Enceladus! The mouse that roars

1. Enceladus!The mouse that roars Dominic Fortes APEX October 12th 2006

2. Enceladus basics Mean radius: 252.3 km Mass: 1.081x1020 kg Density: 1.606 g cm-3 Orbital semi-major axis: 238,040 km Orbital eccentricity: 0.0047 Orbital period: 1.370 days Mean surface temperature: 75 K Enceladus has the most reflective surface of any solar system body. Spectroscopy indicates a surface composed of pure water ice. The bulk density implies a ‘rock’ to ‘ice’ ratio of roughly 50:50

3. Voyager 2 image acquired August 26th 1981 Smooth uncratered plains Faulting Folding Grooved terrain (cf. Ganymede) Relaxed crater topography Heat flow must have been higher in the past. Resurfacing must have occurred within last 100 million – 1 billion years.

4. Enceladus’ near neighbour Mimas Mean radius: 198.3 km Mass: 3.752x1019 kg Density: 1.152 g cm-3 No sign of surface activity Mimas preserves an ancient heavily cratered surface

5. Beyond the main ring system is another diffuse ring (the E-ring) It consists of ice grains (mean diameter 1 mm) Extends from 3 – 8 Saturn radii, with a peak density near 3.95 RS The lifetime of E-ring particles is order 101 - 102 of years Sputtering of the ring feeds an enormous torus of neutral OH To sustain the E-ring requires a water flux from Enceladus of order 1027 molecules per second

6. Since Saturn orbital insertion in July 2004, the Cassini space-craft has transformed our views of Enceladus. First results are compiled in a special issue of Science (Science, vol. 311, March 10th 2006) Flybys to date: Feb. 17 2005 1264 km Mar. 09 2005 500 km Mar. 29 2005 64,000 km May 21 2005 93,000 km July 14 2005 168.2 km Oct. 12 2005 49,000 km Dec. 24 2005 94,000 km Jan. 17 2006 146,000 km Sep. 09 2006 40,000 km • Magnetometry / Plasma • Stellar occultation • UV-visible-IR spectroscopy • Particle spectroscopy • Imaging

7. A number of Cassini instruments sense the response of the subsonic plasma flow to Enceladus: Magnetometer (MAG) Cassini plasma spectrometer (CAPS) Radio and plasma wave science (RPWS) Ion and neutral mass spectrometer (INMS) These reveal a significant disturbance to the plasma flow around the south polar region of Enceladus.

8. Stellar occultation Cassini used its UV imaging spectrometer (UVIS) to observe the occultation of two stars; Shaula (l-Scorpii) on February 17th 2005, and Bellatrix (g-Orionis) on July 14th 2005. Evidence for an atmosphere was only seen during the Bellatrix ingress.

9. Cassini ISS mosaic acquired March 9th 2005. Tectonic disturbances are concentrated around the south polar region. This crater-free terrain is crossed by a series of fractures dubbed ‘Tiger Stripes.’ These appear bluish in this highly stretched colour image due to a difference in grain size and crystallinity.

10. Cassini CIRS* data (long-wavelength thermal emission) Data collected during July 14th 2005 flyby The south polar ‘hot spot’ represents a power source of 4 – 7 GW *Composite infrared spectrometer

12. The south polar plumes of Enceladus in forward-scattered sunlight. Cassini ISS wide angle image (March 9th 2005). The ring system in forward-scattered sunlight Mosaic of Cassini ISS wide-angle images acquired on September 15th 2006.

13. Cassini flew through the south polar plume on July 14th 2005 at an altitude of 168 km. The INMS* instrument directly sampled the particles in the plume. • Water ice 91 % • Carbon dioxide 3.2 % • Nitrogen or carbon monoxide 4 % • Methane 1.7 % No evidence of ammonia, either in the plume or on the surface. The abundances of CO2, CO or N2, and CH4 are an order of magnitude larger than their solubility in water. But, consistent with the explosive decomposition of clathrate hydrates in pure water or brine. *Ion and neutral mass spectrometer

14. The south polar plumes are producing 150 - 350 kg s-1, (5x1027 - 1028 molecules s-1) The plume velocity is 300 – 500 m s-1 (escape velocity is 241 m s-1) Estimated resurfacing rate around the south pole is 1 mm yr-1. But where is the energy coming from? Enceladus is too small to be generating sufficient heat from radioactive decay. The only plausible source is tidal dissipation (tricky to model)

15. Enceladus has a very small orbital eccentricity and is in a 2:1 resonance with Dione. But Mimas has much larger eccentricity and is closer to Saturn. Dissipation inside Mimas ought to 25x larger than Enceladus. However, there is a 3:1 spin orbit resonance (i.e., a periodic nutation) which may be significant (see Wisdom, 2004; Astron. J. 128, 484-491). Nimmo and Pappalardo (Nature 441, 614 [2006]) believe that a convection plume (either in the rocky core or the ice shell) resulted in global reorientation. This resulted in the thermal anomaly becoming aligned with the spin axis (in this case, the south pole). What about the possible role of clathrates?

16. The future… Terrain on the as-yet poorly imaged hemisphere looks even more bizarre than seen to date. Cassini will make one more very close flyby (in the primary mission) on March 12th 2008 at an altitude of less than 25 km. Improved measurement of the shape and gravity field will constrain the internal structure and range of permissible tidal dissipation models. Upcoming flybys: Nov. 09 2006 (95,000 km): Jun. 28 2007 (90,000 km) Sep. 30 2007 (98,000 km): Mar. 12 2008 (23 km).