On Jan. 3, 2019, the Chinese spacecraft Chang’e 4 (above) safely landed on the Far Side floor of the Moon’s Von Kármán crater, naming the site, “Milky Way Base.” Located within an even larger four-billion-year-old impact crater known as the South Pole–Aitken basin roughly 2,500 kilometers in diameter and 13 kilometers deep –the largest impact crater in the Solar System –it now appears that the “Milky Way Base” sits atop of an abnormally massive blob buried deep below.
The lunar feature, with a mass five times the size of Hawaii’s Big Island, was discovered by researchers using data from NASA’s 2011 Gravity Recovery and Interior Laboratory, or GRAIL, mission and mapping information from the Lunar Reconnaissance Orbiter. By combining both datasets, researchers found that the abnormal mass is located 180 miles underneath the South Pole-Aitken basin. The exact reason why this anomaly exists in unclear, according to a new study in the journal Geophysical Research Letters.
The mysterious large mass may contain metal from an asteroid that crashed into the Moon and formed the crater, according to a Baylor University study. The dense mass — “whatever it is, wherever it came from” — is weighing the basin floor downward by more than half a mile, said lead author Peter B. James at Baylor University who has served on the science team of NASA’s Lunar Reconnaissance Orbiter (LRO) and the Gravity Recovery and Interior Laboratory (GRAIL).
James called the basin “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.”
“Imagine taking a pile of metal five times larger than the Big Island of Hawaii and burying it underground. That’s roughly how much unexpected mass we detected,” said James who specializes in the use of spacecraft data to study the crusts and mantles of planets and moons in our solar system.
The false-color graphic below shows the topography of the far side of the Moon. The warmer colors indicate high topography and the bluer colors indicate low topography. (NASA/Goddard Space Flight Center/University of Arizona) The South Pole-Aitken (SPA) basin is shown by the shades of blue. The dashed circle shows the location of the mass anomaly under the basin.
The oval-shaped crater, as wide as 2,000 kilometers, was formed when some large space rock with a heavy metal core smashed into the lunar surface billions of years ago, says Maya Wei-Haas at National Geographic. When that happened, the asteroid drilled through layers of the moon’s crust while losing mass of its own. Molten rock partially refilled the impact area, melting chunks of the asteroid’s busted metal core along the way.
Providing more evidence for this theory, there appears to be what’s called a central depression on the basin’s floor that is about half a mile deeper than the rest of the crater, suggesting that something beneath it has enough gravitational pull to tug the area inward.
To measure subtle changes in the strength of gravity around the Moon, researchers analyzed data from spacecrafts used for the NASA Gravity Recovery and Interior Laboratory (GRAIL) mission.
“When we combined that with lunar topography data from the Lunar Reconnaissance Orbiter, we discovered the unexpectedly large amount of mass hundreds of miles underneath the South Pole-Aitken basin,” said lead author Peter B. James, Ph.D., assistant professor of planetary geophysics in Baylor’s College of Arts & Sciences. “One of the explanations of this extra mass is that the metal from the asteroid that formed this crater is still embedded in the Moon’s mantle.”
“We did the math and showed that a sufficiently dispersed core of the asteroid that made the impact could remain suspended in the Moon’s mantle until the present day, rather than sinking to the Moon’s core,” James aded.
Another possibility is that the large mass might be a concentration of dense oxides associated with the last stage of lunar magma ocean solidification.
James said that the South Pole-Aitken basin is the largest preserved crater in the solar system. While larger impacts may have occurred throughout the solar system, including on Earth, most traces of those have been lost.
Last month, researchers released data showing that China’s Chang’e-4 mission, which explored part of the basin in January, may have found rocks from the moon’s mantle on the surface, which could provide scientists new insights into the processes that formed the moon.
The research was supported through the NASA Gravity Recovery and Interior Laboratory (GRAIL) science team. The study — ”Deep Structure of the Lunar South Pole-Aitken Basin” — is published in the journal Geophysical Research Letters.
The Daily Galaxy, Max Goldberg, via Baylor University