Jetting Activity and Thermal Emission across the South Polar Terrain of Enceladus: Observations and Comparisons with Shear-Heating Models

Kurzfassung

Introduction: Cassini ISS Enceladus images reveal a forest of jets of fine icy particles erupting from the moon?s south polar terrain (SPT) and feeding a giant plume that extends thousands of kilometers into space [1] eventually forming Saturn?s E ring. Cassini infrared spectrometer (CIRS) observations have also shown the SPT to be anomalously warm [2], and the comparison of high resolution images of the SPT with the highest resolution thermal measurements has shown a coincidence between the hottest measured temperatures in the SPT and the ?tiger stripe? fractures which straddle the region [1,2]. The most plausible mechanism for heat production on the fractures is tidally driven frictional heating along faults due to cyclical shearing. The time-averaged absolute shear stress on each fracture segment can be calculated [3] and will be affected by location and orientation, and proportional to the local frictional heat production rate (assuming a constant coefficient of friction). Initial triangulation measurements of the jets seen in ISS images spanning 1.3 years (from late 2005 to early 2007) and taken over a broad range of image scales (1 to 14 km/pixel), phase angles (148? to 178?), and viewing directions with respect to the surface revealed the source locations for the 8 most prominent jets [4]. All 8 fell, within 2? measurement uncertainties, on the fractures, and about half of these sources were found to be coincident, within uncertainties, with the CIRS hot spots initially reported in 2006. Moreover, ISS jet locations were used to successfully predict the discovery of additional hot spots [4]: eg, the prominent hot spot on Baghdad Sulcus near Jet #I. Results: We have taken new, more precise jet positional measurements in the 62 highest resolution images (down to 40 m/pixel spatial scale) taken from late 2005 to early 2011 at phase angles of 130? to 165?. We now find over 80 distinct, measureable jets. Nearly all of them fall on one of the four main fractures, most on Cairo and Baghdad; only a few fall off a main fracture and on a nearby branch. To within measurement errors, all 8 previously measured jets have been recovered, with the notable distinction that at very high spatial scale, most of these 8 resolve into multiple narrower jets. Some jet sources that were prominent earlier in the mission appear to be less so in more recent images, indicating some degree of time variability. The most robust jetting activity still appears to be correlated with the hottest locales on the fractures. We will use this correlation between jetting activity and thermal power to produce a proxy thermal map of the SPT that we will then compare with (i) the best available thermal maps of the south polar terrain obtained by CIRS over the last 6 years and (ii) a map of predicted shear-heating along the south polar fractures.