The Lunar Reconnaissance Orbiter, launched in 2009, was sent to investigate the Moon and to help identify potential landing sites for future missions. One of the instruments onboard the spacecraft, the Lunar Exploration Neutron Detector (LEND), was designed to create maps of hydrogen concentrations on the Moon. Scientists are interested in mapping the hydrogen on the Moon because areas with significant amounts of hydrogen indicate the possible presence of water.

How does LEND look for hydrogen? Atoms consist of three types of particles (electrons, protons, and neutrons) organized into two regions. The nucleus—or core—of the atom contains protons and neutrons, and electrons surround the nucleus. On the Moon, neutrons break free from atoms when cosmic rays (high-energy charged particles that originate in outer space) strike the atoms in the lunar soil. Most cosmic rays are simple protons that likely came from processes on the Sun or from supernova remnants. These high-energy particles constantly bombard the Moon's surface, releasing neutrons, which are similar in mass to protons.

LEND measures neutron flux—a measure of the quantity of neutrons that pass through an area—to detect the presence of hydrogen in the soil. The speed of a neutron depends on what it collides with as it escapes. Some of the neutrons released by cosmic rays collide with nuclei of other atoms in the soil, and as a result, the neutrons slow down. For example, the most common type of hydrogen atom is protium, which has a nucleus consisting of a single proton; a neutron loses about half of its energy when it collides with protium, slowing it down. Neutrons can also be absorbed by certain atomic nuclei so that they do not escape the surface. LEND actually consists of nine different detectors that are designed to measure neutrons of different energies and different speeds. By counting the distribution of neutrons that escape the surface of the Moon, scientists can calculate the composition of the soil.

Oxygen is known to exist in lunar soil. Hydrogen that is detected in the soil of the Moon could exist in different forms, such as water molecules or as hydroxyl compounds (which contain an oxygen atom and a hydrogen atom) bound to minerals.

Based on data provided by LEND, a spacecraft impacted a promising region of the Moon's surface in 2009 to create debris that could be analyzed for the presence of hydrogen. Using spectrometers—instruments that examine the wavelengths of light absorbed by a material—scientists were able to identify the presence of water. Finding water there changes our view of the Moon. It leads to further investigations to understand where the water came from and what kinds of processes are currently acting on it. In addition, lunar water would be an important resource for future exploration, such as for a space station on the Moon. The water could potentially be used as drinking water, and hydrogen and oxygen could be extracted from it for use as rocket fuel.

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