The Galileo spacecraft, which is investigating the giant planet and its moons, seems to have detected the presence of complex molecules containing the essential elements on both Ganymede and Callisto, the two largest moons of Jupiter.
Astronomers already suspect that there may be life in warm water lying beneath the frozen surface of Europa, the smallest of Jupiter's four principal moons, based on observations by Galileo of patterns of meteor impacts on its surface and calculations about tidal heating of the moon's core. It is thought to have a crust of ice five miles thick, and an ocean of liquid water 60 miles deep, warmed by the hot inner core. Some experts think Europa's hidden ocean could be teeming with life.
However, the new observations are the first to suggest that life may exist, or previously have done, on both Ganymede and Callisto. The new evidence comes from analysis of the light reflected from the moons. When light hits a molecule, it excites the molecular bonds, which "bounce" like a spring at their own characteristic frequency.
The frequency depends on the atoms in the molecule and the number of bonds. They then re-emit that energy as light at that frequency. When this is analysed it indicates what molecules are present. Four previously undiscovered substances turned up in infra-red spectrometry analysis of light from Ganymede and Callisto, scientists said at a meeting in Houston, Texas, of the Lunar and Planetary Institute conference last month.
Three were no surprise, comprising water-bearing minerals, sulphur dioxide and ice. But the scientists were excited by the discovery of organic cyanides, comprising carbon and nitrogen linked by a triple bond.
These are unusual molecules to find in inorganic reactions, Thomas McCord, of the University of Hawaii, told New Scientist magazine. "We're saying that CN is the best candidate. That doesn't mean that there aren't others."
Molecules containing carbon and nitrogen have also been detected in the core of the Hale-Bopp comet, which has sparked further interest in the idea that comets provide an early precursor for life to develop by providing an environment in which such molecules can form.
Ganymede is unusual in that it has its own powerful magnetic field - suggesting that it has its own iron core, rather like the Earth. This could also generate enough heat to start life near the centre of the moon, despite being so remote that it gets only a tiny fraction of the sunlight that the Earth does.