Guest geological note by David Middleton
Chang’e-4: Chinese rover ‘confirms’ Moon crater theory
By Paul Rincon
Science editor, BBC News website
15 May 2019
The Chinese Chang’e-4 rover may have confirmed a longstanding idea about the origin of a vast crater on the Moon’s far side.
The rover’s landing site lies within a vast impact depression created by an asteroid strike billions of years ago.
Now, mission scientists have found evidence that impact was so powerful it punched through the Moon’s crust and into the layer below called the mantle.
Chang’e-4 has identified what appear to be mantle rocks on the surface.
It’s something the rover was sent to the far side to find out.
Chunlai Li, from the Chinese Academy of Sciences in Beijing, and colleagues have presented their findings in the journal Nature.
The rover landed inside a 180km-wide impact bowl called Von Kármán crater. But that smaller crater lies within the 2,300km-wide South Pole Aitken (SPA) Basin, which covers nearly a quarter of the Moon’s circumference.
Early results from the rover’s Visible and Near Infrared Spectrometer (VNIS) suggest the rocks contain minerals known as low-calcium (ortho)pyroxene and olivine.
They fit the profile of rocks from the lunar mantle and suggest that the ancient impact that created the SPA drove right through the 50km-deep crust into the mantle.
In other Lunar geology news…
The Moon appears to have active, relatively young, thrust fault systems.
The moon is still geologically active, study suggests
May 13, 2019
We tend to think of the moon as the archetypal “dead” world. Not only is there no life, almost all its volcanic activity died out billions of years ago. Even the youngest lunar lava is old enough to have become scarred by numerous impact craters that have been collected over the aeons as cosmic debris crashed into the ground.
Hints that the moon is not quite geologically dead though have been around since the Apollo era, 50 years ago. Apollo missions 12, 14, 15 and 16 left working “moonquake detectors” (seismometers) on the lunar surface. These transmitted recorded data to Earth until 1977, showing vibrations caused by internal “moonquakes”. But no one was sure whether any of these were associated with actual moving faults breaking the surface of the moon or purely internal movements that could also cause tremors. Now a new study, published in Nature Geoscience, suggests the moon may indeed have active faults today.
Another clue that something is still going on at the moon came in 1972 when Apollo 17 astronauts Gene Cernan and Jack Schmitt inspected a step in the terrain, a few tens of metres high, that they called “the Lee-Lincoln scarp”. They, and their team of advisers back on Earth thought it might be a geological fault (where one tract of crustal rock has moved relative to another), but they weren’t sure.
It is now widely agreed that these are thrust faults, caused as the moon cools down from its hot birth. As it does, “thermal contraction” causes its volume to shrink and compresses the surface. That means that the moon is shrinking slightly. However, thrust faults don’t necessarily have to be active and moving, causing more further tremors. The same thing has been happening on Mercury on a far grander scale, where the planetary radius has shrunk by 7km during the past 3m years. There, the biggest scarps are nearly a hundred times larger than those on the moon.
Analysis shows that these faults are relatively young, not older than about 50m years. But are they active and still moving today? In the new study, Tom Watters of the Smithsonian Institution in the US and colleagues employed a new way to pinpoint the locations of the near-surface moonquakes in the Apollo data more precisely than was previously possible.
The team discovered that of the 28 detected shallow quakes, eight are close to (within 30km of) fault scarps, suggesting these faults may indeed be active.
To capitalize, or not to capitalize
Capitalize the names of planets (e.g., “Earth,” “Mars,” “Jupiter”). Capitalize “Moon” when referring to Earth’s Moon; otherwise, lowercase “moon” (e.g., “The Moon orbits Earth,” “Jupiter’s moons”). Capitalize “Sun” when referring to our Sun but not to other suns. Do not capitalize “solar system” and “universe.” Another note on usage: “Earth,” when used as the name of the planet, is not preceded by “the”; you would not say “the Neptune” or “the Venus.” When “earth” is lowercased, it refers to soil or the ground, not the planet as a whole. Do use “the” in front of “Sun” and “Moon” as applicable.NASA
And… It’s NASA, not Nasa… 😉