UVIS Satellite surfaces update Amanda R. Hendrix UVIS Team Meeting Berlin 8-10 June 2009

H2O ice Flat in visible Well-known 1.5, 2.0, 3.0 m features Strong absorption edge in far-UV, ~0.165 m Start by observing the reflectance spectrum of H2O ice. (varies of course in some ways with grain size and temperature)

Enceladus : previous work - 1 Buratti 1984 Voyager data “the spectrum of Enceladus is flat, consistent with an almost pure H2O ice surface”

Enceladus: previous work - 2 Verbiscer et al. 2005 HST data Consistent with Buratti 1984 results One additional data point at 275 nm, suggesting a dropoff in reflectance

Enceladus: previous work - 3 Cruikshank et al. 2005 Ground-based data “Consistent with the work of other investigators, we have detected no spectral features other than those of H2O ice”

Enceladus: previous work - 4 Emery et al. 2005 Ground-based data “The spectrum of Enceladus shows activity near 2.25 m that we interpret as a possible signature of NH3 ice.” NH3 ice feature at 2.235 m Verbiscer et al. 2006: possible detection of NH3 hydrate feature at 2.21 m

Enceladus: disk-integrated UVIS reflectance spectrum We simultaneously noted 2 characteristics: the UVIS spectrum is darker than 1) the visible and 2) pure H2O ice model

Enceladus is much darker than pure H2O model We simultaneously noted 2 characteristics: the UVIS spectrum is darker than 1) pure H2O ice model and 2) visible wavelengths. The H2O ice edge is measured at 165 nm -- but it’s still dark 170-190 nm

Enceladus is much darker in UV than in visible UVIS disk-integrated observation at ~2° compared with ground-based observation at ~2° Composite spectrum with UVIS + Enceladus data from Verbiscer et al 2005, 2006

what darkens Enceladus’ spectrum in the UV? Not H2O Must be something that is bright in the visible featureless in the visible featureless (nearly) in the NIR

candidate species CO2: tholins: The CO2 absorption edge occurs (in models) at ~175 nm -- too short for what we need tholins: Spectral mixing models with H2O ice & tholins did not suitably match UV data

NH3 NH3 is an attractive candidate because it has such a strong UV edge that you only need a little bit in your spectral model to be effective. And it’s bright and featureless throughout the visible.

mixture models: H2O + NH3 Blue line: intimate mixture of (95:5:1 1 m H2O: 12 m NH3:2 m tholin) Light blue line: areal model of 20% 50 m H2O and 80% (86:1:13 1 m H2O: 16 m NH3:2 m tholin) Red line: areal model of 23% 1 m H2O and 77% (99:1 1 m H2O: 100 m NH3) Orange line: intimate mixture of (99:1 1 m H2O: 10 m NH3) • So far, the models do not exactly replicate the Enceladus spectrum. • They do mimic the spectral characteristics including the FUV “notch.” • We do not have H2O ice or NH3 ice optical constants for appropriate temperatures.

more recent NH3 data Dawes et al. 2007

one interpretation There is a growing body of evidence that NH3 is present in small amounts (~1%) in the Enceladus plume, on the surface and in the environment INMS (Waite et al.) CDA (McBride et al. 2007) CAPS (Smith et al. 2008) NH3 is ejected through the plume into the system (E-ring) Enceladus is bathed by E-ring grains, so the small amount of NH3 on the surface of Enceladus is continually replenished also helps to continually keep the surface so visibly bright Two options: Redeposited globally through continual E-ring grain deposition/bombardment We expect the other inner icy moons to exhibit similar spectral effects Redeposited mainly in south pole region in plume fallout Should see spectral variations with latitude on Enceladus

preliminary Mimas, Tethys composite spectra * VIMS * VIMS LH TH Spectral modeling still to be done … but certainly something is absorbing strongly in the 0.2-0.3 m region

preliminary Dione, Rhea composite spectra * VIMS * VIMS Ground-based (Noll/Roush et al.) Ground-based (Noll/Roush et al.) HST (Noll et al.) HST (Noll et al.) Dione & Rhea might have less-steep slopes in the 0.2-0.5 m region than Enceladus, Mimas, Tethys (more tholin, less NH3?)

Currently working up phase curves for the other moons …

So do we have evidence for NH3 on other moons? Not sure yet.

So do we have evidence for more NH3 in Enceladus south polar region than elsewhere on surface?

Enceladus : disk-resolved example #1 004EN_ICYMAP010

Enceladus : disk-resolved example #2 011EN_ICYMAP015 Why do these spectra appear to have dip near 1830 Å while previous ones don’t?

disk-resolved: northern hem Measured I/F spectrum is consistent with disk-integrated at ~115° (ratty though)

So far, we don’t seem to have any evidence for large long-wavelength brightness variations across the surface of Enceladus that could suggest variations in NH3 amounts.