Occultation observations of Saturn's rings with Cassini VIMS

Nicholson, P. D.; Ansty, T. M.; Hedman, Matthew M.; Creel, Douglas; Ahlers, Johnathon; Harbison, Rebecca A.; Brown, R. H.; Clark, R. N.; Baines, K. H.; Buratti, B. J.; Sotin, C.

doi:10.1016/j.icarus.2019.06.017

Cited by 8 publications

(5 citation statements)

References 52 publications

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“…Navigation information, such as geographic position (latitude and longitude position at the pixel's center and at the four corners of the pixel), corresponding illumination and viewing conditions (incidence, emergence and solar phase angles at each pixel's center) and spatial resolution, were retrieved using reconstructed SPICE kernels (Acton, 1996). The pixel location is calculated as the intersection between Enceladus mean radius (252.1 km) and the VIMS pixel boresight (Nicholson et al, 2019). Our projected VIMS cubes are geometrically consistent with the latest ISS global map (Bland et al, 2018) as it can been on a few examples on Fig.…”

Section: Vims Dataset and Methods For Computing Global Mapsmentioning

confidence: 99%

Photometrically-corrected global infrared mosaics of Enceladus: New implications for its spectral diversity and geological activity

Robidel

Mouëlic

Tobie

et al. 2020

Icarus

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Between 2004 and 2017, spectral observations have been gathered by the Visual and Infrared Mapping Spectrometer (VIMS) on-board Cassini (Brown et al., 2004) during 23 Enceladus close encounters, in addition to more distant surveys. The objective of the present study is to produce a global hyperspectral mosaic of the complete VIMS data set of Enceladus in order to highlight spectral variations among the different geological units. This requires the selection of the best observations in terms of spatial resolution and illumination conditions. We have carried out a detailed investigation of the photometric behavior at several key wavelengths (1.35,1.5, 1.65, 1.8, 2.0, 2.25, 2.55 and 3.6 µm), characteristics of the infrared spectra of water ice. We propose a new photometric function, based on the model of Shkuratov et al. (2011) When combined, corrected mosaics at different wavelengths reveal heterogeneous areas, in particular in the terrains surrounding the Tiger Stripes on the South Pole and in the northern hemisphere around 30 • N, 90 • W. Those areas appear mainly correlated to tectonized units, indicating an endogenous origin, potentially driven by seafloor hotspots.

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Section: Vims Dataset and Methods For Computing Global Mapsmentioning

confidence: 99%

Photometrically-corrected global infrared mosaics of Enceladus: New implications for its spectral diversity and geological activity

Robidel

Mouëlic

Tobie

et al. 2020

Icarus

View full text Add to dashboard Cite

show abstract

“…, where the transparency T is the ratio between the occulted and unocculted signal (e.g., Nicholson et al 2020). Furthermore, the normal optical depth τ n is a representation of what the optical depth would be if the rings were viewed faceon, accounting for any slanted ray paths:…”

Section: Hmentioning

confidence: 99%

Identifying Shadowing Signatures of C Ring Ringlets and Plateaus in Cassini Data from Saturn’s Ionosphere

et al. 2022

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For orbits 288 and 292 of Cassini’s Grand Finale, clear dips (sharp and narrow decreases) are visible in the H 2 + densities measured by the Ion and Neutral Mass Spectrometer (INMS). In 2017, the southern hemisphere of Saturn was shadowed by its rings and the substructures within. Tracing a path of the solar photons through the ring plane to Cassini’s position, we can identify regions in the ionosphere that were shadowed by the individual ringlets and plateaus (with increased optical depths) of Saturn’s C ring. The calculated shadowed altitudes along Cassini’s trajectory line up well with the dips in the H 2 + data when adjusting the latter based on a detected evolving shift in the INMS timestamps since 2013, illustrating the potential for verification of instrument timings. We can further estimate the mean optical depths of the ringlets/plateaus by comparing the dips to inbound H 2 + densities. Our results agree well with values derived from stellar occultation measurements. No clear dips are visible for orbits 283 and 287, whose periapsides were at higher altitudes. This can be attributed to the much longer chemical lifetime of H 2 + at these higher altitudes, which in turn can be further used to estimate a lower limit for the flow speed along Cassini’s trajectory. The resulting estimate of ∼0.3 km s−1 at an altitude of ∼3400 km is in line with prior suggestions. Finally, the ringlet and plateau shadows are not associated with obvious dips in the electron density, which is expected due to their comparatively long chemical (recombination) lifetime.

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“…The high signal-to-noise in these regions was due to the occultations occurring at low ring opening angles(Nicholson et al 2020). Unfortunately, this also meant that the plateaux were nearly opaque during these occultations, so they could not be included in the full wavelet analysis.…”

mentioning

confidence: 99%

Kronoseismology. VI. Reading the Recent History of Saturn’s Gravity Field in Its Rings

et al. 2022

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Saturn’s C ring contains multiple structures that appear to be density waves driven by time-variable anomalies in the planet’s gravitational field. Semiempirical extensions of density wave theory enable the observed wave properties to be translated into information about how the pattern speeds and amplitudes of these gravitational anomalies have changed over time. Combining these theoretical tools with wavelet-based analyses of data obtained by the Visual and Infrared Mapping Spectrometer on board the Cassini spacecraft reveals a suite of structures in Saturn’s gravity field with azimuthal wavenumber 3, rotation rates between 804° day−1 and 842° day−1, and local gravitational potential amplitudes between 30 and 150 cm2 s−2. Some of these anomalies are transient, appearing and disappearing over the course of a few Earth years, while others persist for decades. Most of these persistent patterns appear to have roughly constant pattern speeds, but there is at least one structure in the planet’s gravitational field whose rotation rate steadily increased between 1970 and 2010. This gravitational field structure appears to induce two different asymmetries in the planet’s gravity field, one with azimuthal wavenumber 3 that rotates at roughly 810° day−1 and another with azimuthal wavenumber 1 rotating three times faster. The atmospheric processes responsible for generating the latter pattern may involve solar tides.

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Occultation observations of Saturn's rings with Cassini VIMS

Cited by 8 publications

References 52 publications

Photometrically-corrected global infrared mosaics of Enceladus: New implications for its spectral diversity and geological activity

Photometrically-corrected global infrared mosaics of Enceladus: New implications for its spectral diversity and geological activity

Identifying Shadowing Signatures of C Ring Ringlets and Plateaus in Cassini Data from Saturn’s Ionosphere

Kronoseismology. VI. Reading the Recent History of Saturn’s Gravity Field in Its Rings

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