Cassini Observes the Active South Pole of Enceladus

1. Cassini Observes the Active South Pole of Enceladus Porco, C. C., et al. Presented by Patrick Harner

2. www.nasa.gov

3. Pre-Cassini • Voyager high resolution (~1km/pixel) of the northern hemisphere • Albedo of 1.4 • Reflectance spectrum dominated by pure water ice • Morphologically distinct regions • Insufficient heat budget • Eutectic temperature of NH3 + water = 175K • Potential 1:3 spin/orbit resonance, necessary libration apod.nasa.gov

4. Cassini • Composite Infrared Spectrometer (CIRS) • Imaging Science Subsystem (ISS) • Ultraviolet Imaging Spectrograph (UVIS) • Visible Infrared Mapping Spectrometer (VIMS) • Cosmic Dust Analyzer (CDA) • Magnetometer (MAG) www.nasa.gov

5. Cassini Flybys • Three close flybys planned for 2005 • February 17th – 1259 km over Saturn-facing equatorial region • March 9th – 497 km over anti-Saturn equatorial region • July 14th – 168 km over southern polar region apod.nasa.gov

6. February Flyby • Tenuous atmosphere (MAG) • Surface dominated by water and simple organics (VIMS) • Prominent fractures in the south polar region. • Smooth plains observed by Voyager are finely fractured (ISS) • No observed surface NH3

7. March Flyby apod.nasa.gov • Confirmed atmospheric source from SPT (MAG)

8. July Flyby • High resolution images up to 4m/pixel (ISS) • Large boulders scattered throughout the southern terrain • Carved with tectonic features, virtually no impacts • Prominent 130-km ‘tiger stripe’ features • 114-157 K graybody temperatures in south polar region (CIRS) • Anomalously warm compared to the rest of Enceladus • Coincide with tiger stripes • Plume of water vapor and icy particles emanating from south polar region (no gaseous NH3) (INMS)

9. November ISS Imaging • High phase angle, high-resolution to analyze southern plumes • Large plume forming over the southern polar region sourced by multiple jets of fine particles • Higher phase angle revealed more near surface jets

10. South Polar Terrain • Interior (below of 55°S) • Covers 70,000 km2 (~9% of surface) • Unusual albedo and color patterns • Geologically young • Source of atmospheric particles • Cross-cutting tiger stripes

11. Surface • Bright surface of fine grained particles • Highest resolution show hummocky or block-covered surfaces between cross-cutting fractures • High fraction of surface covered by blocks 20-80 meters not-likely associated with craters • Complex terrain predating tiger stripes with 10-100m relief

12. South Polar Terrain • Separated by a continuous, sinuous chain of scarps and ridges • Boundary is interrupted by ‘Y-shaped’ discontinuities that taper northward and confine parallel chains of convex ridges and troughs • Discontinuities, interpreted as fold belts, are hundreds of meters higher than surrounding terrain

13. Tiger Stripes • Linear depressions 500 m deep, 2km wide, 130km in length • Surrounded by 100m high ridges on both sides • Spaced ~35km apart, with approximately parallel orientation and shape • Strike direction 45° offset from direction of Saturn • Terminate in prominent hook-shaped bends in the anti-Saturnian hemisphere, and bifurcate in dendritic patterns in the sub-Saturnian hemisphere • Associated with the highest temperatures

14. Spectra of SPT • South polar terrain plains are 10% brighter than the average of Enceladus • Tiger stripe dark material extends outside of the feature on both sides a few km • Greatest contrast in brightness on Enceladus exists between stripes and surrounding material • Thin bands of spectrally distinct material on valley floors • Broadband spectra of all material is consistent with pure water ice

16. Cratering • Highest variety in crater count among Saturnian objects • Heaviest cratering in isolated areas outside of SPT surrounded by troughs and fracturing • Lowest cratering within SPT, with no craters >1km • Scaled impactor flux from Iapetus to Enceladus • Two scenarios: both show discrete ages of different terrains

