Enceladus Dual Star Occultation C. J. Hansen 6 January 2012

The Structure of Enceladus’ Plume from Cassini Occultation Observations Gave AGU Talk, Dec. 2011 C. J. Hansen, L. W. Esposito, B. Buffington, J. Colwell, A. Hendrix, B. Meinke, D. Shemansky, I. A. F. Stewart, R. A. West

UVIS Observations of Enceladus’ Plume Cassini’s Ultraviolet Imaging Spectrograph (UVIS) observes occultations of stars and the sun to probe Enceladus’ plume Composition, mass flux, and plume and jet structure Four stellar and one solar occultation observed to-date 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 Oct. 2011 – epsilon and zeta Orionis dual occultation What do we observe

The Occultation Collection 2005 - gamma Orionis Occultation The Occultation Collection 2007 - zeta Orionis Occultation 2011 occ was a horizontal cut through the plume also 2010 - Solar Occultation Point is that we now have two horizontal cuts through the plume, as it turns out almost orthogonal to each other

Orion’s Belt Dual Occultation Geometry Rev 155 Dual stellar occultation by Enceladus’ plume, E15, 19 October 2011, of epsilon Orionis (blue) and zeta Orionis (white) Horizontal cut through plume

Dual Occultation Eps Ori (Alnilam, B star) Zeta Ori (Alnitak, O star) 16.5 km at closest point HSP centered on eps Ori Dimmer star in uv by ~2x Zeta Ori (Alnitak, O star) 37.9 km at closest point

The Plume: Water Vapor Column Density zeta Orionis Ratio of occulted signal to unocculted signal: I/I0 From average of data records above FWHM Compare to water vapor Cross-sections from Mota, 2005 Same as we used for 2007 zeta Orionis occ Don did f(time) Best fit at H2O column density of 1.35 x 1016 cm-2 Larry got 1.3 x 1016 cm-2 Don’s plot of column depth as a function of time: Deepest absorption just after closest approach Similar situation in 2007, 2x difference

The Plume: Water Vapor Column Density eps Orionis Same technique by CJH gave column density of 1.6 x 1016 cm-2 But Larry calculated column density of 1.4 x 1016 cm-2 I think my value is too high, re-doing this is on my to-do list… Don did f(time) As a function of time: Deepest absorption just after closest approach

All Groundtracks In all occultations we look through the plume Basemap from Spitale & Porco, 2007 Zeta Ori 2011 Solar occ In all occultations we look through the plume The groundtrack is the perpendicular dropped to the surface from the ray to the star Zeta Ori 2007 Blue => zeta Orionis 2007 Red => Solar occ 2010 Green => zeta Orionis 2011

Estimate of Water Source Rate from Enceladus = 200 kg/sec S = flux = N * x * y * vth = (n/x) * x * y * vth = n * y * vth Where N = number density / cm3 x * y = area y = vlos * t at FWHM vth = thermal velocity = 45,000 cm/sec for T = 170K n = column density measured by UVIS 2011: vlos = 7.48 km/sec y x v Year n (cm-2) Uncert-ainty +/- y (x 105 cm) vth (cm / sec) Flux: Molecules / sec Flux: Kg/sec Fraction of orbit from periapsis 2005 1.6 x 1016 0.15 x 1016 80 (est.) 45000 5.8 x 1027 170 0.27 2007 1.5 x 1016 0.14 x 1016 110 7.4 x 1027 220 0.70 2010 0.9 x 1016 0.23 x 1016 150 6 x 1027 180 0.19 2011 1.35 x 1016 (prelim) 135 8.2 x 1027 240 Important message – flux has not changed much in 5 years. (Deviation is only 15%, not factors of 2) Width (y) is at FWHM Vlos is the line-of-sight velocity of the star across the plane of the sky Given uncertainties our best estimate of water flux from Enceladus is 200 kg/sec 10

One more comparison to tidal energy model Position of Enceladus in its orbit at times of stellar occultations, and solar occultation Hurford et al 2007 model predicts tidally-controlled differences in eruption activity as a function of where Enceladus is in its eccentric orbit Expect fissures to open and close Substantial changes are not seen in the occultation data, although they would be predicted, based on this model Taken from Hurford et al, Nature 447:292 (2007) True Anomaly (deg) Fraction of orbit from Periapsis Position in Orbit Stress 105 Pa Source rate Kg/sec 0.0 Periapsis 0.3 0.186 May 18, 2010 180 90 0.25 One quarter -0.8 97.76 0.27 July 14, 2005 -0.77 170 0.5 Apoapsis -0.4 254.13 0.7 2007 and 2011 0.4 220, 240 270 0.75 Three quarter 0.6

Caveats How constant is the source rate? Is the source rate modulated by the position of Enceladus in its orbit? From Larry: The previous derivations use slightly different approaches to find the column density N: 2005 gOri: Last FUV spectrum measured, closest to surface. Best fit for N. Use scale height H to estimate L. 2007 zOri: Two 5-second FUV spectra that span the entire occultation. Best fit for N. 2010 Solar: 42 seconds of summed EUV spectra, covering the FWHM of the occultation. Best fit for N. 2011 eOri, zOri: Mean of photometric analysis (match total attenuation with H2O alone) of spectra within FWHM. This resembles DES approach for Solar occ. To get the mass flux, multiply by the mass of H2O molecule and FWHM. This gives zOri: 1.3 E16 * 134km (9 spectra) = 236kg/sec eOri: 1.4 E16 * 120km (8 spectra) = 227kg/sec Given the variety of approaches, the different channels, IP and occultation tracks, it may be fortuitous that all 5 UVIS results give 208 ± 28 kg/sec. That is, a constant flux with standard deviation of 15%.

