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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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