Cassini Observes the Active South Pole of Enceladus

Porco, C. C., et al.

Presented by Patrick Harner

www.nasa.gov

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

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

Cassini FlybysThree 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

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

March Flyby

•Confirmed atmospheric source from SPT (MAG)

apod.nasa.gov

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)

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

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

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

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

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

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

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

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)

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

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°

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

Discussion

Heat Balance?

Plume Origin?

Libration?

Water?

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

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

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

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

Plume Salt

Postberg et al., 2007

•Three compositional types previously detected in the E-ring

•Discovered in plume during a 21km flyby

•Lack of observed sodium in vapor

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)