Water & Ices on the Moon

Composite of astronaut on the Moon surface with a timeline superimposed.

What’s big, covered in water, yet 100 times drier than the Sahara Desert? It’s not a riddle, it’s the Moon! For centuries, astronomers debated whether water exists on Earth’s closest neighbor. In 2020, data from NASA’s SOFIA mission confirmed water exists in the sunlit area of the lunar surface as molecules of H2O embedded within, or perhaps sticking to the surface of, grains of lunar dust. Here is a brief history of the discoveries leading up to the confirmation of water on the Moon.

Maria on the Moon (1645)

When early astronomers looked up at the Moon, they were struck by the large, dark spots on its surface. In 1645, Dutch astronomer Michael van Langren published the first-known map of the Moon referring to the dark spots as “maria” – the Latin word for “seas” – and putting into writing the widely-held view that the marks were oceans on the lunar surface. Similar maps from Johannes Hevelius (1647), Giovanni Riccioli and Francesco Grimaldi (1651) were published over the next few years. We now know these spots to be plains of basalt created by early volcanic eruptions, but the nomenclature of ‘maria’ (plural) or ‘mare’ (singular) remains.

Drawing of the Moon with labelled names of various geographic formations.
A map of the Moon, featuring the maria depicted as oceans, published by astronomer Michael Van Langren in 1645.
Creative Commons

Totally Dry Moon (1892)

American astronomer William Pickering made measurements in the late 1800s that led him to conclude the Moon essentially has no atmosphere. With no clouds and no atmosphere, scientists generally agreed that any water on the lunar surface would evaporate immediately. Pickering’s measurements led to a widespread view that the Moon was devoid of water.

View of an opened book view with 2 pages open; left page is a picture of the Moon, right page is the title "The Moon"
An open page of astronomer William Pickering’s book “The Moon: A Summary of Our Satellite with a Complete Photographic Atlas.” Pickering’s observations of the Moon lead to the belief that with such little atmospheric pressure, any ice on the lunar surface would near instantly sublimate.
Internet Archive/Knopf Doubleday (This image is in the public domain)

Ideas of Water (1960s)

As scientists made headway in understanding the behavior of substances that are prone to vaporize at relatively low temperatures – called volatiles – theoretical physicist Kenneth Watson published a paper in 1961 describing how a substance like water could exist on the Moon. Watson’s paper first popularized the idea that water ice could stick to the bottom of craters on the Moon that never receive light from the Sun, while sunlit areas on the Moon would be so hot that water would evaporate near-instantly. These lightless areas of the Moon are called “permanently shadowed regions.”

Photo of Moon's surface showing dark shadow in cratered regions
The Shackleton Crater at the Moon’s south pole in a permanently shadowed region of the Moon.

Apollo Landings (1969 – 1972)

The Apollo era brought humans to the lunar surface for the very first time, giving researchers the opportunity to directly look for signs of water on the Moon. When tested, soil samples brought back by Apollo astronauts revealed no sign of water. Scientists concluded that the lunar surface must be completely dry, and the prospect of water wasn’t seriously considered again for decades.

Photo of astronaut driving a vehicle on the surface of the Moon
Apollo 17 astronaut Eugene Cernan drives the Lunar Roving Vehicle.
Photo of astronaut on the Moon digging into the surface.
Apollo 15 astronaut Jim Irwin uses a scoop to collect soil samples on the lunar surface.

Possible Frozen Water in Shadowed Craters (1994, 1998)

NASA’s Clementine mission launched in 1994 to orbit the Moon for two months and collect information about its minerals. Clementine data suggested there was ice in a permanently shadowed region of the Moon. The Lunar Prospector Mission focused on permanently shadowed craters to look deeper into the discovery and in 1998 found that the largest concentrations of hydrogen exist in the areas of the lunar surface that are never exposed to sunlight. The results indicated water ice at the lunar poles. However, the images were low resolution so no strong conclusions could be made.

Photo of the Moon, center-focus on the south pole
A mosaic image of the south pole of the Moon as taken by NASA’s Clementine spacecraft.

Revisiting Apollo Samples (2008)

Capitalizing on major advances in technology since the Apollo era, researchers from Brown University revisited the Apollo samples. They found hydrogen inside tiny beads of volcanic glass. Since no volcanoes are erupting on the Moon today, the discovery presented evidence that water had existed in the Moon when the volcanoes erupted in the Moon’s ancient past. Additionally, the preserved hydrogen provided clues to the origins of lunar water: if it emerged from erupting volcanoes, it must have come from within the Moon. The discovery suggested that water was a part of the Moon since its early existence – and perhaps since it first formed.

