# Presentation: 250901

## Magnetic Portraits of the Icy Satellites of Saturn

**Magnetic Portraits of the Icy Satellites of Saturn**

**K. K Khurana (1) M.K. Dougherty (2),  P. Travnicek (3), C.T. Russell (1), F. M. Neubauer**
**(1) Institute of Geophysics and Planetary Physics, UCLA, Los Angeles, CA, 90095, USA, **_**kkhurana@igpp.ucla.edu**_
**(2) Imperial College, Department of Physics, London SW7 2AZ.**
**(3) Institute of Atmospheric Physics, Czech Academy of Sciences, Prague, Czechoslovakia, **
**(4) Institut f ̈ur Geophysik und Meteorologie, ****University of Cologne, Germany**

## Slide 2

**Notes:**
There are 49 known satellites of Saturn:
Titan radius 2575 km, Enceladus radius 250 km. I will talk about five of them today.

## The regular portraits of the icy satellites I will discuss today(not to scale)

- Enceladus

## Physical Properties of Saturn’s icy satellites

| Satellite | Mean Radius
(km) | Saturn distance (RS) | Orbital
Eccentricity | Orbital Inclination | Density
gm/cm3 | Visual Albedo | Mean Surface Temp.
(K) |
| --- | --- | --- | --- | --- | --- | --- | --- |
| Mimas | 198.5 | 3.08 | 0.02 | 1.53 | 1.17 | 0.5 | 64 |
| Enceladus | 249.4 | 3.95 | 0.0045 | 0.02 | 1.61 | ~ 1.0 | 75 |
| Tethys | 529.9 | 4.89 | 0 | 1.86 | 1.26 | 0.9 | 86 |
| Dione | 560.0 | 6.26 | 0.0022 | 0.02 | 1.44 | 0.7 | 87 |
| Rhea | 764 | 8.74 | 0.0010 | 0.35 | 1.24 | 0.7 | 76 |
| Hyperion | 143 | 24.575 | 0.1042 | 0.43 | 0.6 | 0.3 | ?? |

**Physical Properties of Saturn’s icy satellites**

- Source: http://nssdc.gsfc.nasa.gov/planetary/factsheet/saturniansatfact.html

## Field and Plasma conditions near icy satellites

**Saturn has a strong magnetic field (surface equatorial strength = 0.2 Gauss) which is surprisingly axis-symmetric but tilted in the opposite sense to the Earth’s magnetic field.**
**All of the major satellites of Saturn with the exception of Iapetus are located inside the magnetosphere of Saturn.**
**The corotating plasma of Saturn’s magnetosphere overtakes the moons and preferentially sputters neutrals from their lagging sides.**

- Source: Bagenal, 1994

## Single Particle Motion in a Dipole FieldImage courtesy:  Windows to the universehttp://www.windows.ucar.edu/

*m d****v****/dt = q****v********B***

- Particle Gyration

- Particle Bounce

- Particle *B*** **drift

**F ****|| ***= -***||B

***1⁄2****m *v*┴**2**/*B

**r****L****= mv***┴**/q *B         = A few km to tens of km for a 1 keV O+ near the icy satellites.

## Field and Plasma Properties near the Icy moons

##      Trajectories and coordinate                                              		systemX along the plasma flow direction.Y positive towards SaturnZ axis oriented along the rotation axis of Saturn

## MHD interaction of a mass loading/conducting moon with flowing plasma

**Notes:**
The interaction of a moving plasma with a “conducting” moon is very well understood. The field lines cannot cross through the conductor and therefore drape around the moon (figure on the right). The field lines are slowed down ahead of the moon (left figure) but their ends at large Z distance move unperturbed. This causes a bend in the field lines. The bend propagates along the field line at the Alfven speed while being carried downstream by the flowing plasma. Notice that upstream of the moon (where all three encounters took place), above the moon, the Bx component is negative, whereas below the moon it is positive.

