Cassini Exploration of the Planet Saturn: A Comprehensive Review
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Cassini Exploration of the Planet Saturn: A Comprehensive Review Andrew P. Ingersoll1
Received: 10 October 2019 / Accepted: 10 October 2020 © The Author(s) 2020
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. Keywords Saturn · Jupiter · Cassini · Atmosphere · Interior · Giant planet
1 Introduction The planets themselves, Saturn and Jupiter, were an important scientific objective of the Cassini mission, and they are the focus of this review. The review is taken from Volume 1 of the Cassini Final Report (https://pds-rings.seti.org/cassini/). That document contains instrument-specific chapters, other discipline chapters beside the planets themselves, and a
B A.P. Ingersoll
[email protected]
1
Division of Geological and Planetary Sciences, California Institute of Technology, 1200 East California Blvd, Pasadena, CA 91125, USA
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science summary with a science assessment for the complete mission. For setting priorities and planning, Cassini had five discipline working groups: Icy satellites, magnetosphere, rings, Saturn, and Titan. The report is taken from the Saturn chapter. The overlap region, where the upper atmosphere of the planet merges with the near-vacuum of space, is covered in the magnetosphere chapter if charged particles are the dominant medium, and in the Saturn chapter if neutral gases are the dominant medium The focus is on Cassini and the discoveries it made. The report is not a complete review of giant planet atmospheres and interiors. The reference list was compiled from the Cassini instrument reports and from literature searc
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