From Sky & Telescope : “Two Impacts-Not Just One-May Have Formed The Moon” 

From Sky & Telescope

October 14, 2021
Asa Stahl

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In this image, the proposed hit-and-run collision is simulated in 3D, shown about an hour after impact. Theia, the impactor, barely escapes the collision. A. Emsenhuber / The University of Bern [Universität Bern](CH) / The Ludwig Maximilians University of Munich [Ludwig-Maximilians-Universität München](DE).

Scientists have long thought that the Moon formed with a bang, when a protoplanet the size of Mars hit the newborn Earth. Evidence from Moon rocks and simulations back up this idea.

But a new study suggests that the protoplanet most likely hit Earth twice. The first time, the impactor (dubbed “Theia”) only glanced off Earth. Then, some hundreds of thousands of years later, it came back to deliver the final blow.

The study, which simulated the literally Earth-shattering impact thousands of times, found that such a “hit-and-run return” scenario could help answer two longstanding questions surrounding the creation of the Moon. At the same time, it might explain how Earth and Venus ended up so different.

The One-Two Punch

“The key issue here is planetary diversity,” says Erik Asphaug (The University of Arizona (US)), who led the study. Venus and Earth have similar sizes, masses, and distances from the Sun. If Venus is a “crushing hot-house,” he asks, “why is Earth so amazingly blue and rich?”

The Moon might hold the secret. Its creation was the last major episode in Earth’s formation, a catastrophic event that set the stage for the rest of our planet’s evolution. “You can’t understand how Earth formed without understanding how the Moon formed,” Asphaug explains. “They are part of the same puzzle.”

The new simulations, which were published in the October Journal of Planetary Sciences, put a few more pieces of that puzzle into place.

The first has to do with the speed of Theia’s impact. If Theia had hit our planet too fast, it would have exploded into an interplanetary plume of debris and eroded much of Earth. Yet if it had come in too slowly, the result would be a Moon whose orbit looks nothing like what we see today. The original impact theory doesn’t explain why Theia traveled at a just-right speed between these extremes.

“[This] new scenario fixes that,” says Matthias Meier (Natural History Museum, Switzerland), who was not involved in the study. Initially, Theia could have been going much faster, but the first impact would have slowed it down to the perfect speed for the second one.

The other problem with the original impact theory is that our Moon ought to be mostly made of primordial Theia. But Moon rocks from the Apollo missions show that Earth and the Moon have nearly identical compositions when it comes to certain kinds of elements. How could they have formed from two different building blocks?

“The canonical giant-impact scenario is really bad at solving [this issue],” Meier says (though others have tried).

A hit-and-run return, on the other hand, would enable Earth’s and Theia’s materials to mix more than in a single impact, ultimately forming a Moon chemically more similar to Earth. Though Asphaug and colleagues don’t quite fix the mismatch, they argue that more advanced simulations would yield even better results.

Earth vs. Venus

Resolving this aspect of the giant-impact theory would be no mean feat. But Asphaug’s real surprise came when he saw how hit-and-run impacts would have affected Venus compared to Earth.

“I first thought maybe there was a mistake,” he recalls.

The new simulations showed that the young Earth tended to pass on half of its hit-and-runners to Venus, while Venus accreted almost everything that came its way. This dynamic could help explain the drastic differences between the two planets: If more runners ended up at Venus, they would have enriched the planet in more outer solar system material compared to Earth. And since the impactors that escaped Earth to go on to Venus would have been the faster ones, each planet would have experienced generally different collisions.

This finding flips the original purpose of the study on its head. If Venus suffered more giant impacts than Earth, the question would no longer be “why does Earth have a moon?” but “why doesn’t Venus?”

Perhaps there was only one hit-and-run event, the one that made our Moon. Perhaps there were many, but for the same reason that Venus collected more impacts than Earth, it also accreted more destructive debris, obliterating any moon it already had. Or perhaps the last of Venus’ impacts was just particularly violent.

Finding out means taking a trip to Venus. That would provide “the next leap in understanding,” Meier says. If Earth and Venus both had hit-and-runs, for example, then the surface of Venus ought to be more like Earth’s than previously expected. If Venus has the same chemical similarities as the Moon and Earth, that would throw out the giant-impact theory’s last remaining problem.

“Getting samples from Venus,” Asphaug concludes, “is the key to answering all these questions.”

See the full article here .

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Sky & Telescope, founded in 1941 by Charles A. Federer Jr. and Helen Spence Federer, has the largest, most experienced staff of any astronomy magazine in the world. Its editors are virtually all amateur or professional astronomers, and every one has built a telescope, written a book, done original research, developed a new product, or otherwise distinguished him or herself.

Sky & Telescope magazine, now in its eighth decade, came about because of some happy accidents. Its earliest known ancestor was a four-page bulletin called The Amateur Astronomer, which was begun in 1929 by the Amateur Astronomers Association in New York City. Then, in 1935, the American Museum of Natural History opened its Hayden Planetarium and began to issue a monthly bulletin that became a full-size magazine called The Sky within a year. Under the editorship of Hans Christian Adamson, The Sky featured large illustrations and articles from astronomers all over the globe. It immediately absorbed The Amateur Astronomer.

Despite initial success, by 1939 the planetarium found itself unable to continue financial support of The Sky. Charles A. Federer, who would become the dominant force behind Sky & Telescope, was then working as a lecturer at the planetarium. He was asked to take over publishing The Sky. Federer agreed and started an independent publishing corporation in New York.

“Our first issue came out in January 1940,” he noted. “We dropped from 32 to 24 pages, used cheaper quality paper…but editorially we further defined the departments and tried to squeeze as much information as possible between the covers.” Federer was The Sky’s editor, and his wife, Helen, served as managing editor. In that January 1940 issue, they stated their goal: “We shall try to make the magazine meet the needs of amateur astronomy, so that amateur astronomers will come to regard it as essential to their pursuit, and professionals to consider it a worthwhile medium in which to bring their work before the public.”