Nomenclature Plume Jets “Plume” refers to the broad cloud of dust and gas emanating from the south pole of Enceladus “Jets” are the highly collimated streams of ice particles (detected by ISS) and gas UVIS observes the gas component

The Jets – Past Occs 2007 2010 a b c d e f In the past we have identified collimated jets of gas from enhanced absorption features in the HSP (2007 zeta Ori occ) and the EUV (2010 solar occ) Features in the 2007 HSP data were validated by Bonnie Meinke using her F ring statistical test techniques Features in the 2010 solar occ were identified by looking for matching absorptions in the two windows, and making the argument that it was unlikely to be shot noise if they matched Assume Poisson distribution Calculate number of events one would expect to occur by chance in entire data set, for several bins

Solar Occ Geometry gave us well-separated jets Spacecraft viewed sun from this side Ingress Egress Minimum Altitude  Geometry gave us well-separated jets Don’t dwell too much on this – just point out ground track and location of enhanced absorptions (big blue dots) Basemap from Spitale & Porco, 2007

The Jets – 2011 HSP Data This time the HSP data was lower snr Assume Poisson distribution Calculate number of events one would expect to occur by chance in entire data set, for several bins This time the HSP data was lower snr Eps Ori instead of zeta Ori no features passed the rigorous statistical tests applied Rely on FUV data, cross-correlation of absorptions in same place / shifted in time Plot note: elapsed times are incorrect

Data Bonnie’s Analysis

Visual inspection Human eye is good at picking out features

Data

Data Only one time integration wide Interesting hole

Binned Data N Y ? ? ?

Optical Depth

M Values

No Significant Features From Bonnie: Smallest m values are on the order of a few Optical depths are below 0.4 Nothing passes more stringent tests of repeated significance NOTE: Attenuation in plume ~5%, was ~10% last time Perhaps this is more diffuse overall compared to 2008 zet Ori occ’n Bonnie’s conclusions were verified by Bob West with a different technique

FUV Data Two sec integrations Zeta Ori trails eps Ori by ~4 sec Data is summed over all wavelengths, all spatial pixels Zeta Ori trails eps Ori by ~4 sec

Groundtracks 2011 Eps Ori is blue, zeta Ori is green Three matching features: “Split end” on Baghdad fissure Crossing Baghdad fissure Damascus jets In addition, eps Ori likely distinguishes Baghdad I S/C

Eps and Zeta Orionis Comparison Signal of gas from Baghdad fissure (B-f), though no dust jet nearby Damascus jets (DII and DIII) and BI identified Feature at BVII? Weak feature at “?” is not located at a published dust jet, but ISS and CIRS have reported enhanced activity here DII&III BI B-f BVII? ? Average computed for each star Then ratios computed for each Time shifted to align enhanced absorption feature at B-f because geometry clearly correlated with fissure-crossing (4 sec)

Direct Comparison Clear correlation at fissure-crossing DII&III BI B-f ? Direct Comparison S/C Clear correlation at fissure-crossing Slow return to unocculted signal may be activity between BI and BVII

HSP Now compare HSP (targeted to eps Ori) to FUV for eps Ori 0.008 sec integration summed to 2 sec to match FUV (plot time ok now) Although features did not pass our statistical tests we can compare to the FUV data set Pretty good agreement with eps Ori – more work to do on this to resolve timing issues Then, bootstrap back to less-summed HSP data to get better time resolution Time correct on HSP

Summary Mass flux determined, comparable to other occs Work to do to better quantify uncertainties Jets tougher to identify because of low snr Features in HSP data did not pass statistical tests Geometry of occ along rather than across fissures may also have an effect? Determination of spreading at the two altitudes also limited by temporal resolution of the FUV (2 sec integration time) 2 sec x 7.48 km/sec line-of-sight velocity = 15 km That is the approx. width of the jets derived in earlier occultations Work in progress to better constrain jet structure HSP bootstrap from FUV

Back-up

Jets vs. Tiger Stripes Spacecraft viewed sun from this side Ingress Egress Minimum Altitude  As before, gas jets appear to correlate to dust jets Feature Altitude* (km) Dust Jet a 20 Alexandria IV Closest approach 19.7 b 21 Cairo V and/or VIII c 27 Baghdad I d 30 Baghdad VII e 38 Damascus III f 46 Damascus II Don’t dwell too much on this – just point out ground track and location of enhanced absorptions (big blue dots) * Altitude of ray to sun from limb Basemap from Spitale & Porco, 2007

2007 - Plume Structure and Jets 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 New solar occ data is better resolution, gives us better numbers for all of these results…

Jet Structure 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 The 3.4% is for all 6 of the jets

Solar Occultation Jets

Comparison to INMS results from E7

Jet Properties Feature Altitude of ray relative to limb Z0: Altitude of ray relative to jet source FWHM: full width half max(km) Mach number ~ 2 * Z0 / FWHM Associated Dust Jet Excess attenu-ation at the jet (%) – for density calc* a 21.3 21.6 7 6 Alexandria IV 27 Closest approach 20.7 b 22 24 9 5 Cairo V and/or VIII 17 c 28.4 29 10 Baghdad I 19 d 31.2 36 Baghdad VII 12 e 39 40 8 Damascus III 13 f 47.5 49.7 14 Damascus II *Average attenuation =17%

CJH To Do List Re-do calculation of eps Ori column density Really rigorous determination of error bars for every occultation we have observed Re-do summed FUV vs. time for just spatial pixels 2-4, calibrated properly Plot HSP with eps Ori – now that timing issues are resolved! Then go back to higher time resolution HSP data