There is a high probability that life spread from Earth on the Moons of the outer planets via Lithopanspermia. An artist’s conception of a rock fragment colliding with Europa’s icy surface. Image © NASA/JPL.
During the period, about 4.1 billion to 3.8 billion years ago, of heavy impacts of asteroids and comets on Earth, rock fragments would have been ejected and may have carried the basic ingredients for life to other solar system bodies.
According to the study from Pennsylvania State University, supporting Lithopanspermia, sometimes referred to as interstellar panspermia, these meteoroids encase hardy enough organisms, and they could seed life on another planet or moon.
“Material from the surface of a planet can be ejected into space by a large impact, and could carry primitive life forms with it. We performed n-body simulations of such ejecta to determine where in the Solar System rock from Earth and Mars may end up. We find that, in addition to frequent transfer of material among the terrestrial planets, transfer of material from Earth and Mars to the moons of Jupiter and Saturn is also possible, but rare. We expect that such transfer is most likely during the Late Heavy Bombardment or during the next one or two billion years. At this time, the icy moons were warmer and likely had little or no icy shell to prevent meteorites from reaching their liquid interiors.”