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 suggesting that some PSRs may grow and shrink slightly as temperatures on the moon fluctuate. The largest measure tens of kilometers across inside giant craters, such as Shackleton crater at the lunar south pole, which is twice as deep as the Grand Canyon. They located just a handful of PSRs, but subsequent studies have identified thousands. “We simulated the direction of sunlight and used our topographic maps to identify regions that were permanently shadowed,” Margot said. They used a radar dish in the Mojave Desert in California to make topographic maps of the lunar poles. In 1999, Jean-Luc Margot at Cornell University and colleagues pinpointed PSRs on the moon that could contain ice. In 1994, using a radar instrument on NASA’s Clementine spacecraft, scientists detected an enhanced signal over the moon’s south pole that was consistent with the presence of water ice. Scientists debated the possibility of ice in PSRs until the early 1990s, when radar instruments detected signs of ice at the poles of Mercury, which was also thought to have permanently shadowed craters. “There should still be detectable amounts of ice in the permanently shaded areas of the moon,” they wrote. Nightside temperatures on the moon were known to plunge to minus 150 degrees Celsius Watson and two colleagues argued that this meant ice would get trapped in the coldest places, despite the exposure to space. Then in 1961, the geophysicist Kenneth Watson of Lawrence Berkeley National Laboratory theorized that ice could persist inside PSRs. However, Urey believed that any ice in these sunless locations would have been “rapidly lost” because of the moon’s lack of atmosphere. This means the sun’s rays strike its poles nearly horizontally, and the rims of polar craters will block light from directly reaching their depths. He observed that, whereas Earth orbits the sun with its rotational axis tilted by 23.5 degrees, the moon orbits at a mere 1.5-degree tilt. “Near its poles there may be depressions on which the sun never shines,” he wrote. Speculation about PSRs dates back to 1952, when the American chemist Harold Urey first hypothesized their existence on the moon. “That’s the coolest thing.” Water, Water, Everywhere “I don’t know what we’re going to see,” said Robinson, the lead scientist for next year’s robotic mission. What will we find lurking in the shadows? On the eve of this new era of moon landings, a slew of fresh studies of PSRs have revealed that these shadowed regions are even stranger than scientists imagined. By the decade’s end, NASA plans to send humans to explore in person. Next year, robotic vehicles will enter the bewildering icy depths of PSRs for the first time, revealing what the interiors of these shadowed craters look like. Studies so far have provided a tantalizing glimpse at best. It could also be a resource for future human activities on the moon. Studying the ice’s chemical composition should reveal how it was delivered to the moon, in turn illuminating the origin of water on Earth, or indeed any rocky world around any star. This means ice on or below the lunar surface in PSRs won’t necessarily melt instead it might have survived there for billions of years. “Some PSRs are colder than the surface of Pluto,” said Parvathy Prem, a planetary scientist at the Johns Hopkins University Applied Physics Laboratory in Maryland. Inside, temperatures can drop below minus 170 degrees Celsius. PSRs are of immense interest to scientists. “They’re in permanent darkness,” said Valentin Bickel, a planetary scientist at the Max Planck Institute for Solar System Research in Germany. These are craters like Cabeus into which the sun can’t reach, because of the geometry of the moon’s orbit. Most of this ice resides in peculiar features at the moon’s poles called permanently shadowed regions (PSRs). Scientists now think there’s not just a bit of water ice on the moon there are 6 trillion kilograms of it. Yet about 25 years ago, spacecraft began to detect signatures of hydrogen around the moon’s poles, hinting that water might be trapped there as ice. Its lack of atmosphere and extreme temperatures should cause any water to almost instantly evaporate. “It’s really weird when you stop to think about it,” said Mark Robinson, a planetary scientist at Arizona State University. The moon isn’t an obvious reservoir of water.
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