News Notes Editor's note: this story first appeared online on May 10, 2004.
Planetary geology
Big impact for a small Moon mineral

Without atmospheres to protect them, planets, moons and other orbiting bodies are open to all manner of assault — from the impact of a kilometer-wide rock to the steady barrage of high-energy protons from the solar wind. Even the smallest fragments of space debris can inflict damage. And when micrometeorites are traveling at speeds over 100,000 kilometers per hour, the force can be dramatic.

Now researchers have identified a new mineral from the Moon that may have been produced during such a micrometeorite attack. Dubbed hapkeite, after planetary geologist Bruce Hapke of the University of Pittsburgh, the crystalline material made of iron and silicon reveals new details about space weathering processes and may help researchers better understand remote observations of the Moon and its brethren.

The research team identified the mineral within a meteorite recently discovered in the Dhofar region of Oman. Led by Mahesh Anand of the University of Tennessee in Knoxville, the team proposes in the May 4 Proceedings of the National Academy of Sciences that hapkeite forms when small volumes of lunar soil melt and various elements vaporize during micrometeorite impacts. Although the ultimate temperature at impact is unknown, the researchers believe the value far exceeds 2,000 degrees Celsius.

“With such high velocities, even a minute mass of particle turns into a real high-powered kinetic projectile,” says Larry Taylor, petrology professor at the University of Tennessee in Knoxville and co-author on the study, funded by NASA and a Russian Foundation for Basic Research grant. As the iron- and silicon-rich vapor cools, the mineral assembles atom by atom. The unique nature of the iron and silicon mix allows the mineral phase to form instead of quenching into glass, the usual product from similar melting events.

Planetary geologists strive to understand details of space weathering because many remote-sensing studies of planetary chemistry and mineralogy rely on the spectra of light reflecting off of planetary body surfaces; the nature of the body’s surface affects the spectral signatures. “Hapkeite helps us to better understand the space-weathering processes that occur on the Moon and on all airless bodies,” Taylor says.

However, hapkeite is still rare, says Allan Treiman, Senior Staff Scientist at the Lunar and Planetary Institute in Houston, Texas, and he would like to see additional research before drawing conclusions about the formation process and the impact of the mineral on spectra. “The hapkeite was found in this meteorite fragment but has not yet been seen in lunar soil samples, so it seems that the mineral is probably rare.” Without more data, he says, it would be a “big jump to get from these samples to reflectance data.”

Joshua Chamot
Geotimes contributing writer

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