Is there organic materials on Callistos surface?
Intriguingly, near-infrared spectra show not only water ice and hydrated minerals, as expected, but also four unusual absorption features near a wavelength of four microns. One appears to be carbon dioxide trapped in the surface, perhaps as inclusions in icy particles or bubbles produced by radiation damage to the surface. Two other spectral features probably represent sulfur in the surface, which may originate in Io’s volcanic eruptions. The fourth spectral feature is the strangest. Its wavelength corresponds to that absorbed by carbon-nitrogen bonds. In fact, laboratory spectra of complex organic molecules called tholins by the late Carl Sagan are similar. Tholins are thought to resemble organic material in the solar nebula; clouds of interstellar ice grains have comparable spectra. Taken together, the data provide the first direct evidence that icy satellites contain the carbon, nitrogen and sulfur compounds common in primitive meteorites and comets. These materials are also some of
