If NASA's Curiosity Mars rover successfully lands on Sunday, it will use a neutron detector to scan for hydrogen below the Red Planet's surface.
The rover, part of the $2.5 billion Mars Science Laboratory mission, will aim to search for signs that Mars is, or ever was, habitable. Since one of the key requirements of habitability is thought to be the presence of liquid water, Curiosity will seek signs of water buried beneath the Martian surface.
To do this, the rover will shove neutrons underneath the planet's surface in hopes that the particles bump against hydrogen, one of the two types of atoms that make up water molecules. Neutrons are subatomic particles that have no electrical charge. When a neutron hits a hydrogen atom, the neutron will slow to a near-stop because the two particles are about the same size.
“The goal is in about 20 minutes of pulsing and returning and detecting the signal, [the rover] can build up a fairly good understanding of how much water there is below the surface,” said Ashwin Vasavada, MSL's deputy project scientist. [11 Amazing Things NASA's Huge Mars Rover Can Do]
Neutrons have already been used on Mars to find what are believed to be ice reservoirs. In 2002, a high-energy neutron detector aboard the orbiting Mars Odyssey spacecraft found robust evidence of hydrogen on the higher latitudes of the Red Planet, lurking just underneath the surface.