Saturn's F Ring: Observations by Cassini UVIS HSP
Nicole Albers, Miodrag Sremčević, and Larry W. Esposito
Nicole.Albers@lasp.colorado.edu
Laboratory for Atmospheric and Space Physics, University of Colorado at Boulder, USA
Ultra Violet Imaging Spectrograph (UVIS) High Speed Photometer (HSP) [1]
Abstract
Since the Saturn orbit insertion in July 2004, the Ultraviolet Imaging Spectrograph (UVIS) aboard the Cassini spacecraft obtained >59 stellar occultations by Saturn's F ring with spatial resolutions ranging between a few and hundred meters. The data covers various times, longitudes, and angles. Structural changes are apparent and confirm the F ring's reputation as one of the most dynamic structures observed in the rings.
HSP is one of three channels of the UVIS instrument and sensitive to incoming photons of wavelengths ranging between 110 nm and 190 nm which for stellar occultations implies type O or B stars
●
Sampling time intervals are on average 2 ms leading to spatial resolutions of a few to hundred meters in the ring plane.
●
Photometer is basically “counting” incoming photons and its statistical properties are well described by Poissonian statistics.
●
New Orbit Fit – Comparison to Bosh et al. (2002) [2]
Optical Depth & Co
The F ring appears differently in each and every single occultation cut without apparent dependence on either ring plane elevation, azimuth, or time. Prior to Cassini, strands were unresolved in occultations but, if present, are clearly visible now. Using stellar I stellar
and I back
background levels we obtain an optical depth profile for each occultation cut according to:
[
I counts −I background
 = − log
I stellar
]
We fit a Lorentzian to the radial optical depth profile yielding peak optical depth  max
, half­
width at half­maximum (HWHM), equivalent r core
width , and core position .
W =∫ r  dr
W Lorentz
HWHM
r  =
2
2

 HWHM  r−r core 
Material is not conserved azimuthally: W ≠const
● Ring is not governed by self­gravity alone, 
since its width does not correlate with radius r
● No other obvious correlations are revealed in the data
● On average: max ~0.1−0.3 , W ~10 km
● Core appears self­similar on scale of 20 km
max ∝W r ±10 km
●
radial distance [1.39,1.41]x1e5 km
Figure 1a Exemplary optical depth profiles
core
Figure 1b Equivalent width as measured by various instruments. Despite a general trend W is highly variable pointing to the ring's azimuthal asymmetry.
0 ≤ max ≤ 3.59
4.8 km ≤ W ≤ 50.8 km
2.3 km ≤ HWHM ≤ 124.1 km
W ≤20 km
max ≤0.5
87% of occultations: and Figure 2a Core positions in Cassini UVIS data compared to the predicted orbit solution [2]. Residuals reach up to 80 km and reveal a general periodic trend.
Figure 2b Core positions in Cassini UVIS data compared to the new orbit fit. In general, residuals are km but do reach up to ±20
40 km. However, these are at least several times larger than any uncertainty in either the geometry solution or core position determination.
Semimajor Eccentricity Long. of Precession Rate Inclination Long. of ascending axis [km] x 1e­3 pericenter [deg] [deg/day] [deg] node [deg] Bosh et al. (2002,#3): 140223.7±2.0
2.54±0.05
24.1±1.6
2.7001±0.0004
0.0065±0.0007
16.1±3.6
New elements: 140215.3±1.2
2.33±0.01
23.2±0.5
2.7003±0.0002
0.0082±0.0005
4.2±4.1
Note: The new fit is close to #2 in [2] where no data points were excluded, pointing to a subset influence on the orbit solution.
360 degrees longitude
Figure 3 Radial optical depth profiles as seen in various occultations by Saturn's F ring in Cassini UVIS data. Spanning 360 deg in longitude the F ring core is “clearly” visible describing an elliptical, precessing orbit. F Ring: A Split Personality?
Summary
F ring core appears highly variable in time and longitude. Optical depth is changing between 0 and 3.6
● Despite its variability the determined core position can still be fitted by a precessing ellipse. Deviations from the elliptical motion are on the order of 40 km and are probably due to the influence of shepherds and, perhaps, also yet unobserved tiny moonlets. ● Following the new orbit fit Prometheus will not collide with the main F ring core but have its closest approach of about 200 km in late 2009. It, however, will not “miss” the second, core­like feature that recently occurred, if it turns out to be persistent. ●
Beginning 2008 (first coverage on 2008­026 in UVIS data) a second core­like feature appeared. Occasionally optically thicker than the core, it lies 200 km interior and outside of a radial range of where the otherwise optically thin strands usually occur. A preliminary orbital fit yields a semimajor axis of 140,020 km and an eccentricity of about 0.002. This rough orbit indicates that the second “core” approached Prometheus to about 40 km.
“Und so sehen wir betroffen. Den Vorhang zu und alle Fragen offen.” Berholt Brecht, Der gute Mensch von Sezuan
References: [1] Esposito et al. (2004, Space Science Reviews, 115, p. 299), [2] Bosh et al. (2002, Icarus, 157, p. 57)