Begging a crust
NASA has a lot of moon rock, but is it enough?
There's an unassuming building at the Johnson Space Center in Houston, Texas, that is home to some exceedingly precious rubble: more than 300 kilograms of lunar rock.
The grey fragments of Moon range from fine grains of sand to rocks the size of basketballs. They were brought back between 1969 and 1976, mainly by the US Apollo missions, along with a tiny amount from Russia's robotic Luna missions.
Since their arrival, these rocks have helped scientists to understand much of what we know about the Moon today. Data from them were crucial in supporting the idea that the Moon was formed by a giant impact with the early Earth, for example.
You might think that after 30 years of analysing and reanalysing these samples mission scientists have learned all they could possibly want know.
Not so, says Randy Korotev of Washington University in St Louis. "People are developing new techniques all the time, which are helping us to pinpoint the dates of pivotal events in the Moon's history," he says. In 2005, for example, fragments of Apollo rock were used to pin down the exact age of the Moon1. Analysis of the elements halfnium and wolfram in the rock showed that the Moon is 4.527 billion years old, give or take 10 million years an improvement over previous datings of 4.56 to 4.29 billion years.
lunar curator at the Johnson Space Centre
In total, 382 kilograms of rock have been brought back from the Moon, 99.9% of it by the Apollo missions. About 85% of this is still in storage (mostly at Johnson, with a small amount elsewhere for safe keeping) and has never been exposed to Earth's air. The United States has given away only a tiny proportion of its store (see 'Scattered fragments'), and of the rock that has been loaned out for educational purposes or research, only 1% or so has been destroyed.
The samples in storage are kept in a dry nitrogen atmosphere to prevent degradation. And, after a decision made in the 1970s, only three materials - stainless steel, aluminium and Teflon have ever been allowed to come in contact with them: these materials can be easily identified and separated from the lunar rock.
Nevertheless, just 1% of the rock at stored at the Johnson Space Center matches up to the strict definition of 'pristine'.
"Pristine rocks are those that have not been altered or changed by the process that broke them free from the Moon's surface" explains Gary Lofgren, lunar curator at Johnson. That rules out most samples simply because they are volcanic, and having been belched from a volcano are no longer original crust, or because they have been smashed around by meteoric impacts.
In truly pristine samples, says Lofgren, "the minerals within the rock have not been disordered; it is an original piece". This enables scientists to get a true picture of the processes that formed the Moon. And whenever new tests are developed, scientists are keen to try them out on pristine rock.
"In the next five to ten years the scarcity of pristine lunar material will be a real problem; we could easily be in the situation of turning down good research proposals," Lofgren warns.
The right variety
It's not just quantity and quality that lunar geologists want, they need a better range of rocks too.
The Apollo missions landed in a relatively small area of the Moon near the equator, on the Earth-facing side. Scientists have since discovered that this area has an unusually high concentration of radioactive chemicals, making the samples unrepresentative of the Moon as a whole.
Lunar meteorites can help to provide information about other parts of the Moon, but these have been altered by whatever process chipped them from the Moon's surface and their entry into our atmosphere. And one cannot tell exactly where such bits of rock have come from.
When the Apollo missions were launched, "all we had to guide sample collection were photographs and some spectrum data from Earth-based telescopes", says Korotev. "Now we are a lot more aware of where the interesting sites are".
The South Pole-Aitken basin is one tempting site: photographic evidence suggests it is the oldest visible meteorite impact2. Measuring the age of the rock here could help to put a date on when the Moon's crust formed. And samples from the lunar poles would reveal whether there is or is not any water on the Moon.
So do President George W. Bush's plans to return to the Moon and head on to Mars offer hope to geologists? Korotev isn't overly optimistic.
"NASA's plan for planetary exploration is not science driven, it's driven by the aim of getting to Mars. It is true that samples from anywhere not near the Apollo landing sites would give us new information but they wouldn't necessarily help answer a specific question," he says. It may take a long time to tick off Korotev's wish list.
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- Kleine T., Palme H., Mezger K.& Halliday A. N. Science, 310. 1671 - 1674 (2005).
- Shoemaker E. M., Robinson M. S. , Eliason E. M. , et al. Science, 266. 1851 - 1854 (1995).