## Enceladus

- Discovered by William Herschel on August 28, 1789.
- Mean radius of 252.1 km and density 1.61 g/cm3.
- May be fully differentiated with an icy crust and rocky core.
- Currently in a 2:1 orbital resonance with Dione.
- Wide range of surface types, from heavily cratered old to relatively crater-free young regions near the south pole.
- Enceladus is tectonically active today where eruptions have been observed
- (Other such moons are Jupiter’s Io and Neptune’s Triton.)
- Analysis of outgassing material is consistent with a subsurface source of liquid water.
- Particles from Enceladus populate the E-ring (~4 - 10 RS) and are the main population source for the magnetosphere of Saturn.

## Enceladus

## Enceladus deep color view of transitional terrain between smooth plains and cratered terrain 

- 150 meters/pixel

## Tiger Stripes 

**Notes:**
covered in tectonic fractures and ridges. < 10-100 My surface age.
Results from the Visual and Infrared Spectrometer (VIMS) instrument suggest that the green-colored material surrounding the tiger stripes is spectrally distinct from the rest of the surface of Enceladus. VIMS detected crystalline ice in the stripes, suggesting that they are quite young (likely less than 1000 years old). Vims also detected organic compounds in Tiger Stripes.

## Slide 14

## Modeling of Alfven wing current system

**Notes:**
We use a computational model of the Alfven wings developed for the studies of Io’s Alfven wing. In these models, the currents flow inwards toward the moon along the Alfven characteristics from both above and below on the Saturn-ward side and exit along the Alfven Characteristics on the side away from saturn.  By using nominal values of plasma conditions we find that for E3 the current flowing through each of the wings is ~ 4X10^4 Amps.

## Shifted mass-loading region:  E3

- I = 105 Amp, Mass pick-up rate = ~ 2kg/s

**Notes:**
And here is a fit to the E3 data with the same southward shift of the mass-loading region. The reasonable nature of the fit suggests that our conjecture that the plume is the source of mass-loading is correct.

## Modeling of E4 as a cloud centered at Enceladus fails

- Notice that even though the spacecraft was below Enceladus nearest the closest approach (Z is negative), the observed Bx component was negative.

**Notes:**
Modeling of E4 which was a closer flyby by using the same parameters and the technique used for E3 fails completely. Notice that even though the spacecraft was below Enceladus nearest the closest approach (Z is negative), the observed Bx component was negative. For an Alfven wing a positive Bx component would be expected. This shows that there is some different physics operating closer to the moon.

## Enceladus’s plume interacts with plasma

## Fountains of Enceladus

## Shifted mass-loading region:  E4

** ****Imaging Observations have shown that a plume near the south pole spouts dust and gas into space. Thus the mass-loading region would be expected to be shifted below Enceladus. Here we model the E4 signature with a mass-loading region located 4 R****E**** below Enceladus. **

## Shifted mass-loading region:  E3

**And here is a fit to the E3 data with the same southward shift of the mass-loading region. The reasonable nature of the fit suggests that our conjecture that the plume is the source of mass-loading is correct.**

## Shifted mass-loading region:  E11

**And here are the fits to the E11 data. The phasing of the signature is correct but the poor fit suggests that we need to improve upon the specifics of the mass-loading region.**

## Rate of Mass loading

**After Chris Goertz**

**X**

**Z**

**J****y**

**The rate of mass-loading can be related to the current passing through the mass-loading region. Surprisingly, the total amount of mass loading implied is quite low.**

## Dione

- Discovered by Giovanni Cassini on March 21, 1684.
- Mean radius of 560 km and surprisingly high density of 1.5 g/cm3.
- Probably differentiated with rocky core making  1/3rd of the moon’s mass.
- Currently in a 2:1 orbital resonance with Enceladus.
- Terrain types consist of heavily cratered terrain, moderately cratered plains, lightly cratered plains, and wispy material.
- Several regions show evidence of past tectonic activity.
- The wispy material may have arisen from eruptions along cracks and then deposited on the surface.

## Dione

## Dione Fractures and wispy terrain

## Dione

**Dione**

- A close upstream flyby.
- CA distance 1.89 RDI
- A mass-loading type interaction.
- Field is draped around the mass loading region.
- X, Y and Z? signatures are consistent with a mass-loading interaction.
- Mirror modes have counterparts in plasma wave data.

