Secrets of the Moon’s Permanent Shadows Are Coming to Light

On October 9, 2009, a two-ton rocket smashed into the moon. It exploded in a shower of dust and heated the lunar surface to hundreds of degrees, causing a crater to fill with light for the first time in billions of years.

The crash was intentional. The LCROSS mission was to see what would be kicked up from the lunar shadows by the impact. NASA's lunar observatory observed from afar as a spacecraft flew through the dust to sample it. 155 kilograms of water vapor was detected in the experiment. Anthony Colaprete of NASA's Ames Research Center said that they had found water on the moon.

Mark Robinson is a planetary scientist at Arizona State University. Its lack of atmosphere and extreme temperatures will cause water to evaporate quickly. The moon's poles have been detected with signatures of hydrogen, suggesting that water may be trapped there. This theory was proved by LCROSS. There are 6 trillion kilograms of water ice on the moon, according to scientists.

The moon's poles are called permanently shadowed regions and they contain most of the ice. These are craters that the sun can't reach because they are in permanent darkness.

Scientists are interested in PSRs. Some PSRs are colder than the surface of Pluto, according to a planetary scientist. Ice on or below the lunar surface might have survived for billions of years, because this means it won't necessarily melt. The chemical composition of the ice should reveal how it was delivered to the moon, in turn illuminating the origin of water on Earth. It could be used for future human activities on the moon.

Studies have provided a glimpse at best. That is about to change. Next year, robotic vehicles will enter the icy depths of PSRs for the first time, revealing what the interiors of these shadowed craters look like. NASA plans to send humans to explore in person by the end of the decade.

The shadowed regions of PSRs have been found to be even stranger than scientists had thought. In the shadows, what will we find?

Robinson, the lead scientist for next year's robotic mission, said he didn't know what to expect.

Water, Water, Everywhere

Harold Urey, an American chemist, first speculated about the existence of PSRs on the moon in 1952. He observed that the moon's tilt is less than the Earth's tilt, which is 23.5 degrees. The sun's rays strike its poles nearly horizontally, and the polar craters block light from reaching their depths. Urey believed that the sunless locations would have lost ice because of the lack of atmosphere on the moon.

In 1961, Kenneth Watson of Lawrence Berkeley National Laboratory proposed that ice could persist inside PSRs. There should still be ice in the shaded areas despite the exposure to space.

There were signs of ice at the poles of Mercury in the early 1990s, which was thought to have been caused by shadowed craters. Scientists detected an enhanced signal over the south pole of the moon in 1994 that was consistent with the presence of water ice. The hunt was going on.

In 1999, Jean-Luc Margot and colleagues at Cornell University discovered PSRs on the moon. They used a radar dish to make maps of the lunar poles.

They only found a few PSRs, but subsequent studies have identified thousands. The deepest inside giant craters are the Shackleton crater at the lunar south pole, which is twice as deep as the Grand Canyon. The smallest span is just a few centimeters. At the Lunar and Planetary Science Conference held in Houston in March, a planetary scientist at NASA presented research suggesting that some PSRs may grow and shrink as temperatures on the moon fluctuate.

Patrick O Brien, a graduate student at the University of Arizona, presented evidence for the idea that some craters have double-shadowed regions. The reflected light from the crater can cause ice to melt. Secondary craters inside PSRs don't get reflected light, so they are called double-shadowed.

Icy Secrets

The double-shadowed regions are cold enough to freeze carbon dioxide and nitrogen. The chemical composition of these and of the water ice inside PSRs could reveal how water got to the moon. The moon is a museum of the solar system, whereas Earth's past has been scrambled by geological processes.

There are three different theories about how water got to the moon. The first thing that happened is that it came from an asteroid or comet impact. When the solar system formed, the water in the hot inner solar system was blown away by the solar wind and only the water in the frigid outskirts could accumulate into icy bodies. The bodies bombarded the solar system with water. The second theory is that volcanic eruptions on the moon created a thin, temporary lunar atmosphere that caused ice to form at the poles. The hydrogen could have been transported by solar wind to the moon.

