A&A 412, 305-316 (2003)
DOI: 10.1051/0004-6361:20031123
F. Poulet 1 - D. P. Cruikshank 2 - J. N. Cuzzi 2 - T. L. Roush 2 - R. G. French 3
1 - IAS, Bâtiment 121, Université Paris-Sud, 91405 Orsay Cedex, France
2 -
NASA Ames Research Center, MS 245-3, Moffett Field, CA 94035-1000, USA
3 -
Astronomy Department, Wellesley College, Wellesley, MA 02481, USA
Received 6 May 2003 / Accepted 24 July 2003
Abstract
We used the NASA IRTF spectrograph SpeX to obtain
near-infrared spectra (0.9-5.4 m) of Saturn's rings,
achieving spectral resolution
of about 2000.
The spatial resolution (about 1 arcsec) is sufficient to distinguish the
three main ring components (A, B and C rings) from one another. These new
observations of Saturn's rings are the first to combine an extended
spectral range with high spectral resolution and good spatial resolution.
We combined these data with recent photometric observations acquired by
HST in the 0.3-1.0
m range. The spectra of the A band B rings are dominated by
strong features due to crystalline water ice. The shape and the depth of these
absorptions differ for each ring, which indicates different water ice grain
sizes and abundances. No spectral evidence for volatile ices other
than water ice has been detected. Both the lower albedo and the less blue
slope in the near-infrared reflectance of the C ring indicate a concentration
of dark material different from that in the A and B rings.
The broader triangular Fresnel reflection peak at 3.1
m may support the
presence of some amount of amorphous ice. The C ring spectrum
exhibits bands centered at 1.73 and 3.4
m which agree in position quite
well with the C-H bands. Although the detection is probable, it requires
confirmation. With a radiative transfer model, we constrain the grain sizes and the relative
abundances of water ice, a dark colorless component (amorphous
carbon) to adjust the albedo and a second contaminant to reproduce the
reddening in the UV-visible range represented here by organic tholins.
The dark component of the C ring spectrum is included as
an intra-mixture only. The cosmogenic implications of the inferred
compositions are discussed.
Key words: planets: rings - radiative transfer
Saturn's ring particles have been shown to contain water ice
(Lebofsky et al. 1970; Clark 1980). The discovery of a reddening
between 0.3 and 0.8 m indicated the presence of some additional
non-icy material (Lebofsky & Fegley 1976). Color images of the
rings obtained with Voyager imaging (Smith et al. 1981) clearly
showed that the C ring and the Cassini division
to be less reddish than the A and B rings. Recent reinvestigations of
Voyager images suggest that the color of rings varied qualitatively on
a radial scale of few hundred kilometers (Estrada & Cuzzi 1996).
These broadband photometric measurements were analysed by
Cuzzi & Estrada (1998) with a
modified Hapke-van de Hulst model of their construction. An important
result was obtained: the color of rings
could be best modeled by a small amount of a reddish absorber
such as Titan tholin. Microwave and radiometry
measurements indicated an icy surface with certainly less than 10%
by mass of non-icy material, but still unidentified (Epstein et al. 1984).
A hint of an impurity band near 0.85
m was reported by
Clark (1980).
Recently, the composition of the rings has been studied by modeling a
composite spectrum (0.3-4.0 m) of the overall main rings with a
Shkuratov-type albedo model by Poulet & Cuzzi (2002). This study confirmed that the reddish absorber likely is
represented by tholins which are molecularly mixed with water ice particles.
Three different typical sizes of water ice grain (typically 10, 100 and
1000
m) are required to reproduce the near-infrared part of the composite
spectrum, and a dark material is intimately mixed
("salt-and-pepper'' mixture) with
water ice grains to lower the albedo. An attempt to reproduce a possible
0.85
m weak absorption with various sets of laboratory data was unsuccessful.
Since the Voyager 2 encounter, only a few ground-based observations have been
made. Ring color analysis from Hubble Space Telescope (HST) photometric
images revealed a color relation with the small inner satellites
(Poulet al. 1999). Cuzzi et al. (2002) studied the color differences at high spatial
resolution from new HST images between 0.3 and 1.0 m. They suggest the
presence of a number of compositionally distinct unidentified materials
with different radial distributions.
