Ahead of this new era of moon landings, a series of new PSR studies have revealed that these shadowed regions are even stranger than scientists had imagined. What will we find in the shadows?
“I don’t know what we’re going to see,” said Robinson, lead scientist for next year’s robotic mission. “It’s the sweetest thing.”
Water, water, everywhere
Speculation about PSRs dates back to 1952, when American chemist Harold Urey first hypothesized their existence on the Moon. “Near its poles there may be hollows on which the sun never shines,” he wrote. He noted that while the Earth orbits the Sun with its rotational axis tilted by 23.5 degrees, the Moon orbits at a tilt of only 1.5 degrees. This means that the sun’s rays strike its poles almost horizontally, and the rims of the polar craters will block light from directly reaching their depths. However, Urey believed that any ice in these sunless locations would be “rapidly lost” due to the lack of an atmosphere on the moon.
Then in 1961, geophysicist Kenneth Watson of Lawrence Berkeley National Laboratory theorized that ice could exist within the PSR. Nighttime temperatures on the Moon are known to drop to minus 150 degrees Celsius; Watson and two colleagues argued that this meant the ice would remain trapped in the coldest places, despite exposure to space. “There should still be detectable amounts of ice in the permanently shadowed regions of the Moon,” they wrote.
Scientists debated the possibility of ice in the 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 the craters. In 1994, using a radar instrument on NASA’s Clementine spacecraft, scientists detected an enhanced signal over the southern half of the moon that was consistent with the presence of water ice. The hunt was on.
In 1999, Jean-Luc Margot of Cornell University and colleagues identified a PSR on the Moon that could contain ice. They used a radar antenna in the Mojave Desert in California to make topographic maps of the moon’s poles. “We simulated the direction of sunlight and used our topographic maps to identify regions that were permanently shaded,” said Margot.
They located only a few PSRs, but subsequent studies identified thousands. The largest measure tens of kilometers in diameter inside giant craters, such as Shackleton Crater on the south pole of the Moon, which is twice as deep as the Grand Canyon. The smallest range 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 suggesting that some PSRs may wax and wane slightly as lunar temperatures fluctuate. “These are very dynamic cold regions,” Ahrens said in an interview. “They don’t stagnate.”
The new research shows that some craters also contain doubly shadowed regions, or “shadows within shadows,” said Patrick O’Brien, a graduate student at the University of Arizona, who presented evidence for the idea in Houston. Although PSRs do not experience direct sunlight, most receive reflected light that bounces off the rim of the crater, and this can melt the ice. The double-shaded regions are secondary craters within the PSR that do not receive reflected light. “Temperatures can be even lower than permanent shadows,” O’Brien said; they reach up to minus 250 degrees Celsius.
The double shadow regions are cold enough to freeze more exotic ices, such as carbon dioxide and nitrogen, if present. Scientists say the chemical composition of these and the water ice inside the PSR could reveal how water got to the Moon — and, more importantly, to Earth and to 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, she said, is, “When and how did the conditions suitable for life on Earth arise?” While Earth’s past was punctuated by geological processes, the Moon is a museum of solar system history; its ice is thought to have remained largely intact since his arrival.
There are three dominant theories about how water got to the Moon. The first is that it arrived via an asteroid or comet impact. In this scenario, when the solar system formed, water molecules in the hot inner solar system evaporated and were blown away by the solar wind; only water in the cold peripheries could condense and accumulate into ice bodies. These bodies subsequently bombarded the inner solar system, including the moon, delivering water. Another theory is that volcanic eruptions on the Moon sometime in its middle ages formed a thin, temporary lunar atmosphere that caused ice to form at the poles. Or the solar wind could have carried hydrogen to the Moon that mixed with oxygen to form ice.