Recently a major gravitational anomaly in a crater on the moon detected by astronomers. Information gathered by NASA’s Lunar Reconnaissance Orbiter on the far side of the moon finds something huge is buried beneath it.
The Moon is home to one of the Solar System’s biggest known impact craters. At 2,500 kilometers (1,550 miles) across, nearly a quarter of the lunar surface is covered by the South Pole-Aitken Basin on the far side of the Moon-and something huge is buried beneath it.
From here on Earth, we can’t see it. Thorough measurements produced using lunar orbiters suggest that under that crater there is something big enough to cause a major gravitational anomaly.
“Imagine taking a stack of metal five times bigger than Hawaii’s Big Island and burying it underground,” said Baylor University’s geophysicist Peter James.
“That’s about how much we detected an unexpected mass.” The anomaly was found in two information sets. The first was from the GRAIL mission of NASA. A pair of spacecraft orbiting that mapped the gravitational field of the Moon in 2011 and 2012. It was an attempt to shed some light on its interior structure.
This information had already shown a gravitational anomaly. The basin had higher than average density relative to the remainder of the lunar surface. This was ascribed by the team to its iron-rich surface structure.
But when the team contrasted these findings with the lunar topography information gathered by NASA’s Lunar Reconnaissance Orbiter, the results showed something else. A mass of approximately 2.18 quintillion kilograms (that’s 18 zero numbers), extending more than 300 kilometers (184 miles) below the ground.
This mass, scientists think, weighs down the basin floor by more than 800 meters. About 10 percent of its complete depth. Explaining a depression earlier ascribed to the contraction in the bottom of the basin.
“One of the explanations for this extra mass is that the asteroid-forming metal is still embedded in the mantle of the Moon,” said James.
If conditions are right, the iron-nickel core of an impacting asteroid can be dispersed between the Moon’s crust and core, according to computer simulations.
That’s what might have occurred four billion years ago when the object that formed the basin crashed into the moon.
“We did the maths and showed that an adequately distributed nucleus of the asteroid that produced the effect could stay suspended in the mantle of the Moon until today. Instead of sinking into the heart of the Moon,” James said.
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Another possible explanation relates to volcanism, which was once a hotbed for the Moon. In the lunar mantle, there is a large concentration of titanium oxides. It is believed to have been generated by lunar magma ocean cooling and solidification.
These oxides have a lot of mass that could have been focused under the South Pole-Aitken Basin. Although it’s still to be explored ‘somehow’.
Whatever the explanation, some interesting things about Moon’s interior are revealed by the mass. We understand, for example, that the mass is not molten enough to sink to the center.
This implies an upper-temperature limit of around 1,480 degrees Celsius for the latter half of the lifespan of the Moon consistent with estimates based on seismology if the mass is from around the same time as the impact that made the basin.
This also means that the Moon’s lifespan has lost a lot of thermal energy, the team said. Perhaps the Yutu2 rover from China, presently creeping across the South Pole-Aitken Basin, could shed more light on the issue.
The study was released in the Geophysical Research Letters.