UVIS Observations of Enceladus’ Plume Note notes from UVIS telecon on 24 October!! C. J. Hansen 7 January 2009

Outline Nature paper Discrete or continuous sources? Local oxygen variability

Enceladus Plume Occultation of zeta Orionis October 2007 In 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

Nature Paper Rev 51 occultation results: Published November 2008: Horizontal probe of plume structure Four discrete gas jets identified Velocity is supersonic, consistent with gas accelerated in nozzles as postulated by Schmidt et al. 2008 Water content of plume higher than in 2005, inconsistent with Hurford et al. 2008 (but motivates new model incorporating libration) CO ruled out (again) Published November 2008: C. J. Hansen, et al, “Water vapour jets inside the plume of gas leaving Enceladus”, Vol 456, 27 November 2008, doi:10.1038/nature07542 Lots of media interest when this came out!

Absorption Features, Compared to Dust Jet Locations a. Cairo (V) d. Damascus (III) c. Baghdad (VI) Closest point b. Cairo (VIII) + Baghdad (I) Ingress Egress

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

Discrete vs. Continuous Sources Is the plume generated by discrete jets or by continous gas release along the tiger stripes?

Simulations w/ multiple sources Bonnie’s analysis using T. Fian’s model

Dust Jets Numerous dust jets are observed coming from the tiger stripes There are 8 clusters of activity, with numerous small jets Suggests we may also be seeing numerous small sources along the tiger stripes Insufficient solid mass (compared to vapor) -> need bigger eruption Implies temporal variability

Plume Variability Ongoing data collection: ICYATMs

Inspiration: Changes in System Oxygen Content Units are Rayleighs Point out Enceladus orbit, E ring

Detecting Temporal Variability Since the oxygen in the system comes from Enceladus UVIS may be able to remotely monitor Enceladus’ activity levels by monitoring the system oxygen level A series of mini - system scans (ICYATMs) in the vicinity of Enceladus were added to the Cassini science observation plans starting in September 2006 Initially used 15 min dwell time per footprint Now using 60 min These observations are often obtained when the spacecraft is far from Enceladus If this approach pays off, we will be able to monitor Enceladus’ eruptive activity even when the spacecraft is not close to Enceladus, thus opening up many more observation opportunities As Cassini extended mission timelines have been planned we have more deliberately surrounded close flybys with remote observations, to more closely tie together the state of eruptive activity on Enceladus with the state of the oxygen in its vicinity (During the primary mission, observation blocks were added in available times, not with a systematic approach) Team will have rayleigh tutorial - these are the units I got from Kris Larsen - need to evaluate

Enceladus mini- System Scan

Comparative Spectra 2007 DOY 102

Comparative spectra 2006 DOY 272

Results To-date and Future Plans Initial efforts to map the oxygen around Enceladus were hampered by the very low snr, so 15 minutes integration time is not adequate. Observations now use a longer time at each location, and sacrifice some spatial coverage. At this time, 40 ICYATMs have been carried out - data processing is underway Data is flat-fielded and calibrated to physical units (kR/pixel) using cube generator Sum individual 200 sec integrations over time at individual footprint using cube_merger Data is plotted to visualize the atomic oxygen content in the vicinity of Enceladus, and the precise pixel containing Enceladus is identified using geometer Divide by number of individual integrations to get average, sum over spectral channels 237 to 246 to get oxygen emission at pixel where Enceladus is

Today’s Status Units and processing issues resolved Just to get sense of magnitude of variability I’ve just been looking at the signal at Enceladus itself The oxygen signal at Enceladus has been observed to vary from 3 to 8 Rayleighs in the incomplete re-processing of older data and processing of new data

2008 DOY035 Brightest pixel is just below Enceladus Summing over 10 spectral pixels (full width of oxygen emission feature) Sum over entire 1 hr observation Brightness = 9 R 2008 DOY035

Backup

Variability of Oxygen Emission Feature Note: Units are Rayleighs / 10 pixels / sec because I summed spectral channels 237 to 246 - do not need to divide by 10 because summing over the emission feature This plot shows the variability observed in the observations analyzed to-date. The range is 0.3 to 2.3 Rayleighs/pixel/sec. If no need to divide by 10 then these units go back up to 3 to 23, units = ?

