IT鈥橲 time to go back to the Moon. The call is common enough, but now there鈥檚 a new, intriguing reason for returning. It could be harbouring secrets of the origin of life on Earth and even Mars.
That鈥檚 the verdict of planetary scientist John Armstrong at the University of Washington in Seattle and colleagues. They say rock from Earth, Mars and Venus could dot the Moon鈥檚 thin upper crust, having crash-landed there after impacts with the planets billions of years ago kicked up showers of meteoroids.
That could make the Moon the best place to search for the remains of early terrestrial or Martian life. The Moon lacks the weather and volcanic activity that has ground up the rock on both planets since the ancient impacts. 鈥淭he early history of the Solar System could be preserved in rocks found nowhere else,鈥� says Armstrong.
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The only natural force powerful enough to blast rocks into space is a cosmic crash. The early Solar System frequently saw such collisions, including the Mars-sized rock thought to have carved the Moon away from the Earth. Less than 0.1 per cent of the debris would ever reach the Moon. But the researchers calculate that there could still be up to 20 tonnes of Earth rock scattered across every 100 square kilometres of the Moon鈥檚 surface. The same area would yield 180 kilograms of rock from Mars, and a mere 1 to 30 kilograms from Venus, they will report in the journal Icarus.
The amount of Venusian rock is so small because the planet is closer to the Sun, so gravity is likely to pull fragments in. The planet鈥檚 thick, soupy atmosphere would also drag debris down, but the team鈥檚 calculations show that an asteroid 10 to 200 kilometres wide hitting the planet would punch a hole in the atmosphere through which up to 10 per cent of the asteroid鈥檚 mass could escape as outbound meteors.
A Venusian meteorite would be 鈥渁 real prize鈥� since none has been discovered so far, says planetary scientist Jay Melosh from the University of Arizona in Tucson. A rock from Venus would be easy to spot, he says, because it would contain unusually low levels of argon. Identifying terrestrial rocks might be more difficult, because Earth and Moon share the same isotope signatures. But while the Moon is dry, terrestrial rocks contain water, which could be detected with infrared scanners.
Much of the rock would get pulverised on impact. But fragments could survive, as large as the 15-centimetre-long Martian meteorite ALH 84001 found in Antarctica in 1984. Some believe the meteorite holds evidence of ancient microbes. If so, lunar finds might contain similar traces.
Even if lunar missions don鈥檛 manage to unearth alien fossils, biochemical traces in the rocks would still shed light on when life on Earth began, says team member Llyd Wells. Earth was bombarded by asteroids 3.8 to 4.1 billion years ago, and knowing when the last big one hit would be important in figuring out when Earth became habitable.
But a dedicated robot mission to prospect for extra-lunar rocks is unlikely, according to Melosh. Any fossil secrets locked away on the Moon will have to wait for a larger project, perhaps a 鈥渓unar mine鈥� like the one China is dreaming up.
In the meantime, Armstrong and his colleagues are studying some of the Moon rocks brought back to Earth by the Apollo missions in the 1970s. They calculate that the 380 kilograms of lunar material collected in total should contain 2 to 3 grams from Earth.