Your Guide to Water on the Moon | The Planetary Society

Your Guide to Water on the Moon
Facts worth sharing
The Moonâs poles have permanently shadowed regions that never
receive sunlight, where spacecraft have detected large amounts of water
ice.
Future astronauts could use the ice for air, water, and propellant.
Starting with missions like VIPER, NASA is embarking a sustained exploration program of the lunar poles, part of a global interest in the said regions.
When was water on the Moon discovered?
Scientists have long suspected water might exist on the Moon, but
exactly how much there was and where it was located has been the source
of much debate. In a 1967 paper, lunar scientist Harold Urey mocked his colleagues
for believing the Moonâs river-like channels were caused by anything
other than water, such as âlava, dust-gas or possibly even vodka.â
The definitive discovery of Moon water came in 2008, when Indiaâs
space agency ISRO launched the Chandrayaan-1 spacecraft to lunar orbit.
Chandrayaan-1 carried with it a NASA-provided science instrument called
the Moon Mineralogical MapperâM3 for shortâthat observed how the surface
absorbed infrared light. Using this data M3 determined that previously
suspected water molecules were ice inside the Moonâs polar craters.
A year later in 2009, NASA launched the Lunar Reconnaissance Orbiter (LRO)
and an impact probe called LCROSS. The upper stage from the rocket used
to launch LRO and LCROSS was deliberately smashed into a dark crater on
the Moonâs south pole. LCROSS flew through the debris plume and
detected 155 kilograms of water before it too crashed into the Moon.
Since then, many other Moon missions
have seen hints of water in different forms and in different regions.
Though these discoveries are vital incremental steps in the scientific
process, they are occasionally over-hyped by the media.
How can water exist on the Moon?
The existence of lunar water may seem strange, considering the Moon
has no atmosphere and its surface is exposed to the vacuum of space.
Daytime temperatures reach 120 degrees Celsius (248 degrees Fahrenheit).
Any surface water in sunlit regions evaporates and then floats away.
Earth is tilted 23 degrees away from the plane in which planets go
around the Sun. This tilt changes the amount of sunlight our planetâs
northern and southern hemispheres receive each year, creating our
seasons. It also plunges the north and south poles into periods of
constant darkness and constant sunlight.
Our Moon, however, has less of a tilt. At the poles the Sun hovers
close to the horizon. Large craters with terraced rims block sunlight
from ever reaching inside. These areas are called permanently shadowed
regions, or PSRs. PSRs are known to exist on other low-gravity, airless
worlds too, like Mercury and Ceres.
Temperatures inside the Moonâs PSRs can drop to -250 degrees Celsius
(-418 degrees Fahrenheit). Thatâs colder than Pluto! The Moonâs poles
have hundreds of PSRs
where spacecraft have detected substantial water ice. Itâs easy to see
why the water stays there, but where did it come from in the first
place?
Erlanger Crater This image of Erlanger crater near the Moon's north pole was captured by NASA's Lunar Reconnaissance Orbiter. Only the upper rims of the crater, which measures about 10 kilometers wide, ever receive sunlight.Image: NASA/GSFC/Arizona State University
Where did water inside the Moonâs permanently shadowed craters come from?
Water-bearing comets and asteroids
have bombarded our inner solar system multiple times throughout
history. Some of these small worlds smashed into the early Earth and
Moon, depositing water. Some of the Moonâs water may also have come from Earth
if our theory that the Moon was created when a large object smashed
into Earth is indeed correct. While most of the Moonâs water evaporated
into space, some of it found its way into PSRs, where it has been
preserved for billions of years.
In the Moonâs non-polar regions, the solar windâmore specifically,
protons emitted by the Sunâconstantly bombard the surface. Some of these
protons interact with oxygen molecules in the lunar soil to produce
water. This water isnât anything like what you could drink, though: itâs
in such small amounts that the lunar soil is still hundreds of times
drier than Earthâs deserts.
How much water is on the Moon?
Based on remote observations by radar instruments aboard Chandrayaan-1 and LRO, the lunar poles have over 600 billion kilograms of water ice.
Thatâs enough to fill at least 240,000 Olympic-sized swimming pools.
This is a low-end estimate because the amount of water detected is
limited by the strength of spacecraft radar. New missions with radars
that penetrate deeper will likely find more water ice.
In 2020, researchers using LRO data identified PSRs inside craters that were smaller than a kilometer. These micro-cold traps increase the expected amount of water ice on the Moon by at least 10 to 20 percent.
