Neutron map of Mars by the HEND instrument on the Mars Odyssey
Igor Mitrofanov is the Principal Investigator, Space Research Institute, Moscow, Russia
The Dynamic Albedo of Neutrons investigation, or DAN, can detect water bound into underground minerals along Curiosity’s path.
The DAN instrument shoots neutrons into the ground and measures how they are scattered, giving it a high sensitivity for finding any hydrogen to a depth of about 50 centimeters (20 inches) directly beneath the rover.
The DAN instrument is sensitive enough to detect water content as low as one-tenth of 1 percent in the ground beneath the rover.
The neutron generator is mounted on Curiosity’s right hip, a pair of neutron detectors on the left hip. Pulses last about one microsecond and repeat as frequently as 10 times per second. The detectors measure the flow of moderated neutrons with different energy levels returning from the ground, and their delay times. Neutrons that arrive later may indicate water buried beneath a drier soil layer. The generator will be able to emit a total of about 10 million pulses during the mission, with about 10 million neutrons at each pulse.
The most likely form of hydrogen in the ground of the landing area is minerals filled with water. These are minerals with water molecules or hydroxyl ions bound into the crystalline structure of the mineral. Molecules can retain water from a wetter past when all free water has gone.
DAN may also detect water that comes and goes with the Martian seasons, such as soil moisture that varies with the atmospheric humidity. Together with Curiosity’s cameras and weather station, DAN will observe how the sparse water cycle on Mars works in the present. DAN also could detect any water ice in the shallow subsurface, however scientist do not expect much of that at Curiosity’s Gale Crater landing site.
The word “Albedo” in the name of the instrument means reflectance — in this case, how high-energy neutrons injected into the ground bounce off of atomic nuclei in the ground.
Neutrons that collide with hydrogen atoms bounce off with a characteristic decrease in energy, like one billiard ball hitting another. By measuring the energies of the reflected neutrons, DAN can detect the fraction that was slowed in these collisions, and from there can calculate the amount of hydrogen.
The science objectives of the DAN instrument:
1) Detect and estimate the amounf of the hydrogen in the soil right beneath Curiosity during the full mission
2) Investigate the upper half meter (0.5 m = 20 inches) of the upper soil and determine the possible layering structure of hydrogen-bearing materials in that soil.
3) Track the variability of hydrogen content in the upper soil layer (~1 m) during the mission by periodic analysis
4) Track the variability of neutron radiation background (neutrons with energy < 100 keV) during the mission by periodic analysis
Scientist are planning to use DAN during short pauses in drives and while the rover is parked. It will check for any changes or trends in subsurface hydrogen content from place to place along the path Curiosity is taking. The instrument can be used to search for places worthy for examination with Curiosity’s other tools. Also, rock formations that Curiosity’s cameras view at the surface may be traced underground by DAN, which helps scientists to understand the geology of the Gale Crater.
Since 2001 nuclear technology to detect water on Mars from orbit is used by the Mars Odyssey satellite and DAN will now use that technology on the surface of Mars.
The Russian Federal Space Agency contributed DAN to NASA as part of a broad collaboration between the United States and Russia in the exploration of space. Russia’s Space Research Institute developed the DAN instrument in close cooperation with the N.L. Dukhov All-Russia Research Institute of Automatics, Moscow, and the Joint Institute of Nuclear Research, Dubna.
Igor Mitrofanov, principal investigator of DAN, is also the principal investigator for a Russian the high energy neutron detector (HEND) instrument on NASA’s Mars Odyssey orbiter, This instrument measures high energy of neutrons coming from Mars.
In 2002, it and companion instruments on Odyssey detected hydrogen interpreted as abundant underground water ice close to the surface at high latitudes.
Water, whether liquid or frozen, slows and 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.
The neutron detectors on Odyssey rely on galactic cosmic rays hitting Mars as a source of neutrons. DAN can work in a passive mode relying on cosmic rays, but it also has its own pulsing neutron generator for an active mode of shooting high-energy neutrons into the ground.
Oil prospectors use this technology in instruments lowered down exploration holes to detect the hydrogen in petroleum. Space explorers have adapted it for missions to the moon and Mars, where most hydrogen is in water ice or in water-derived hydroxyl ions.