Dynamics and Evolution of SO 2 Gas Condensation around Prometheus-like Volcanic Plumes on Io as Seen by the Near Infrared Mapping Spectrometer

Abstract

We analyze a series of spectral image cubes acquired by the Galileo Near Infrared Mapping Spectrometer (NIMS) over the Prometheus region of Io. We use SO 2 frost, a volatile compound ubiquitous on the surface, as a tracer to understand various thermodynamic and volcanic processes acting in this equatorial region. Here we develop a new method to derive, from the 12-wavelength NIMS products, the distribution and physical properties of solid SO 2. This method is based on the inversion of a bidirectional reflectance model on two observed spectral ratios sensitive to (1) the areal abundance of SO 2 and (2) its mean grain size. As a result, reliable and consistent maps of SO 2 abundance and granularity are obtained which can be correlated to distinguish four different physical units. The distribution of these SO 2 units indicates zones of condensation, metamorphism, and sublimation linked with the thermodynamic and volcanic processes of interest. Our maps depict equatorial plains undisturbed by any kind of vigorous volcanic activity over 35–40% of their surface. Elsewhere, 10–20% of the equatorial plains display abnormally low frost coverage which may imply the recent presence of positive thermal anomalies with temperatures in the range 110–200 K. Hot-spots such as Prometheus, Culann, Surya, and Tupan (to mention the most persistent) emit a great variety of gases, some of which will condense at Io's surface near their source regions. Associated fields of freshly condensed SO 2 are easily observed, and deposits of more refractory compounds with higher (e.g., S 8) or lower (e.g., NaCl) molecular weight must also be present (although their exact nature is unknown). Three different mechanisms of emission are proposed for the volatile compounds and supported by the distribution maps. These are (a) the interaction between flowing lava and preexisting volatile deposits on the surface, (b) direct degassing from the lava, an d (c) the eruption of a liquid aquifer from underground. The geometric elongation of the Prometheus SO 2 deposition ring being related to the development of a 95-km-long lava field is the best illustration of mechanism (a). Details of the progressive emplacement of the SO 2 ring by the associated plume are examined by the development of a semiempirical model of material deposition based on a ballistic transfer from the sources to the surface. This model shows that lava emission may have been occuring at Prometheus at a fairly constant rate since Voyager. Mechanism (b) may operate at the hot-spot Surya, which presents a noticeable field of fresh SO 2 frost but no extended lava flow. Finally, we have noted on the northwestern flank of the volcanic edifice Emakong the existence of an extremely deep ν 1+ν 3 SO 2 absorption which is indicative of abundant, pure, and perhaps icy SO 2 deposits. These could be the result of the eruption of an SO 2 liquid aquifer (mechanism (c)).