Possible detection of hydrazine on Saturn’s moon Rhea
In a new report on Science Advances, Mark Elowitz, and a team of scientists in physical sciences, optical physics, planetary science and radiation research in the U.S., U.K., India, and Taiwan, presented the first analysis of far-ultraviolet reflectance spectra of regions on Rhea’s leading and trailing hemispheres—as collected by the Cassini ultraviolet imaging spectrograph during targeted flybys.
In this work, they specifically aimed to explain the unidentified broad absorption feature centered near 184 nanometers of the resulting spectra. Using laboratory measurements of the UV spectroscopy of a set of molecules, Elowitz et al. found a good fit to Rhea’s spectra with both hydrazine monohydrate and several chlorine-containing molecules. They showed hydrazine monohydrate to be the most plausible candidate to explain the absorption feature at 184 nm. Hydrazine was also a propellant in Cassini’s thrusters, however, in this instance, the thrusters were not used during icy satellite flybys and therefore the signal was assumed to not rise from spacecraft fuel. The scientists then detailed how hydrazine monohydrate may be chemically produced on icy surfaces.
Saturn’s moon Rhea
Knowledge of the geology and surface topography of Saturn’s second largest moon Rhea had advanced greatly by several flybys during the Cassini-Huygens mission. The surface of Rhea is heavily cratered with geomorphological features to indicate endogenic activity such as large impact craters in the north-south direction. The surface temperature of Rhea can change from about 40 to 100 K, with high visible geometric albedo. The albedo, i.e., the amount of light reflected from a celestial object, was consistent with a surface composed of water-ice, typically supported by the measurement of infrared (IR) absorption features. In general, Rhea orbits Saturn at a distance approximating 8.75 Saturn radii with a velocity of 8.5 Km/s, where its traveling hemisphere is irradiated by plasma traveling at approximately 57 Km/s. The E-ring grains of Saturn could bombard and coat much of the surface of Rhea, and such bombardments from different sources could cause chemical changes in the irradiated surface to synthesize a rich surface chemistry. However, the surface composition of Rhea currently remains largely unknown. In this work, Elowitz et al. used four Cassini ultraviolet imaging spectrograph/far ultraviolet (UVIS/FUV) disk-resolved observations of Rhea. To reduce the noise in the data, the researchers applied a smoothing filter. They noted the spectra to be dominated by water-ice absorption features, as noted in previous icy satellites. They explored explanations for broad absorption spectra across the wavelength range approximating 179-to-189 nm in the UVIS spectra of Rhea.
Possible detection of hydrazine on Saturn’s moon Rhea
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