Are There Shadows on the Moon?

Shadows on the Moon are not only present but far more dramatic than anything you would encounter on Earth. Without an atmosphere to scatter sunlight and soften edges, lunar shadows are pitch-black, knife-sharp, and can persist for billions of years in certain polar craters. The interplay between harsh direct sunlight and absolute darkness is one of the defining features of the lunar environment, and it shapes everything from how astronauts perceive the terrain to where scientists expect to find water ice.

Why Lunar Shadows Look Nothing Like Shadows on Earth

On Earth, the atmosphere acts as a giant diffuser. Sunlight bounces off gas molecules, water droplets, and dust particles in every direction, which is why you can still see detail inside a shadow on a sunny day. The air fills shadows with scattered light, giving them soft edges and a dim bluish tone. On the Moon, none of that happens. There is essentially no atmosphere to redirect photons, so when sunlight hits a rock or a crater rim, the far side drops into near-total blackness. The boundary between lit ground and shadow is so abrupt it looks almost like a line drawn with a marker.

This creates serious perceptual problems. A study from MIT noted that the lack of atmosphere on the Moon inhibits the use of aerial perspective as a distance cue and causes the formation of deep cast shadows, both of which are expected to distort how humans estimate slope, distance, and height on the surface.¹ On Earth, haze and atmospheric scattering help your brain judge how far away an object is. On the Moon, a boulder ten meters away and a hill a kilometer away can look equally crisp, and a shadowed crater can look like a bottomless pit or a flat dark patch depending on the sun angle.

The lunar soil itself contributes to this harsh visual environment. The regolith, that fine powdery layer covering the Moon, does not reflect light the way a smooth surface would. Instead, it scatters light predominantly back toward the source. This is partly why the Moon appears brightest at full phase when the Sun is directly behind us: sunlight bounces almost straight back rather than spreading evenly in all directions. At the particle level, tiny shadows between individual grains of regolith play a measurable role in how bright the surface appears at different viewing angles.² The shadow-hiding effect, where microscopic shadows between grains vanish when you look straight down the line of illumination, is a major component of the Moon’s characteristic brightness surge at low phase angles.

Permanently Shadowed Regions at the Poles

Some lunar shadows never move. Near the Moon’s north and south poles, the Sun hangs so low on the horizon that the floors of deep craters never see direct sunlight. The Moon’s spin axis is tilted only about 1.5 degrees relative to the plane of its orbit around the Sun, meaning the solar elevation at the poles is always extremely small.³ A crater rim a few kilometers high can block every direct ray of sunlight from reaching its interior, not for a season but for eons.

These permanently shadowed regions, or PSRs, are among the coldest places in the solar system. Temperature measurements from the Diviner instrument aboard the Lunar Reconnaissance Orbiter revealed widespread cryogenic zones where surface temperatures drop far below what you need to trap water ice, and these cold areas actually extend beyond the boundaries of the visible shadow itself.⁴ Some spots stay below 100 Kelvin, which is roughly minus 170 degrees Celsius. At those temperatures, not only water but also other volatile compounds can remain frozen on the surface essentially forever.

The size and distribution of these regions vary. Large craters near the south pole, like Shackleton and Cabeus, contain extensive PSRs, but smaller cold traps exist too, some only meters across, tucked into the shadows of boulders and small depressions.⁵ These micro cold traps could collectively hold meaningful amounts of frozen material spread across the polar landscape in a pattern far more complex than just “big dark craters.”

Not Completely Dark Inside

You might imagine a permanently shadowed crater as a place of absolute blackness, and by human visual standards it nearly is. But “permanently shadowed” does not mean “zero light.” Most PSRs receive secondary illumination: sunlight that first hits an illuminated crater wall or nearby ridge and then bounces down into the shadowed floor.⁶ This reflected light is faint compared to direct sunlight, but it is real and measurable.

How much secondary light reaches the floor depends on the season. During lunar summer, when the Sun climbs as high as it ever does at the poles, the illuminated walls surrounding a PSR reflect enough light that the interior can reach luminance levels comparable to those of a well-lit building for several Earth days each month. In winter, peak lighting inside these shadows drops to roughly six times dimmer, and in areas where regional shadows overlap, illumination can be even weaker.⁷ That seasonal variation matters for planning robotic missions and eventual human expeditions: there are windows when cameras and solar panels could function inside PSRs, and windows when they cannot.

Beyond reflected sunlight, there is another light source that illuminates the lunar surface during its roughly two-week-long night: earthshine. The Earth, from the Moon’s perspective, is a brilliant object in the sky, appearing about 20 to 30 times brighter at visible wavelengths than the Moon appears from Earth, thanks to its larger apparent size and higher albedo. Earthshine serves as the dominant source of natural illumination during lunar night and reaches some permanently shadowed polar regions as well, with broadband irradiance approaching about 150 milliwatts per square meter near zero phase, or roughly one ten-thousandth of direct sunlight.⁸ That is far too dim for a solar panel but enough for a sufficiently sensitive camera to capture useful images.

Water Ice Hiding in the Shadows

The reason permanently shadowed regions attract so much scientific and commercial interest is water. Spectroscopic observations have confirmed that water ice is trapped and accumulates in PSRs, and in some locations it sits exposed at the surface rather than buried under layers of regolith.⁹ Reflectance measurements from the Lunar Orbiter Laser Altimeter showed a sharp increase in surface brightness at temperatures below about 110 Kelvin near the south pole, a behavior consistent with exposed water ice deposits.¹⁰

How did the water get there? The leading idea is ballistic transport. Water molecules released from the surface by solar wind interaction or delivered by comet impacts hop across the Moon in a series of ballistic arcs. Each hop exposes the molecule to sunlight and high temperatures, which destroy most of them, but a small fraction make it to the cold poles before being lost. Simulations suggest that roughly 2% of the water released from the surface through desorption eventually reaches polar regions.¹¹ Once a water molecule lands in a PSR where temperatures stay below about 110 Kelvin, it stays put, potentially for billions of years.