In this paper, we present the spectral analysis of new near-infrared
observations of
Saturn's rings, obtained with the spectrometer SpeX mounted on the NASA
IRTF Telescope, Mauna Kea. These new observations have a
spectral resolution much higher than any previous observations.
The 0.8-5.5 m spectral range of the instrument covers diagnostic
spectral features of a variety of interesting candidate materials: iron-bearing
materials at 0.85
m, organic materials which could present
C-H and/or C-N stretch features in the 1.7-1.8, 2.2-2.4,
3.2-3.4 and 4.3-4.7
m regions,
hydrates, which present
features near 1.5, 2.2 and 2.4
m.
Water ice itself also has a strong 3.0
m absorption feature in which very small
quantities of dark non-icy material may reveal their presence. Another kind of
constituent
about which nothing is known is represented by the volatile ices other than water.
Because of their proximity to the Sun, the ring particles are unlikely to retain
the most volatile ices (
,
,
), but the high
spectral resolution of these data could reveal absorptions due to
or
ice (Brown 2000).
In addition to their spectral resolution, these data have sufficient
spatial resolution to distinguish the three main components of the rings (A, B
and C rings), allowing us to test how the composition vary among
them.
The phase angle dependence of spectrocopic data has not been well
characterized. Cuzzi et al. (2002) and Poulet et al.
(2002a) show how the wavelength-dependent reflectivity in the UV-visible
range can significantly vary with phase even over 0-6 .
However, no data exist for longer wavelengths.
A strong phase angle dependent spectrum could complicate the interpretation
of spectroscopic data. We thus studied the effect of phase angle on the
shape of spectra.
The paper is organized as follows. We first present the observations and their reduction. The second part is dedicated to the analysis of Saturn's ring spectra. In particular, modeling of observations is performed with the Shkuratov-type albedo model (Shkuratov et al. 1999; Poulet et al. 2002b) in order to improve our understanding of ring compositional variability in quantitative terms. The consequences of the results on the origin and the evolution of rings are then briefly discussed.
The observations reported here were made with the spectrograph SpeX of
the NASA InfraRed Telescope Facility (IRTF) on Mauna Kea
during three nights: 6 October 2000, 7 October 2000 and 8 December 2001. The
circumstances of the observations are given in Table 1.
Note that the phase angle
is equal to 4.7
in October 2000,
and close to opposition (
)
in December 2001.
The October (resp. December) nights had an average seeing of 0.9 arcsec (resp.
1.1 arcsec).
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Figure 1:
One IRTF image of Saturn taken in J band on October 6th, 2000, showing
the position of the slit (
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Figure 2:
A part of the mode 1 coverage of the 1 ![]() ![]() |
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Table 1: Observation log.
SpeX is a medium-resolution 0.8-5.5 m cryogenic spectrograph, which
provides simultaneous wavelength coverage by using
prism cross-dispersers at spectral resolutions of
approximately 2000 across 0.8-2.5, 2.0-4.2, or 2.4-5.5
m
(Rayner et al. 2003). SpeX uses
an Aladdin II 1024 X 1024 InSb array in its spectrograph and an
Aladdin II 512 X 512 InSb array in its IR slit-viewer at 0.12 arcsec/pixel.
The two cross-dispersed modes were used:
1- The 0.8-2.5 m cross-dispersed mode (mode 1 hereafter); most of this range is
covered simultaneously.
2- The 2.4-5.5 m cross-dispersed mode (mode 2 hereafter); with a small rotation of
the grating turret, the range 2.0-4.2
m is also covered simultaneously (mode
2bis hereafter). For these two modes, a slit of
arcsec2 was positioned
on the rings as shown in Fig. 1.
In October 2000, spectra resulting from six co-added 10 s exposures for mode 1 and from one exposure of 25 s for mode 2bis were measured for each ansa. For mode 2, spectra of three co-added 10 s exposures were obtained for the east ansa only. Then, we combined different spectra to get a spectrum of 300 s for mode 1, 300 s for mode 2 and 350 s for mode 2bis. For the observations near opposition (December 2001), spectra of 80 s (ten co-added 8 s exposures) for mode 1 and 60 s (three co-added 20 sec exposures) for mode 2 were obtained for each ansa. Then, we combined different images to get spectra of 400 s for mode 1 and 600 s for mode 2 for each ansa. The overall quality of data was improved thanks to the tip-tilt system (an active secondary mirror to reduce the tip-tilt components of seeing and telescope tracking) that was working during the October 2000 observations only.
