Rev 131 Enceladus’ Plume Solar Occultation LW Esposito and UVIS Team 14 June 2010

Solar Occultation Geometry 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 Characteristics 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 binned to two channels of 27 rows each “Window 0” => Rows 5 to 31 “Window 1” => Rows 32 to spectral elements

Occultation is clearly visible 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: T05:51:44.45 Observation end time: T06:10:36.45 Ingress: T06:00:40.45 Egress: T06:02:59.45 Velocity of sun across plane of sky ~ 2.75 km/sec Data shown is summed over wavelength

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

Composition 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 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%

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

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

Water in the Plume H 2 0 fit to absorption spectrum

Water Vapor Abundance 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 cm -2 Overall amount of water vapor is comparable to previous two (stellar) occultations –2005: 1.5 x cm -2 –2007: 1.3 x cm -2 (maximum value of 2.6 x cm -2 at center) Total water flux constant within +/- 50%

Ground Tracks 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

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

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

Jet Locations 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 Minimum altitude

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

Altitude of Sun above Limb Minimum ray height plotted Closest approach between a and b a b dc e f

Gas Velocity 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 h w/h = tan ß = v thermal / v vert = 7 / 27 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

Outlier Absorption outside plume, may be solids

Analysis underway Improve H 2 O cross-sections (some absorption features not attributed to H 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