CONTRASTING ALTITUDINAL TRENDS IN LAUNCH VELOCITIES OF ENCELADUS' PLUME PARTICLES

Water-ice particles in Enceladus’ plume hold key information about the formation and dynamics of the plume activity. We have determined trends in the plume particle launch velocities with respect to altitude (5 km – 100 km from the source vents) using high spatial and spectral resolution observations by Cassini’s Visible and Infrared Mapping Spectrometer (VIMS). The low altitude trends (5 km – 35 km from the source vents), determined for the first time in this work, exhibit a steeper slope than the high altitude trends (> 35 km). This slope contrast has been observed at different particle sizes (sampled through different wavelengths), across observations as well as for individual fissure eruptions (i.e. eruptions along Cairo, Baghdad and Damascus).

The trends at low altitudes do not match those expected based on extrapolations of previous observations at higher altitude (1, 2). One potential cause for the different particle launch velocity trends is the presence of more than one particle population in Enceladus’ plume. In such a scenario, at low altitudes (< 35 km), the particle population may be dominated by larger particles (> 5 µm diameter) that are launched at low velocities and would fall off rapidly with altitude giving a steeper slope for launch velocity. In contrast, the particle population at higher altitudes (>35 km) may be dominated by smaller particles (< 5 µm diameter) which would fall off much more gently giving a shallow slope w.r.t altitude. Such a stratified nature of the plume is consistent with previous ideas based on independent datasets (3).

1. Hedman M.M. et al. (2009) Astrophys. J., 693, 1749-1762

2. Hedman M.M. et al. (2013) Nature, 500, 182-184

3. Postberg F. et al. (2011) Nature, 474, 620-622