Because Saturn was too big to be autoguided, guiding was done using manual corrections from the hand-paddle of the telescope control system. The observations were done by switching the slit by 30 arcsec between 2 positions A (planet) and B (sky). Corresponding pairs of images planet/sky were differenced, allowing a first-order subtraction of the sky. The planet/sky images were then flat-fielded, corrected for spatial and spectral distortion using the IDL-based spectral reduction tool called Spextool (Vacca et al. 2003). This program flatfields, extracts and wavelength calibrates our data.
In order to extract the spectra, we averaged the signal along
the modes shown in Fig. 2 according to a radial width that
depends on the ring:
- width of 1.6 arcsec (9500 km) centered in the middle of A ring (122 170-136 780 km);
- width of 3.4 arcsec (20 000 km) centered in the middle of B ring (92 000-117 580 km);
- width of 2.0 arcsec (12 000 km) centered in the middle of C ring (74 510-92 000 km).
We calibrated the profiles radially using Saturn's limb and/or the Cassini Division as pointing reference.
Residual background was determined from a linear fit to pixels located outside the A ring and inside the C ring, and then subtracted to eliminate the contribution of the planet and the sky. Observations of G-type SAO stars (Table 1) were
performed before and after
each run of observation of the rings in order to provide atmospheric and photometric
calibrations for each mode. The flux- and wavelength- calibrated spectral segments
were then merged to provide contiguous spectra between 0.81 and 5.40 m.
Our analysis was performed in two steps. The first step consisted of a description
of the spectral characteristics of the three main rings and an assignment
of absorption bands. This operation allowed us to select the sets of optical constants
for the spectral modeling. The second step consisted of a global modeling of
the 0.9-5.4 m region spectrum, accompanied by the UV/visible
spectrophotometric data obtained with HST by Cuzzi et al. (2002)
in order to enable a comprehensive analysis of the dark material. Our knowledge of
the composition derived from the direct analysis step is thus improved, because
quantitative information about the abundance and the grain size of different
components can be obtained. Eventually, the implications for the
origin and the evolution of rings are discussed.
Figure 3 shows our scaled, full-resolution
spectra of the A and B rings at the two phase angles. The signal-to-noise ratio (SNR)
of the data taken at 4.7
was slightly better between 0.9 and 4.2
m (SNR
100-500) and much better beyond 4.5
m than at 0.6
,
which provides
the first measurement of this part of the spectrum even though the
signal is noisy (
1-10). The spectra of the two main rings are
similar to the spectrum obtained by Clark & McCord (1980).
They resemble those of the higher albedo satellites of Saturn
in the 1.0-3.0
m region: they are characterized by a strong blue
slope. However, the major water ice features at 1.04, 1.25,
1.50, 1.65, 2.02 and 3.1
m are much more clearly defined than in
earlier studies. Moreover,
the 1.31 and 1.56
m bands are readily apparent in the 4.7
spectra.
Both the temperature-sensitive band at 1.65
m and the fundamental
absorption in the 3.0-3.2
m region, which appears as a reflection peak
because the imaginary index is so large, are in favor of the presence of
water ice in its crystalline phase only.
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Figure 3:
Spectra of B and A rings at two different phase angles. The water ice absorption bands in the 0.9-2.5 ![]() |
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Figure 4:
Upper: Smoothed ratio of spectra of B/A. The ratio is normalized to 1 at
1 ![]() ![]() |
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Figure 5:
Spectrum of C ring obtained at 4.7 ![]() ![]() |
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Figure 6:
Comparison of the 2.95-3.2 ![]() |
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Regarding the presence of ices other than water ice,
no spectral evidence for ices of
(major absorption bands at
1.75, 1.78, 2.20, 2.31, 2.37
m),
(2.35),
(2.14),
(2.27),
(2.20) or
(1.43, 2.01, 2.07) was found. The large noise in
the 0.8-0.9
m region, which is just at the edge of our spectral window,
prevents the confirmation of a proposed absorption at 0.85
m.
