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The South Pole-Aitken Basin – the largest crater in the solar system – is a gigantic impact structure located on the other side of the Moon. NASA lunar satellite data indicate significant excess mass – about 2.18 * 1018 kg – in the lunar mantle under the pelvis. According to new research, this mass anomaly could contain metal from a massive asteroid that crashed into the moon and formed the crater.
The South-Aitken Pole Basin, located on the other side of the moon, is one of the largest and oldest impact elements of the solar system. This is easily seen in the altitude data. The center down is dark blue and purple. The mountains on its edge, the remains of the outer rings, are red and yellow. Image Credit: NASA / GSFC / University of Arizona.
"Imagine taking a pile of metal five times larger than the Big Island of Hawaii and burying it underground. That's about the unexpected mass we've detected, "said Dr. Peter B. James, a planet scientist at Baylor University and Lunar and Planetary Institute.
The South Pole-Aitken basin is oval and can reach 2,500 km deep and 13 km deep. Despite its size, it can not be seen from the Earth because it is on the other side of the Moon.
To measure the subtle changes in gravitational force around the Moon, James and his colleagues analyzed data from NASA's GRAIL (Gravity Recovery and Interior Laboratory) mission.
"When we combined this data with lunar topography data from NASA's Lunar Reconnaissance Orbiter, we discovered an unexpected mass of hundreds of kilometers under the South Pole-Aitken Basin," said Dr. James.
"One of the explanations for this extra mass is that the metal of the asteroid that formed this crater is still embedded in the mantle of the moon."
"The dense mass – whatever it is, from wherever it comes – weighs the bottom of the basin more than one kilometer down."
Computational computer simulations of large asteroids suggest that, under good conditions, an iron-nickel nucleus of an asteroid can be scattered in the upper lunar mantle during an impact.
"We did the math and showed that a sufficiently dispersed asteroid nucleus that caused the impact could remain suspended in the lunar mantle until today, instead of sinking into the lunar core," he said. Dr. James.
Another possibility is that the large mass is a concentration of dense oxides associated with the last stage of solidification of the ocean by lunar magma.
"The South Pole-Aitken Basin, which is thought to have been created about 4 billion years ago, is the largest preserved crater in the solar system," said Dr. James.
"This is one of the best natural laboratories for studying catastrophic events, an ancient process that has shaped all the planets and rocky moons we see today."
The results appear in the log Geophysical Research Letters.
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Peter B. James et al. Deep structure of the south lunar basin of the Aitken pole. Geophysical Research Letters, published online June 5, 2019; doi: 10.1029 / 2019GL082252
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