ShadowCam Images Dark Regions Of The Moon’s South Pole

With the success of NASA’s Artemis I mission and the agency recently naming the Artemis II crew, progress towards humanity’s first-ever journey to the lunar South Pole region during Artemis III is well underway.

To prepare, NASA scientists and engineers are learning as much as possible about this shadowy region that promises to yield scientific discoveries that can help us learn about our place in the universe and venture farther than ever before.

One way scientists are gathering information is through a hypersensitive optical camera called ShadowCam. The NASA instrument is flying with five other Korean instruments aboard KARI (Korea Aerospace Research Institute)’s KPLO (Korea Pathfinder Lunar Orbiter), also known as Danuri, which launched in Aug. 2022.

Developed by Malin Space Science Systems and Arizona State University (ASU), ShadowCam is significantly more light-sensitive than comparable lunar cameras. It acquires high-resolution images of permanently shadowed regions, which never receive direct sunlight, in service of science and exploration planning for Artemis missions and robotic missions.

Since Danuri entered lunar orbit last December, ShadowCam has been routinely capturing images of the lunar North and South Pole regions. Highlighted below are some standout images so far and what they reveal.

Shackleton Crater in Unprecedented Detail

One of ShadowCam’s first images from lunar orbit, pictured here in greater detail than ever before, is the permanently shadowed wall and floor of Shackleton crater, which is found near the South Pole. The level of detail in this image is possible thanks to ShadowCam’s ability to operate in extremely low-light conditions – it is 200 times more sensitive than the Lunar Reconnaissance Orbiter Narrow Angle Camera.

The arrow marks the track of a boulder that rolled down the crater wall. The observation of these trails helps scientists characterize the boulder shape and velocity and regolith features, furthering our understanding of the geotechnical properties of the Moon.

The Earthshine Test

ShadowCam was designed to offer views into shadowed areas near the poles. This image, however, was taken under earthshine in the equatorial region of the Moon as part of an instrument sensitivity test. It reveals the interior of Bruce crater and bright streamers that formed from soil sliding down the crater’s walls.

ShadowCam captured this image just after a new Moon. During a new Moon, at the same time that we’d see a thin lunar crescent from Earth, a person on the Moon would see a near-full Earth. Just as a full Moon can provide illumination on Earth, a full Earth can provide illumination on the Moon – this is referred to as earthshine.

While earthshine is about ten times dimmer than the illumination available in the average permanently shadowed region from sunlight reflected off lunar geologic features, ShadowCam was still able to image the surface using earthshine, indicating the instrument’s ability to see into the dimmer areas of the South Pole.

The Power of Reflection

Two types of secondary lighting allow ShadowCam to capture images in areas that do not receive direct sunlight. The first is earthshine, which illuminates the Moon’s surface away from the poles with sunlight reflected off the Earth. The second is the illumination that results from sunlight reflected off nearby geologic features such as mountains and crater walls at the poles that rise high enough above the surface to reflect direct sunlight.

The image above, captured with the latter type of illumination, shows the rim of Marvin crater, about 16 miles (26 kilometers) from the South Pole. There is variation of more than 90 degrees in the lighting direction in the small craters on the rim compared to that in the small interior craters because the secondary illumination emanates from a broad arc rather than a point light source.

The image below shows a broader area surrounding Marvin crater. The white area on the left side is where the surface was in direct sunlight – the source of secondary illumination within the shadowed aeras. ShadowCam was designed to operate in low-light, so the sunlit areas are saturated (indicated by the white areas).

Using Earthshine During a New Moon

Although ShadowCam was primarily designed to use secondary illumination from lunar geologic features for imaging, this image, showing Aristarchus crater central peak (left), was captured using earthshine.

ShadowCam will not be able to image Artemis astronauts walking on the surface of the Moon if they are in direct sunlight because the powerful light would render the images saturated. This image, however, shows that it may be possible using earthshine, if astronauts are spacewalking during lunar night.

In this image, the shadow cast by the Aristarchus central peak is from the earthshine that resulted from the Earth being 35 degrees above the horizon at the time. The different tones in the central peak are thought to represent distinct rock types.