Enceladus Plume Update C. J. Hansen, I. Stewart, L. Esposito, A. Hendrix June 2009

Work-in-Progress Plume Composition Individual jets vs. gas release along tiger stripe

Background INMS detects a species with atomic mass = 28 Previously thought that this must be CO or N 2 New idea (consistent with other INMS data) is that it could be ethylene –C 2 H 4 = 2 * = 28

Ethylene Absorption Cross-sections in FUV Don supplied absorption cross-sections for C 2 H 4 from Wu, 06/2004, data acquired at T = 150K

Water only, Rev 11 Best Fit Rev 11 gamma Orionis occultation Water only, uses Mota cross-sections Column density = 1.6 x cm -2

Ethylene-only at 10% H 2 O Column Density Rev 11 gamma Orionis occultation Ethylene only Column density = 1.6 x cm -2 This is just for illustration, not really this much ethylene!

Ethylene + Water at 3% H 2 O Column Density Rev 11 gamma Orionis occultation Ethylene plus water C 2 H 4 column density = 4.8 x cm -2 H 2 O column density = 1.6 x cm -2

Ethylene at 3% H 2 O Column Density compared to H 2 O only Rev 11 gamma Orionis occultation Ethylene plus water compared to water only C 2 H 4 column density = 4.8 x cm -2 H 2 O column density = 1.6 x cm -2 Water only is still best fit to occulted spectrum although there are some interesting matches to small dips with ethylene added in

Summary and Future Work Data has too much scatter to definitively say yes or no to presence of ethylene Star does drift during the course of the occultation - can we pull out a better I 0 with more careful selection of the records? Need to run Ian’s regression routines to quantitatively compare the fit of water-only vs. water + ethylene

FUV analysis Occ is easy to detect Star drifted from pixel 13 to pixel 12 over the course of the observation

Methanol Not likely to be detectable, doesn’t look like a good fit anyway…

Individual jets vs. broad outgassing Debate about whether gas was coming primarily from jets or all along tiger stripes Implications for being able to compare the 2005 occ to the 2007 occ

Zeta Orionis Occultation 2007 FUV and HSP data collected FUV: 5 sec integration HSP: 2 msec sampling zeta Orionis Horizontal density profile True anomaly = gamma Orionis Vertical cut through plume True anomaly = 98 Key results: Dominant composition = water vapor Plume column density = 1.6 x /cm 2 Water vapor flux ~ 180 kg/sec

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 Occultation Blue line is groundtrack Roman numerals correspond to ISS dust jet sources

Gas Jets a. Cairo (V) d. Damascus (II) c. Baghdad (VI) Closest point b. Alexandria (IV) Ingress Egress Density in jets is twice the background plume Gas jet typical width = 10 km at 15 km altitude

Absorption Features, Compared to Dust Jet Locations Plotted here are: Altitude above Enceladus' limb of the line-of-sight from Cassini to the star Attenuation of the HSP signal, scaled by a factor of 300 Projections of the 8 jets seen by the ISS into the plane of the figure Jets assigned a length of 50 km (for purposes of illustration) C/A marks the closest approach of the line-of-sight to Enceladus. The times and positions at which the line-of-sight intersected the centerlines of the jets are marked by squares. The slant of the jets at Baghdad (VII) and Damascus (III) contribute to the overall width of the plume bacd

Groundtrack of Ray

2005 HSP data HSP data can be fit by an exponential Look for departures due to jets Appear to see real features

2005 Jets Jets mapped to increases in opacity In this occ we do not see B7 (star is occulted by limb before crossing B7) Is it OK to compare 2005 and 2007? IF individual jets are only source of plume then no If gas from entire tiger stripe probably ok

Compare 2007 to HSP 2005 attenuation <6% at 15 km 2007 attenuation at same altitude ~10% Overall attenuation clearly higher in 2007 compared to 2005 The ratio of the opacity from 16 to 22 km between 2007 and 2005 is 1.4 +/- 0.4.

2007 Plume Simulation Ian has modeled plume water vapor He now agrees that gas needs to come from along tiger stripe, not just jets

Backup slides

Gas Jet Model Gas Jets are idealized as sources along the line of sight with thermal and vertical velocity components Source strength is varied to match the absorption profile. The ratio of thermal velocity (vt) to vertical velocity (vb) is optimal at vt / vb = Higher thermal velocities would cause the streams to smear together and the HSP would not distinguish the two deepest absorptions as separate events. At least 8 evenly-spaced gas streams are required to reproduce the overall width of the absorption feature (there may be more). Key Result: V thermal / V bulk = 0.65 Flow is supersonic

Water Column Density: FUV comparison to HSP FUV integrations are 5 sec duration FUV spectrum shows gas absorption in time records 89 and 90 Higher time resolution of HSP data shows that the peak column density is about 2x higher FUV time record 89 FUV time record 90

High Speed Photometer (HSP) Data HSP is sensitive to 1140 to 1900Å Statistical analysis applied to find features that are probably real –Assumes signal is Poisson distribution –Calculate running mean Six different bin sizes employed, absorptions compared, persistence of feature is part of test m is the number of such events one would expect to occur by chance in the data set m<<1 are likely to be real events Possible real features: 1 (a) m = (b) m = (c) m = (d) m = 0.026

Rev 3 CIRS_003EN_FP1FP3MAP 001 (CIRS is prime) T00:15 UVIS observation: UVIS_003EN_ICYLON004_ CIRS Range: km Subs/c lat: 0 Subs/c lon: 299 These values are for the start times of the observation CIRS IFOV is shown

Rev 11 CIRS_011EN_FP3GLOBAL T13:54 UVIS observation: UVIS_011EN_ICYLON003_CIR S Range: 198,220 km Subs/c latitude: -37 Subs/c longitude: 144 CIRS IFOV shown

Rev 11 CIRS_011EN_FP3REGION T15:24 UVIS observation: UVIS_011EN_ICYLON006 Range: 141,252 km Subs/c latitude: -41 Subs/c longitude: 158 CIRS IFOV

Rev 61 CIRS_061EN_FP34MAP T16:36 UVIS observation: UVIS_061EN_ICYLON003_ CIRS Range: 126,677 km Subs/c latitude: 69 Subs/c longitude: 112 This is CIRS IFOV, so our slit will extend well off limb, but probably too far north

Rev 61 UVIS 061EN_ICYMAP002_PR IME (our observation) T17:40 Range: km Subs/c latitude: 69 Subs/c longitude: 121 Too far north Start End

Rev November 2009 CIRS is driver T08:03 UVIS observation: UVIS_120EN_ICYMAP00 2 Range: 9473 km Subs/c latitude: -1 Subs/c longitude: 163 Note that the observation following this one is our observation of the plume with Saturn in the background

Enigmatic Enceladus High density dust jets Are there corresponding high density gas streams?

Future Plans Regression analysis

Ethylene at 10% H 2 O Column Density, reduced by ~10% Rev 11 gamma Orionis occultation Ethylene plus water C 2 H 4 column density = 1.6 x cm -2 H 2 O column density = 1.5 x cm -2

Ethylene at 10% H 2 O Column Density Rev 11 gamma Orionis occultation Ethylene plus water C 2 H 4 column density = 1.6 x cm -2 H 2 O column density = 1.6 x cm -2