Nature Communications published a re-analysis of the LCROSS data in February that said the ice in Cabeus crater is most likely cometary origin. The best explanation for the frozen nitrogen, sulfur, and carbon was comets, according to Kathleen Mandt of the Applied Physics Laboratory.

If the ice on the moon was delivered by comets, the same could happen on Earth. It's possible rocky worlds must experience impacts to accumulate the water necessary for life to flourish. It's too early to say if Mandt's research holds true for all ice on the moon.

If lunar ice is determined to be volcanic origin, this would suggest worlds have an innate ability to generate water from their interiors rather than relying on impacts.

Scientists want to measure the water ice's proportion of deuterium, a heavier isotope of hydrogen, in order to find exotic ice in the PSRs. Less of it would point to solar wind than it would be if it was substantial deuterium. There is a volcanic origin in the middle. Paul Hayne, a planetary scientist at the University of Colorado, Boulder, said that ice from volcanoes should contain sulfur from the lunar interior.

Into the Abyss

The Apollo landings took place near the moon's equator at a time when knowledge of PSRs was still in its infancy. The Chang e-4 lander and rover did not target PSRs when it touched down at the south pole.

President Trump signed a directive to NASA to return humans to the moon. Ahead of the first crewed Artemis landings in the mid-2020s, which may include the first sorties into the moon's permanently shadowed craters, NASA is paying commercial companies to conduct initial robotic exploration.

The first company to explore a PSR will be Houston-based Intuitive Machines. The Nova-C lander will land on a ridge near the Shackleton crater, a possible location for future human exploration. The Micro-Nova Hopper is a suitcase-size vehicle. The Hopper will be able to jump across the lunar surface up to hundreds of meters at a time, and in three hops, it will reach the edge of the 100 meter-wide Marston crater. The Hopper will descend into the depths after it fires itself above Marston.

The lander has cameras and lights, but they are not sure what it will see. Robinson, the mission's lead scientist, said that sheets of surface ice are possible, but that it's more likely that the vehicle's lights will reflect off ice crystals mixed in with the lunar soil. It may not show up in images at all if there is minimal ice on the surface. The view will be historic regardless of the case.

The Hopper's dip into Marston will last 45 minutes, and the primary goal is to demonstrate that the hopping approach works. We won't have long to wait for a more thorough dive into the moon.

Drilling Down

The first launch of NASA's new Space Launch System rocket will carry several small satellites that will study PSRs from the moon. ShadowCam is a purpose-built NASA instrument designed to image PSRs and will be carried by a Korean orbiter in August.

The most important moment in robotic PSR exploration will come in late 2023, when a golf cart-size rover called VIPER will head to the moon. After exiting its landing vehicle, VIPER will drive into three of the moon's permanently shadowed regions and drill into the ground.

If there is a block of ice, the rover will drill up to a meter into it. The team expects to do up to 50 drilling sessions.

The knowledge of these regions will berevolutionized by the use of the VIPER. The ratio of deuterium to hydrogen and the presence of carbon dioxide or nitrogen will be revealed by the use of the spectrometers. Colaprete said that there will be a quantum leap in our understanding of where the moon's ice comes from.

Drops to Drink

The coattails of a different project will bring the scientific advances. NASA wants astronauts to use ice as either drinking water or fuel if it is accessible on or near the surface. The first crewed Artemis landing will be near a PSR so that the astronauts can see for themselves how viable the idea is.

Jim Green, NASA's former chief scientist, said that this is not the Apollo program. He said that the concept of acquiring materials and having habitats on the moon is feasible.

Kevin Cannon, a space resource expert at the Colorado School of Mines, said that various proposals for how to extract and utilize water ice are under development. The water from the excavated lunar soil would be taken from the sun or oven. Cannon said that one idea is to skip the excavation step and use a tent to heat the ground.

Confirmation of accessible ice on the moon could come by the start of next year, with the first images from inside a permanently shadowed lunar crater. We may know by the end of the year how it got there.

There are many fundamental things we don't know.