Posted: October 08, 2008
Tomorrow, 9 October, Cassini will make the closest approach yet of Enceladus at just 25 kilometres, with a follow-up look from 196 kilometres on 31 October.
The goals of the flybys are to learn more about the composition of the icy plumes that jet out from giant fractures - nicknamed tiger stripes - in the moon’s icy shell, to understand what powers these geysers and to try and shed light on whether a liquid water ocean exists beneath surface. Trace amounts of organics have already been detected in the plumes, raising tantalizing possibilities about the moon's habitability.
Cassini will swoop past the southern hemisphere of Enceladus tomorrow at an altitude of just 25 kilometres to taste the organic rich plumes that have been observed to jet out from the tiger stripe fractures that characterise this icy moon. Credit: NASA/JPL/Space Science Institute.
While Cassini's optical imaging systems were the focus of the 11 August flyby, the spacecraft's fields and particles instruments will be the stars of the show on 9 October, taking bigger mouthfuls of the ejected particles and gases than ever before.
"We know that Enceladus produces a few hundred kilograms per second of gas and dust and that this material is mainly water vapour and water ice," says Tamas Gambosi, Cassini scientist at the University of Michigan. "The water vapour and the evaporation from the ice grains contribute most of the mass found in Saturn's magnetosphere. One of the overarching scientific puzzles we are trying to understand is what happens to the gas and dust released from Enceladus, including how some of the gas is transformed to ionized plasma and is disseminated throughout the magnetosphere."
For the 31 October flyby, the cameras and other optical remote sensing instruments will be back in the limelight, to take more images of the tiger stripes. It is hoped that these two flybys will augment findings from the most recent Enceladus flyby, which hinted at possible changes in the surface temperatures of the moon. In the 11 August flyby, Cassini instruments measured the temperature of one of the tiger stripe fractures called Damascus Sulcus as 20 degrees lower than the 180 degrees Kelvin measured in March. Results from Cassini's magnetometer instrument also suggested a difference in the intensity of the plume compared to earlier encounters.
"We don't know yet if this is due to a real cooling of this tiger stripe, or to the fact that we were looking much closer, at a relatively small area, and might have missed the warmest spot," says John Spencer, Cassini scientist on the composite infrared spectrometer, at the Southwest Research Institute, Boulder.
Information from the next two flybys will help scientists understand these observations, and a further four more Enceladus flybys are planned for the next two years, bringing the total number to seven during Cassini's new mission phase, the Cassini Equinox Mission. Another double hit is scheduled for 2 and 21 November 2009, giving scientists another attempt at understanding what powers this enigmatic moon of Saturn.