At a press conference in the UK at the Cassini PSG (Project Science Group) meeting, additional details from the July flyby of Enceladus were revealed. The briefing focused on the cryovolcanic activity observed in the south polar region observed by a number of Cassini's instruments, including ISS, VIMS, CIRS, INMS, UVIS, and CDA. There is a lot of ground to cover on this and I will be posting several times today on this story.
First, INMS and CDA observed asymmetries in their data that have been plotted on a graph as well as shown on a polar projection map of Enceladus with the ground-track and the locations of the CDA and INMS peaks, and the CIRS hotspot, indicated. Previously, CDA reported that their data was consistent with impact-generated dust, not from endogenic activity. If you look at the graph, it is pretty close to a bell-shape curve, which is what you would see in an impact-generated dust scenario. However, the curve is shifted in time from closest approach, with the peak occuring 70 seconds before C/A. In the impact-generated dust scenario, the peak would be right at closest approach, so this shift would not be consistent with that scenario, but with one where dust is ejected up to a certain height from a volcanic vent. So it now appears that the south polar region of Enceladus is the source of the very small particles in the south polar region. UVIS data from two star occultations in February and July also seem to suggest that the "atmosphere" produced by the venting of water vapor and micron-sized dust is not global and localized to the south polar region.
VIMS was also at the press conference, showing a view of the surface of Enceladus taken by their instrument. This image shows the distribution of crystalline ice on the surface, which under Enceladus conditions quickly degrades to amorphous ice. VIMS found that there was quite a bit of crystalline ice in the area surrounding each fracture collectively known as the tiger stripes (because of their appearance and the way they standout in the normally very bright south polar region). Considering the conditions at Enceladus, crystalline ice is expected to convert to amorphous ice over a period of several decades so geologic activity along the stripes must have occured between 10 and 1000 years ago (though more recent episodes are certainly possible, and likely, as I will discuss in a post later today).
In my next post, I will discuss the ISS releases as well as the possible models for activity.