On the eve of this new era of moon landings, a new set of studies on the PSR shows that these shadowy regions are stranger than scientists thought. What will we find lurking in the shadows?
“I don’t know what we’re going to see,” said Robinson, chief scientist for next year’s robotic mission. “It’s the coolest thing.”
water, water, everywhere
Speculation about PSRs dates back to 1952, when American chemist Harold Urey first hypothesized that they existed on the moon. “Near its poles, there may be depressions that the sun will never reach,” he wrote. He observed that while the Earth’s axis of rotation around the Sun is tilted by 23.5 degrees, the Moon’s orbit is tilted by only 1.5 degrees. This means that the sun’s rays hit its poles almost horizontally, and the edges of polar craters block light from reaching their depths directly. However, Yuri believes that any ice in these sunless places would “disappear quickly” due to the moon’s lack of an atmosphere.
Then in 1961, Lawrence Berkeley National Laboratory geophysicist Kenneth Watson speculated that ice could persist inside the PSR. Lunar nighttime temperatures are known to drop to -150 degrees Celsius; Watson and two colleagues believe this means ice is trapped in the coldest places, albeit exposed to space. “A detectable amount of ice should still exist in permanently shadowed regions of the Moon,” they wrote.
Scientists debated the presence of ice in the PSR until the early 1990s, when radar instruments found signs of ice at the poles of Mercury, which is also thought to have permanently shadowed craters. In 1994, scientists using radar instruments on NASA’s Clementine spacecraft detected an enhanced signal at the moon’s south pole consistent with the presence of water ice. The hunt has begun.
In 1999, Jean-Luc Margot of Cornell University and colleagues identified PSRs on the moon that may contain ice. They used a radar antenna in California’s Mojave Desert to create a topographic map of the moon’s poles. “We simulated the direction of sunlight and used our topographic map to identify areas of permanent shade,” Margot said.
They found only a handful of PSRs, but subsequent studies have identified thousands. The largest craters are tens of kilometers inside, such as the 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 in Houston in March, research presented by planetary scientist Caitlin Ahrens of NASA’s Goddard Space Flight Center suggests that some PSR may increase slightly as the moon’s temperature fluctuates and shrink. “These are vibrant cold regions,” Ahrens said in an interview. “They’re not standing still.”
The new study shows that some craters also contain areas of double shadows, or “shadows within shadows,” said Patrick O’Brien, a graduate student at the University of Arizona who provided evidence for this idea in Houston. While PSRs don’t receive direct sunlight, most receive some reflected light bouncing off the crater rim, which melts the ice. The double-shaded areas are secondary craters inside the PSR that do not reflect light. “The temperature could be colder than a permanent shade,” O’Brien said. They have temperatures as low as minus 250 degrees Celsius.
The double-shaded area is cold enough to freeze more exotic ices, such as carbon dioxide and nitrogen, if they exist there. These and the chemical composition of the water ice within the PSR could reveal how water got to the Moon—and, more importantly, Earth, and the rocky world in general, scientists say. “Water is essential to life as we know it,” said Margaret Landis, a planetary scientist at the University of Colorado Boulder. The question, she says, is, “When and how did the conditions favorable for life on Earth formed?” Earth’s past has been disrupted by geological processes, and the moon is a museum of the history of the solar system; its ice is thought to have been nearly Not affected.
There are currently three dominant theories about how water got to the Moon. The first is that it arrives via an asteroid or comet impact. In this case, when the solar system formed, the hot water molecules inside the solar system were evaporated and blown away by the solar wind; only the water in the frigid suburbs could condense and accumulate into an icy body. These objects then bombarded the inner solar system, including the moon, delivering water. The second theory is that volcanic eruptions created a thin, temporary lunar atmosphere sometime during the lunar middle age, causing the poles to freeze. Or the solar wind could carry hydrogen to the moon, where it mixes with oxygen to form ice.