Where the Moon's surface ice Is This image shows the locations of water ice at the Moon's south pole (left) and north pole (right) as indicated by NASA's M3 instrument onboard India's Chandrayaan-1 spacecraft. The ice lies in permanently shadowed regions.Image: NASA
Why do we care about water on the Moon?
The high volume of water ice on the Moon has attracted the attention of space agencies and private companies around the world. They envision mining the water ice to produce air, drinking water, and propellant, fueling the needs of lunar habitats and even entire lunar industries in the future.
For scientists, the water ice and other chemicals in the PSRs offer a pristine record of cometary and asteroid bombardment from the solar systemâs early days. By studying the water, we can learn more about the origin of the Earth and Moon, and by extension how life arose on Earth. Realizing the high value of these PSRs, NASA has designated them as sensitive locations subject to strict contamination protections.
How can we explore the Moonâs water ice directly?
Exploring the PSRs firsthand is no small challenge. The first hurdle is developing an ability to land with precision. Whereas regions near the Moonâs equator have large expanses of flat terrain, the polar regions are often rocky and sloped.
The lunar poles also require spacecraft that can survive very cold temperatures. While PSRs are extremely cold, their sunlit crater rims donât get very much direct sunlight, keeping them cooled to an average of about -50 degrees Celsius (-58 degrees Fahrenheit).
Another challenge is power and communication. A rover venturing into a PSR will lose its line-of-sight with Earth, disabling communications, while the lack of sunlight means it will have to pack powerful batteries or rely on nuclear power.
This content is hosted by a third party (vimeo.com), which uses marketing cookies. Please accept marketing cookies to watch this video.
Light and Shadow at the Moon's South Pole The Moon's north and south poles have high points that are almost always in sunlight, and low spots that are permanently dark. This NASA visualization starts on the side of the Moon we see from Earthâwith the Apollo landing sites labeledâand moves down to the south pole, where a timelapse shows changing illumination conditions for an entire year. The large crater in the center, Shackleton crater, lies in permanent darkness, but its raised rims are nearly constantly illuminated. High-sunlight areas are good locations for human habitats because they stay warm and can draw near-continuous power from solar panels, while nearby permanently dark areas harbor water ice that could be mined for air, water, and rocket propellant.Video: NASA's Scientific Visualization Studio
How will future missions study water on the Moon?
Space missions
in the last decade have been acquiring more information about PSRs so
they can be explored with landers and rovers. Scientists using data from
LRO and Japanâs Kayuga orbiter have identified favorable landing sites for future missions. LRO data has also been used to make an extensive atlas of PSRs, which includes high-resolution altitude and slope maps, and even imagery based on dim light bouncing off the upper walls of lunar craters.
ISROâs Chandrayaan-2 orbiter,
launched in 2019, is using its advanced radar to map the water ice at
greater depths and quantify it. These efforts will be aided by NASAâs Lunar Trailblazer orbiter launching in 2023, as well as the agencyâs ShadowCam instrument onboard South Koreaâs first lunar orbiter
launched in 2022. Chinaâs Changâe-6 mission may bring samples from the
Moonâs south pole back to Earth. These samples would allow us to
precisely date different chemicals within the ice, tracing their origin
story to unlock fundamental mysteries about the solar system.
NASAâs VIPER
mission, slated for launch in 2024, would drive into PSRs to make
high-resolution maps of the water ice and other chemicals, and drill the
ice to unravel what is hidden in its pristine depths.
VIPERâs findings will set the stage for NASAâs Artemis program,
which envisions an eventual long-term human presence on the lunar
surface. As part of the identified science priorities for Artemis III, the first crewed Artemis landing, NASA intends to cryogenically sample and bring precious volatiles from PSRs to Earth for meticulous studies. The lunar poles are also central to human and robotic
exploration plans of commercial companies and many other nations,
including China, India, Japan, Europe, and Russia.
PSRs add to the long list of reasons to explore our Moon. Samples from PSRs and the lunar poles will be studied by laboratories worldwide to precisely date the ages of volatiles and materials within, trace their origin, and unlock fundamental mysteries about the Solar System.
VIPER lunar rover Artist's concept of NASAâs Volatiles Investigating Polar Exploration Rover, or VIPER. VIPER is a mobile robot that will roam around the Moonâs south pole looking for water ice.Image: NASA Ames / Daniel Rutter
Become A Member
When you become a member, you join our mission to increase discoveries in our solar system and beyond, elevate the search for life outside our planet, and decrease the risk of Earth being hit by an asteroid.