Somewhat lost in the current panspermia revival is the intriguing flip-side of the "ubiquitous life" idea: If life could have come here from somewhere, why couldn't an Earth rock have been dislodged long ago, sending life to another planet or star system?
"It is possible that there are small fragments of the Earth out there in space today, some with microorganisms, that were blasted off the Earth's surface many millions of years ago," Genge says. "These could reach the Jovian moons and through extreme good fortune seed the water oceans with microorganisms."
Okay. How likely?
"The chances of this happening in reality are probably similar to someone finding their way home after being blindfolded and airlifted to another continent."
Genge and others say the more plausible scenario for the transfer of life — if it has ever occurred and given the scant solid evidence currently available — is that it started on Mars and came to Earth.