UVIS Icy Satellites Update Amanda Hendrix January 2007

topics Phoebe results Iapetus results Hyperion results In the works FUV water ice paper Enceladus plume against Saturn disk? H2O2 - VIMS/UVIS comparison Future work

General procedure Do this for entire image (1024,i,j) - scattered light data_cal = smooth(data(*,i,j) x FF(*,i)/int.per.,3,/NaN) x 99cal(*,i) x redpatch(*,i) x1.05 x0.91 Do this for entire image (1024,i,j) Use nightside rows (if possible) as background and subtract off Need to test scattered light correction; may need to add this step

Spectral Models Hapke Model Bidi. ref=(w/4)(0/(+0))[(1+B)*p+H()*H(0)-1]*R() Emphasis on how to correctly model single-scatter albedo w For H2O, non-H2O species Use Enceladus as object against which we compare the H2O model -- but need to make sure we get Enceladus reflectance absolutely right Depends on FF Depends on scattered light correction (?) Apply to Phoebe, Iapetus, inner icies

Update on Phoebe paper Submitted to Icarus in September Reviews back, revisions needed Focus on this when Iapetus paper is submitted (next 1-2 wks) Hope to apply new red patch before final submission Main results in paper: Demonstration of Hapke models Areal, intimate H2O + carbon, tholins, kerogen, poly-HCN etc. Intimate mixture models work better overall But it’s pretty hard to distinguish meaningfully between the different non-ice species Disk-resolved spectra show compositional variations across surface More H2O ice in high southern latitude region Possibly corresponding to rim of a large crater Upper limits on O, C; H distribution off-body (or lack thereof)

Sample Phoebe models average Phoebe reflectance

Phoebe disk-resolved reflectance spectra High S. latitude bright region - more H2O mid-latitude dark region

Update on Iapetus paper For submission to Icarus as part of special issue Hopefully within next 1-2 weeks Will apply new red patch later (after reviews) Main results in paper: We can use VIMS models (Buratti et al. 2005) for average light terrain and average dark terrain to match UVIS spectra Light terrain: 78% H2O + 22% Triton tholin Dark terrain: 5% H2O + 55% poly-HCN + 40% Triton tholin Within dark material, Apex region (centered on 90°W) is spectrally different from non-apex (more H2O ice?) Could suggest compositional differences related to emplacement of dark material Or could suggest compositional differences due to meteoritic bombardment of dark material Comparisons with Phoebe, Hyperion

Iapetus images 3-color Long-

Analysis of different regions on Iapetus LH

Intimate Mixture Model Fits to Iapetus Bright Terrain and Average Dark Terrain New red patch should help here (make reflectance flatter) Intimate mixture model fits to light terrain and average dark region are consistent with VIMS results (Buratti et al., 2005)

Spectral variations within dark terrain Apex: 45-135°W longitude Spectral models are in progress; it appears that more H2O ice is present in apex region compared to non-apex region

That darn star Light from way off-limb star (rows 26-27) contaminated data from disk of Iapetus (32-33)

Update on Hyperion Results Nature paper in preparation (Cruikshank et al.) Some UVIS results were given to Dale C. Awaiting status of manuscript

Models of Hyperion spectra Intimate Mixture Model results: Bright: ~50% H2O plus ~50% organic material Average: ~35% H2O plus ~65% organic material Dark: ~10% H2O plus ~90% organic material Cannot pull out a dark terrain spectrum that is H2O-free

In progress: Water Ice paper Possible improvement of optical constants over Warren (1984) Include new data since Warren (1984) Dependence in optical constants on hexagonal vs amorphous ice Hapke modeling of water ice in FUV Comparisons to icy satellites My models (so far) generally need H2O grain sizes much smaller than those modeled by VIMS Is this real? Two (+) populations of H2O grain sizes? Or does UVIS detect asperities on larger H2O grain surfaces? New red patch will help at longest wavelengths Need to try Bill’s scattered light correction, especially for Enceladus

Models of H2O for different grain sizes

Use Enceladus reflectance spectrum to check H2O spectral models New red patch should help here Is refl really this bright here? Do scattered long- (>1700 Å) photons contaminate short- <1500 Å) spectrum?

(From July 2006 team meeting) Enceladus scattering issue for filled slit, bright source, does signal at bright long  contaminate short ? **At the end of this observation, when we are very close and the slit is ~filled, there is a lot of scattering of light at the shorter wavelengths and the reflectance shape is not accurate. **

Is it possible to detect Enceladus plume against Saturn disk? Or do we only measure variations in Saturn reflectance? 3-color image 004EN Icymap007_cirs 2 3 1 LW image Geometer image

Need more integration time to clean up spectra before we can do this right

H2O2 - VIMS/UVIS comparison Preliminary results from Sarah Newman/Bonnie Buratti

H2O2 at FUV wavelengths This is what H2O2 gas looks like - in the NUV, H2O2 in H2O ice (e.g. Europa) looks like H2O2 gas. So we may want to look for this signature in icy satellite reflectance spectra

Future Work Io paper!! (with Don) Look into images of Io torus neutrals Icy satellite phase curves, models Surface structure Any spatial variability in surface structure? Icy satellites - EUV spectra