An international team of scientists has found an unusual feature in the spectrum of light reflected from Saturn’s icy moon Rhea. Astronomers attribute it to the presence of hydrazine monohydrate and several chlorine-containing molecules of unknown origin. Articles by astronomers have been published in the journals Science Advances, Science and Nature. This is reported in a press release on Phys.org.
Researchers analyzed data from the Cassini probe, which captured an ultraviolet image of the satellite. Absorption spectra were determined with broad peaks in the wavelength range from 179 to 189 nanometers. To determine what caused them, scientists measured the absorption spectra of several chemicals in laboratory conditions. It turned out that under the layer of water ice Rhea contains such molecules as hydrazine monohydrate (N2H4 * H2O) and trichloromethane (CHCl3).
Astronomers have also found signs of carbon tetrachloride (CCl4), above which is a layer of fresh ice falling from Saturn’s ring.
The existence of chlorine-containing compounds on the surface of Rhea is difficult to explain by chemical reactions, since this would require the presence of an under-ice ocean, since the molecules lower the freezing point of ice and cause the formation of the water layer. Most likely, these substances were delivered by chondritic meteorites, and then distributed over the surface by charged particles from the magnetosphere of Saturn. Hydrazine monohydrate could have formed due to chemical reactions involving water ice and ammonia, or entered Rhea from Titan’s nitrogen-rich atmosphere.