While the red slopes of the B and A rings in the
UV/visible can vary up to 10% between 0.6
and 4.7
of phase angle
(Poulet et al. 2002a), our data show that only slight variations in
the spectra are seen in response to
differing solar phase angles for
between 0.9 and 2.6
m,
except between 1.1 and 1.4
m where the 0.6
data present an unexplained
odd peak. For the larger
wavelengths, the brightness of the B ring is noticeably lower at 0.6
phase angle
than 4.7
phase angle. This effect is especially strong for the A ring, while
no data were obtained for the C ring near the opposition because of the bad SNR.
Because we have no instrumental or observational explanations for
such large differences, we believe that the 0.6
phase angle data are suspicious and no reliable conclusion on the phase angle dependence can be therefore made from the present data.
The A and B ring spectra are compared using their ratio in Fig. 4. They are very similar, with small differences arising from slightly stronger water ice absorption bands in the B ring.
The major water ice absorptions are detected in the C ring spectrum.
The 0.95 m band is especially strong. To see it so clearly suggests long path lengths in the ice and is consistent with the strong 1.04 and 1.25
m bands.
The overall shape of the spectrum is different from the A and B rings
(Figs. 4
and 5). In particular, its slope in the near-IR
reflectance is less blue. The blue slope is typical of
the pure water ice and depends on the size of surface grains. If dark material
is mixed with water ice, the reflectance of
the mixture increasingly approaches that of the dark material as its abundance is
increased. Thus, both the brightness
and the NIR slope of the C ring
compared to A and B rings indicate a concentration of dark material
different in terms
of abundance. On the other hand, the low signal in the 3.0
m region
implies that the dark non-icy contaminant is likely incorporated as an
intra-mixture (Sect. 3.2) and not as an intimate mixture
as in the case of Iapetus (Owen et al. 2001).
The 1.65 m band clearly implies the presence of crystalline ice. However,
its depth in the C ring is much smaller than in the A and B rings.
Using the strong temperature-dependent strength of this feature
(Grundy & Schmitt 1998), we infer an ice physical temperature of about
K. This value is much larger than the estimate of 90 K range
from IRIS measurements (Esposito et al. 1984). Although Voyager temperatures
were obtained when the rings were almost edge-on to the Sun, this overestimate
of the C ring ice temperature might be also due
to the presence of modest quantities of amorphous water ice for which the
1.65
m band disappears. A more convincing argument to probe lattice order
is related to the triangular Fresnel reflection peak
at 3.1
m. As shown in
Fig. 6, the peak is broad and low, and its poor resemblance to
crystalline ice may also support the presence of amorphous ice.
Solid
at low pressure
can exist in two phases: amorphous ice in high and low density configurations,
and crystalline cubic and hexagonal phases. The temperature
of the phase change from amorphous to crystalline ice is in the range 140-150 K
(Jenniskens & Blake 1). Amorphous
ice is not commonly found in the Solar System objects even on the very cold surfaces
of Charon (Brown & Calvin 2000) and the uranian satellites (Roush et al. 1998;
Grundy et al. 1999) where it could be stable.
The C ring spectrum exhibits one band centered at 1.73 m. It is likely not
a spectral artifact because it does not appear in B and A ring spectra.
We compared the spectrum with laboratory transmission spectra of major pure ices
(
,
,
,
or
). This comparison
indicates that this band could be assigned to a band of
ice
(
)
only. However, pure
ice has several
other strong bands in the K filter region (at 2.20, 2.31, 2.37
m) which
are not detected. Other potential candidates could
be minerals (silicates, salts, and carbonates). We have investigated the mineral
hypothesis using the USGS data base (Clark et al. 1993), but none of
the main known minerals
possess this feature at 1.73
m without other strong features. For
salts and carbonate materials, the strong features are in the 3-5
m range.
The B and C ring spectra are compared using their ratio in Fig. 4. Much of
the structure is due to the greater water ice band strengths in the B ring
compared to the C ring. However, there are two regions at 1.73 and 3.4 m
where the differences cannot be explained by the water ice bands. These bands
agree in position quite well with the aliphatic C-H streching and bending
bands of the spectrum of asphaltite,
a natural solid oil bitumen (Moroz et al. 1998). The absorption at 2.3-2.5
m is not clearly visible in the C/B ratio, in part because of
the interference of the water ice band stronger in B than in C.
