Although the Moon has not been hit by a large meteorite for about 3.8 billion years, it has been continually bombarded since then by much smaller interplanetary rocks called micrometeorites. These tiny objects, which can be just tens of microns across, have been responsible for altering the lunar surface over time.


Micrometeorites travel at speeds of up to 100 000 kilometres per hour, and the heat generated when they crash into the Moon can flash-melt the silicate rock particles present on the lunar surface. Splashes of this molten material can then vaporize and dissociate into their constituent elements before coating other fragments of rock in a process is known as "space weathering" (see figure). This process is virtually unknown on Earth.

Lawrence Taylor and Mahesh Anand from the University of Tennessee, together with co-workers at the Vernadsky Institute of Geochemistry in Moscow and the Carnegie Institution of Washington, analysed a piece of rock from the Moon that had been found in Oman in 2000. They expected to find grains of pure iron in their sample, but instead found that the rock contained hapkeite – a mineral made up iron and silicon in a ration of two-to-one – and other iron-silicon phases.

The discovery was completely unexpected, says Taylor. At first he thought that the surface of the sample was tarnished as a result of oxidation. “I asked Mahesh Anand to perform electron microprobe analyses on these grains to see how much nickel and cobalt they contained,” he told PhysicsWeb. “Little did I know that the grains were iron silicides - the first report of such minerals in lunar samples.”

According to Taylor, the minerals were formed when elemental iron and silicon re-combined from the vapour state. He believes they provide direct evidence for the space-weathering process, and show how important it is for creating the soil on the Moon. The team named the mineral hapkeite after Bruce Hapke of the University of Pittsburgh, who predicted some 30 years ago that vapour-deposited coatings would be found on lunar grains.