Photo of astronaut on the Moon digging into the surface.
Apollo 17 astronaut Eugene Cernan preparing to collect samples.
Image of small irregular shapes in green, yellow and dark yellow, scattered,
Glass beads produced by ancient volcanic eruptions on the Moon and collected by Apollo astronauts.

Signs of Hydration (2009)

Chandrayaan, Cassini, Deep Impact

A suite of spacecraft enabled exciting discoveries in 2009. None were designed to look for water on the Moon, yet the Indian Space Research Organization’s Chandrayaan-1 and NASA’s Cassini and Deep Impact missions detected signs of hydrated minerals in the form of oxygen and hydrogen molecules in sunlit areas of the Moon. Researchers couldn’t determine whether they were seeing hydration by hydroxyl (OH) or water (H2O). They also debated whether the amount of hydration depended on the time of day.

Illustration of a spacecraft in space

Artist’s depiction of NASA’s Cassini spacecraft.

Observations of Lunar Debris Reveal More (2009 – 2019)

The Lunar Crater Observation and Sensing Satellite (LCROSS) spacecraft and Lunar Reconnaissance Orbiter (LRO) launched together in 2009. Later that year, LCROSS intentionally discharged a projectile into a crater believed to contain water ice, and flew through the debris from the projectile’s impact. Four minutes later, LCROSS itself intentionally impacted the Moon while LRO observed. The combined observations showed grains of water ice in the ejected material. The LRO and LCROSS findings added to a growing body of evidence that water exists on the Moon in the form of ice within permanently shadowed regions. LRO continues to orbit the Moon and provide data used to characterize and map lunar resources, including hydrogen.

Illustration of a spacecraft discharging a projectile into the surface of Moon
Artist rendering of the LCROSS satellite.
Close-up photo of the Moon's surface with a circled aread superimposed with blue blotches.
Areas of the Moon’s south pole with possible deposits of water ice, shown in blue. The map is based on data taken by NASA’s Lunar Reconnaissance Orbiter.

Confirmation of Moon Water – Shadowed Regions (2018)

Data from Moon Mineralogy Mapper (M3), carried by ISRO's Chandrayaan-1, provided scientists with the first high-resolution map of the minerals that make up the lunar surface. The NASA instrument flew aboard India’s Chandrayaan-1 mission in 2009. An analysis of the full set of data from M3, announced in 2018, revealed multiple confirmed locations of water ice in permanently shadowed regions of the Moon.

Digitalized bands of gradient colors superimposed over the Moon's surface, within a circle area.
A composite image using data from NASA’s Moon Mineralogy Mapper. The blue shows areas of confirmed water ice on the lunar surface.
ISRO/NASA/JPL-Caltech/Brown University/USGS

Confirmation of Moon Water – Sunlit Surface (2020)

In 2020, NASA announced the discovery of water on the sunlit surface of the Moon. Data from the Stratospheric Observatory for Infrared Astronomy (SOFIA), revealed that in Clavius crater, water exists in concentrations roughly equivalent to a 12-ounce bottle of water within a cubic meter of soil across the lunar surface. The discovery showed that water could be distributed across the lunar surface, even on sunlit portions, and not confined to cold, dark areas.

Composite illustration and photo of an airplane in the sky, and the Moon, and visualization of water molecule embedded in a rock.
An illustration of H2O molecules on the lunar surface. NASA’s SOFIA confirmed the existence of water on the sunlit surface of the Moon in 2020.

First Detailed, Wide-Area Map of Water on the Moon (2023)

In 2023, a new map of water distribution on the Moon provided hints about how water may be moving across the Moon’s surface. The map, made using SOFIA data, extends to the Moon’s South Pole – the intended region of study for NASA’s Artemis missions, including the water-hunting rover, VIPER.

Image of the Moon's surface up close, with enhanced colored bands to signify scanned areas with water signals detected from SOFIA.

More to Discover

Researchers have confirmed that water exists both in the sunlit and shadowed surfaces of the Moon, yet many questions remain. Lunar scientists continue to investigate the origins of water and its behavior. There is evidence that the water on the Moon comes from ancient and current comet impacts, icy micrometeorites colliding on the lunar surface, and lunar dust interactions with the solar wind. However, more research is needed to understand the full history, present, and future of water on the Moon.

Writer: Allison Gasparini and Molly Wasser
Science Advisors: Casey Honniball, Tim Livengood, NASA's Goddard Space Flight Center