English: Physics of How DAN on Curiosity Checks for Water, Part 1

This diagram and the one at PIA16917 illustrate how the Dynamic Albedo of Neutrons (DAN) instrument on NASA's Curiosity Mars rover detects hydrogen in the ground beneath the rover. Detected hydrogen is interpreted as hydroxyl groups or water molecules, such as those bound into the structure of hydrated minerals.

DAN shoots neutrons into the ground and measures the timing and energy levels of neutrons reflected back up. This diagram depicts the case of a neutron that does not collide with any hydrogen atoms before it reaches DAN's detector. It is detected in a characteristically short time -- about one millisecond -- after being emitted by DAN's neutron generator, and with a characteristic energy. The companion diagram illustrates the case of a neutron that does collide with hydrogen in the ground.