## Dione perturbation consistent with mass-loading

- Cassini trajectory from left to right.
- Both x and y perturbations consistent with mass-loading.

## Phase space density in a supersonic plasma|| and refer to directions w.r.t.  fieldAssume vflow  B

**Mass absorbing super-magnetosonic interaction**

## Mass absorbing super-magnetosonic interaction

## Phase space density in a subsonic plasma|| and refer to directions w.r.t.  fieldAssume vflow  B

## Mass absorbing sub-magnetosonic interaction

## Tethys

- Discovered by Giovanni Cassini on March 21, 1684.
- Mean radius of 530 km and density of 1.21 g/cm3.
- Internal structure unknown but mostly pure ice.
- Currently in a 4:2 orbital resonance with Mimas.
- Terrain mostly heavily cratered. Surface age several billion years.
- Two minor moons Telesto and Calypso inhabit the same orbit and are located within Tethys' Lagrangian points L4 and L5, 60 degrees ahead and behind Tethys in its orbit respectively.

## Tethys

**Notes:**
Major features are Ithaca Chasma and Odysseus crater a large shallow crater.

## Tethys

- No internal field.
- A close downstream flyby.
- CA distance 3.83 RTe
- Shows an inert moon type “empty” wake which extends to high latitudes where some plasma has been absorbed upstream.
- Both Y and Z signatures are consistent with an absorbing moon model.

## Tethys

- Cassini trajectory from right to left.
- Both Y and Z signatures are consistent with an absorbing moon model.
- Field enhances in the wake to maintain pressure balance.
- IC waves are present in the wake.

## Rhea

- Discovered by Giovanni Cassini on December 23, 1672.
- Mean radius of 764 km and density of 1.24 g/cm3.
- Internal structure unknown but mostly pure ice.
- Currently in a 5:3 **near** orbital resonance with Dione.
- Terrain mostly heavily cratered. Like Dione shows leading/trailing hemisphere differences in cratering.
- A major resurfacing event occurred during its formation.
- Also shows wispy features like Dione but not associated with cracks.

## Rhea

## Rhea wispy features on the trailing side

## Rhea

- No internal field.
- A close downstream flyby at 1.65 RRH.
- Shows a classical Lunar type wake where plasma has been absorbed upstream.
- Field enhances in the wake to maintain pressure balance.

## Rhea

- Spacecraft trajectory from left to right.
- Field enhances in the wake to maintain pressure balance.
- Both Y and Z signatures are consistent with an absorbing moon model.
- X signature may be due to subsonic regime interaction.

## Hyperion

- Discovered by William Cranch Bond, George Phillips Bond, and William Lassell in September 1848.
- Diameter of 360×280×225 km and density of 0.6 g/cm3.
- Internal structure unknown but mostly pure ice.
- Currently in a 4:3 orbital resonance with Titan.
- Terrain mostly heavily cratered. Surface age > 4 billion years.
- Has one of the lowest albedos in the solar system suggesting it is covered by at least a thin layer of dark material.
- Because of the large eccentricity and resonance with Titan does not have a fixed rotation period or axis (it tumbles chaotically).

## Slide 43

## Hyperion

- Not a very close downstream flyby.
- CA distance 4.44 RHy
- No discernible features associated with Hyperion.

## Conclusions

- No internal magnetic fields in any of the satellites visited.
- Enceladus interacts with the corotating plasma of Saturn’s magnetosphere over an extended region (~ 6 RE).
- The plasma pick-up region is below Enceladus (~ 4 RE) and the total mass pick-up rate ~ 2 kg/s.
- Dione also mass-loads the corotating plasma. A hidden plume? We should look for it.
- The pick-up rates at Dione are at least an order of magnitude lower than those observed at Enceladus.
- Tethys and Rhea absorb the incoming plasma and form a plasma depleted wake.
- So far upstream trajectories showed evidence of plasma pick-up and downstream trajectories showed evidence of plasma depleted wakes. Coincidence? Feature of interaction? Need more flybys.