If a meteor strikes Earth, it sends material flying into the air. Generally friction with the air and Earth's high escape velocity prevent that material from departing from the Earth into space. If living material were flung up fast enough to escape, it would be vaporized on its way up, and anything that somehow survived the voyage into and through space would find no Earth-like environment where it could live in the whole solar system.

Ganymede's gravity field is much weaker and there is no air to slow the material being ejected. Thus, some can make it into space, and it isn't heated by an atmosphere on the way. It is thus reasonable to expect that spores and microbial life could make its way still living into space. Perhaps this is the origin of some of the rare carbonaceous chondrite meteors that strike Earth. Since it started from an airless world, it is conceivable that occasional clumps of soil with living material could find their way across the void to strike down on some other airless world. If that other world had sufficiently hospitable conditions, the Ganymedean life might survive and reproduce.

There are two other airless worlds that have complex organic 'tholins' as Ganymede does. The first is Callisto. It is not a big surprise that Ganymedean life might live on Callisto. Callisto is only a little smaller than Ganymede, the same distance from the sun, in a quiet space zone... and it's not very far from Ganymede. Indeed, no other two planets are as close together or so alike. Unfortunately, if there is vegetation on Callisto, the plant species were too small to be identified even at 10 meters per pixel or so scale - the surface just looks dark, an oddly "fluffy" looking blanket. Color images might have told a different story (and certainly would have painted a fascinating picture of Ganymede!), but the Galileo had no color camera except a clunky system that could take colors at various wavelengths, but only at great distances as the different filters had to take images one after another, during which time the spacecraft had moved on to another scene whenever it got near a world.

A mystery of the solar system for many decades has been why the leading hemisphere of tiny Iapetus, orbiting Saturn at a considerable distance, is very dark, while the tail side is very bright. (The polar areas are also very bright - "polar caps" evidently with CO2 ice. The bright hemisphere is largely water ice like most of Saturn's small moons.) The Cassini has shown the dark side of Iapetus has tholins as well as cyanogen polymers, and carbon dioxide said to be probably bound up in something else (tholins?) and of photochemical origin. Callisto also has notable CO2. "Photochemical origin" could be pointing a finger towards photosynthesis. It is a surprise that any life of Ganymede could possibly survive on this miniature world, almost twice as far from the Sun as Ganymede, yet there's those telltale spectral results. And "Because large craters within the dark terrain appear to be evenly colored with the dark material (no craters appear to break up the dark material, exposing bright underlying terrain), this suggests the emplacement of the dark material is relatively new or ongoing." [I A P E T U S 0 4 9 I A MISSION DESCRIPTION, September 2007, Jet Propulsion Laboratory, California Institute of Technology] Life is an obvious ongoing process. New dark plants could cover over meteor craters.

Soon Cassini will bring us some closer images of Iapetus than the 750+ meters per pixel ones already taken, but there is no sufficiently close pass planned to be sure of identifing even very gigantic vegetation, and like Galileo, Cassini becomes color blind and takes only monochrome images when it does get close enough to a world to identify vegetation even en mass, in groves or fields. And it is possible Iapetus, like Callisto, has nothing big enough to see from space anyway - just some Ganymedean equivalent of "algae" and very small, perhaps only microscopic, animal life - if any.

My highly tentative conclusion, based on the spectra, is that both Callisto and Iapetus have life. Since life is said to be only initiated on worlds where it can evolve into a human race, my surmise is that that the life would have come from Ganymede.

Written: Craig Carmichael, Independent Researcher, February 12th 2007.
Rev: Sept 7 2007: added quote about recency of dark material.