Cassini UVIS Icy Satellites: Update and Progress on Analysis Amanda Hendrix & Candy Hansen UVIS team meeting January 2006 *** This version of the presentations was modified as noted after the team meeting, based on discussion at meeting.***

Since last UVIS team meeting… 4 icy satellite flybys Tethys (September 2005) Hyperion (September 2005) Dione (October 2005) Rhea (November 2005)

TETHYS 015TE_ICYLON019_CIRS 015TE_EWSCAN001_CIRS crater Ithaca Chasm long- image (1710-1900 Å) R= 49658 km Longitude= 347°W Phase= 21°

HYPERION 015HY_ICYMAP008_UVIS Ly- image (1710-1900 Å) long- image (1710-1900 Å) R= 55,679 km Phase= 51.7°

HYPERION 015HY_ICYMAP013_UVIS long- image (1710-1900 Å) R= 17,883 km Phase= 126.3°

DIONE 016DI_icymap003_prime long- image (1710-1900 Å) (no ISS rider) R= 103,024 km Longitude= 160°W Phase= 22.9°

DIONE 016DI_icymap005_iss (part 30; 17:16:21) 016DI_targflyby001_iss - part 4 ~250°W, 20°N 240°W, 7°S start end R= 64,749 -> 3585 km Longitude= 166°W Phase= 22.7°

RHEA 018RH_ICYMAP006_CIRS 330T19:03 018RH_FP3MAP002_CIRS long- image (1710-1900 Å) 1304Å image R= 87,493 km Longitude= 170°W Phase= 18.8°

spectral analysis -- history We’ve known for a long time (since Phoebe) that the shape of the icy satellite (and rings) spectra are consistent with H2O ice, due to upturn in brightness for >~1650 Å. In trying to get the Phoebe paper out, we’ve been struggling to interpret the shape of the spectrum as well as the magnitude of the spectrum We look at the other icy satellites to see if that helps (their spectra may be more easily understood - more similar to pure H2O ice) We have been carefully analyzing the data to make sure we’ve got everything right - in terms of factors of , 4, 2 (low-res vs high-res slits) - to make sure that the absolute reflectance (or albedo) is well-understood We’ve been looking into different spectral modeling types (i.e., checkerboard, intimate) and using accurate representations of candidate species (now using single-scatter parameters from Gary Hansen at many grain sizes and wavelengths to get out the bidirectional reflectance of the model parameters) for direct comparison with UVIS data

Imeas=p()()(F/2)4e-6 Imeas (cts/sec) = p()() Raw spectrum Flat-fielded 1 row average cts/sec Simulated solar spectrum; Flat-fielded; 1 mrad filled Output in cts/sec (input was (F/r2)4e-6) Note shape of reflectance spectrum From Tomasko, Imeas=p()()(F/2)4e-6 Imeas (cts/sec) = p()() solaruvis (cts/sec) = geometric albedo at some phase angle

Raw spectrum/simulator output= I/F=reflectance Flat-fielded 1 row average cts/sec Simulated solar spectrum; Flat-fielded; 1 mrad filled Output in cts/sec (input was (F/r2)4e-6) Note shape of reflectance spectrum Raw spectrum/simulator output= I/F=reflectance

More examples of reflectance spectra Notice these are >1 at longest  This is OK. This is the geometric albedo and therefore this means that the satellites do not scatter like Lambertians. BUT: if this is right, then geometric albedo at 1900 Å may be larger than at longer  Could be interpreted to mean that EN and DI have similar H2O grain sizes and Dione has more non-ice material (more scooped out)

More examples of reflectance spectra Notice these are >1 at longest  This is NOT OK. This is I/F and NOT the geometric albedo. Could be interpreted to mean that EN and DI have similar H2O grain sizes and Dione has more non-ice material (more scooped out)

Phoebe, Iapetus, Hyperion And more examples of reflectance spectra Phoebe, Iapetus, Hyperion

H2O spectra flatten out shortward of 1900 Å H2O reflectance at different grain sizes for a particular set of , i, e angles

Could the difference in shape between H2O and icy satellites be due to instrument response to NUV solar and H2O (both bright)?? Evidently not.

Comparisons with Longer Wavelengths Compare with HST at NUV wavelengths to see if they agree Rhea, Dione, Iapetus (Noll et al.) Use a sub-pixel, disk-integrated UVIS observation for comparison with HST When comparing UVIS and HST, must consider filling factor and phase correction This comparison should really help us understand whether we’ve got the correct magnitude, and all factors of 2, , 4 or other accounted for.

Disk-Integrated Phase Curve: All longitudes at 1820 Å

Disk-Integrated Phase Curve: All longitudes at 1900 Å

Test Case 1: Rhea 004RH_icylon003 (targeted using UVIS_FUV)

Test Case 2: Rhea 010RH_zerophase

Rhea at 0° phase (geometric albedo) HST UVIS We are clearly not comparing apples and apples - so the UVIS spectrum is likely not geometric albedo after all, and we need to scale UVIS data to match up with HST.

Rhea at 0° phase (geometric albedo) The UVIS spectrum (sub-pixel) needs to be adjusted properly for comparison with HST HST UVIS We are clearly not comparing apples and apples - so the UVIS spectrum is likely not geometric albedo after all, and we need to scale UVIS data to match up with HST.

Rhea: FUV - NIR (disk-integrated) something other than H2O is present 1 um H2O, scaled arbitrarily UVIS + HST + Clark ground-based

Rhea: FUV - NIR (disk-integrated) 1 um H2O + tholin, scaled arbitrarily UVIS + HST + Clark ground-based

Rhea: FUV - NIR (disk-integrated) 1 um H2O + tholin, scaled arbitrarily UVIS (scaled) + HST

Spectral Models - Intimate Mixtures

Spectra of Candidate Species - 1 CO2 H2O NH3

Spectra of Candidate Species - 2