C. J. Hansen, L.
Esposito, D. Shemansky,
A. I. F. Stewart, A.
Hendrix
23 May 2011

UVIS occultations gives indicate the composition and structure of the gas flowing from Enceladus’ tiger stripe fissures
• Plume Results
– Composition
– Mass flux
– Temporal variability
• Gas Jets
– Structure
– Mach number
Plume
Jets

UVIS has 4 separate channels
For stellar occultations:
• Far UltraViolet (FUV)
– 1115 to 1915 Å
– 2D detector: 1024 spectral x 64 onemrad spatial pixels
• Binned to 512 spectral elements
– 5 sec integration time
• High Speed Photometer (HSP)
– 2 or 8 msec time resolution
– Sensitive to 1140 to 1915 Å
•
Hydrogen-Deuterium Absorption Cell
(HDAC) not used
•
For the solar occultation:
Extreme UltraViolet (EUV)
• 550 to 1100 Å solar port
• 2D detector: 1024 spectral x 64 one-mrad spatial pixels
• No spatial information because signal from sun is spread across the detector
(deliberately)
• Spatial rows 5 - 58 binned to two windows of 27 rows each
• 1 sec integration

• UVIS occultations of stars and the sun probe Enceladus’ plume
• Three stellar and one solar occultation observed to-date
Zeta Orionis
• Feb. 2005 - lambda Sco
• No detection (equatorial)
• July 2005 - gamma Orionis
• Composition, mass flux
• Oct. 2007 - zeta Orionis
• Gas jets
• May 2010 - Sun
• Composition, jets

2005 - gamma Orionis Occultation
2007 - zeta Orionis Occultation
2010 - Solar Occultation

FWHM
Total duration of Solar Occ:
1min 35sec
Duration for full-width half max:
53 sec
Line of sight velocity: 2.85 km/sec
Width of plume at FWHM:
56 sec * 2.85 = 150 km
Compare to zeta Orionis Occ
– Zeta Orionis occultation lasted just 10 sec
– Line of sight velocity = 22.5 km/sec
– Width of plume at FWHM = 110 km
– HSP data summed to 200 msec so 50 samples
Zeta Orionis occultation

Plume
UVIS Ultraviolet Spectra provide constraints on:
• Composition, from absorption features
• Column density
– Mass Flux
• Plume and jet structure
Jets
Terminology:
• Plume - large body of gas and particles
• Jets - individual collimated streams of gas and particles

Two science objectives enabled by solar (rather than stellar) occultation:
1. Composition of the plume
New wavelength range: EUV
H
2
O and N
2 have diagnostic absorption features at EUV wavelengths
The primary goal was to look for N
2
, on basis of
INMS detecting a species with amu=28
2. Structure of the jets and plume
Higher time resolution, better snr

Navy is unocculted solar spectrum, with typical solar emissions
Red is solar spectrum attenuated by Enceladus’ plume

• H
2
0 fit to absorption spectrum
• Column density
= 0.9 +/- 0.23 x
10 16 cm -2
• No N
2 absorption feature: N
2 upper limit of 5 x 10 13 cm -2

Actual
• No dip is seen at all at 97.2 nm
• Upper limit < 0.5%
Consequences of no N
2 models of the interior for
• High temperature liquid not required for dissociation of NH
3
(if there is NH
3 in the plume)
• Percolation of H
2
O and NH
3 through hot rock is not required
Predict
• N
2 feature at 97.2 nm fortuitously coincides with strong lyman gamma emission so lots of signal available
• Very sensitive test!
• Clathrate decomposition is not substantiated for N plume propellant
2 as the

• Spectra are summed during the center 60 sec of the occultation, then divided by a 650 sec average unocculted sum to compute I /
I
0
• The extinction spectrum is well-matched by a water vapor spectrum with column density = 0.9 +/- 0.23 x 10 16 cm -2
• Overall amount of water vapor is comparable to previous two
(stellar) occultations
– 2005: 1.6 x 10 16 cm -2
– 2007: 1.5 x 10 16 cm -2 (maximum value of 3.0 x 10 16 cm -2 at center)
• Lower value in 2010 is due to the viewing geometry – the H
2 flux is in family with the previous results
O

