It is the largest preserved crater in the solar system.
When the research team combined the data with lunar topography results from the Lunar Reconnaissance Orbiter, the large mass was discovered, one which is big enough to weigh down the basin floor by a little less than a kilometre. According to the published study, "Plausible sources for this anomaly include metal from the core of a differentiated impactor or oxides from the last stage of magma ocean crystallization", which hypothesizes the moon's surface was once a molten liquid ocean of magma.
The South Pole-Aitken basin is a huge crater produced by an ancient impact on the Moon, whose longest axis would span from New York City to Omaha, Nebraska.
Yet its size and the fact that the anomaly appears to be located about 186 miles (300 km) down also offers scientists an intriguing idea: the moon's insides can't be all that gooey; if they were, the moon's gravity would pull the massive patch into the lunar centre.
The view of the South Pole-Aitken basin from Chang'e 4.
"Imagine taking a pile of metal five times larger than the Big Island of Hawaii and burying it underground", lead study author Peter James, assistant professor of planetary geophysics at Baylor University's College of Arts & Sciences, said in a statement. Beneath this basin lies a unusual anomaly-an excess of mass extending at least 300 kilometers down, more than 10 times the depth of the Earth's crust.
The other mission was the Gravity Recovery and Interior Laboratory (GRAIL), which involved two spacecraft - GRAIL A and GRAIL B - working in tandem to detect variations in the strength of the moon's gravitational field.
This odd dense mass is causing the basin floor to go down by more than half a mile and according to computer simulations of big asteroid impacts, it's possible that under the right conditions, an iron-nickel core of an asteroid can be distributed into the upper mantle (the layer sandwiched between the Moon's crust and core), during an impact.
An image of the lunar surface showing its various basins. If the mass was produced by an impact, its location around 400 kilometers southeast of the crater's center could help improve our knowledge of how impacts form craters, according to the paper.
If the mass is a metallic asteroid core, it didn't get stuck inside the moon intact; instead, computer simulations suggest it could have spread out as it struck.
Whatever the mass is, the team believes it can explain some of the features of the South Pole-Aitken basin-specifically that the central depression of the crater is being weighed down by this mass, rather than being caused by the contraction of the "melt sheet"-found where impacts take place".
Alternatively, the mass might be a dense region caused by the Moon's magma ocean solidifying as our satellite cooled and aged.
If the mass is from around the same time as the impact that made the basin, this implies an upper temperature limit of around 1,480 degrees Celsius for the latter half of the Moon's lifespan, consistent with estimates based on seismology.