Moon Meteor Truly Extraterrestrial

micrograph
Micrograph of meteorite section showing iron phase Credit: PNAS/UTK.edu

Scientists have discovered a new lunar mineral in a golf ball-size meteorite that landed on Earth in 2000. The lunar meteorite was collected in the Dhofar region of Oman but originated from an impact on the moon thousands of years ago. The particular mineral phase discovered in the meteorite has never been seen in terrestrial rocks.

The discovery by Mahesh Anand of the University of Tennessee and colleagues suggests that the deposition of metals as hot vapors is important in altering the surface characteristics of airless planets and moons. Airless bodies in the solar system, such as the Moon, Mercury, and asteroids, have an inorganic soil made of crushed rocks, or regolith, formed by the impact of micrometeorites traveling at velocities up to 100,000 kilometers per hour.

Heat generated by these impacts melts and vaporizes metals, which are then redeposited on rock fragments as tiny, randomly scattered beads in a glassy coating. Previously, scientists discovered pure iron nanoparticles in rock samples returned from the Moon.

The new mineral, dubbed hapkeite, results from the deposition of iron and silicon in a 2:1 ratio.

Earth and Moon
Cartoon showing impact to deposition cycle.
Credit: PNAS/utk.edu

The authors named hapkeite after Bruce Hapke, who predicted the presence and importance of vapor-deposited coatings on lunar soil grains some 30 years ago. These coatings affect the reflectance spectra recorded by spacecraft, and raise questions about the ability to accurately determine the soil composition of airless bodies.

The airless impact of a meteor carries its full blow directly to the surface, unlike terrestrial meteorites which are slowed and burn up while entering the atmosphere. "It’s like a miniature nuclear explosion," Hapke said.

The authors described that weathering in the absence of air or water–known as ‘space weathering’ — is not comparable to anything that might be observed on Earth. "The formation of regolith and soil on airless planetary bodies, such as the Moon, Mercury, asteroids, is the result of processes virtually nonexistent on Earth." This weathering is caused by large impacts which pulverize rocks to soil, combined with melting to a glassy state for smaller impacts. When heated vapor recondenses on the rock’s surface and surrounding soil, a thin iron- and silica-rich layer is deposited.

Earth and Moon
Image of the Earth and Moon taken by Galileo spacecraft.
Credit: NASA

On a polished section from the meteorite, what originally struck the scientists as a bit of tarnish turns out to be a new iron-silicide mineral never seen before on Earth.

What’s Next

In September 2003, SMART-1 , Europe’s first science spacecraft designed to orbit the Moon, launched successfully to demonstrate propulsion technologies like a solar-powered ion engine for efficient travel on long space missions. Using infrared detectors, SMART-1 will map lunar materials and look for water and carbon dioxide ice in permanently shadowed craters.

Towards the end of 2004, the Japanese Lunar-A, Lunar Mapping Orbiter and Penetrator, will fire two bullets 3 meters into the lunar soil near Apollo 12 and 14 sites


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