Today, scientists announced that the spin axis of the moon is about 6° different than it was billions of years ago, based on evidence of off-kilter polar ice deposits. Science first reported on this story last year when the scientists presented the research at the Lunar and Planetary Science Conference in The Woodlands, Texas.
THE WOODLANDS, TEXAS—What little ice remains on Mercury and Mars is mostly confined to the planets’ poles, as one would expect, because the sun shines the least in those regions. Not so on the moon. Much of the moon’s ice, which lurks beneath the surface, is found in an area 5.5° away from the north pole and in a matching region 5.5° from the south pole, scientists announced here this week at the Lunar and Planetary Science Conference. The data suggest that in the past, the moon’s axis of rotation—and hence its poles—shifted.
“It turns out these enhanced concentrations are exactly opposite each other—they’re antipodal,” says Matthew Siegler, a planetary scientist at the Planetary Science Institute who is based in Dallas, Texas. “The easiest explanation is: There used to be poles there.” Siegler and his colleagues have suggested a cause for the “polar wander”: a 3.5-billion-year-old hot spot beneath the moon’s surface. If the story holds up, it means the moon's water is nearly as ancient as the orb itself.
The researchers relied on data from NASA’s Lunar Prospector mission, which orbited the moon from 1998 to 1999. One of the spacecraft’s instruments measured neutrons emitted from the surface. Slower, less energetic ones indicate the amount of hydrogen lurking within a meter of the surface, and on the moon, hydrogen is a proxy for water. Although scientists had noticed before that the water was not centered at the current poles, no one had noticed this precise off-axis, antipodal relationship. “Everyone is basically kicking themselves and saying, ‘Why didn’t I notice this?’ ” Siegler says.
He and his colleagues assumed that when the ice was deposited, it was centered on the poles. But what kind of event could have moved the poles by 5.5°? Known asteroid impacts were too small or in the wrong location to do the job. Instead, the team hypothesizes that a 3.5-billion-year-old hot spot could have nudged the poles to their present-day position. Pouring out enormous amounts of lava, that hot spot created Oceanus Procellarum, the vast dark spot on the near side of the moon. The Procellarum region is known to have high concentrations of radioactive elements that would have been hot in ancient times. The research team theorizes that this heat would have created a less dense lens in the moon’s mantle that would have caused the axis to wobble into today’s position.
If that idea is correct, then it implies that the moon’s water is mostly ancient—contrary to scientists who have argued that water was delivered more recently by asteroid impacts or even produced by a hail of protons known as the solar wind. “That ice might be primordial from the beginning of the moon,” Siegler says.
“It’s a terrific idea,” says Oded Aharonson, a planetary scientist at the Weizmann Institute of Science in Rehovot, Israel. But Aharonson isn’t sure the ice could have persisted for so long. At other times in its ancient past, the moon’s poles are thought to have wandered far enough to bring polar regions near the sun-drenched equator. Moreover, ice could be destroyed by large asteroid impacts. For the ice to survive, the 5.5° tilting event would need to occur after those cataclysms, he says. “It’s really critical not to do this [tilting] too early in lunar history,” he says.
But the research team says there are ways for the ice to persist through the ages. Some of the water may be locked up as hydrated minerals in rocks. And some of it may be protected by an insulating layer of regolith, says Richard Miller, a planetary scientist at the University of Alabama, Huntsville, and a collaborator on the research. “If it gets buried and moves to depth, some fraction can survive for a long period of time.”
*Update, 23 March, 2:00 p.m.