The Jets of Enceladus: Locations, Correlations with Thermal Hot Spots, and Jet Particle Vertical Velocities

Kurzfassung

High resolution images of Enceladus and its south polar jets taken with the Cassini ISS cameras in the last year have provided an opportunity for detailed study of the jetting phenomenon and its relationship to features and thermal hot spots on the moon’s south polar terrain. We have identified ~ 30 individual jets in a series of images, ranging from 43 to 100 m per pixel, taken in November 2009. All jets are found to be erupting through `tiger stripe’ fractures that cross the south polar terrain. The most intense jetting activity generally corresponds to the hottest regions on the fractures. One of the brightest, most prominent jets observed in this image series vents from a region on the Damascus Sulcus fracture that was imaged at 16 m/pixel during Cassini’s August 13, 2010 flyby; it is also one of the hottest places found so far on the south polar region. Several jets were selected for dynamical modeling. These were jets whose source regions were on the limb as seen from Cassini, allowing extraction of brightness profiles down to a few hundred meters of the surface. We infer the velocity distribution of the particles as they leave the surface by modeling the integrated brightness vs. altitude. The particles are assumed to follow ballistic trajectories, and their contribution to the brightness in each thin layer is proportional to the time that they spend in the layer. We find slow jets, fast jets, and jets in between. After a rapid ~ 2-km-scale-height decrease near the surface, the most prominent jet (mentioned above) extends with constant integrated brightness to the edge of the image 25 km above the surface; some of the particles in this jet appear to have mean velocities that exceed the 235 m/sec escape speed from Enceladus. Further analysis of higher-altitude images from the November flyby is in progress to verify this result. The integrated brightness of slow jets falls off with a scale height of 5 km or less, implying mean vertical velocities of order 30 m/s or less, much less than either the escape speed or the thermal speed for a temperature of 273 K. From the collimation of the vapor in the jets, the Cassini UVIS team infers vertical velocities of 1000 m/s or more [Hansen et al. (2008) Nature 456, 477-479]. Schmidt et al. [(2008) Nature 451, 685-688] account for the slow particle speeds by invoking collisions with the walls of the vent. Ingersoll and Pankine [(2010) Icarus 206, 594-607] invoke short distances during which the gas velocity is high; the particles don’t have time reach escape speed. The third possibility is that the particles are so large that the gas cannot accelerate them to escape speed. This possibility is testable with Cassini ISS high-resolution images, which span phase angles up to 176 degrees and wavelengths from UV to near-IR. Our ultimate goal is to test models of how the jets form. The particles form either by condensing directly from vapor, by spallation from the icy walls of the vent, or by freezing of liquid water droplets. Images collected by Cassini thus far will help us choose among the possibilities.