This implies that the C-H may be present in the C ring only, but it needs
confirmation.
We use the model recently developed by Shkuratov et al. (1999) to calculate
the albedo of a ring particle. This model, based on the geometrical optics
approximation as in the more familiar Hapke (1981) model, provides
the spectral albedo of powdered surfaces. The degree of physical realism has
been studied in Poulet et al. (2002b). In addition to common intimate
("salt-and-pepper'') mixture, an interesting type of mixture
can be formulated: some individual large particles (water ice in the case
of ring particles) must themselves contain a mixture of different
materials visualized as small inclusions ()
in the bulk. This mixture
belongs to the type previously referred to as "intra" or "molecular" mixing of
constituents (Cuzzi & Estrada 1998). The Shkuratov model was
successfully used to reproduce a composite
spectrum of Saturn's rings (Poulet & Cuzzi 2002), and more details on the model
and our representation of a particle's surface can be found in that paper.
We combined our spectra with UV/visible spectrophotometric data obtained with
the HST by Cuzzi et al. (2002) in order to get composite spectra of the B and A rings
from 0.25 to 5.4 m (Figs. 7 and 8) and the
C ring from 0.25 to 4.0
m (Fig. 9).
For each ring, the mixture has to satisfy the following constraints:
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Figure 7:
Spectra of the B ring taken at 4.7 ![]() |
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Figure 8:
Spectrum of the A ring taken at 4.7 ![]() |
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Figure 9: Spectrum of the C ring compared with three models (red, green and blue curves). Red: best model; blue: same as red except for one water ice grain made of amorphous water ice; green: Same kind of mixture than that used for the spectra of the B and A rings. |
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Table 2: B ring: Relative abundances and grain sizes.
Table 3: A ring: Relative abundances and grain sizes.
We simultaneously fit the data for all wavelengths using the IDL simplex
minimization algorithm.
For the A and B rings, the best-fitting model consists of a salt-and-pepper
mixture of four components (Figs. 7 and 8):
water ice grains of three different sizes (typically 10, 100 and 1000 m,
probably merely indicating a grain size distribution)
and amorphous carbon grains (Tables 2 and 3).
In order to reproduce the red slope, the water ice grains
have to be contaminated by a reddish tholin compound. As
expected from previous albedo measurements (Doyle et al. 1989; Dones et al.
1992), the amount of dark matter (carbon) is larger for the A ring (8% vol) than
for the B ring (2% vol). From a spectrum integrated over the B ring,
the Cassini division and a part of A ring, Poulet & Cuzzi
(2002) found a relative abundance of 7%. Note that we cannot constrain the
diameter l of carbon particles, because no absorption band exists. A
lower limit can be given only as explained in Poulet & Cuzzi. (2002).
We also tried to incorporate the amorphous carbon as both an intramixture
and an intimate mixture, but the fitting procedure converged to a
minimal solution where no intramixed amorphous carbon is present.
For the C ring, the best-fitting model consists of a salt-and-pepper mixture of crystalline water ice grains of three different sizes intra-mixed with both ice tholin and amorphous carbon inclusions (red line in Fig. 9, Table 4). If the amorphous carbon is intimately mixed as found for the A and B rings, we fail to reproduce the C ring spectrum (green line in Fig. 9): the model is too bright in general and the bands are too shallow. In other words, the carbon, if intimately mixed, seems to provide too large a reflectivity from front-surface grain reflection in the deep water absorptions. By contrast, in the case of amorphous carbon intramixtured, the water ice component can be on the top of amorphous carbon making the deep bands dark enough. We also tested the presence of a small amount of amorphous ice by incorporating a grain made of dirty amorphous ice. However, this worsens the fit to the positions of the water ice absorption bands (blue line in Fig. 9).
Table 4: C ring: Relative abundances and grain sizes (red model in Fig.9).
In order to better identify the discrepancies between the model and the data, we divide the data by the best fit model (Figs. 10 and 11).
There is a significant discrepancy in the UV range (F255W and F336W HST filters).
This can be interpreted in two ways: 1- the specific tholins we adopted absorb
too strongly in UV, or 2- the rings being a strong red source, the
steeply changing intensity across the filter band pass could move the band center
to a longer wavelength relative to the band center theoretically
defined for a white light source. We reiterate that
our fitting procedure consisted of finding the best fit of the overall spectrum.
