Under the South Pole-Aitken (SPA) basin, near the south pole of the Moon, something massive — and unknown — is hiding beneath the crust. A recently-discovered gravitational anomaly reveals the presence of a massive deposit of material sitting within the mantle of the Moon, undetected before now. Baylor University researchers discovered this mass in the mantle of the Moon, between the crust and core of our planetary neighbor. The team theorizes it may be the remains of an asteroid that crashed long ago, forming the massive Aitken Crater.
The far side of the Moon, seen in a topgraphic map. Blue areas are lower elevations, while mountains are seen in red. The dashed circle highlights the site of the gravitational anomaly. Image credit: NASA/Goddard Space Flight Center/University of Arizona
“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 Dr. Peter James, assistant professor of planetary geophysics in Baylor’s College of Arts and Sciences.
There’s No Crater Place The South Pole-Aitken basin stretches 2,000 kilometers (1,250 miles) in length, from the Aitken Crater to the South Pole of the Moon, providing the feature with its name. If placed on the United States, this feature would reach from Washington, D.C. to Waco, Texas. Around four billion years ago, a massive impact created an impact site on the Moon reaching more than eight kilometers (five miles) into the lunar surface, four times deeper than the Grand Canyon. Unfortunately for skygazers here on Earth, this feature is out of sight for us, as the massive crater sits on the far side of the Moon. Utilizing data complied by the Gravity Recovery and Interior Laboratory (GRAIL) mission, combined with a lunar topographic map, researchers uncovered evidence of an enormous — something — beneath the lunar crust. This unexplained mass is weighing the basin down, lowering its depth by approximately 800 meters (half a mile).
Dr. Peter James, assistant professor of planetary geophysics at Baylor University. Image credit: Baylor University
“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. 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,” James stated. This hypothesis is consistent with computer simulations suggesting significant portions of the core of a nickle/iron asteroid might be left behind in the lunar mantle during a powerful impact between an asteroid and the Moon.
A monochrome mosaic showing the Aitken Crater, relative to the south pole of the Moon. Image credit: NASA/GSFC/Arizona State University
This Story Needs a Prequel The South Pole-Aitken basin is four billion years old, roughly as old as life on Earth. Impacts more powerful than the one which formed the crater have impacted planets and moons around our solar system in the past. However, few of these (as far as astronomers are aware) have been as well-preserved as these structures. Investigation of this powerful ancient impact could lead to new insights on the early days of our solar system, at a critical time during the formation of the Solar System. “Lunar samples suggest that most of the major basins on the moon formed around 3.9 billion years ago in a period called the late heavy bombardment. By this time most of the large debris within the solar system should have already accreted to form the planets, so such a large number of big impacts occurring at nearly the same time may have been due to unusual gravitational dynamics in the early Solar System,” The Lunar Reconnaissance Orbiter (LRO) team explains. Analysis of the region could lend evidence — or help refute — the possibility of a massive reign of bodies colliding with the planets of the inner solar system long ago. Such an event would have likely had a significant influence on the early formation of Earth, as well as the young Moon. A giant impactor may not be the only explanation for this new discovery, researchers suggest. A second idea holds that this large deposit in the lunar mantle may be left over from the ancient solar system. The detection of this unexplained mass in the lunar mantle could also be evidence of oxides created from the solidification of ancient seas of magma on the Moon, investigators postulate.
I Think I’ve Seen this Movie Before… Currently, NASA is aiming to place human beings on the Moon once again, a goal scheduled for completion by 2024. The next crewed mission to the Moon for the agency is scheduled to touch down near the south pole of our lunar companion.
At least it’s not an obelisk. Probably. Video clip from 2001: A Space Odyssey, Warner Bothers Productions
“…Moon-Watcher felt the first faint twinges of a new and potent emotion. It was a vague and diffuse sense of envy — of dissatisfaction with his life. He had no idea of its cause, still less of its cure; but discontent had come into his soul, and he had taken one small step toward humanity.” ― Arthur C. Clarke, 2001: A Space Odyssey
Nations and corporations are vying to be the next to land humans on the lunar surface. NASA is preparing the Space Launch System (SLS), Orion spacecraft, and the Lunar Gateway. Their plan aims for long-term human habitation of the Moon, readying for the next step of landing people on Mars. Recently, China has announced plans to build shelters for travelers to the south pole of the Moon, using robotic 3D printers. Meanwhile, corporations like SpaceX have their own plans to place people on the Moon and Mars. As humans journey to space, the Moon is a necessary first step. Without reliable transportation to the lunar surface, there is no colonization of Mars, or the rest of the Solar System. The Moon still holds some mysteries for those inquisitive enough to look to the future, but this study could bring us a little but closer to our new home.
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