On the eve of this new era of lunar landings, a number of new studies of PSRs have revealed that these shaded areas are even more alien than scientists had imagined. What will we find lurking in the shadows?
“I do not know what we are going to see,” said Robinson, lead scientist for next year’s robot mission. “That’s the coolest thing.”
Water, water, everywhere
Speculation about PSRs dates back to 1952, when the American chemist Harold Urey first assumed their existence on the moon. “Near its poles there may be depressions on which the sun never shines,” he wrote. He observed that while the Earth orbits the sun with its axis of rotation tilted by 23.5 degrees, the moon orbits with only a slope of 1.5 degrees. This means that the sun’s rays hit its poles almost horizontally, and the edges of the polar craters will block the light from reaching their depths directly. Urey, however, believed that all the ice in these sunless places would have been “quickly lost” due to the moon’s lack of atmosphere.
Then in 1961, geophysicist Kenneth Watson from Lawrence Berkeley National Laboratory theorized that ice could exist inside PSRs. Night temperatures on the moon were known to drop to minus 150 degrees Celsius; Watson and two colleagues argued that this meant that ice would be trapped in the coldest places, despite the exposure to space. “There should still be detectable amounts of ice in the permanently shaded areas of the moon,” they wrote.
Researchers discussed the possibility of ice in PSRs until the early 1990s, when radar instruments discovered signs of ice at Mercury’s poles, which was also thought to have permanently shaded craters. In 1994, scientists used a radar instrument on NASA’s Clementine spacecraft. detected an improved signal above the lunar south pole, which was consistent with the presence of water ice. The hunt was on.
In 1999 Jean-Luc Margot at Cornell University and colleagues designated PSRs on the moon that could contain ice. They used a radar dish in the California Mojave Desert to make topographic maps of the lunar poles. “We simulated the direction of sunlight and used our topographic maps to identify areas that were permanently in shadow,” Margot said.
They located only a handful of PSRs, but subsequent studies have identified thousands. The biggest measure dozens of kilometers across inside giant craters, such as the Shackleton crater at the moon’s south pole, which is twice as deep as the Grand Canyon. The smallest span just inches. At the Lunar and Planetary Science Conference held in Houston in March, Caitlin Ahrensa planetary scientist at NASA’s Goddard Space Flight Center, presented research suggesting that some PSRs may grow and shrink slightly as the temperatures on the moon fluctuate. “These are very dynamic cold areas,” Ahrens said in an interview. “They are not stagnant.”
New research suggests that some craters also contain double-shaded areas, or “shadows in shadows,” said Patrick O’Brien, a graduate student at the University of Arizona who presented evidence for the idea in Houston. While PSRs do not experience direct sunlight, most receive some reflected light bouncing off the crater rim, and this can melt ice. Double-shaded areas are secondary craters inside PSRs that do not receive reflected light. “Temperatures can be even colder than the permanent shadows,” O’Brien said; they reach as low as minus 250 degrees Celsius.
Ice cold secrets
The double-shaded areas are cold enough to freeze more exotic ice, like carbon dioxide and nitrogen, if any were to exist there. Scientists say that the chemical composition of these and the water ice inside PSRs can reveal how water came to the moon – and more importantly, to Earth and to rock worlds in general. “Water is essential to life as we know it,” he said Margaret Landis, a planetary scientist at the University of Colorado, Boulder. The question is, she said, “When and how did the favorable conditions for life on Earth arise?” While the Earth’s past has been distorted by geological processes, the moon is a museum of the history of the solar system; its ice cream is believed to have remained untouched since its arrival.
There are three prevailing theories about how water came to the moon. The first is that it arrived via asteroid or comet impact. In this scenario, when the solar system was formed, water molecules in the hot inner solar system were evaporated and blown away by the solar wind; only water at the cold edge could condense and accumulate into icy bodies. These bodies subsequently bombarded the inner solar system, including the moon, and supplied water. The second theory is that volcanic eruptions on the moon sometime in its Middle Ages formed a thin, temporary lunar atmosphere that spawned ice formation at the poles. Or solar wind could have transported hydrogen to the moon, which mixed with oxygen to form ice.