17. Shape • Determined from 23 limb profiles • Departure from mean ellipsoid ~2km • Longitudinally averaged limb heights range from 400m below the mean at the south pole, and 400m above the mean at 50°S • If homogenous, it is close to an equilibrium ellipsoid (ideal difference between long and short axis of 8.5, real difference of 8.3)

18. Density • Uses measurements taken from Cassini, Earth based telescopes, Hubble, and Voyager • Density = 1608.3 ± 4.5kg m-3 • 4 Scenarios modeled • Model 1: Homogenous • Model 2: 10.6 km ice crust, 1700kg m-3 core • Model 3: 20.5 km ice crust, 1800kg m-3 core • Model 4: 2700kg m-3 core

19. Orbit • Dense, small core requires relaxation at a higher rotation rate (Model 4 requires relaxation 0.87-0.95 of current orbit) • Standard orbit evolution models do not allow for more than 5% change from current orbit • Outward orbital evolution would push shape toward more hydrostatic form, but tectonic patterns suggest movement towards a more oblate body • Libration frequency/spin frequency ε = [3(B-A)/C]1/2 ~ 0.25 for all models • No present libration detected using 1375 measurements of 190 control points in 129 images, with uncertainties allowing for a maximum libration of 1.5°

20. Particle Plume • One large plume was discovered prior to November ISS imaging • Higher angle imaging discovered numerous near surface jets supplying a much larger fainter plume • Particles visible only at high angle indicate fine forward-scattering particles • Absolute brightness from ISS imaging determined particle density with altitude and particle escape rate • Best fit has mean velocity of 60m s-1 • ~1% of upward moving particles supply E-ring • Particles supplying the E-ring have a mean velocity of 90m s-1 upon leaving Enceladus

21. Discussion Libration? Heat Balance? Plume Origin? Water?

22. Plume Origin: Sublimation • Can occur above or below ground • Can occur below 273K • Mass of water vapor measured (UVIS) compared to the mass of ice calculated (ISS) estimates a high ice/gas ratio • Ice unlikely to condense out of vapor: ~20x entropy change for vapor to condense than expand. • Ice could be entrained in vapor (as in a comet) but this should create a dark crust, not a bright surface

23. Plume Origin: Reservoir • Requires a liquid subsurface reservoir at >273K • Liquid can freeze into ice out of the vent into the plume • Source cannot contain NH3 • Assuming 7m depth for a reservoir (to achieve triple point pressure), when pressure is released volume per mole of vapor becomes 24,000x that of liquid water

24. Heat Balance • 2:1 mean motion resonance with Dione tidal heating rate of 1.2 x 105 ergs s-1 • Maximum allowed libration would yield heating rate of 1.8 x105 ergs s-1 • Radiogenic heating based on condritic composition provides ~ 3.2 x 105ergs s-1 • Total of 4-6 x 105ergs s-1, ~10% total power of SPT • Previous heating required for present heat balance to explain plumes

25. Libration • 2:1 mean motion Dione resonance does not allow for a past eccentricity greater than current • 1:4 secondary libration with 22° amplitude yields heating 100x present rates • Insufficient internal heating could allow for oblate shape and non-differentiation but still dampen the resonance • Possible non-uniform relaxation • Symmetry on the surface shows a change in tectonic stresses • Problems • Absence of similar circumpolar features in the northern hemisphere • Lack of a plausible mechanism for increased flattening

26. Plume Salt • Three compositional types previously detected in the E-ring • Discovered in plume during a 21km flyby • Lack of observed sodium in vapor Postberg et al., 2007

27. References • R. H. Brown et al., Science 311, 1425 (2006). • G. Neukum, B. A. Ivanov, W. K. Hartmann, Space Sci. Rev. 96, 55 (2001) • G. Neukum, Adv. Space Res. 5, 107 (1985). • C. C. Porco et al. Science 311: 1393-1401. (2006) • F. Postberg et al. Nature 474: 620-622. (2011). • N. M. Schneider et al. Nature 459: 1102-1104. (2009). • J. Wisdom, Astron. J. 128, 484 (2004)