Moon's Interior Was Pulverized By Early Impacts
December 5, 2012

GRAIL Mission Reveals A Tragic Past For The Moon’s Crust

[ Watch the Video: GRAIL's Gravity Tour of the Moon ]

Lee Rannals for - Your Universe Online

Scientists using data from NASA's GRAIL mission have determined that the Moon's interior is nearly completely pulverized.

The latest finding from the Gravity Recovery and Interior Laboratory (GRAIL) mission suggests that during the Moon's first billion years, it may have endured more fracturing from massive impacts than previously thought.

Planetary scientists have stitched together a high-resolution map of the Moon's gravity using GRAIL's measurements. The map reveals an interior gravitational field consistent with an incredibly fractured lunar crust.

“It was known that planets were battered by impacts, but nobody had envisioned that the [Moon´s] crust was so beaten up,” said MIT´s Maria Zuber, who leads the GRAIL mission and is the E. A. Griswold Professor of Geophysics in the Department of Earth, Atmospheric and Planetary Sciences. “This is a really big surprise, and is going to cause a lot of people to think about what this means for planetary evolution.”

The scientists reported in the journal Science that GRAIL's lunar gravity map also revealed numerous structures on the Moon's surface that were unresolved by previous gravity maps of any planet.

Scientists determined that the Moon's crust is much thinner than previously suspected. The crust beneath some major basins is nearly nonexistent, which indicates that early impacts may have excavated the lunar mantle.

The twin GRAIL probes measure the changing distance between themselves as they orbit in tight formation around the Moon.

As one probe flies over a large mass, the stronger local gravity will pull that probe ahead, widening the space between the two spacecraft. Scientists translate this changing distance into a gravitational map that represents the gravity produced by both the surface structures and the interior.

The team used topographic measurements from one of their instruments to help find the gravitational field for the Moon's interior. They calculated the gravitational field expected to be produced by the Moon's topography, then subtracted that field from the field measured by GRAIL.

“It´s essentially like removing a veil to reveal the gravity due to the inside of the planet,” Zuber said. “And when we saw those maps, we were just speechless.”

The map of the interior looked smooth when compared to the surface. The team found that most of the local variations in the Moon's gravity are due to surface features like crater rims and mountains.

The Moon's upper crust lacks dense rock structures, and is likely made of porous, pulverized material, according to the map.

The interior map reveals long, linear structures of denser material, which the team believes to be buried lunar dikes. These dikes represent evidence for expansion of the Moon in its earliest history. Overall, 98 percent of the lunar crust is fragmented, according to the findings.

“This is interesting for the Moon,” Zuber said. “But what it also means is that every other planet was being bombarded like this.” The resulting fractures, she says, affect the way a planetary body loses heat and also provide a pathway for the transport of interior fluids."