Has Earth life colonised the Solar System?

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During the Late Heavy Bombardment, collisions with large asteroids were far more frequent than they are now.

Earth is hit by a comet or asteroid with a diameter in the tens of kilometres once every 100 million years or so. But during a more hazardous epoch of the Solar System’s history soon after the birth of the planets, known as the Late Heavy Bombardment, Earth and its neighbours are believed to have been pummelled by such giants much more frequently.

While that’s obviously rather bad news for life in the immediate region of any impact, as well as a general inconvenience globally, such events are also likely to have flung biologically laden material off the planet. After a looping interplanetary journey, a certain fraction of such terrestrial meteorites may have fallen onto another world in the Solar System, and potentially even delivered Earth life intact. This is the idea behind ‘lithopanspermia’: it is possible that if we do detect life on Mars or Europa it may not be native, but a descendent of such space migrants.

Many papers have looked at different aspects of this transfer process, and Mauricio Reyes-Ruiz and colleagues have now contributed the most complete study yet of the orbital dynamics of the ejecta cloud from a massive impact. They ran simulations of over 100,000 test particles being sneezed off Earth, tracking their trajectories as they swirled through the Solar System. They limited the simulation length to 30,000 years – after that any microbes are likely to have been sterilised by cosmic radiation.

Once a splinter of the crust has been ejected off Earth, where is it most likely to end up? The authors found that the answer depended on how fast the incoming object is and where it strikes the planet. For most scenarios, a significant fraction of the ejected material – a few per cent of the total number of particles – falls back down to Earth within less than 10,000 years and thus is relatively unexposed to space conditions. The importance of this is that even if a cataclysmic collision wiped all life off the early Earth, the planet could have simply repopulated itself with returning meteorites. And encouragingly for the prospects of early terrestrial life seeding other worlds, Reyes-Ruiz and colleagues found that the transfer rate to Mars is about 10 times higher than previously calculated.

Hurling material all the way out to Jupiter or Saturn and their icy moons, though, is only possible under particular conditions, the paper says. The object that hits Earth in the first place must be travelling at least 30km/s and must slam into the planet’s leading face, to give the ejecta extra orbital velocity around the Sun. Nonetheless, up to about one rock in 2,000 may make this long-haul journey successfully.

The next step, say the researchers, is to focus on the probability of terrestrial material actually colliding with Europa, Ganymede, Titan and Enceladus – and thus to calculate the odds that Earthly life of some kind may already have visited these potentially habitable worlds.

This article first appeared in the November 2012 issue of Sky at Night Magazine.


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