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Two-stage process formed moon, simulations suggest

Chemicals missing from lunar rocks may be hiding under crust

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11:55am, November 13, 2015
illustration of planets colliding

ONE-TWO PUNCH  A two-stage moon formation, following a collision between Earth and a smaller planet (illustrated), might explain the absence of certain elements in lunar rocks.

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OXON HILL, Md. — Rocks on Earth and the moon are nearly identical — except when they’re not. Now new computer simulations might be close to figuring out why lunar samples are in many ways chemically identical to counterparts on Earth and yet missing a few key ingredients.

Easily vaporized elements, known as volatiles, are largely missing from moon rocks but might be sequestered deep in the lunar interior. This core is hidden beneath a crust that accumulated in a second phase of moon formation, planetary scientist Robin Canup reported November 11 at a meeting of the American Astronomical Society’s Division for Planetary Sciences.

Volatiles such as sodium and zinc were long assumed to have been blasted away in Earth’s collision with a Mars-sized planet that formed the moon roughly 4.5 billion years ago. “People made this assertion for decades,” said Canup, of the Southwest Research Institute in Boulder, Colo. “But it doesn’t work very well.”

Even in the disk of molten and vaporized rock that hung around after the impact, the temperatures weren’t high enough to launch volatiles away from Earth, Canup said. The atoms must have instead stayed local but somehow avoided the moon.

“We are in a strange situation where some things match perfectly and some things do not match at all,” says Sébastien Charnoz, a planetary scientist at the Institute of Earth Physics of Paris. “So you have to find a strange scenario to explain it all.”

Canup and colleagues ran computer simulations that tracked how Earth’s temporary ring evolved over time. They found that, initially, material in the outer part of the ring, far enough away to not be strongly affected by Earth’s gravity, quickly cooled and condensed into a ball roughly half as massive as the moon. Gravitational interactions sent this protomoon slowly drifting away from Earth, a journey it’s still on today.

The inner part of the ring is a bit more complicated. “It’s a very beautiful system,” says Charnoz. A molten river of rock nearly as bright as the sun encircled the planet, sandwiched between layers of gas. The liquid ring would have tended to spread while the gas — where the volatile elements ended up — stayed put. “This is a key idea,” Charnoz says. “You have a physical mechanism to separate the two things.”

As the liquid disk spread away from Earth, the outer edges cooled and formed volatile-depleted pebbles, which were then snagged by the moon’s gravity. Drawing from the molten ring, the moon slathered itself in a crust, possibly hundreds of kilometers thick, devoid of the elements that are still missing today.

By the time the remainder of the disk cooled enough for the gas and liquid to intermingle, Canup says, the moon had drifted too far from Earth to harvest the remaining pieces. Those, presumably, fell back to Earth — though Charnoz notes that the true fate of the ring is still a mystery.

Charnoz developed similar simulations with different underlying algorithms. “We both decided — independently — to tackle this problem,” Charnoz says. “We seem to converge to a similar solution but with different tools.”

Some lunar samples seem to support the idea that the missing volatiles are buried inside the moon, Canup says. A few rocks brought back during Apollo 15 and 17 contain volcanic glass (SN: 6/29/13, p. 8). These rocks, dredged up from deep within the moon, contain traces of a well-known highly volatile molecule: water.

Citations

R.M. Canup et al. Lunar volatile depletion due to incomplete accretion within an impact-generated disk. 47th meeting of the American Astronomical Society’s Division for Planetary Sciences, Oxon Hill, Md., November 11, 2015.

S. Charnoz, L. Bugnet and C. Michaut. Moon formation coupled with the protolunar disk. 47th meeting of the American Astronomical Society’s Division for Planetary Sciences, Oxon Hill, Md., November 11, 2015.

Further Reading

H. Thompson. The moon is about as old as we thought it was. Science News Online, April 17, 2015.

A. Grant. Moon's water may have earthly origins. Science News. Vol. 183, June 29, 2013, p. 8.

R.M. Canup et al. Lunar volatile depletion due to incomplete accretion within an impact-generated disk. Nature Geoscience. Published online November 9, 2015. doi: 10.1038/ngeo2574.

S. Charnoz and C. Michaut. Evolution of the protolunar disk: Dynamics, cooling timescale and implantation of volatiles onto the Earth. Icarus. Vol. 260, November 1, 2015, p. 440. doi:10.1016/j.icarus.2015.07.018.

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