BBC News (August 23, 2011) says that, according to “vast computer simulations,” hit impacts are more likely to have spread life from Earth than to Earth ( which our UD author Rob Sheldon has been saying):
The hardiest life forms of Earth could have made it to Jupiter or its moons
Asteroid impacts on the Earth may have scattered more life-bearing debris to Mars, Jupiter or beyond our Solar System than previously thought.
Vast computer simulations of debris thrown up from Earth impacts show 100 times more particles end up on Mars than prior studies have shown.
The highest-energy impacts drive debris all the way to Jupiter, which hosts two moons that may be amenable to life.
Faint hopes, but hey ..,