Variability of Oxygen Emission Normalized This plot shows the variability observed in the observations analyzed to-date, normalized for distance, ratioed to 2006 DOY 272 Do not need to normalize for distance when IFOV is filled

C. Hansen, L. Esposito, J. Colwell, A. Hendrix, B. Meinke, I. Stewart Observations of Enceladus’ Plume from Cassini’s UltraViolet Imaging Spectrograph (UVIS) June 2008 C. Hansen, L. Esposito, J. Colwell, A. Hendrix, B. Meinke, I. Stewart

Overview 2007 Occultation of Zeta Orionis - new results Overall plume shape and density Significant events are likely gas jets UVIS gas jets correlate with dust jets in images Previous Monte Carlo model updated with new data We characterize jet widths, opacity, density Ratio of thermal velocity to vertical velocity = 0.65, supersonic Water vapor abundance derived from new FUV spectra, no CO Comparison of 2005 to 2007 occultations does not substantiate tidally-controlled energy-source models Paper submitted, in review by Nature In a nutshell the 2005 results Won’t dwell on these except to compare / contrast to 2007 results

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

Enceladus Plume Occultations FUV and HSP data collected FUV: 5 sec integration HSP: 2 msec sampling 2007 - zeta Orionis Horizontal density profile True anomaly = 254 2005 - gamma Orionis Vertical cut through plume True anomaly = 98 Key results: Dominant composition = water vapor Plume column density = 1.6 x 1016 /cm2 Water vapor flux ~ 150 kg/sec

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 = 0.032 2 (b) m = 0.000008 3 (c) m = 0.00056 6 (d) m = 0.026 Statistical analysis is applied to determine which features are likely to be real. The analysis assumes the signal is a Poisson distribution. A running mean is calculated over 1001 integration periods or binned equivalent time [6]. The mean is the baseline for point i. Six different bin sizes were employed. C is the binned stellar signal at a particular bin. The probability that the signal would be ≤C at that bin is given by the sum of the distribution over values ≤C. This step is performed for each bin, i, in the data set to find Pi = P(mi, ≤Ci). Pi is multiplied by the number of bins in the data set, N. This product gives us a value m, where m=NPi [7], or the number of such events one would expect to occur by chance in the data set.

Enhanced HSP absorption features a, b, c, and d can be mapped to dust jets located by Spitale and Porco (2007) along the tiger stripes Take time to explain this!

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

Gas Jets Density in jets is twice the background plume Gas jet typical width = 10 km at 15 km altitude Egress Ingress Because the dimension of the two prominents jets is about 1/8 the width of the largest plume, their gas density needs to be twice that of the surrounding plume at 15 km to yield the 12% opacity increase seen in the jets. a. Cairo (V) d. Damascus (II) c. Baghdad (VI) b. Alexandria (IV) Closest point

Gas Jet Model Key Result: Vthermal / Vbulk = 0.65 Flow is supersonic 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 = 0.65. 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). In the plot the gas streams are shown as dashed vertical lines. Key Result: Vthermal / Vbulk = 0.65 Flow is supersonic

Tilted Jets Opening angle of plume derived from 0.65 ratio of thermal to bulk velocity, projected to altitude of occ, includes off-vertical tilt of B7 and D3 jets May not need additional arbitrary jets Work in progress because timing not consistent with previous plot

Comparison to tidal energy model Position of Enceladus in its orbit at times of stellar occultations Hurford et al 2007 model predicts tidally-controlled differences in eruption activity as a function of where Enceladus is in its eccentric orbit Substantial changes are not seen in the occultation data, although they would be predicted, based on this model Expect fissures to open and close Taken from Hurford et al, Nature 447:292 (2007) True Anomaly (deg) Fraction of orbit from Periapsis Position in Orbit Stress 105 Pa 0.0 Periapsis 0.3 90 0.25 One quarter -0.8 97.76 0.27 July 14, 2005 -0.77 180 0.5 Apoapsis -0.4 254.13 0.7 October 24, 2007 0.4 270 0.75 Three quarter 0.6

Compare 2007 to 2005 - 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.

