New study complicates theory that ancient impact pierced Moon's crust

New study complicates theory that ancient impact pierced Moon’s crust
Images show wrinkles in the Aitken Crater. Credit: NASA/GSFC/Arizona State University

The moon's largest and oldest impact crater likely doesn't have minerals from below the lunar crust on its surface, complicating a theory that an ancient massive impact event pierced the Moon's crust during the crater's formation, a new study finds.

A study published earlier this year analyzed the way reflect light to determine that a basin-forming impact that formed an ancient massive , the South Pole-Aitken basin, caused minerals from deep inside the Moon's mantle to rupture the Moon's surface. If mantle materials breached the lunar crust, studying them could yield significant clues about the Moon's history.

Now, new research in the AGU journal Geophysical Research Letters reexamined the same data, acquired by the Chinese spacecraft Chang'E 4's rover, which landed in the crater in January 2019. The new study finds the crater's crust mainly consists of a common lunar crustal not detected in earlier analyses. The new results suggest the basin floor may not have exposed lunar mantle material as previously reported.

"We are not seeing the mantle materials at the landing site as expected," said Hao Zhang, a planetary scientist at the China University of Geosciences, Wuhan, China, and a co-author of the new study.

The new study complicates theories about how the oldest, largest crater on Moon formed, adding to the body of knowledge about the Moon's history.

Dating the South Pole-Aitken basin

The South Pole-Aitken basin is considered one of the largest craters in the Solar System and the oldest on the Moon. The basin is 2,500 kilometers (1,553 miles) in diameter and runs roughly 13 kilometers (8 miles) deep. The basin resides on the Moon's far side, the enigmatic area facing away from Earth. It was untouched until Chang'E 4's landing in the crater in January 2019.

Although scientists haven't radiometrically dated the basin's age yet, some estimates place its formation at 4.2 billion years ago.

Scientists theorized the South Pole-Aitken basin-forming event ruptured the lunar crust, because of how deep the basin is today. Crustal topographic maps estimate the crust only extends 30 kilometers (19 miles) beneath the crater, whereas the rest of the lunar crust is 40 kilometers (25 miles) thick on average.

The Moon was once covered in molten magma oceans. Over time, these cooled and separated into crust and mantle layers distinguished by many characteristics, including their . Clinopyroxene, orthopyroxene, and olivine are all minerals associated with the Moon's mantle. They occasionally appear on the surface of the Moon, but large concentrations of them in a region could signal that the mantle once punctured the crust.

Testing the crustal composition

Spectroscopy is the study of how matter interacts with light. Minerals absorb specific wavelengths of light and color, which gives them unique signatures. Astrophysicists perform different types of spectroscopy to determine the composition and concentration of different materials on planetary bodies and their regions, based on these unique signatures.

Previous research published in May in the journal Nature found concentrations of clinopyroxene, orthopyroxene, and olivine in the crater—amounts high enough to seemingly confirm the theory that the mantle had once breached the crust. The Nature study analyzed spectroscopic soil data from Chang'E 4 and processed the data using a series of functions. This process allowed them to identify the mathematically best fitting mineral for each's spectra compositions.

Zhang and his colleagues also analyzed spectroscopic data acquired by instruments on Chang'E 4's rover after the spacecraft landed in the crater. They used a technique that compared the rover's documented reflections of light and color from the lunar surface to a database of known minerals. The database accounted for minerals' , the way the minerals interact with light, and how they respond to space weathering—changes to the soil surface caused by solar wind irradiation and bombardment from tiny particles that the Moon's surface experiences.

This different process allowed the researchers to detect and measure the amount of plagioclase in the crater. Plagioclase is a mineral created from cooling lava. It's also one of the most common rocks on the Moon's surface. The results showed plagioclase made up 56-72% of the crater's composition, making it the majority mineral. The high concentration of plagioclase suggests the was not pierced by an ancient impact.

The new study also found the on the crater had concentrations of 9-28% orthopyroxene, 4-19% clinopyroxene, and 2-12% olivine. Although The three minerals are in the basin, they are not present at high enough amounts to prove an impact event once broke the , according to the study's authors.

The new study complicates the certainty of earlier findings and points towards a need for continued research on the far side's lunar surface, according to Zhang.


Explore further

China's Chang'E 4 mission discovers new 'secrets' from the far side of the moon

More information: Xiaoyi Hu et al. Mineral Abundances Inferred From In Situ Reflectance Measurements of Chang'E‐4 Landing Site in South Pole‐Aitken Basin, Geophysical Research Letters (2019). DOI: 10.1029/2019GL084531
Journal information: Geophysical Research Letters , Nature

This story is republished courtesy of AGU Blogs (http://blogs.agu.org), a community of Earth and space science blogs, hosted by the American Geophysical Union. Read the original story here.

