Detecting Subsurface Water

Water, whether liquid or frozen, absorbs neutrons more than other substances. The Detector of Albedo Neutrons on the Mars Science Laboratory rover will use this characteristic to search for subsurface ice on Mars. Image credit: NASA/JPL-Caltech/Russian Federal Space Agency

One way to look for water on Mars is to look for neutrons escaping from the planet's surface. Cosmic rays from space constantly bombard the surface of Mars, knocking neutrons in soils and rocks out of their atomic orbits. If liquid or frozen water happens to be present, hydrogen atoms slow the neutrons down. In this way, some of the neutrons escaping into space have less energy and move more slowly. These slower particles can be measured with a neutron detector.

Scientists expect to find hydrogen on Mars in two forms: water ice and minerals that have molecules of water in their crystal structures. At the request of the Russian Federal Space Agency, the Mars Science Laboratory rover will carry a pulsing neutron generator called the Dynamic Albedo of Neutrons that will be sensitive enough to detect water content as low as one-tenth of 1 percent and resolve layers of water and ice beneath the surface. Albedo is a scientific word for the reflection or scattering of light. Funded by the Russian government, the instrument will focus a beam of neutrons on the Martian surface from a height of 2.6 feet. The neutrons are expected to travel 1 to 2 meters (3 to 6 feet) below the surface before being absorbed by hydrogen atoms in subsurface ice.

Scientists estimate that, near the martian poles, water ice makes up 30 percent to 50 percent of shallow subsurface deposits. If the beam of neutrons encounters a layer of water ice beneath the surface, DAN will detect a relatively greater number of slower neutrons reflected at the surface. If there are no ice layers or water-logged minerals beneath the surface, DAN will detect a relatively greater amount of faster neutrons reflected at the surface.