Most of the lunar surface consists of irregular dust grains called regolith. Particles striking the moon should get trapped in the spaces between these grains and absorbed into the regolith. When protons are absorbed in this manner they should interact with the oxygen present to produce hydroxyl and water.
SARA has confirmed the creation of hydroxyl and water, but discovered that approximately one out of every five protons rebounds into space rather than being absorbed by the regolith. When this happens, the proton bonds with an electron to form a hydrogen atom. The resulting hydrogen escapes from the moon at about 200 kilometers per second and, because it’s electrically neutral, flies in a straight line like photons rather than being deflected by magnetic fields as charged particles are. Because the path taken by each observed hydrogen atom is well known, the atoms can be traced back to their origins on the moon to create an image of the surface.
Although the hydrogen atoms do not interact with magnetic fields, the protons can be deflected by magnetized moon rock prior to bonding with an electron. Areas containing these rocks show as darker areas on the hydrogen image since protons do not travel outward directly from these spots. Solar winds can also bring streams of protons to the surface of the moon since it has no atmosphere to reflect them. Expectations are that many of these additional protons are deflected outward as hydrogen atoms and included in the new images.
ESA’s upcoming BepiColombo mission to Mercury will include two instruments based on SARA and will allow scientists to compare the hydrogen reflection levels of the two bodies. Scientists expect Mercury to reflect even more protons than the moon because solar winds are more concentrated closer to the Sun.
TFOT has previously reported on other ESA missions including an overview of the Herschel Space Observatory designed to observe far infrared and submillimeter wavelengths, the first images returned from the Herschel Space Observatory, the Herschel Space Observatory’s search for the source of the Cosmic Microwave Background (CMB), and the launch of the Gravity Field and Steady State Ocean Circulation Explorer (GOCE) that’s also part of the Living Planet program.
Read more about the Sub KeV Atom Reflecting Analyser on the Chandrayaan-1 website and about the use of hydrogen atoms to study the moon in this ESA press release.