Specifically, the
amount of tholin has been derived from the best fit of the reddening from 0.25 to 0.8
m. Matching a slope is not a satisfying approach for
proving a surface composition. It is suited for proving
that something is not present, but not so good for establishing what is present.
However, the tholins provide a far better fit than any other known
materials (Poulet & Cuzzi 2002), and do not introduce or interfere with spectral features at other wavelengths. We also note that the red
reflectance slope in the same spectral region has been taken as evidence for the
presence of organic solid material in the surface materials of other Solar
System bodies, notably the Centaur object 5145 Pholus (Cruikshank et al. 1998; Poulet et al. 2002b), because minerals and ices alone do not provide an appropriate
absorption in the violet and ultraviolet regions.
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Figure 10: Data/model ratio for B (upper) and A rings (lower). |
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Between 1 and 2.5 m, the fits agree reasonably well, even if some
variations of ratios are due to differences in water ice band shapes (2.0
m for example).
A significant mismatch is observed at wavelengths longer than 2.2
m in the case
of A and B rings. Water ice alone does not appear to account for the spectra.
Some other component is needed to achieve the blue slope. This problem has
been already noted in the attempts of modeling of the spectra of the
Saturn's bright satellites (Benedix et al. 1998; Dalle Ore et al. 1999;
Cruikshank et al. 1999) and Charon (Buie & Grundy 2000). The possible but
unidentified constituent seems to be similar to what is
seen on other icy bodies of the outer Solar System.
Obviously, the wavelength region longer than 2.8 m is less well reproduced
by the models than the shorter wavelength region.
This spectral region has not been a subject of extensive analysis for Solar System
objects because of the lack of data with sufficient SNR.
Moreover, modeling analysis is not easy because of the combination of
the lack of accurate optical constants and the strong dependence on grain size.
In spite of these limitations, we attempt to explain the anomalous spectral
features below.
A major discrepancy occurs in the 3.0-3.2 m where the water ice has very large imaginary refractive index. The peak comes from the Fresnel
reflection off the facets of water
grains in the surface, and is thus from zero depth. The saturnian satellites
for which data exist in this region, also present this
peak (Fig. 6).
In any case, the Shkuratov model fails to reproduce the shape of the absorption
peak. The presence of frost made of very small grains (
)
might
reproduce this peak (Hansen, personal communication). However, we recall that the
Shkuratov theory, which is based on geometrical optics, is not suitable
to simulate the contribution of such a fine frost.
Beyond 3.2 m, the shape of the bump strongly depends on water ice grain size
and possible presence of unidentified non-icy impurities. Again, the rise to the
peak at 3.6
m is higher than the model, indicating further that very fine
icy grains and/or some impurities might exist. The entire region beyond 3.6
m is bad because the model overestimates the signal. The presence of the
non-icy components used in our modellings is not responsible for the greater signal.
Larger grains have a lower albedo beyond 3.6
m but larger grains are
ruled out by the shapes of the H2O absorption bands at 1.6 and 2.0
m.
However, it seems quite possible that a component of the absorber may simply be
H2O with absorptions distorted by close association of other elements or
ions. The precise details of the bandshapes depend on the composition, temperature,
thermal history, and equilibria established between radiation, erosional
and photochemical processes. The optical constants of ring water ice
will therfore surely differ from those of laboratory water ice if their conditions
of formation or evolution were different.
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Figure 11: Data/model ratio for C ring. |
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These new observations of Saturn's rings are the first to combine an extended spectral range, high spectral resolution, and spatial resolution. The analysis presented here yields new information on the composition of the surfaces of the ring particles in A, B and C rings separately:
The amorphous carbon impurities mixed intimately with water ice in the A and B rings could come from meteoritic bombardment, as proposed by Cuzzi & Estrada (1998). A discussion about this dark material is presented in Poulet & Cuzzi (2002). It is well known that darkening due to meteoroid material that becomes mixed with the ring material appears to give ages much shorter than the Solar System (Doyle et al. 1989; Cuzzi & Estrada 1998). However, uncertainty in the meteoroid flux could allow the rings to be as old as the Solar System (Dones 1991; Cuzzi & Estrada 1998).