Icy Satellites Update
Amanda Hendrix
With contributions from
Tim Cassidy
Candy Hansen
Bonnie Buratti
Ben Teolis
Chris Paranicas
UVIS Team Meeting, Boulder
6 January 2012
Since the last team meeting…
• Mimas paper was submitted to Icarus (Aug.)
– Resubmitted 4 Jan 2012
• Zastrow Enceladus paper submitted to
Icarus
• UVIS data (Mimas, Tethys) presented:
–
–
–
–
MOP
DPS
PSG
AGU
• Flybys (E14, E15, E16, D3)
“lenses” on Mimas and Tethys
(Schenk et al. 2011)
Mimas: ISS ratio (IR/NUV)
Mimas: electron contours
Mimas: ISS color (IR-green-NUV)
Tethys: ISS color (IR-green-NUV)
comparison with CIRS
• Thermal anomaly:
leading hemisphere
region has high
thermal inertia
related to highenergy electrons
(Howett et al., 2011)
the E-ring
Eccentric E-ring grains hit primarily Mimas’ trailing hemisphere and Tethys’
leading hemisphere (Hamilton & Burns, 1994)
Saturn
r
B
Typical “cold” ions
Mimas

Hot electrons
Co-rotation
Saturn
TETHYSr
B
Typical “cold” ions
Trailing
Hemisphere
E-ring grains & neutrals
Bombardment produces distinct patterns
Tethys
Leading
Hemisphere

Leading
Hemisphere
MIMAS
Trailing
Hemisphere
E-ring grains & neutrals
Hot electrons
Co-rotation
126MI_ICYLON003
I/F
m0
Mimas: the UV albedo on the anti-
Saturnian – leading hemisphere is higher
(x~1.5) than on the leading hemisphere
This may be consistent with E-ring grain bombardment,
brightening the trailing hemisphere compared to the
leading hemisphere – consistent with VIMS results.
Mimas
E-ring grains & neutrals
Typical “cold” ions
Hot electrons
Co-rotation
Mimas: 170-190 nm
E-ring grains & neutrals
Typical “cold” ions
Brightening by E-ring grains
on trailing hemisphere
Hot electrons
Co-rotation
Mimas: 170-190 nm
E-ring grains & neutrals
Typical “cold” ions
Brightening by E-ring grains
on trailing hemisphere
Hot electrons
Co-rotation
Hot electrons do NOT
produce a significant
“lens” shape on the
leading hemisphere
Mimas: 170-190 nm
E-ring grains & neutrals
Typical “cold” ions
Brightening by E-ring grains
on trailing hemisphere
Hot electrons
Co-rotation
Hot electrons do NOT
produce a significant
“lens” shape on the
leading hemisphere
Mimas: 170-190 nm
E-ring grains & neutrals
Typical “cold” ions
Brightening by E-ring grains
on trailing hemisphere
Hot electrons
Co-rotation
Hot electrons do NOT
produce a significant
“lens” shape on the
leading hemisphere
photolysis.
H 2 O2
H2O2: a UV darkening agent
Carlson et al. 1999
The Mimas observation was made in Feb 2010, not long after
equinox; the southern hemisphere of Mimas had been
experiencing summer
Saturn
r
B
Typical “cold” ions

Hot electrons
Co-rotation
Saturn
TETHYSr
B
Typical “cold” ions
Trailing
Hemisphere
E-ring grains & neutrals
Tethys
Leading
Hemisphere

Mimas
Leading
Hemisphere
MIMAS
Trailing
Hemisphere
E-ring grains & neutrals
Hot electrons
Co-rotation
Bombardment produces distinct patterns;
photolytic products might be expected to appear mostly in the south (for Cassini data so far)
MIMAS
anti-Saturnian hemisphere
February 2010
TETHYS
anti-Saturnian hemisphere
July 2007
normal albedos;
same color scale
MIMAS
TETHYS
1. Tethys gets darker (toward TH) than Mimas
MIMAS
TETHYS
1. Tethys gets darker (toward TH) than Mimas
2. Tethys’ LH (part of it) is as bright as Mimas’s TH
MIMAS
TETHYS
1. Tethys gets darker (toward TH) than Mimas
2. Tethys’ LH (part of it) is as bright as Mimas’s TH
3. Tethys’ bright LH seems to be offset toward north
Tethys
Brightening by E-ring grains
on leading hemisphere
E-ring grains & neutrals
Typical “cold” ions
Hot electrons
Co-rotation
Hot electrons do NOT
produce a “lens” shape on
the leading hemisphere
photolysis.
H 2 O2
H2O2 formation modeling
• For simplicity we analyze the concentration
and time constant on the ice surface to get
a rough idea of the concentrations and
time scales.
• On the Mimas leading hemisphere, from
electrons & ions
– We estimate an average surface H2O2
concentration from electrons and ions of only
~0.008%
– Considering UV photons: ~0.13 % at 45 deg
latitude
Estimating H2O2 concentration and time scales
d(concentration)/dt = creation – destruction
= flux*(creation cross section) flux*(destruction cross section)
creation cross section
(~constant*stopping power)
destruction cross section
(~constant)
fit
For ions and electrons we did not find any statistically significant trend in over the measured range of S,
The creation cross section is proportional to the stopping power - implies H2O2 formation directly from
OH radicals following water dissociation
Uncertain by at least order of magnitude, so time scales are uncertain
H2O2 timescales
• The time constants are
– ~8 years for dark surfaces on the leading hemisphere, and
– ~65 days on illuminated surfaces at 45 deg latitude.
• consistent with slow H2O2 destruction by electrons
and ions in the shadowed northern latitudes during
the ~7 year winter timeframe
– followed by a several month recovery in peroxide during
the transition to summer as surfaces are newly illuminated.
– The months-long recovery time scales would imply a time
lag in the northern latitude albedo, possibly accounting for
the north-south asymmetry seen by UVIS ~6 months after
equinox.
“lenses” on Mimas and Tethys
(Schenk et al. 2011)
Mimas: ISS ratio (IR/NUV)
Mimas: electron contours
Mimas: ISS color (IR-green-NUV)
Tethys: ISS color (IR-green-NUV)
ISS ratio (UV/IR)
at UVIS resolution/perspective
UVIS (170-190 nm)
Dione, Rhea: no latitudinal variations
due to less pure H2O ice on the surface (?)
30°W
129DI_ICYLON001
Normal albedo
180°W
018RH_ICYMAP006
New work on Enceladus
plume fallout
Schenk et al. 2011
Kempf et al. 2010
non-fallout region
60°W
fallout region
Rev 121
We have not done anything about solar variations (diurnal or cycle) OR
photometry – so disregard y-scale.
But spectral SHAPE should be ok.
H2O ice reflectance overplotted
Enceladus I/F
Upcoming activities
• LPSC (abstracts due 10 Jan; special Cassini
session)
• MAPS workshop (Mar 28-30, Cologne)
• Flybys (E17 on 27 Mar 2012; E18 on 14 Apr
2012; E19 on 2 May 2012)
•
•
•
•
•
Icy Sats phase curve analysis (with Buratti)
Io paper – finish (with Shemansky, Steffl)
Icy sats – rings comparisons (with Todd)
Comparisons with VIMS (with Roger Clark)
HST observations (Cuzzi)
Disk-integrated phase curves
What does this say about plume particle size? VIMS sees a
very very bright forward scattering peak at 2 um.
the NUV slope
HST/STIS
Noll, 2008; Hendrix et al. 2011