Groundtrack of Ray 2005 2007

Water column density: FUV Absorption is best fit by water vapor Best fit column density = 1.3 x 1016 cm-2 Error bar: +/- 1.4 x 1015 cm-2 Comparison to 2005 at 15 km altitude 2007 peak column density = 3.0 x 1016 cm-2 2005 = 1.6 x 1016 cm-2 No detection of CO formal 2-σ upper limit is 3.6 x 1014 cm-2 corresponds to mixing ratio with H2O of 3.0 Our nondetection of CO excludes 3% CO in the plume at the 2 sigma level Used Mota (2005) cross-sections I/I0

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

Plume or jets? Attempt to pick comparable “box” between 2005 and 2007 But different jets visible means comparing apples and oranges

Summary of Results PLUME: H2O column density in 2007 = 1.3 x 1016 cm-2 Density at 15 km altitude 2x higher H2O column density in 2007 ~ 3.0 x 1016 cm-2 Attenuation in HSP data ~10% in 2007, ~6% in 2005 Difference contradicts Hurford et al model of fissures opening and closing Plume column density goes as ~ z-2 (z is minimum rayheight) Water vapor flux ~200 kg/sec No detection of CO

Summary of Results JETS: HSP data shows 4 features with m < 0.1 (probability of chance occurrence). Typical half-width: 10 km at z = 15 km. Gas jets can be correlated with dust jets mapped in images on Cairo, Alexandria, Damascus and Baghdad tiger stripes Jet opacity corresponds to vapor density doubled within jets Alternate explanation: no excess gas, with all increase due to dust. Then, dust opacity peaks at 0.05 in the jets. This would give 50x more mass in dust compared to vapor within the jet. Ratio of thermal velocity to vertical velocity in jet = 0.6 5 Gas is supersonic Eight or more jets required to reproduce width and shape of absorption Jet source is approximately 300 m x 300 m Example Calculations T surface = 140 K V thermal = 359 m/sec V vertical = 552 m/sec For Tsurface = 180 K (from CIRS) V thermal = 406 m/sec V vertical = 624 m/sec

Data shown as optical depth (inverted) Point out non-alignment of b

Backup Slides

Gas vs. Ice For reasonable mass ratios of ice to gas in the jets (fI = 1) ice has too little opacity (tau = 0.001/grain radius) to be detected by HSP. Radius in microns If jets are unresolved by HSP or have spread significantly in reaching altitude z=15km, the surface pressure at the vent could be correspondingly higher

Plume Model The results shown here have T surface = 140 K Monte Carlo simulation of test particles given vertical + thermal velocity, particle trajectories tracked under influence of gravity and collisions (Tian et al, 2006) Original model had arbitrary source spacing along the tiger stripes Model now adapted for specific locations of the 8 dust jets identified by Spitale and Porco, actual viewing geometry of these sources as seen from the spacecraft The results shown here have T surface = 140 K V thermal = 359 m/sec V vertical = 552 m/sec For Tsurface = 180 K (from CIRS) V thermal = 406 m/sec V vertical = 624 m/sec Data shown as optical depth (inverted) Point out non-alignment of b

Optical Depth vs. Rayheight Minimum distance of rayheight above limb = 15.6 km S/C velocity = 22.57 km/sec Best fit is tau = 64.4 x z-2.33 - 0.007 Density at jets is ~2x higher than “background” plume