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Telltale Clues: Solar System Impact Bombardment

Meteoritic samples of asteroid Vesta have been used to determine that it was hit by a flurry of other asteroids roughly four billion years ago. This sample of fragmented rock was produced by those impact events when they tore up basaltic lava flows and solidified subsurface magma chambers to produce an impact breccia. The sample is seen on a microscope used to study these types of meteoritic samples of the asteroid belt. Photograph by David Kring/LPI/CLSE.

Howardites and eucrites are meteorite classifications, both of which are providing surprising clues to solar system-wide impact bombardment billions of years ago.

New research unveils an unexpected link between asteroid Vesta and the Moon, providing new means for studying the early bombardment history of the terrestrial planets.

According to the Lunar and Planetary Institute (LPI) in Houston, Texas, researchers there have found that large asteroids — like Vesta — share a surprising history with Earth’s Moon. In fact, the heavily cratered surface of the Moon and that of Vesta were apparently produced by the same population of high-speed projectiles four billion years ago.

The radiometric ages of lunar rocks collected by Apollo astronauts have long been used to study the bombardment history of the Moon. Similarly, the impact reset ages of meteorites have been used to study the collisional history of main-belt asteroids.

In particular, howardite and eucrite meteorites (common species in meteorite collections) have been used to study asteroid Vesta, their parent body. Now, for the first time, an international team of researchers show that the same projectiles responsible for making craters and basins on the Moon were also hitting Vesta at very high velocities, enough to leave behind a number of telltale impact-related ages.

This research has been possible thanks to a multi-disciplinary work, including geochemistry, dynamics, simulations of impact events, and spacecraft observations. The findings were published in the March issue of Nature Geoscience in an article entitled “High Velocity Collisions from the Lunar Cataclysm Recorded in Asteroidal Meteorites.”

Unusually high-speed projectiles

With the aid of computer simulations, the researchers determined that meteorites from Vesta recorded impacts by unusually high-speed projectiles that are now long gone. They deduced that this period of bombardment was related to a possible dramatic time in solar system history four billion years ago when gas giant planets like Jupiter and Saturn were migrating from their original orbits to where we see them today.

The team’s findings support the theory that this repositioning of the gas giant planets destabilized portions of the asteroid belt and triggered a solar-system-wide bombardment of asteroids. This event, called the “lunar cataclysm,” pulled many asteroids into orbits that collided with Earth and the Moon.

The research provides new constraints on the start and duration of the lunar cataclysm, and it demonstrates that the cataclysm was an event that affected not only the inner solar system planets, but asteroid belts as well.

The team’s interpretation of the howardites and eucrites was augmented by recent close-in observations of Vesta’s surface by NASA’s Dawn spacecraft.

In addition, the team used the latest dynamical models of early main-belt evolution to find out the likely source of these high-velocity impactors, and determined that the same projectiles hitting Vesta also had orbits that struck the Moon at high speeds.
The impact study team consisted of S. Marchi, an NLSI postdoctoral researcher with W. Bottke (SwRI-CLOE) and D. Kring (LPI-CLSE); B. Cohen (NASA Marshall Space Flight Center, Huntsville, Alabama); H. McSween (University of Tennessee, Knoxville); D. O’Brien (Planetary Science Institute, Tucson, Arizona); P. Schenk (LPI); C. Raymond (Jet Propulsion Laboratory, Pasadena, California); C. Russell (University of California, Los Angeles, California); and international partners K. Wunnemann (Museum Fur Naturkunde, Berlin, Germany) and M. De Sanctis (Instituto Nazionale d’Astrofisica, Rome, Italy).

The Nature Geoscience paper is available online at:

www.nature.com/ngeo/journal/vaop/ncurrent/pdf/ngeo1769.pdf

By Leonard David via LPI Press Release

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