life on Jupiter
Figure 1. A true-color simulated view of Jupiter composed of 4 images taken by the Cassini spacecraft on Dec. 7, 2000. To illustrate what Jupiter would have looked like if the cameras had a field-of-view large enough to capture the entire planet, the cylindrical map was projected onto a globe. The resolution is about 144 km (89 miles) per pixel. Jupiter's moon Europa is casting the shadow on the planet (NASA/JPL).
Figure 2. Artwork by Paul Calle of hypothetical life in Jupiter's atmosphere.
There has been much speculation that any life on Jupiter, or on other gas giants, might be ammonia-based life. The possibility of "abundant biota" in the upper regions of Jupiter's atmosphere was considered in a 1976 paper by Carl Sagan and Edwin E. Salpeter1, three years after the fly-by of the first Jupiter probe, Pioneer 10. Sagan and Salpeter compared the ecology of the Jovian atmosphere with that of terrestrial seas which have simple photosynthetic plankton at the top level, fish at lower levels feeding on these creatures, and marine predators which hunt the fish. The three hypothetical Jovian equivalents of these organisms, Sagan and Salpeter termed "sinkers", "floaters", and "hunters". They envisaged creatures like giant gas-bags (see bubble life) that move by pumping out helium and calculated that the "hunter" variety might grow to be many kilometers across (and therefore visible from space).
Jovian aerial life-forms like those described by Sagan and Salpeter are portrayed in Arthur C. Clarke's short story "A Meeting with Medusa" (in The Wind From the Sun). Ben Bova refers in his novel Jupiter to:
"[H]uge balloonlike creatures called Clarke's Medusas that drifted in the hurricane-like winds surging across the planet. Birds that have never seen land, living out their entire lives aloft. Gossamer spider-kites that trapped microscopic spores. Particles of long-chain carbon molecules that form in the clouds and sift downward, toward the global ocean below."
Bova speculates further that, in the high-pressure, liquid hydrogen ocean that lies below Jupiter's thick atmosphere, are colossal, city-sized creatures with intelligence. He follows the exploits of one of these sentient giants – Leviathan:
"Predators swarmed through Leviathan's ocean: swift voracious Darters that struck at Leviathan's kind and devoured their outer members."
In recent years, astrobiological interest in the Jovian system has shifted from Jupiter itself to its larger moons, especially Europa, Ganymede, and Callisto. Read also about the possibilities for life on Mars, Venus, and for extraterrestrial life in general.
1. Sagan, C., and Salpeter, E. E. "Particles, Environments and Possible Ecologies in the Jovian Atmosphere," Astrophysical Journal Supplement, 32, 737 (1976).