Estimates of how much ice might be stored vary. Radar observations from Arecibo found that permanently shadowed crater floors do not produce the strong radar echoes you would expect from thick, pure ice sheets, suggesting that any ice present is in the form of distributed grains or thin layers mixed into the regolith rather than solid glacial deposits.¹² On the other hand, analysis of a different class of polar craters using the Mini-RF imaging radar found deposits consistent with relatively clean water ice, and if that interpretation is correct, both poles could contain several hundred million tons of water within the upper couple of meters of the surface.¹³ The discrepancy likely reflects real variation: some craters may hold more ice than others depending on their depth, temperature profile, and history of volatile delivery.

How Scientists Use Shadows as Tools

Shadows on the Moon are not just obstacles to navigate around. Planetary scientists have long used them as measurement instruments. Because sunlight arrives at the Moon essentially as parallel rays from a single point source, the geometry of a shadow inside a crater directly encodes the crater’s shape and depth. By measuring the length and curvature of a shadow cast on a crater floor and knowing the Sun’s angle at the time an image was taken, researchers can calculate how deep the crater is. An improved technique for this approach considers whether the crater is bowl-shaped, conical, or flat-floored, and uses the specific outline of the cast shadow to distinguish between those geometries.¹⁴ This method has been applied not just on the Moon but on other airless bodies throughout the solar system.

Photometric analysis, the study of how surfaces reflect light at different angles, offers another shadow-related scientific tool. The way brightness changes as the Sun angle shifts reveals properties of the surface texture at scales too small to resolve directly. At the Apollo 14, 15, and 17 landing sites, phase-ratio imaging detected photometric anomalies about 170 by 120 meters in size caused by the engine exhaust of the lunar modules, which smoothed the local surface microstructure. Other anomalies with steeper photometric slopes were traced to regolith loosened by astronaut boots and rover wheels.¹⁵ In other words, the subtle shadow behavior of individual soil grains can reveal whether someone walked or drove over a patch of ground decades ago.

What Shadows Mean for Astronauts

The Apollo astronauts described lunar shadows as one of the most disorienting aspects of working on the surface. Step into a shadow and you could barely see your own feet; step out and you were blinded by glare off the bright regolith. This is not a minor inconvenience. The non-Lambertian reflectance of the regolith, meaning it does not scatter light evenly in all directions, combined with the absence of atmospheric scattering and the lack of familiar reference objects, makes it genuinely difficult to judge terrain.¹ A shallow dip can look like a cliff, and a steep slope can look flat, depending on where the Sun is.

For future lunar surface missions, especially those targeting the south polar region where permanently shadowed craters sit beside peaks of near-permanent sunlight, this presents a unique operational challenge. Astronauts and rovers will need to transition repeatedly between blinding illumination and near-total darkness over short distances. Lighting systems, camera sensitivity ranges, and route planning all have to account for the fact that the visual environment changes more abruptly on the Moon than in any terrestrial setting.

Building in the Shadow

The extreme and permanent nature of polar shadows has led to some ambitious engineering proposals. One concept that has been studied for decades is placing a large telescope inside a permanently shadowed crater. The perpetual cold and darkness would be ideal for infrared astronomy, eliminating the thermal noise that plagues even the best Earth-based and orbital observatories. A study explored the feasibility of constructing a 25-meter segmented infrared telescope at the lunar south pole, identifying the temperature environment in permanently shadowed craters, dust contamination, and human-robot interaction as the most significant unsolved challenges.⁸

Water ice in PSRs also figures prominently in plans for sustained human presence on the Moon. If the ice deposits are accessible and abundant enough, they could supply drinking water, oxygen for breathing, and hydrogen for rocket fuel without the enormous cost of launching those materials from Earth. The catch is that extracting resources from the darkest, coldest places on the Moon requires hardware that can operate in extreme cold with only artificial light or faint reflected illumination. Every mission concept targeting these regions has to reconcile two competing facts: the shadows are what preserve the ice, but they also make the ice extraordinarily hard to reach.

Shadows During a Lunar Eclipse

There is one situation where a shadow falls across the entire visible face of the Moon at once: a lunar eclipse, when Earth passes between the Sun and the Moon. From the Moon’s perspective, the Earth blocks the Sun and the surface temperature drops rapidly. Observations of a lunar eclipse at microwave frequencies showed that once the penumbral phase begins, the uppermost layer of the surface cools quickly as solar radiation weakens, but deeper layers, measured at lower microwave frequencies, cool much more slowly because the regolith conducts heat poorly. The disk-averaged brightness temperature began declining about 15 minutes after the start of the penumbral phase.¹⁶ This thermal lag between the surface and the subsurface gives scientists a way to probe the insulating properties of the regolith: the bigger the lag, the fluffier and more insulating the top layer must be.

A lunar eclipse is, in a sense, the one time the Moon gets a taste of what its permanently shadowed craters experience all the time, except it lasts only a few hours instead of an eternity. The surface cools dramatically but never reaches the extreme lows found in polar PSRs, because the shadow passes before the deeper ground can fully equilibrate. It is a useful natural experiment, but it also highlights how special the polar cold traps really are: sustained shadow, not a brief one, is what makes those regions the freezers of the solar system.