Published online 2 November 2002 | Nature | doi:10.1038/news021028-13


Rummaging through Earth's attic for remains of ancient life

Lunar rocks retain memories long since lost down here.

There are 20 tonnes of the Earth on every 100 square kilometres of the Moon.There are 20 tonnes of the Earth on every 100 square kilometres of the Moon.© NASA

The surface of the moon is spattered with over 8 million tonnes of the Earth, astronomers have estimated. A mission to collect and study this planetary shrapnel could provide unique insights into the origins of life and the planets, they say1.

Asteroids and comets have pelted the Earth, Mars and Venus since the Solar System formed. The barrage peaked during a period known as the Late Heavy Bombardment, around 3.9 billion years ago.

The moon "witnessed and recorded all this", says John Armstrong of the University of Washington, Seattle. Our companion formed in a massive collision between the Earth and a Mars-sized object around 4.5 billion years ago.

Armstrong and his colleagues estimated the impacts on the Earth by measuring the number and size of craters on the Moon. They then calculated how much of the Earth these collisions would have hurled into space, and the probability that this rubble hit the Moon.

A 100-square-kilometre patch of the Moon contains about 20 tonnes of Earth fragments, the team reckons. The same area contains around 180 kilograms of Mars and 80 of Venus.

Collecting pieces of the three planets would let us compare their origins. "It could end all the speculation about what the early planets were like," says Armstrong.

Reach for the Moon

The origins of life on Earth are hotly debated, partly because there are no rocks older than about 3.8 billion years. Erosion and continental drift have wiped the slate clean over and over.

But the Moon has remained largely untouched - except by asteroids - since it formed. Pieces of the Earth littering its surface would be of all ages, although most would date from the Late Heavy Bombardment, says Armstrong. They could reveal the chemical signatures of life, and possibly fossilized bacteria.

A cheap, robotic foray such as the European Space Agency's SMART-1 mission, due for launch in 2003, "could achieve tonnes of great science", says Armstrong.

Rocks would have hit the moon at very high speeds, and fragments larger than a grain of sand would be rare, comments Phil Bland, who studies meteorites at Imperial College, London.

But the material would undoubtedly be there. "It's certainly worth a go," Bland says. "You can get a hell of a lot of information from even a one-millimetre grain of stuff."

The form of carbon found in the Earth fragments would indicate whether they once hosted early life, says Bland. 

  • References

    1. Armstrong, J. C., Wells, L. E. & Gonzalez, G. Rummaging through Earth's attic for remains of ancient life. Icarus, 160, 183 - 196, (2002). | Article | ISI | ChemPort |