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

On the eve of this new era of moon landings, a series of new studies on PSRs have revealed that these shadowy regions are even stranger than scientists had imagined. What will we find lurking in the shadows?

“I don’t know what we’re going to see,” said Robinson, the lead scientist for next year’s robotic mission. “That’s the coolest thing.”

Water, water, everywhere

Speculations about PSRs date back to 1952, when American chemist Harold Urey first suggested their existence on the moon. “Near its poles there may be depressions where the sun never shines,” he wrote. He observed that while the Earth orbits the Sun at a 23.5 degree tilted axis of rotation, the Moon orbits at only a 1.5 degree tilt. This means that the sun’s rays hit the poles almost horizontally, and the rims of polar craters block light from reaching directly into their depths. However, Urey believed that any ice in these sunless locations would have been “quickly lost” due to the moon’s lack of atmosphere.

American chemist Harold Urey received the 1934 Nobel Prize in Chemistry for the discovery of deuterium. He also worked on the Manhattan Project, conducting groundbreaking research on the origin of life, paleoclimatology, and the origin and properties of the moon.Photo: US Department of Energy

Then, in 1961, geophysicist Kenneth Watson of Lawrence Berkeley National Laboratory theorized that ice might exist in PSRs. Nightside temperatures on the moon were known to drop to minus 150 degrees Celsius; Watson and two colleagues argued that this meant ice would be trapped in the coldest places, despite exposure to space. “There should still be detectable amounts of ice in the permanently shadowed areas of the moon,” they wrote.

Scientists debated the possibility of ice in PSRs until the early 1990s, when radar instruments detected signs of ice at Mercury’s poles, which were thought to be permanently shadowing craters. In 1994, scientists using a radar instrument on NASA’s Clementine spacecraft detected an enhanced signal over the moon’s south pole consistent with the presence of water ice. The hunt was on.

In 1999, Cornell University’s Jean-Luc Margot and colleagues located PSRs on the Moon that might contain ice. They used a radar dish in California’s Mojave Desert to create topographic maps of the lunar poles. “We simulated the direction of sunlight and used our topographic maps to identify regions that were permanently in shadow,” Margot said.

They only found a handful of PSRs, but subsequent studies have identified thousands. The largest measure tens of kilometers inside giant craters, like Shackleton Crater at the moon’s south pole, which is twice as deep as the Grand Canyon. The smallest span is only a few centimeters. At the Lunar and Planetary Science Conference held in Houston in March, Caitlin Ahrens, a planetary scientist at NASA’s Goddard Space Flight Center, presented research that suggests some PSRs can grow and shrink slightly as temperatures on the moon change vary. “These are very dynamic cold regions,” said Ahrens in an interview. “They don’t stand still.”

Patrick O’Brien and a colleague recently identified doubly-shaded regions on the moon that are cold enough to keep exotic ice frozen.Courtesy of Patrick O’Brien

New research suggests some craters also contain double-shaded regions, or “shadows within shadows,” said Patrick O’Brien, a graduate student at the University of Arizona who presented evidence supporting the idea in Houston. While PSRs don’t experience direct sunlight, most get some reflected light bouncing off the crater rim, and this can melt ice. Double-shaded regions are secondary craters within PSRs that do not receive reflected light. “Temperatures can be even colder than the permanent shadows,” O’Brien said; they reach up to minus 250 degrees Celsius.

Icy Secrets

The double-shaded regions are cold enough to freeze more exotic types of ice, such as carbon dioxide and nitrogen, should they be present. Scientists say the chemical composition of this and the water ice in PSRs could shed light on how water got to the moon — and, more importantly, to Earth and rocky worlds in general. “Water is essential to life as we know it,” said Margaret Landis, a planetary scientist at the University of Colorado, Boulder. The question is, she said, “When and how did the conditions favorable for life on Earth form?” While Earth’s past has been muddled by geological processes, the moon is a museum of the history of the solar system; Its ice is believed to have remained mostly untouched since its arrival.

There are three prevailing theories about how water got to the moon. The first is that it came from asteroid or comet impacts. In this scenario, as the solar system formed, water molecules in the hot inner solar system were vaporized and blown away by the solar wind; Only water in the frigid outskirts could condense and accumulate into icy bodies. These bodies subsequently bombarded the inner solar system, including the moon, providing water. The second theory holds that volcanic eruptions on the moon sometime during its middle age created a thin, transient lunar atmosphere that caused ice formation at the poles. Or the solar wind could have transported hydrogen to the moon, which mixed with oxygen to form ice. Secrets of the Moon’s Permanent Shadows Are Coming to Light

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