Far-ultraviolet photometric characteristics of the icy moons of
Saturn Royer, E.M. 1 & Hendrix, A.R. 2 1 JPL/Caltech, Pasadena,
CA ; 2 Planetary Science Institute, Tucson, AZ 04/18/2013, JPL
2013. All rights reserved. Slide 2 E-ring Satellites in the Saturn
System Slide 3 The further from Saturn, the bigger Radius: Mimas
(198 km), Enceladus (252 km), Tethys (531 km), Dione (561 km) and
Rhea (764 km) Enceladus is the source of the E-ring, presence of a
plume The further from Enceladus, the darker Albedo geometric at
0.55 m: Mimas (0.96), Enceladus (1.37, 0.99 for Bond), Tethys
(1.23, 0.80 for Bond), Dione (0.99), Rhea(0.95) Mainly composed of
water ice + an UV absorber: density close to 1 Slide 4 Affecting
the scattering properties and/or the chemistry, the structure of
the surface, we have: Charged particle (ion and electrons)
bombardment: affect the surface chemistry by radiolysis, leading to
products that absorb efficiently in the UV but not in the visible
(e.g. SO2, O3, H2O2); Bombardment by plasma and energetic electrons
have a darkening effect on the surface. Plasma impact the trailing
and energetic electrons impact the leading (retrograde motion). Ion
bombardment, produce defects in ices (voids and bubbles) and trap
gases which can produce spectral absorption features. E ring grain
coating/bombardment: have been suggested to have a brightening
effect (coating and/or sandblasting) BUT the surface of satellites
are UV dark, implying that if E- ring grains dominate the optical
properties, then in FUV we are seeing a non-ice component of the
E-ring (H2O is bright in UV). Photolysis, breaks up water ice and
produces species as H2O2 and O3, O2 Bombardment by incoming dark
material (micrometeorites), garden the surface and produce finer
particles Exogenic processes Slide 5 Direction of motion Leading
Trailing Mimas Tethys & Dione Leading Trailing Energetic
electrons Plasma E-ring grains Slide 6 Cassini-UVIS instrument FUV
Spectrograph EUV Spectrograph HSP (High Speed Photometer) HDAC
Microprocessor LOGIC Assembly Ultraviolet Imaging Spectrograph
(UVIS) Far-Ultraviolet: 111.5 to 191.2 nm Low resolution slit: FOV
of 1.5mrad Pixel format: 1024 (spectral) x 64 (spatial) Slide 7
Disk-integrated observations Slide 8 The phase angle is the angle
between the Sun, the moon and the observer A phase angle of 0
represents the full moon. It means that shadows disappear, which
increase the brightness of the moon Phase Angle ~ 0 180 ~ 90 Slide
9 LeadingTrailing Mimas: 40 observations Dione: 54 observations
Tethys: 40 observations Disk-integrated UVIS observations Slide 10
Data sets/Calibration We analyze data in term of reflectance : r =
I/F The measured signal depends not only of the albedo but also of
the photometric properties of the surface I use solar data measured
by SOLSTICE on the SORCE spacecraft on the days of the
observations. The background (RTG and other) have been subtracted
from the data and then they have been calibrated Usually I sum data
on two pixels and average it on several samples (integration time
is usually 120 sec) Slide 11 Spectrum This is a good representation
of the spectra of all the icy satellites of the E-ring. We note an
absorption band of H2O around 165 nm. The spectrum is very dark at
wavelengths shorter than 165 nm. 180 nm is a good wavelength,
bright enough to create a phase curve. Disk-average reflectance
spectrum of Enceladus at solar phase = 2 Hendrix et al., 2010 Slide
12 Tethys disk-integrated phase curve Slide 13 Dione
disk-integrated phase curve Slide 14 Mimas disk-integrated phase
curve Slide 15 Modeling Hapke model: radiative transfer-based
photometric model for light scattering in a semi-transparent porous
medium. It is the analytical model that has the widest application
to icy surfaces. Buratti model is a combination of the generalized
Lommel-Seeliger law and the Lamberts law. Shkuratov model:
semi-empirical description of the photometric function of a
surface. It is a useful way to describe the reflectance
empirically, but the physical interpretation of the parameters is
unclear. It uses fractals to describe roughness, cratering
characteristics Lummel-Bowell model takes into account multiple and
single scattering but does not include CBOE. Slide 16 Opposition
effect It is the fact that an object appears brighter at very low
phase angle. It is a backscattering peak near 0 phase angle. The
first historical record of it is from an Italian sixteenth- century
artist, Benvenuto Cellini. He noted that he often saw a glow around
shadow of his head on the grass. Since he did not see it around the
shadow of anyone else, he took this as an evidence that he was
especially favored by God! 2 mechanisms to explain it: - coherent
backscatter and - shadow hiding Slide 17 Coherent backscatter
Backscatter is the reflection of waves back to the direction from
which they came. Particles can be smaller or larger than the
wavelength. The light is scattered two or more times before exiting
the medium at small phase angle (less than 2). For every such path
another portion of the same wave will traverse the same path within
the medium but in the opposite direction. At large phase angle
there is not coherence between the two wavelets. Slide 18 Shadow
hiding Shadow hiding consists in particles near the surface that
cast shadows on the deeper grains, so the shadows become invisible.
