Scientists spot 'anomaly' beneath the moon's largest crater

Federico Mansilla
Junio 12, 2019

"When we combined [the gravitational data] 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", Dr James said. Instead, the researchers believe the dense mass is actually leftover material from the object that struck the Moon and created the colossal crater. "That's roughly how much unexpected mass we detected", said lead author Peter B. James, Ph.D., assistant professor of planetary geophysics in Baylor's College of Arts & Sciences. Despite its size, it can not be seen from Earth because it is on the far side of the Moon.

When it comes to the South Pole-Aitken basin, the topography is particularly striking.

It sits 180 miles beneath the South Pole-Aitken basin - one of the solar system's largest impact craters, and the moon's oldest, at over 4 billion years - a massive dent spanning some 1,550 miles on the far side of the moon.

James is one of a handful of U.S. scientists who announced their discovery in a study published in the journal Geophysical Research Letters. It is one of the biggest impact craters of the known solar system but, because it is located on the far side of the moon, can not be viewed directly from Earth. 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 dashed circle shows the location of the mass anomaly under the basin.

The mass of the metal "anomaly" beneath the moon's largest crater is five times greater than the big island of Hawaii, and according to a new study from scientists at Baylor University, it could contain metals remaining from an ancient asteroid impact, weighing in at around 4.8 quintillion pounds.

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The gravitational force of "whatever it is, wherever it came from", James said, is so great that it drags down the floor of the basin by more than 800m.

The mystery mass, circled, is beneath an impact crater on the far side of the moon.

An image of the lunar surface showing its various basins.

The team ran complicated computer simulations of large asteroid impacts which suggested that - under the right conditions - an asteroid which had an iron-nickel core could have dispersed into the moon's upper mantle during an impact.

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". Or, intriguingly, the extra mass could suggest the presence of an enormous metal core deposited in the Moon mantle, left over from the impact.

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