DENVER — For the first time in decades, astronomers have identified a new rock type on the moon. Tucked away on the lunar farside, unseen until a space probe spotted its odd mineralogy, are a few deposits of what is probably ancient material that originated deep inside the moon.
“These are very unusual areas,” said Carle Pieters, a planetary geologist at Brown University in Providence, R.I., who reported the finding November 2 at a meeting of the Geological Society of America.
Pieters has dubbed the new rock type OOS, because it is rich in the minerals orthopyroxene, olivine and spinel. Lunar scientists are particularly intrigued by the amount of spinel in the rock; every other part of the moon has only trace amounts. On Earth, in larger chunks, spinel is a gemstone prized in such collections as the British crown jewels.
The discovery comes from the Chandrayaan-1 mission, an Indian probe that orbited the moon for 10 months in 2008–2009. It carried a NASA instrument, the Moon Mineralogy Mapper or M3. By studying how much light was reflected from various parts of the lunar surface, and at what wavelength, M3 analyzed the moon’s mineralogical makeup in unprecedented detail.
“It is the first fully capable spectrometer flown to the moon,” said lunar scientist Thomas McCord of the Bear Fight Institute in Winthrop, Wash. Last year, using M3, Pieters and her colleagues reported finding spectral evidence of water on the moon.
Deep within a basin called Moscoviense, M3 spotted areas where light was being strongly absorbed at a wavelength of 2 micrometers. That particular absorption is an almost certain sign that magnesium-rich spinel is present, said Pieters.
How the material got there remains something of a mystery. Other images of the same part of Moscoviense show that the surface looks undisturbed, with smooth soil all around. The absence of any impact craters suggests that the spinel-rich areas must have been on the surface for a long time.
Most researchers think that the moon formed when a Mars-sized object hit the infant Earth 4.5 billion years ago, soon after the solar system was born. The impact kicked a slurry of material into Earth orbit, which cooled and coalesced to form the moon.
The newly identified, spinel-rich areas could be a remnant from those early days — a part of the moon that cooled early on, deep in the crust, and was later brought to the surface through some kind of geologic action, Pieters said.
The last time a new rock type was identified on the moon, she said, was in the 1970s, when otherwise typical basalt rocks brought back by Apollo astronauts were found to contain very low amounts of the element titanium.
Pieters’ team is now looking for more of the spinel-rich rock in other parts of the moon. They have already spotted the rock in one location other than Moscoviense, she said.