Moons of Saturn

14 October 2013

Iapetus

Mimas

Enceladus

Most large Jovian Planet satellites are smaller than our moon. Based on the geological principles controlling Terrestrial Planets, we expect cold, dead worlds,

covered by craters…

Io Europa Triton Enceladus Titan (Jupiter) (Neptune) (Saturn)

NOT SO!

Instead, we got Io (left) Enceladus (right) and other active moons.

Enceladus… the next Io???

Enceladus --- the next Io?

Magnetic perturbation-> local ionization

Stellar occultation -> gas in plumes

So, what is cryovolcanism?

Enceladus’ Cryovolcanic Style

Enceladus’ Cryovolcanic Style

• Enceladus jets: water escapes at ~200 kg/sec!

• Io’s eruptions don’t reach escape velocity!

• Why the difference?

UVIS

UVIS has 4 separate channels:

• Far UltraViolet (FUV)• 110 to 190 nm

• 3 slit widths => 2.8, 4.8, 24.9 nm spectral resolution

• 2D detector: 1024 spectral x 64 one-mrad spatial pixels

• Extreme UltraViolet (EUV)• 55 to 110 nm

• 3 slit widths => 2.8, 4.8, 19.4 nm spectral resolution

• 2D detector: 1024 spectral x 64 one-mrad spatial pixels

• Solar occultation port

• High Speed Photometer (HSP)• 2 - 8 msec time resolution

• Hydrogen – Deuterium Absorption Cell (HDAC)

For the occultations we used the • HSP with 2 msec time resolution• FUV with 512 spectral channels (1.56 nm resolution), 5 sec integration time

UVIS

UVIS has 4 separate channels:

• Far UltraViolet (FUV)• 110 to 190 nm

• 3 slit widths => 2.8, 4.8, 24.9 nm spectral resolution

• 2D detector: 1024 spectral x 64 one-mrad spatial pixels

• Extreme UltraViolet (EUV)• 55 to 110 nm

• 3 slit widths => 2.8, 4.8, 19.4 nm spectral resolution

• 2D detector: 1024 spectral x 64 one-mrad spatial pixels

• Solar occultation port

• High Speed Photometer (HSP)• 2 - 8 msec time resolution

• Hydrogen – Deuterium Absorption Cell (HDAC)

For the occultations we used the • HSP with 2 msec time resolution• FUV with 512 spectral channels (1.56 nm resolution), 5 sec integration time

Plume Composition is Water Vapor

The absorption spectrum of water is shown compared to Enceladus’ plume spectrum (I/I0) for a water column density of n = 1.5 x 1016 cm-2

I=I0 exp (-n*)

I0 computed from 25 unocculted samples

n = column density

= absorption cross-section, function of wavelength

Estimation of Enceladus Water Flux

• S = flux = N * h2 * v = n/h * h2 * v = n * h * v

WhereN = number density / cm3

h2 = areav = velocityn = column density measured by UVIS

Estimate h from plume dimension, = 80 km

Estimate v from thermal velocity of water molecules in vapor pressure equilibrium with warm ice (600 m/sec for surface temperature ~ 180K – note that escape velocity = 230 m/sec)

S = 1.5 x 1016 * 80 x 105 * 60 x 103 = 0.7 x 1028 H2O molecules / sec

= 200 kg / sec

h

v

Plume Structure (2005)

Water vapor abundance calculated from each 5 sec spectrum.

The 2005 water profile is best fit by an exponential curve.

The best fit scale length is 80 km

Enceladus Plume Occultation of zeta Orionis October 2007

• In October 2007 zeta Orionis was occulted by Enceladus’ plume• Perfect geometry to get a horizontal cut through the plume and detect density variations indicative of gas jets• Objective was to see if there are gas jets corresponding to dust jets detected in images

Groundtrack of Ray

2005 2007

Enhanced HSP absorption features a, b, c, and d can be mapped to dust jets located by Spitale and Porco (2007) along the tiger stripes

Plume or jets?

• The plume of gas and dust from Enceladus includes a number of individual jets seen by Cassini camera and by UVIS

Best fit of 8 sources from Spitale & Porco to match UVIS occultation profile

Brightness of water vapor over Enceladus South pole from UVIS 8-jet model

Tiger Stripes

IR images -> Temperature:Tiger Stripesare warm.

Tiger Stripes close-up

Even closer!