Lunar Peaks May Be Spattered With Exotic Asteroid Fragments

Olivine and spinel have been found in the central peaks of the 58-mile-wide lunar crater Copernicus. See closeup photo below. Credit: Paolo R. Lazzarotti

Minerals seen in some of the moon’s craters may not belong to the moon at all but instead were likely delivered by asteroids or possibly even the Earth. Unusual minerals like spinel (ruby-like red gemstone) and olivine (olivine-green gemstone) have been found on the floors and especially in the central peaks of several larger lunar craters including the familiar Tycho, Copernicus and Theophilus using instruments like NASA’s Moon Mineralogy Mapper. Scientists assumed they were seeing material excavated from deep below the moon’s surface.

A bird’s eye view of some of the Copernican mountain peaks photographed by the Lunar Reconnaissance Orbiter. Peaks form during rebound of the crater’s floor after impact. Copernicus is about 900 million years old. Click to enlarge. Credit: NASA

Maybe not. These very same minerals are also common in meteorites. A recently published study in the journal Nature Geoscience by a team of scientists from the U.S. and China used computer models to fling simulated asteroids and meteorites at the moon with speeds under 26,820 mph (43,160 kph). Some 30% of asteroidal debris striking the moon travels below that speed according to the study.

Tycho, one of the most prominent craters on the moon, is 51 miles (82 km) across and formed when a small asteroid struck the moon relatively recently – only 110 million years ago. Like Copernicus, olivine has been found atop its central peak. Credit: William Wiethoff

When these slower-moving space rocks slam into the moon, the researchers found that fragments survived the impact. If the newly formed crater was 12 miles (20 km) wide or larger, asteroid material sent flying outward toward the crater’s rim would later fall back through gravity into the crater’s central peak. Peaks form in big craters when material that’s crushed and compacted by the incoming asteroid rebounds or rises back up in the crater’s center after impact.

An LRO photograph of Tycho’s 1.2-mile-high (2 km) central peak. The mountain complex measures 9.3 miles (15 km) from side to side. Resting near the summit is a large boulder 400 feet (120 m) wide. Click to enlarge. Credit: NASA Goddard/Arizona State University

“This observation may explain recent observations of exotic Mg-rich spinels and olivine in the central peaks of craters too small to have excavated the deep crust or mantle of the Moon,” they wrote. By extension, the team suggests that crater peaks on Mars and Vesta may also preserve remnants of exotic minerals delivered by asteroids.

A tighter view of the boulder in the photo above. The scene is 3/4 mile (1.2 km) wide. How such a large object ended up intact atop a mountain isn’t know for certain, but it probably rolled out of the impact debris forming the rising summit. Click to enlarge. Credit: NASA Goddard/Arizona State University

It’s generally assumed meteorites vaporize upon impact and leave only tiny fragments in crater floors, but if the impactor moves below a critical speed, the results of the study show it can leave bigger pieces. That means scientists must be cautious when deciding if the rock in the moon’s peaks really do represent samples excavated from deep down in the moon’s mantle or whether they’re alien rocks left by potshot asteroids.

Steps in the formation of a crater’s central peak. Small impact make simple, bowl-shaped craters; larger ones have peaks. Credit: JAXA

More intriguing is the possibility that some of those olivines and other exotic minerals came from Earth. Our planet got whacked as much or more than the moon several billion years back. More than 170 named lunar meteorites have been found on Earth, and studies have shown that delivery of “Earth meteorites” to the moon via impact is easily accomplished. You never know – there may even be the hardened, glassy remains of stromatolites, one of the planet’s earliest life forms dating from as early as 3.5 billion years ago, sparkling atop some lunar mountain. In simulations, materials leaving Earth would have melted on the outside but remained intact within.

1 Response

  1. aduluthian

    Extremely interesting! That little boulder looks awfully guilty sitting in the middle of the crater like that…

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