Dynamic Albedo of Neutrons (DAN)
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 uses this characteristic to search for subsurface ice on Mars. 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 carries a pulsing neutron generator called the Dynamic Albedo of Neutrons that is 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 focuses a beam of neutrons on the Martian surface from a height of 2.6 feet (0.79 meters). The neutrons are expected to travel 3 to 6 feet (1 to 2 meters) 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.
Detectors of Instrument for Detecting Water-Bearing Minerals
A Russian-built, neutron-shooting instrument on the Curiosity rover of NASA's Mars Science Laboratory mission checks for water-bearing minerals in the ground beneath the rover. Credit: NASA/JPL-Caltech
Curiosity Blasts Ground with Neutrons
NASA's Curiosity rover pinged the ground with neutrons for the first time, a process called active neutron sounding, on Aug. 17, 2012.