Astronomers led by Peter B. James from Baylor University discovered the hidden feature by combining data from NASA's GRAIL lunar orbiter mission and the Lunar Reconnaissance Orbiter to look at where regions of high gravity - and therefore mass - overlap with surface features like craters.
A previously unknown deposit of an unidentified physical substance larger than the size of Hawaii has been discovered beneath the surface of the moon.
We can't see it from here on Earth, but detailed readings made using lunar orbiters indicate there is something huge enough under that crater to be causing a significant gravitational anomaly.
However, we won't know for sure whether this is the case or not until we actually put some boots (or wheels) on the Moon to study the unknown mass in situ.
The Chinese lander Chang'e-4 and its Yutu-2 rover are now exploring the Von Karman crater within the South Pole-Aitken basin, and NASA also wants to target the South Pole for future exploration.
The research study titled "Deep Structure of the Lunar South Pole-Aitken Basin", was published last month in the journal Geophysical Research Letters. James and his team surmise it could be metal embedded in the Moon's mantle from the asteroid impact that caused the crater some estimated 4 billion years ago. Computer simulations of massive asteroid impacts back this up. 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".
"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", said James.
Another source of the unexpected mass, researchers say, could be a concentration of oxides that could have formed when the magma ocean that existed on the moon solidified. These impact basins are said to control the moon's geology.
Whatever formed the basin almost 4 billion years ago remains a mystery, but the blow was so strong that it likely punched all the way through the moon's crust and tossed part of the lunar mantle - a deeper geologic layer - onto the surface.
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