Friday, October 4, 2013

Flying Over an Enceladean Geyser

Enceladus is a small icy moon of Saturn with a diameter of 500 km. An interesting discovery from the Cassini spacecraft in orbit around Saturn is the presence of high-velocity jets of water emanating from four prominent fissures at Enceladus’ South Pole region. Some of the water, in the form of ice grains, escape Enceladus’ gravity and populate Saturn’s tenuous E-ring. Enceladus is currently only the third known object in the Solar System after Earth and Jupiter’s moon Io that is sufficiently geologically active for its internal heat to be detected by remote sensing.

Figure 1: With a diameter of only 500 km, Saturn’s moon Enceladus is small enough to fit within the length of the United Kingdom. Credit: NASA/JPL/Space Science Institute.

On 14 April 2012, the Cassini spacecraft flew over Enceladus’ South Pole region, approaching as close as 74 km from the surface. The flyby velocity was 7.5 km/s. At this speed, the entire diameter of Enceladus was traversed in 67 s. During the flyby, Cassini’s Visual Infrared Mapping Spectrometer (VIMS) recorded a thermal emission spectrum as the instrument’s field-of-view crossed over Baghdad Sulcus, one of the four major fissures. The flyby took place when much of Enceladus’ South Pole region was not illuminated by the Sun. This provided optimum conditions for detecting any endogenic thermal emission against an otherwise cold and dark background.

Figure 2: Heat emanating from four fissures at Enceladus’ South Pole region. From left to right, the four fissures are named Damascus Sulcus, Baghdad Sulcus, Cairo Sulcus and Alexandria Sulcus. The measurements were obtained by Cassini’s Composite Infrared Spectrometer during a close flyby on 12 March 2008. Credit: NASA/JPL/GSFC/SwRI/SSI.

The “colour” temperature that best fits the thermal emission spectrum recorded by VIMS is 197 ± 20 K. In contrasts, the mean surface temperature on Enceladus is 75 K. The thermal emission signature is believed to originate from the vapour-heated ice walls of a portion of a fissure that has an estimated width of 9 m. A total mass loss rate of ~200 kg/s of water vapour is believed to supply the plumes emanating from the fissures at Enceladus’ South Pole region. This corresponds to an average mass loss rate of ~0.4 kg/s of water vapour per km of fissure length, over a cumulative fissure length of 500 km.

For a temperature of 197 K, the mass loss rate for the portion of the fissure observed by VIMS is estimated to be 1.21 kg/s per km of fissure length. This rate is 3 times the average mass loss rate and suggests that the VIMS measurement sampled an active portion of the fissure that is emanating water vapour at a greater than average rate. Additionally, at 197 K, the most probable speed for water molecules is 426 m/s. This is consistant with ballistic models of Enceladus’ plumes where water molecules feeding the plumes have near surface velocities of 300 to 500 m/s.

J.D. Goguen et al., “The temperature and width of an active fissure on Enceladus measured with Cassini VIMS during the 14 April 2012 South Pole flyover”, Icarus 226 (2013) 1128-1137