Shadow Games: Exposing a Hidden World on the Moon

August 26, 2022

Imagine a world where the Sun never passes overhead and, instead, forever moves in a circle along the horizon, casting long shadows that rotate across the landscape. That world is the lunar south polar region, soon to be explored by Artemis astronauts.

Because the Sun hovers near the horizon, the sunken floors of impact craters in the region never see sunlight and lie in perpetual shadow. Those permanently shadowed regions are incredibly cold, less than 100K (or less than -280°F) and approaching absolute zero. At those temperatures, water vapor and other volatile substances may be frozen in lunar soils, even though that soil is exposed to the vacuum of space.

The potential for ice makes those shadowed crater floors intriguing sites to explore. The ice may harbor clues about the delivery and processing of water to the Earth-Moon system. The ice may also provide resources to be used by astronauts for consumption, radiation shielding, and rocket propellant.

But designing exploration plans into such regions is difficult. What lurks in those shadowed regions? Where within them will our astronauts venture?

That puzzling problem was recently solved by an international team of scientists that developed a method for seeing into those dark regions of the Moon. Their work is published in the current issue of Geophysical Research Letters. The paper is led by Valentin Bickel, a former graduate student intern at the Lunar and Planetary Institute (LPI), Houston, and now a postdoctoral researcher at ETH Zurich, Switzerland. Dr. Bickel's former LPI mentor, Dr. David Kring, the principal investigator of the LPI-NASA Johnson Space Center (JSC) Center for Lunar Science and Exploration, co-authored the study along with Dr. B. Moseley (University of Oxford, Oxford, United Kingdom), Dr. E. Hauber (German Aerospace Center, Berlin, Germany), Dr. M. Shirley (NASA Ames Research Center, Mountain View, California), and Dr. J.-P. Williams (University of California, Los Angeles).

Areas that were once dark are made visible by using a physics-based, deep learning-driven post-processing tool to produce high-signal and high-resolution Lunar Reconnaissance Orbiter Narrow Angle Camera (NAC) images that efficiently capture photons bounced into the shadowed regions from adjacent mountains and crater walls. This allows the team of scientists to provide the world with images of potential exploration regions. As LPI's Dr. Kring explains, "Visible routes into the permanently shadowed regions can now be designed, greatly reducing risks to Artemis astronauts and robotic explorers." The spacesuit being developed for Artemis astronauts will allow them to spend at least two hours in those shadowed terrains. Using the new images, mission planners can direct astronauts to boulders to be sampled in the shadowed domains and locations where trenches can be dug in the soil to evaluate the distribution of any ices.

The authors applied their new technique to images collected by the Lunar Reconnaissance Orbiter Camera, which has been documenting the Artemis exploration zone for over a decade. The team used those enhanced images to determine that water ice is not visible in sheets covering those shadowed areas. Dr. Bickel says, "There is no evidence of pure surface ice within the shadowed areas, implying that any ice must be mixed with lunar soil or underneath the surface." Dr. Bickel also notes that this work has an immediate impact on the mission delivering NASA's PRIME-1 payload, "We detect an ~50-meter-wide crater and other surface features in a shadowed region that could alter the location where the hopper lander, Micro-Nova, may touch down next year."

The results published in the new paper are part of a comprehensive investigation of potential Artemis landing sites and exploration options on the lunar surface conducted by the LPI-JSC Center for Lunar Science and Exploration. Thus far, the team has examined more than a half-dozen potential landing sites for Artemis astronauts and complementary robotic missions.

Dr. Kring's work was supported by NASA's Solar System Exploration Research Virtual Institute (SSERVI).

Shackleton Crater oblique view

Oblique view of the lunar south pole and potential Artemis landing sites 001 and 004. This is the region where a new technique was developed to see into the shadowed interiors of impact craters. This novel three-dimensional perspective of the lunar surface was produced by draping Lunar Reconnaissance Orbiter Camera images over Lunar Orbiter Laser Altimeter topography data. Credit: ETHZ\LPI\Valentin T. Bickel and David A. Kring.

Shackleton Crater detailed view

The authors can now peer into the shadowed interiors (left) of impact craters in the south polar region and see the surfaces that Artemis astronauts and robotic assets may explore. Within the now-visible interior of the permanently shadowed interior (right), the authors detected small impact craters, boulders, and different soil patterns. Credit: V. T. Bickel, B. Moseley, E. Hauber, M. Shirley, J.-P. Williams, and D. A. Kring.

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