What are permanently shadowed regions on the Moon?

NASA and ISRO made headlines when they discovered water on the Moon in 2009, the efforts of which span at least two decades. But the Moon has no atmosphere and its surface is exposed to the vacuum of space. Daytime temperatures reach 120 degrees Celsius. Any surface water in sunlit regions evaporates and then floats away due to the Moon’s low gravity.

How then does the Moon have water?

The answer lies in the Moon’s geometry with respect to the Sun. Earth is tilted 23 degrees away from the plane in which planets go around the Sun. This tilt changes the amount of sunlight our planet’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.

The Moon is tilted to the plane in which the planets go around the Sun by only 1.5°. Credit: NASA

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. The Moon has hundreds of PSRs on its poles.

The rim of the 10-kilometer-wide Erlanger crater on the Moon’s north pole. The crater floor is in permanent darkness. Credit: NASA LRO

Colder than Pluto

Lying six billion kilometers from the Sun, Pluto receives 1500 times less heat on average than Earth or the Moon, resulting in surface temperatures as low as -240° Celsius. And yet permanently shadowed regions on the lunar poles can get even colder, going as low as -250° Celsius. That’s a mere 23 degrees above the coldest possible temperature in the Universe.

Individual image credits – Pluto: Most accurate natural color image of Pluto taken by NASA’s New Horizons spacecraft in 2015. Credit: Sources: NASA / APL / Alex Parker, PSR: Erlanger crater on the Moon’s north pole; the crater floor is in permanent darkness. Credit: NASA LRO

The Moon doesn’t have an atmosphere to warm up these regions either. Such exceptionally cold environments make PSRs excellent traps not just for water ice but also other volatiles like carbon dioxide, methane, ammonia and more. PSRs are known to exist on other low-gravity, airless worlds too, like Mercury and Ceres.

Why explore PSRs?

The accumulation of water ice in the lunar PSRs over millions and millions of years has been substantial. Based on remote observations by radars onboard ISRO’s Chandrayaan 1 orbiter and NASA’s Lunar Reconnaissance Orbiter (LRO), scientists estimate the lunar poles to host at least 600 billion kilograms of water ice, enough to fill at least 240,000 Olympic-sized swimming pools.

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 system’s early days. By studying the water, we can learn more about the origin of the Earth and Moon.

Enabling all of those things would be technologies to explore the challenging conditions on the lunar poles. While PSRs exist on Mercury and Ceres too, the Moon’s proximity makes it an accessible place to explore them. Realizing the high value of these PSRs, NASA has designated them as sensitive locations subject to strict contamination protections. Missions there will have to adhere to the same kind of strict guidelines as ones to Mars.

How do we plan to explore PSRs

Exploring the PSRs firsthand is no small challenge. Space missions in the last decade have been acquiring more information about PSRs so they can be explored with landers and rovers.

The first hurdle to exploring the lunar poles is developing an ability to land with precision. Most of the polar terrain, even if not a PSR, experiences shadows for long periods of time, making survival difficult for a spacecraft. There are but a few patches of elevated areas––like crater rims and mountain tops––that enjoy near-continuous sunlight for months. These patches are surrounded by steep slopes, making it quintessential for spacecraft to land within said patches or face the mission being jeopardized.

Aggregated solar illumination map of the Moon’s south pole, as made from LRO’s stacked observations over multiple six-month periods. The brightest spots are near-permanently sunlit areas, whereas the pitch black ones are the permanently shadowed regions. Credit: NASA

Scientists using data from LRO and Japan’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.

Elevation and slope maps near the Moon’s south pole. The permanently shadowed regions are overlaid on the elevation map to the left. Credit: LPI

The lunar poles also require spacecraft that can survive very cold temperatures. While PSRs are extremely cold, their sunlit crater rims don’t get very much direct sunlight, keeping them cooled to an average of about -50 degrees Celsius.

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. One possible solution is building communications relay stations on crater rims that could also reflect sunlight to probes inside.

NASA’s VIPER mission, slated for launch in 2023, 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.

An artist’s impression of NASA’s VIPER rover exploring the Moon’s south pole, scouting for water ice. Credit: NASA

VIPER’s findings will set the stage for NASA’s Artemis program which envisions an eventual long-term human presence on the lunar surface. 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 the Moon. Samples from PSRs will be studied by laboratories worldwide to precisely date the age of volatiles within, trace their origin and unlock fundamental mysteries about the Solar System.


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This article is inspired in part by and shares some words with my article ”Your guide on water on the Moon” written for The Planetary Society.