The sun was occulted by Enceladus’ plume 18 May 2010
Two science objectives enabled by solar (rather than stellar) occultation:
1. Composition of the plume
New wavelength range
2. Structure of the jets and plume
Higher spatial resolution

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 or 8 msec time resolution
• Hydrogen –Deuterium Absorption Cell
(HDAC)
For the solar occultation we used the EUV solar port
• 1 sec integration, 1132 time records
• No spatial information because signal from sun is spread across the detector (deliberately)
• Spatial rows 5 - 58 binned to two channels of 27 rows each
• “Window 0” => Rows 5 to 31
• “Window 1” => Rows 32 to 58
• 1024 spectral elements

• Window 0 has higher counts, but overall shape is the same
– Position of sun was slightly offset from center, but not an issue
•
Observation start time: 2010-138T05:51:44.45
• Observation end time: 2010-138T06:10:36.45
• Ingress: 2010-138T06:00:40.45
• Egress: 2010-138T06:02:59.45
• Velocity of sun across plane of sky ~ 2.75 km/sec
• Data shown is summed over wavelength

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

• H
2
O and N
2 have diagnostic absorption features at
EUV wavelengths
• The primary goal was to look for N
2 detecting a species with amu=28
, on basis of INMS
• No N
2
(upper limit = 3 x 10 13 , so < 0.3%)
– AMU = 28 detected by INMS is not N
2
– It is not CO in the plume (or UVIS would have seen it in our stellar occs) or it is < 3%

Actual
• No dip is seen at all at 97.2 nm
• Upper limit < 0.3%
Predict
• N
2 feature at 97.2 nm fortuitously coincides with strong lyman gamma emission so lots of signal available
• Very sensitive test!
Consequences of no N
2 the interior for models of
• High temperature liquid not required to dissociate NH the plume)
3
(if there is NH
3 in
• Percolation of H
2
O and NH hot rock is not required
3 through
• A catalyst for decomposition is not required at lower temperatures
• No evidence for clathrate decomposition to propel the plume

No N
2 at 97.2 nm
Absorption is due solely to water vapor
N
2
< 0.3%

2

• To calculate water vapor abundance in the plume 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
– I
0 computed at two different times, results were the same
• The extinction spectrum is well-matched by a water vapor spectrum with column density = 0.9 +/- 0.1 x 10 16 cm -2
• Overall amount of water vapor is comparable to previous two
(stellar) occultations
– 2005: 1.5 x 10 16 cm -2
– 2007: 1.3 x 10 16 cm -2 (maximum value of 2.6 x 10 16 cm -2 at center)
• Total water flux constant within +/- 50%

Blue ground track is from zeta Ori occ on Rev 51
Orange is solar occ track, ~orthogonal
Since the plume is elongated, the total flux is same as for
2005, 2007 data

Ingress

Egress

FWHM
Total duration of Solar Occ:
2min 19sec
Duration for full-width half max:
56 sec
Width of plume at FWHM:
145.8 km
Compare to zeta Orionis Occ
– Zeta Orionis occultation lasted just 10 sec
– HSP data summed to 200 msec so 50 samples
Zeta Orionis occultation

Density in jets ~2x density in background plume
Plume
Jets

Minimum altitude a b c d e f
• Window 0 and 1 features match => jets
• Repetition of features in window 0 and window 1 shows they are not due to shot noise

• As before, gas jets correlate to dust jets
Spacecraft viewed sun from this side
• Higher spatial resolution because sun’s passage behind the plume was slower
• Most gas between jets, does not peak at minimum altitude
Ingress

Minimum
Altitude
Egress

a b c d e f
• Minimum ray height plotted
• Closest approach between a and b

• In the case of jet c (Baghdad I) the source of the jet is close enough to the occultation ground track to calculate the height and width
• Half-width of jet c = 2.5 sec * 2.75 km/sec = ~7 km ( w ) at ~27 km ( h ) altitude w w/h = tan ß = v thermal
/ v vert
= 7 / 27 h
Mach number = v vert
/ v thermal
= 3.9
• Mach number from 2007 occultation was 1.5
• New estimate for jet vertical velocity: if v thermal
= 400 m/sec (for ~180 K) then v vert
= 1500 m/sec

• Absorption outside plume, may be solids

• Improve H
H
2
O cross-sections (some absorption features not attributed to
2
O may be visible at higher resolution)
• Quantify differences in column density, water flux compared to 2005, 2007
• Quantify partitioning between gas in jets and gas in overall plume
• Model jets, 3D structure