The hottest of the new giant-impact simulations builds an intact Moon in about five hours

Researchers at the Southwest Research Institute and the University of Arizona used computer simulations to examine the impact thought to have occurred about 4.5 billion years ago between the young Earth and a Mars-sized object; the findings were published in The Astrophysical Journal Letters.

For the first time, the simulations account for the material strength of the two ancient worlds.

The new simulations found that some impacts create the familiar disk of debris that later becomes the Moon, while others can produce an intact Moon in only about five hours.

The main factor in the strength of the colliding bodies is temperature; both Earth and Theia were still hot with soft or molten surfaces, properties that would have cushioned the collision, allowing the debris to stay largely intact and enabling the moon to form more quickly.

If the bodies had cooled and become more brittle, their collision would have created a larger field of rocky debris that would have taken longer to coalesce.

The addition of material strength could reshape scientists' understanding of the Moon-forming collision and may also help narrow down when it happened.

The new simulations highlight the importance of material strength in planetary impact modeling, which could change what is known about other ancient collisions, like the one that knocked Jupiter's moon Ganymede off its axis or the one that birthed Saturn's moon Titan.

Adeene Denton is a geologist and planetary scientist at the Southwest Research Institute in Boulder, Colorado, and is first author of the study.

A foundational 2001 study of the giant impact scenario was led by Robin Canup, vice president of SwRI's Solar System Science and Exploration Division in Boulder, Colorado, and Erik Asphaug, a professor at the Lunar and Planetary Laboratory and co-author of the new study.

Those early simulations, along with many later models, did not account for material strength; scientists assumed the collision was so energetic that the rocky bodies could effectively be treated like fluids.

"Because the collision was considered violent enough to melt and vaporize large portions of Earth and Theia, those previous papers assumed that it is okay to approximate them as fluids," Asphaug said.

"Based on our new results, however, we think that it is time to reconsider that," Asphaug said.

The version of smoothed particle hydrodynamics used in the study was developed at the University of Arizona and the University of Bern in Switzerland; its strength model allows simulated planetary material to resist deformation in ways more consistent with real geologic substances, including the rock and metals that likely made up Theia and the proto-Earth, or solid ice.

Researchers simulated the collision using a range of potential temperatures and found that under the most extreme scenario, where the pair were at their hottest, the moon could have formed in just five hours; there is no way of knowing exactly how hot, and therefore how strong, Earth and Theia really were.

Apollo samples indicate the moon is around 4.5 billion years old, meaning the collision occurred roughly 100 million years after the sun was born.

When the moon took shape it was initially covered with a magma ocean that took hundreds of millions of years to solidify.

Neither the University of Arizona announcement nor the Live Science account reports the number of simulation runs, the temperature range used, or whether the intact-Moon outcomes reproduce the Moon's measured mass.

That places the sun's formation at about 4.6 billion years ago.

Live Science states that when the giant impact hypothesis was first proposed, in 2001, researchers initially thought the collision occurred much later, meaning the bodies would have cooled and become more brittle.