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Planetary geology
Mapping water on Mars

After clicking to life the Mars Odyssey spacecraft’s gamma ray and neutron spectrometers in February, scientists became giddy with excitement. The measurements show an abundant amount of hydrogen, evidence of ice-rich soil, just beneath the surface of Mars’ southern polar region — enough water locked in ice to fill Lake Michigan twice over (Geotimes, April 2002).

“We knew water was stable at the poles, but we didn’t know how much water was there and now we know it’s loaded,” says William Feldman, principal investigator for the neutron spectrometer at Los Alamos National Laboratories in New Mexico.

NASA 2001 Mars Odyssey spacecraft revealed this global view of Mars in intermediate-energy, or epithermal, neutrons. Deep blue colors on the map show a low intensity of epithermal neutrons, indicating hydrogen-enriched soil. Progressively smaller amounts of hydrogen are shown in the colors light blue, green, yellow and red. The deep blue areas in the polar regions are believed to contain greater than 35 percent by mass of water, or higher than 50 percent water ice (frozen water, as opposed to carbon dioxide or dry ice) by volume in the upper one meter (three feet) of the soil. A full bucket of this soil might yield more than half a bucket of water ice. NASA/JPL/University of Arizona/Los Alamos National Laboratory

Using the first month of observations, 38 planetary scientists from the United States, Russia, Germany, New Zealand and France collaborated on three separate reports providing maps of Mars’ icy subsurface. The reports, published in the May 30 online journal Sciencexpress, corresponded with presentations that week at the spring meeting of the American Geophysical Union in Washington.

“The results, even after only a month of mapping observations, are stunning,” wrote Jim Bell of Cornell University in a Sciencexpress perspective article.

The spectrometers detect the energy of neutrons in the upper meter of soil. “When galactic cosmic rays hit the surface of Mars, they destroy the nucleus of the molecules that they hit, releasing neutrons at high energies,” Feldman explains. “After the neutrons are released, they bounce around the surrounding material in the planetary surface and eventually slow down.” At this point the neutrons are in an intermediate, or epithermal, energy range. Once their kinetic energy matches the thermal energy of the surrounding material, they gain on average as much energy as they lose in what is called a slow or thermal energy range. The neutrons stay in this slower range until the surrounding material eventually absorbs them. “At that point they are gone,” Feldman says.

“The rate that neutrons lose energy depends on what it is they are bumping into,” Feldman adds. “Some elements are more effective at absorbing energy from neutrons than others.” The rate neutrons lose energy increases if hydrogen is around. A neutron loses half its energy per collision with a hydrogen nucleus. The Odyssey spectrometers measured both the thermal and the epithermal range of neutrons. The intensity of the neutrons is a strong barometer of hydrogen in the soil: high amounts of hydrogen lower the intensity of the intermediate neutrons in the epithermal energy range.

In the South Polar Region, the low intensity seen in this epithermal neutron map shows up as deep blue. But because the neutrons are only detected in the first meter of soil, scientists speculate that the potential amount of buried ice is much greater and that the gamma ray and neutron spectrometers are only seeing the tip of the iceberg. As to how much water ice might be mixed up in the deeper soil of Mars, “my guess is as good as anyone else’s,” Feldman says. “Some people think it could be hundreds of meters to a kilometer, but really nobody knows.”

Originally the gamma ray and neutron spectrometers flew on the Mars Observer spacecraft, which, in 1993, stopped working three days shy of entering Mars orbit. The 2001 Odyssey mission is the second opportunity for these instruments, which this time also carry with them a High-Energy Neutron Detector. “Many Odyssey investigators have been waiting more than 15 years to finally collect these data,” Bell wrote. “One can hardly blame them for their enthusiasm and excitement over their early findings.”

Christina Reed

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