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Cassini’s CDA observes a variety of dust populations just outside Saturn’s F ring

Presentation #112.03 in the session Many Planets, More Rings (Oral Presentation)

Published onOct 23, 2023
Cassini’s CDA observes a variety of dust populations just outside Saturn’s F ring

During its ‘Ring-Grazing Orbits’ (RGO) between the end of 2016 and early 2017, the Cassini spacecraft repeatedly crossed Saturn’s ring plane just outside the F ring, close to the orbits of the moons Janus, Epimetheus and Pandora. During this penultimate phase of the mission, dust particles in this region were sampled for the first time. By producing time-of-flight mass spectra, the Cosmic Dust Analyzer (CDA) [1] revealed the compositions of thousands of dust grains in this region. Here, we focus on the spectra generated by grains sampled within ±15 minutes relative to the ring plane crossings, which impacted the instrument’s target at relative speeds of ~20 km/s.

Comparing the spectra of the detected particles with those of E ring observations [2,3], we investigate if the grains belong to a local population, or if other sources influence the sampled region. We find that the majority of analyzed spectra are similar to those of E ring water ice grains, indicating that the E ring extends inward to at least 2.45 Saturn radii (RS) from Saturn. Compositional modification by space weathering of many E ring grains suggests a significant dynamical age since ejection by Enceladus.

Close to the orbits of Janus and Epimetheus, a higher abundance of larger grains, unlikely to have an E ring origin, is observed, implying that the E ring dust population overlaps with dust from other sources, such as the nearby moons. However, no compositional differences between these two populations can be observed, suggesting that the surfaces of Janus and Epimetheus are covered with E ring dust.

A small fraction (~1%) of the analyzed spectra is attributed to mineral particles, found to be on prograde orbits around Saturn. This contrasts with the dynamics of other mineral particle populations previously detected by CDA in the Saturnian system, which were either on retrograde orbits or were found to have an exogenous origin [4,5]. Among the RGO mineral grains we identify a unique water/silicate mixed type [6], that was not previously observed elsewhere in the Saturnian system and, for which, due to the probable young particle ages [7], we again infer an origin from a local source, such as the F ring or the nearby moons. Another Fe-poor silicate particle population likely originates from Saturn’s main rings.

References

[1] Srama et al. (2004), Space Science Reviews 114, 465.

[2] Postberg et al. (2008), Icarus 193, 438.

[3] Postberg et al. (2009), Nature 459, 1098.

[4] Trieloff et al. (2023), EGU General Assembly, 16469.

[5] Altobelli et al. (2016), Science 352, 312.

[6] Buratti et al. (2019), Science 364, eaat2349.

[7] Jurac et al. (2001), Icarus 149, 384.

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