Analyzing Io's complexion.

Analyzing Io's complexion

Although several researchers have studied the striking palette of colors on Jupiter's moon Io, spectral measurements have left uncertainties about the principal materials there. On a surface known only from data gathered by distant instruments, slight temperature differences and other factors, such as the varied tendencies of different molecules to stick where they first touch the terrain, can often make spectra difficult to read.

Farid Salama and colleagues at NASA's Ames Research Center at Moffett Field in Mountain View, Calif., have now compared details in Io spectra taken as long ago as 1976 with spectra the group derived in the laboratory. They describe their findings in the January ICARUS.

Io's surface probably contains hydrogen sulfide and water mixed with sulfur dioxide, the researchers report. In addition, the Io spectra "are well matched qualitatively" by lab spectra of sulfur dioxide ices containing about 3 percent hydrogen sulfide and 0.1 percent water, condensed onto a cold surface as a mixture of gases. One question has been whether the different ices on Io are mixed or instead were somehow deposited in separate layers. The group concludes that only a mixture can account for observations of solid hydrogen sulfide on Io at temperatures and pressures above which pure hydrogen sulfide would have turned to a gas.