Titan may offer a 'soft' landing for life-carrying boulders blasted from Earth (Image: NASA/JPL)
Boulders blasted away from the Earth's surface after a major impact could have travelled all the way to the outer solar system, new calculations reveal. The work suggests that terrestrial microbes on the rocks could in theory have landed on Saturn's giant moon, Titan. But whether they could have survived once there remains unclear.
The fact that meteorites from the Moon and Mars have landed on Earth confirms that impacts on solar system bodies can launch rocky debris to other planets. And previous studies have suggested that any life on the rocks could have survived the launch blast and the radiation and chill of the journey through space, assuming it lasted less than a few million years.
Such hardiness raises the possibility that life on Earth itself was seeded from space – a concept called panspermia. But now, researchers led by Brett Gladman of the University of British Columbia in Vancouver, Canada, have analysed the reverse situation – that life on Earth seeded other bodies in the solar system. Gladman presented the results on Thursday at the Lunar and Planetary Science Conference in Houston, Texas, US.
He says only boulders at least 3 metres across could punch out through the Earth's atmosphere and escape the planet's gravity, and that only extremely powerful impacts could achieve this. The cause of such impacts would be comets or asteroids between 10 and 50 kilometres wide, Gladman told New Scientist: "The kind of thing that killed the dinosaurs."
The team ran computer models of such giant impacts, estimating that each would send about 600 million boulders into space to orbit the Sun. Some of those launched at relatively high speeds – faster than 6 kilometres per second – got as far as Jupiter and Saturn in about a million years.
In the simulations, about 100 of the boulders from each impact reached Jupiter's moon Europa. But along the way, Jupiter's gravity boosted their speed to an average of 25 km/s, with some moving as fast as 40 km/s. Impacting Europa's icy crust at such speeds would be like "hitting a brick wall," says Gladman. "This must be rather frustrating if you're a bacterium that survived launch from Earth."
But he found a different situation on Saturn's moon Titan, which boasts a thick atmosphere. About 30 boulders from each Earth impact reached Titan, and they slammed into the atmosphere at just 11 km/s – slower than most meteors hit Earth's atmosphere. "Those reaching Titan can aerobrake and drop their fragments onto the surface," says Gladman.
"That kind of entry should be no problem" for life to survive, says Allan Treiman of the Lunar and Planetary Institute in Houston, who notes that researchers recently found bacteria that appear to have survived the break up of the shuttle Columbia when it re-entered Earth's atmosphere in 2003. And Earthly lichen has also survived when exposed to the harsh environment of space.
"I thought the Titan result was really surprising – how many would get there and how slowly they'd land," Treiman told New Scientist. "The thing I don't know about is if there are any bugs on Earth that would be happy living on Titan." Titan's surface temperature is a very cold -179°C and its chemistry is very different from Earth's.
Gladman agrees that life may be unlikely to survive once on Titan. But he says major impacts may have happened "tens of times" throughout Earth's history and that these could have sent Earth rocks to other solar system bodies. "I just set out to answer this question: is it possible to get something there?" he says. "The answer is yes."