The grains must be larger than the wavelength to have shadows. We
only take this mechanism into account in our Hapke model. It
involves only singly scattered light. Slide 19 Only Shadow-Hiding
Opposition Effect (SHOE) / No coherent backscatter The single
scattering function P( ) is the double Henyey-Greenstein function
P( ) = [(1 - c) * (1 b 2 )] / [1 + 2 * b * cos( ) + b 2 ] + [c * (1
b 2 )] / [(1 2 * b * cos( ) + b 2 ) 3/2 ] Hapke models parameters:
, single scattering albedo b, asymmetry parameter (width of the
forward and backward scattering lobes) c, asymmetry parameter
(relatives amplitudes of the scattering lobes) , surface roughness
fix to 20 deg needs phase angles greater than 90 deg B0, amplitude
of the SHOE fix to 0.55 (Verbiscer and Veverka, 1992) h, angular
width of the SHOE fix to 0.066 (Verbiscer and Veverka, 1992) B0 and
h need phase angles lower than 30 deg Ap, geometric albedo Hapke
model Slide 20 Tethys Hapke model (1800 ) 2 minima Slide 21 Hapke
model (1800 ) Leading Slide 22 Hapke model (1800 ) The leading
hemisphere of Tethys is brighter than the one of Dione. This is
logical, Dione being further from Enceladus ad thus receiving a
lower E-ring grain flux Tethys and Dione have a leading hemisphere
brighter than the trailing. Mimas seems to have a trailing
hemisphere brighter than the leading. This is what is expected from
the theory showing that E-ring grains have a retrograde motion at
the orbit of Mimas. The trailing hemisphere of Mimas has the lowest
single scattering albedo and the fewest data points at phase angles
less than 50 Dione has a asymmetry on the amplitude of its
scattering lobes on the trailing hemisphere different from 1,
suggesting a forward scattering component We observe a forward
scattering peak on each satellite: instrumental effect or physical
property? We cannot affirm anything for the opposition parameters
due to a lack of data. B0 is a measure of the regolith grain
transparency; h is strictly a function of porosity. Slide 23 Tethys
trailing phase functions (1800 ) These variations of h do not
significantly modify the fit of the phase curve A variation of B0
from 0.4 to 0.7 induces the same variations in the phase function h
= 0.1 h = 0.04 Slide 24 Derived phase functions (1800 ) Forward
scatterBack scatter P( ) = [(1 - c) * (1 b 2 )] / [1 + 2 * b * cos(
) + b 2 ] + [c * (1 b 2 )] / [(1 2 * b * cos( ) + b 2 ) 3/2 ] Slide
25 Geometric albedo Dione is 2.4 times further from Enceladus, than
Tethys and Mimas, that are approximately equidistant to the source
of the E-ring, the density of the E-ring diminishing with the
distance to Enceladus (Verbiscer, 2007). Slide 26 The forward
scatter peak All observations included in these forward peaks was
taken during a short period of time, between January, 18 th 2006
and September, 13 th 2006 It is difficult to identify the source of
this forward- scattering peak. We have few hypothesis: 1- it could
come from the surface 2- from an possible plume (especially on
Dione) 3- it could be due to scattering from the E-ring grains 4-
it could be a stray light from the Sun close to the FOV. On the ISS
data, a stray light influence the data. The signal increase as the
camera points closer to the Sun. We need to check UVIS data,
looking at spectra when the phase angle is near 160 and when there
is not objects in the field of view. Slide 27 A linear
superposition of a lunar-like scattering law and a Lambert
component that provide an adequate simple representation of the
scattering properties For disk-resolved observations: A is a
parameter such that for a lunar-like purely single scattering, A =
1 and for a diffuse (Lambert) scatterer A = 0. f( ) is the surface
phase function, which express changes in intensity due to factors
such as the single particle phase function and mutual shadowing
among regolith particles. For disk-integrated observations: ( )
represents the flux normalized to a common distance and scaling for
= 0 Buratti model (1983) Slide 28 Summary: UVIS phase curves The
first published phase curves in FUV of Mimas, Tethys and Dione
Brightening effect of the E-ring: the trailing hemisphere of Mimas
is brighter than his leading, and the contrary for Tethys and
Dione. In addition, Dione has a geometric albedo lower than the
Tethys compatible with its distance from Enceladus Surprising
behavior of the trailing hemisphere of Dione We see a FUV forward
scattering peak (small but significant) at high phase angle that
requires more investigations. Slide 29 Perspectives/Future work
Produce results with the Buratti model Investigate about the
forward scattering peak that appear on the phase curves How the
bright structures on Dione impact the results ? Rotational
correction ? Is the contribution of the coherent backscattering
important? Use different versions of the Hapke model. We need more
low phase angle data. The data could be available among the
disk-resolved data The photometric parameters are inputs for a
spectral model in order to investigate the composition and grain
size of the icy surfaces. Slide 30 Questions