Blue ground track is from zeta Ori occ on
Rev 51
Orange is solar occ track, ~orthogonal
Plume is elongated.
If total flux is same then column density will be less by ~ 2/3
Solar occ result:
0.9 x10 16 cm -2 , ~2/3 of value in 2005

Ingress

Egress Basemap from Spitale & Porco, 2007

Estimate of Water Flux from Enceladus =
200 kg/sec
S = flux
= N * x * y * v th
= (n/x) * x * y * v th
= n * y * v th
Where
N = number density / cm 3 x * y = area v th y = v los
* t => FWHM
= thermal velocity = 45,000 cm/sec for T = 170K n = column density measured by UVIS
Year
2005
2007
2010 note that escape velocity = 23,000 cm/sec n
(cm -2)
1.6 x 10
1.5 x 10
0.9 x 10
16
16
16 y
(x 10
80 (est.)
110
150
5 cm) v th
(cm / sec)
45000 5.8 x 10 27
45000 7.4 x 10 27
45000 6 x 10 27 v
Flux:
Molecules / sec x
Flux:
Kg/sec
170
220
180

Minimum altitude a b c d e f
• Window 0 and 1 matching features => jets are real

• As before, gas jets appear to correlate to dust jets
Spacecraft viewed sun from this side
Feature Altitude a
* (km)
20
Dust
Jet
Alexandria
IV
Closest approach b
19.7
21 Cairo V and/or VIII
Baghdad I c d e
27
30
38
Baghdad VII
Damascus
III
Damascus II f 46
* Altitude of ray to sun from limb
Egress
Ingress

Minimum
Altitude
Basemap from Spitale & Porco, 2007

Summary of 2007 results
• Significant events are likely gas jets
• UVIS-observed gas jets correlate with dust jets in images
• Characterize jet widths, opacity, density
• Density in jets ~2x density in background plume
• Ratio of vertical velocity to bulk velocity = 1.5, supersonic
Supersonic gas jets are consistent with
Schmidt et al. model of nozzle-accelerated gas coming from liquid water reservoir

Optical Depth
• Higher SNR enables better measurements of jets’ dimensions – more clearly distinguished from background plume
• Density of gas in jets is twice the density of the background plume
• The jets contribute 3.4% of the molecules escaping from
Enceladus, based on comparison of the equivalent width of the broad plume compared to the jets’ total equivalent width

Comparison to INMS results from E7

• The full width half max (FWHM) of jet c (Baghdad I) is ~10 km at a jet intercept altitude of 29 km (z
0
)
• Estimating the mach number as ~2 z
0
/FWHM the gas in jet c is moving at a Mach number of 6; estimates for the other jets range from 5 to 8
• Previously estimated mach number (from 2007 occultation) was 1.5
• Jets more collimated than previously estimated
• New estimate for vertical velocity: if v sound then v vert
= 1920 m/sec
= 320 m/sec (for ~170 K)
• This is an upper limit because the gas will be cooled in a nozzle

• Composition
– Upper limit on N
2 of 5 x 10 13 cm -2
– H
2
0 column density = 0.9 x 10 16
• In family with previous occultations cm -2
• Suggests that Enceladus has been steadily erupting for past 5 years
• Plume / jet structure
– Flux of water from 3 occultations is ~200 kg/sec
– Jets are more collimated than estimated from
2007 occultation
• Mach numbers of 5 to 8

Supersonic gas jets are consistent with Schmidt et al. model of nozzle-accelerated gas coming from liquid water reservoir
High velocity jets are also consistent with CDA data reported by Postberg et al. showing compositional differences: salt-poor particles reaching the E ring and salt-rich particles in the diffuse component of the plume close to Enceladus
Lack of N
2 in presence of NH
3 means that a relatively cool liquid reservoir such as “Perrier Ocean” proposed by Matson et al. is viable