What is Lunar Water?

Infographic of the lunar water cycle, depicting key processes controlling the presence of ice, molecular H2O, and OH on the surface of the Moon. Credit: Sara Pooley (PCC/Caltech)

The Lunar Trailblazer mission is going to the Moon to study the abundance, distribution, and form of lunar water—but what exactly is “lunar water”? When planetary scientists say that the surface of the Moon is hydrated, they don’t mean that it drinks eight glasses of water a day. Scientists have technical definitions of water that include different configurations of molecules. These different molecules all have oxygen and hydrogen atoms, and they all absorb approximately—but not exactly—the same wavelengths of light, which is how scientists can sense whether water exists on a surface. Here is a quick guide to the different forms of water that may be found on the Moon.

Molecular Water, or H2O

Credit: Lunar Trailblazer

“Molecular water” refers to two hydrogen atoms bonded with a single oxygen atom to make a v-shaped molecule—you’re likely familiar with its chemical symbol, H2O. Molecular water in its liquid form is what most people think of when they think of water. A trillion billion water molecules exist in a 1 milliliter drop of liquid. Water in the liquid form, if present on the lunar surface, would last only seconds in the vacuum of space before boiling. So, when scientists speak of molecular water on the lunar surface, they mean molecules in vapor form within or on the surface, perhaps hopping from warm areas to stick to surfaces of materials in colder areas (a phenomenon called cold-trapping). A few microscopic layers of molecules of water—about 0.01% by weight—might exist on the Moon.

Water Ice

Credit: Lunar Trailblazer

Water ice is defined by many molecules of water forming an organized, hexagonal crystalline structure that makes water in its solid form. Scientists specify “water ice”, rather than referring to just “ice” in order to distinguish it from other ices, such as CO2 (dry ice). In 2010, the Lunar Crater Observation and Sensing Satellite (LCROSS) tested for the presence of water ice in cold, permanently shadowed lunar craters by jettisoning one of its empty propellant tanks in a controlled collision to impact into a permanently shadowed region of the Moon. When the ersatz meteorite hit, it created a plume that was observed by both the Lunar Reconnaissance Orbiter (LRO) and the LCROSS spacecraft as well as telescopes on Earth. Tremendous amounts of data were captured from the observed plume, including evidence of lunar water ice.

Hydroxyl, or OH

Credit: Lunar Trailblazer

As the chemical symbol implies, hydroxyl is made up of one oxygen molecule bonded to one hydrogen molecule. This form of water is found bonded to minerals, especially silicates, which are the most abundant mineral on the Moon (as well as in the Earth’s crust).

Credit: Lunar Trailblazer

While not exactly water, hydroxyl can come from interactions of H2O molecules found in small amounts in magma—as the magma cools, it crystallizes into minerals that eventually form rocks, trapping the OH in the rocks. On the Moon’s surface, which is not shielded by an atmosphere, OH can also form from the solar wind impacting into silicate rocks. The solar wind is a stream of positively-charged hydrogen atoms (also called H ions) from the sun, and the silicate rocks are made of silicon atoms bonded with oxygen (other metal-with-oxygen bonds are present as well). When the H ions impact the silicate rocks, the hydrogen can be attracted to the oxygen, and the silicates will acquire the hydrogen, forming rocks with Si-O-H or other metal-O-H bonds.

Credit: Lunar Trailblazer

Although it comes from a different set of processes, OH can be difficult to distinguish from H2O and water ice in low spectral resolution data, which is why the instruments on Lunar Trailblazer are specifically designed to tell the difference.

What makes Lunar Trailblazer special?

In 2009, researchers reviewing data from three separate spacecraft—Chandrayaan-1, Deep Impact, and Cassini—extracted a water signature on many parts across the lunar surface. This was a surprise to the lunar science community, particularly because this means that water in some form is present on sunlit portions of the Moon, where ​​temperatures can reach well above boiling, up to 400 K, or 260 °F during the lunar day (which lasts about 14 Earth days). However, the instruments gathering this data weren’t designed to look for water, and scientists did not have enough detail in their measurements to distinguish between OH, H2O, and water ice (except for a small number of cold spots where water ice was more directly observed). In addition, the data did not make it clear how much water exists on the lunar surface.

One way scientists are able to determine the composition of a surface is by gathering light reflected from that surface and measuring which wavelengths of light are “missing” from the full spectrum of light. The specific wavelengths of light that are absorbed by the surface are a fingerprint for the specific molecules that are present. But these different absorption signals can be weak and sometimes very close to one another in the spectrum. In the case of OH, H2O, and water ice, these signals nearly overlap in wavelengths of light, and require high spectral resolution—fine spacing of measurements in wavelength—to tell the difference.

Lunar Trailblazer is designed so that the instruments measuring the surface will work with high sensitivity right at the center of water’s key wavelength region in infrared light (from 2.5-3.5 micrometers) with high enough spectral resolution to differentiate between forms of water. The temperature of the surface being measured also affects the apparent strength of the absorption signal of water. Lunar Trailblazer solves for that with an instrument that will measure the surface temperature at the same time as the spectra are gathered, and then will calibrate the spectral data accordingly. Additionally, Lunar Trailblazer will map the Moon day and night to observe how the signals change over time, providing data that will allow scientists to form a working model of where there is water on the Moon, what type, and how much—essentially determining the lunar water cycle.

To learn more about water on the Moon, check out the Planetary Society's guide to water on the Moon.

By Belinda Blakley
Belinda Blakley is a Pasadena City College student and Caltech intern working on science communication for the Lunar Trailblazer mission.