Cassini Exploration of the Planet Saturn: A Comprehensive Review

Abstract

Before Cassini, scientists viewed Saturn’s unique features only from Earth and from three spacecraft flying by. During more than a decade orbiting the gas giant, Cassini studied the planet from its interior to the top of the atmosphere. It observed the changing seasons, provided up-close observations of Saturn’s exotic storms and jet streams, and heard Saturn’s lightning, which cannot be detected from Earth. During the Grand Finale orbits, it dove through the gap between the planet and its rings and gathered valuable data on Saturn’s interior structure and rotation. Key discoveries and events include: watching the eruption of a planet-encircling storm, which is a 20- or 30-year event, detection of gravity perturbations from winds 9000 km below the tops of the clouds, demonstration that eddies are supplying energy to the zonal jets, which are remarkably steady over the 25-year interval since the Voyager encounters, re-discovery of the north polar hexagon after 25 years, determination of elemental abundance ratios He/H, C/H, N/H, P/H, and As/H, which are clues to planet formation and evolution, characterization of the semiannual oscillation of the equatorial stratosphere, documentation of the mysteriously high temperatures of the thermosphere outside the auroral zone, and seeing the strange intermittency of lightning, which typically ceases to exist on the planet between outbursts every 1–2 years. These results and results from the Jupiter flyby are all discussed in this review.

Study of Chemical Processes Initiated by Electrical Discharge in Titan-Related Atmosphere at Laboratory Temperature and Pressure

“…39−41 Lightning is also believed to take place at Neptune. 39,42 Moreover, it was pointed out very recently that the lightning activity in Jupiter's 38 atmosphere is significantly higher than was expected, and thus the same situation can be also assumed in other atmospheres. 43 Besides the classical lightning phenomena, transient lightning events like sprites, jets, and elves were recently also confirmed in the atmospheres of Venus, Mars, 44 and Jupiter.…”

“…26 Lightning, whcih is well-known in Earth's atmosphere, was confirmed in more planetary atmospheres. 21,27,28,35 While the occurrence of lightning on Venus is still a controversial question, 36,37 there is clear evidence of lightning in the atmospheres of the gas giants like Jupiter, Saturn, 38 and Uranus. 39−41 Lightning is also believed to take place at Neptune.…”

“…It is assumed that the electrical discharge phenomena can play an important role in the atmospheric chemistry at lower altitudes . Lightning, whcih is well-known in Earth’s atmosphere, was confirmed in more planetary atmospheres. ,,, While the occurrence of lightning on Venus is still a controversial question, , there is clear evidence of lightning in the atmospheres of the gas giants like Jupiter, Saturn, and Uranus. − Lightning is also believed to take place at Neptune. , Moreover, it was pointed out very recently that the lightning activity in Jupiter’s atmosphere is significantly higher than was expected, and thus the same situation can be also assumed in other atmospheres . Besides the classical lightning phenomena, transient lightning events like sprites, jets, and elves were recently also confirmed in the atmospheres of Venus, Mars, and Jupiter …”

Study of Low-Frequency Electromagnetic Ion-Cyclotron Wave for Ring Distribution in Magnetosphere of Saturn

“…Pioneer 11 discovered ion cyclotron waves for the first time in Saturn's E orbit (6.26 Rs) [6]. Magnet vibrations from 5 to 7 radial lengths were discovered by Voyager 1 [19]. The anisotropic temperature of the O+ population is thought to be connected to these wave measurements.…”

Layer formation in a stably-stratified fluid cooled from above. Towards an analog for Jupiter and other gas giants

“…The Juno and Cassini missions have provided the best observational constraints on the internal structure of Jupiter and Saturn, both from measurements of their gravity fields and from seismology studies of Saturn's rings [see, e.g., [1][2][3][4]. These observations indicate that a large region within the interior of these planets (up to half of their radii) is likely stably-stratified by heavy elements, suggesting that their cores are diluted and more extended, without a sharp core-envelope transition as previously thought.…”