Scientists Find Mystery Mass Under Moon’s Surface

A monochrome mosaic of the moon’s South Pole-Aitken basin, below which scientists found the giant mass. Photo: NASA/GSFC/Arizona State University

The mass of the metal “anomaly” beneath the moon’s largest crater is five times greater than the big island of Hawaii, and according to a new study from scientists at Baylor University, it could contain metals remaining from an ancient asteroid impact, weighing in at around 4.8 quintillion pounds.

It sits 180 miles beneath the South Pole-Aitken basin — one of the solar system’s largest impact craters, and the moon’s oldest, at over 4 billion years — a massive dent spanning some 1,550 miles on the far side of the moon. (It’s also where China landed its Chang’e 4 lunar rover in January.) Publishing in Geophysical Research Letters, the Baylor scientists have two theories for the origin of the huge subterranean blob. It could be the leftovers of dense oxides created in the last years when the moon’s surface was an ocean of magma — a theory that relies on the giant-impact hypothesis, when an impactor the size of Mars may have collided into a magma-covered Earth, ejecting magma into orbit that became the surface of the moon. But speaking with National Geographic, the Baylor team appears to prefer the idea that the mass is the remainder of the iron-nickel core of an ancient impactor that created the South Pole-Aitken basin:

The space rock that formed the moon’s giant basin was likely large enough to have separated into different layers when it first formed, so that like many of today’s planets, it sported a dense, metallic core and rocky outer layers.

On the fateful day of its collision, the energy of the impact carved a deep bowl-shaped crater on the moon, with the impactor’s metallic core smashed up inside. But the original hole didn’t last, and the divot on the moon partially refilled with molten rock. Within it lingered the melty traces of the ancient impactor’s core.

“That’s what I would bet on,” [coauthor Peter] James says.

The mass was found when the team merged data from NASA’s Gravity Recovery and Interior Laboratory mission, known as GRAIL, with topography gathered from the Lunar Reconnaissance Orbiter, a robotic spacecraft launched from Cape Canaveral in 2009. The GRAIL mission was launched in 2011, and used gravitational field-mapping of the moon to determine its below-surface makeup. Combining the two, the Baylor team was able to find the mass that is “weighing the basin floor downward by more than half a mile.”

It might not be as compelling as the most recent space-based national story — the Navy pilots who claim they frequently encountered UFOs off the Eastern coast of the U.S. — but scientists in the field are revved up at the prospects of studying the big metal chunk. “[The basin is] one of the best natural laboratories for studying catastrophic impact events, an ancient process that shaped all of the rocky planets and moons we see today,” James told CNN.

Scientists Find Mystery Mass Under Moon’s Surface