left by ancient lakes.
An artist's impression of the Odyssey spacecraft in orbit around Mars includes the long boom of the gamma ray spectrometer and the thermal blanketing covering even the high-gain radio antenna dish.
Mars is very dry, much drier than the most arid desert on Earth. But a consensus exists among planetary scientists that liquid once flowed on its surface. Dried-up river channels and flood plains are clearly visible in the photographs from the Viking missions of the 1970s. Could these river channels and flood plains have been carved by a mixture of dry ice and rocky debris? If they were formed by water instead, how long was the water present? Were infrequent flash floods provoked by volcanic eruption or meteor impact or did seas and lakes once cover the planet? Different theories exist to explain how Mars is what it is today, with very different implications for the two questions that are at the heart of the search for water there: did life arise on Mars and is it still there now?
To find some answers, the Odyssey will carry an infrared and a gamma ray spectrometer, plus a radiation detector. And to prevent a repeat of earlier losses, its design has been revised.
The hunt is on
One of three experiments on board, the THermal EMission Imaging System (Themis), should provide a wealth of information about the history of Martian water from its examination of the planet's present-day mineralogy.
"The beauty of mineralogy, versus looking at an image, is that in an image I can see a gully," explained the instrument's principal investigator, Philip Christensen of Arizona State University, Tempe. "But I don't know from looking at the image if that gully was carved in a very brief flash flood of water that ran for a week and evaporated or froze [or if it was carved out by a relatively long-lived river]."
Mineral deposits take thousands of years to form and finding minerals associated with water would indicate the presence of standing water for significant periods of time, said Stephen Saunders, the mission scientist at NASA's Jet Propulsion Laboratory (JPL), in Pasadena, Calif. Such water on Mars "must have contained a lot of dissolved mineral matter, just as water on Earth does. When it evaporated, it would have left that matter behind as salts. On Earth that process creates a whole variety of salts and we should be able to see that with Themis."
Themis is a successor to the Thermal Emission Spectrometer (TES) currently on board the Mars Global Surveyor. Both are infrared spectrometers (except that, unlike TES, Themis can also image in the visible wavelengths). Although a cold planet, Mars still emits infrared radiation. A lot of it fits an idealized black body curve for any radiating body at a given temperature. Researchers will use this curve to determine Martian temperatures from Themis data; but of greater interest is where the infrared spectrum deviates from the black body curve.