Citation: New study complicates theory that ancient impact pierced Moon's crust (2019, September 24) retrieved 20 October 2019 from https://phys.org/news/2019-09-complicates-theory-ancient-impact-pierced.html
This document is subject to copyright. Apart from any fair dealing for the purpose of private study or research, no part may be reproduced without the written permission. The content is provided for information purposes only.
208 shares

Feedback to editors

User comments

Sep 26, 2019
More here [ https://www.lives...ned.html ]:

"This mineral makeup complicates the theory that a giant, high-velocity meteor created the South Pole‐Aitken basin billions of years ago, as such an impact almost certainly would have scattered chunks of mantle over the lunar surface.

So, what, then, created the crater? The researchers did not speculate in the new study. However, prior research has suggested that a renegade space rock is still the culprit, but the hit may not have been so direct. A study published in 2012 in the journal Science [ https://science.s...073/1212 ] argued that a slightly slower-moving meteor could have struck the back of the moon at an angle of about 30 degrees and resulted in an appropriately large crater that never disturbed the moon's mantle. However, those researchers had only simulations to go on."

That would explain also the lunar crust magnetic anomalies, see the Science link.

Sep 27, 2019
The similarity in composition of the Earth and Moon has been a challenge to the impactor theory of its formation, from debris of the impactor. But, what if the impactor passed through the Earth and the Aitken basin is the artifact of the point at which they separated. So, imagine the impactor passing through a viscous Earth. As it exits there remains a circular connection of matter that stretches and eventually breaks. That could be mistaken for a giant crater, for which no impact debris exists.

Sep 30, 2019
Further speculation based on previous post and pdf from first post:
- Difference in mantle smoothness and thickness between moon faces. This appears to vary at a gradient relative to an axis that passes through the basins centre, with the current far side affected more by Earth's gravity, thus drawing matter away from the near side. Thus far side thicker and rougher and near side thinner and smoother. Surface opposite the basin, the smoothest and thinnest?
- Dense blob under the basin https://www.natio...e/#close
Think connection matter at this point being from the Earth's denser core. Which eventually sinks below the surface, possibly drawing the surrounding area down with it; smooth deep basin.

Oct 01, 2019
Now, I'm spending way too much time on this. Anyway a couple of links -
https://pdfs.sema...7751.pdf
https://threadrea...072.html

It's "interesting", how, committed everyone is to the existing moon formation hypothesis, despite significant contradicting evidence. Which, they either try to explain away or ignore with astonishment. Such as:
- computer models indicate that an impact of that size should have completely excavated the crust along with a good chunk of the upper mantle.
- we've found a huge mass excess in the Moon's mantle under the southern portion of the SPA basin. That section of the mantle has at least *2 quadrillion metric tons* more mass than normal. We did the math to show that a sufficiently dispersed impactor core could remain suspended in the Moon's mantle until the present day, rather than sinking to the Moon's core.
/tbc...

Oct 01, 2019
/..con't
Well, If the evidence appears to disprove an impact, then perhaps there wasn't one. That unimaginable mass did not break the mantle and sink to the core, because the mantle is probably the solid surface of the impactor that survived its collision with the Earth.

Oct 01, 2019
One final post. I would suggest that the Moon formed out of a solid spherical extrasolar [obviously] object whose solid inner core survived its impact with, and passed through a very young Earth, after orbiting [greater velocity] the Sun. Oval shape and topography of the Aitken basin, suggests a twist in the object as it separated from the Earth. This sounds less farfetched than the convoluted explanations for the current theory and interestingly combines features of it and Darwin's theory.
/The end
/PS If this turns out to be true, then I get naming rights.

Oct 01, 2019
The crater was likely formed by electric discharge processes, that the largest crater is at a pole shouldn't be too surprising.

Oct 02, 2019
Well, I am proposing the object passed through the earth and separated from it at the basin. So, static discharge at that point, I cannot rule out, but I would be sceptical of it playing a dominant role in creating the features.

Oct 10, 2019
Further to my posts above--
The moon's face is approximately the size of Australia. I would therefore suggests that the 2 continent sized regions of rock described here - https://phys.org/...hed.html are remnants of the impactor's surface, that were sheared off as it passed through the primitive Earth. I believe this would better explain their age, size, survival..etc.

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more