spectroscopic variation of water ice abundance - USRA

46th Lunar and Planetary Science Conference (2015)
F. Scipioni1, P. Schenk1, and F. Tosi2, 1Lunar and Planetary Institure, 3600 Bay Area Blvd. 77058 Houston, Texas,
United States, [email protected]; 2INAF-IAPS, Istituto di Astrofisica e Planetologia Spaziali, Via del Fosso del
Cavaliere 100, I-00133 Rome, Italy.
Introduction: The surface of Saturn's regular satellites is largely composed by water ice. The Cassini
spacecraft has observed present-day geologic activity
at the Enceladus’ South Polar region (the so-called
“Tiger Stripes”) in the form of eruptive plumes of volatile gases (H2O, CO2, NH3, CH4) which are the major
source of Saturn’s E-ring. Some of this material falls
onto Enceladus’ surface to form deposits that extend to
the north at ~220°E and ~40°E and whose highest concentration is at the south pole.
Mimas and Tethys are Enceladus’ orbital neighbours, lying inside and outside Enceladus’ orbit respectively. It is therefore likely that Mimas and Tethys surfaces interact with icy particles from the E-ring, resulting in a spectral, color and grain size modification.
The most prominent feature on Mimas surface is
crater Herschel with a diameter of 130 km, one-third of
the moon itself. Mimas has the most uniform surface
among Saturn's principal satellites, with its trailing side
just 10% brighter and redder than the leading side. The
uniformity of Mimas extends on spectral appearance
too. The 1.52 and 2.02 μm H2O-ice absorption bands
are 10% deeper on the trailing hemisphere.
On Tethys' leading hemisphere, the 400-km crater
Odysseus represents 40% of the diameter of the
Here we present results from our ongoing work
mapping the variation of the main water ice absorption
bands and sub-micron ice particles across Mimas, Tethys and Enceladus’ surfaces using Cassini/VIMS data
acquired in the IR range (0.8–5.1 μm).
Data analysis: The Cassini VIMS spectrometer
acquires hyperspectral images (`cubes') in the overall
0.3–5.1 μm spectral range. We selected VIMS cubes of
Enceladus, Mimas and Tethys in the IR range (0.8–5.1
μm), and minimized photometric effects due to different illumination conditions by normalizing all spectra
at 2.23 μm.
For all pixels in the selected VIMS images, we
measured the band depths for water-ice absorptions at
1.25, 1.5 and 2.02 μm and the height of the 3.6 μm
reflection peak, whose value relates to the grain size.
Moreover, we considered the main spectral indicators in the IR range for ice particles smaller than 1 µm
[1]: (i) the 2-µm absorption band is asymmetric and (ii)
it has the minimum shifted to longer λ; (iii) the band
depth ratio 1.5/2.0 µm decreases; (iv) the reflection
peak at 2.6 µm decreases; (v) the Fresnel reflection
peak is suppressed; (vi) I/F at 5 µm decreases relative
to the 3.6 µm peak.
Since the first part of the VIMS-IR spectrun (0.82.5 µm) is sometimes affected by saturation effects, for
each cube of the dataset we performed a pixel-by-pixel
selection of spectral features to be used: for each pixel,
only (totally or partially) unsaturated absorption bands
were selected.
To characterize the global variation of water-ice
band depths, we sampled the three satellites’ surface
with a 1°x1° fixed-resolution grid and then averaged
the band depths and peak values inside each square
Results: For Mimas and Tethys we find that large
geologic features, such as the Odysseus and Herschel
craters, do not correlate with water ice’s abundance
and/or grain size variation.
For Tethys, we found a quite uniform surface on
both hemispheres. The only deviation from this pattern
shows up on the trailing hemisphere, where we notice
two north-oriented, dark areas around 225° and 315°.
For Mimas the selected dataset covers just the leading
side and a portion of the trailing side. From the analysis, the two hemispheres appear to be quite similar in
water ice abundance, the trailing portion having water
ice absorption bands slightly reduced than the leading
No correlation with the visible-color/thermal anomaly features has yet been found.
For Enceladus, water ice bands depth’ variation and
all sub-micron ice particle spectral indicators can clearly map plumes deposits on the surface. The highest
concentrations occur at Enceladus’ South Pole, where
the band depths values are the deepest across the entire
moon’s surface. Our results confirm that plume particles fall in north-oriented patterns at ~40°E and
~220°E, and disappear around ~0°E and ~180°E.
Outside plumes deposits, the value of all spectral
indicators considered decreases, meaning that in this
region the concentration of sub-micron ice particles is
higher and that water ice is more contaminated than on
the plumes deposits: since plumes deposits undergo
darkening processes for less time than surrounding
terrains, they appear brighter meaning that water-ice
absorption bands must be deeper.
46th Lunar and Planetary Science Conference (2015)
An example of spectral maps is reported in Figures
1, 2 and 3 for Tethys, Mimas and Enceladus, respectively. In the top panels the variation of the 2.02 μm
band depth is mapped, while in the bottom panels the
intensity of the 3.1-μm Fresnel peak is traced.
Figure 3: Enceladus 2.0 μm (top) and 3.1 μm (bottom)
bands depth variation.
Figure 1: Tethys 2.0 μm (top) and 3.1 μm (bottom) bands
depth variation.
Conclusions: All spectral indicators considered in
this work clearly outline the E-ring plumes deposits on
Enceladus surface. Moreover, the global variation of
water ice band depths highlighted a bright feature located between 45°-90°W and 30°-60°N, which is not
apparently related to any surface feature.
The analysis of the sub-micron ice grains spectral
indicators revealed that sub-micron ice particles locate
mostly on the trailing hemisphere, while they are not
present on the plumes deposits and in the Tiger Stripes
For Tethys, we found a deposit of crystalline water ice on the trailing hemisphere superimposed on the
dark material deposit, and a gradient of grain size, being bigger on the leading site and on the water ice deposit on the trailing hemisphere.
For Mimas, we found no evidence of the thermal
anomaly feature seen on the basis of ISS images.
[1] Clark, R. et al. (2013in The science of Solar
System ices, Springer Science+Business Media New
Figure 2: Mimas 2.0 μm (top) and 3.1 μm (bottom)
bands depth variation.