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Cassini gradually lifts the veil on Saturn’s rings

NASA's Cassini probe, which orbited Saturn from 2004 to 2017, changed its trajectory during the final phase of its mission. Its objective? To carry out particularly close flybys of the planet and its rings.

Cassini gradually lifts the veil on Saturn’s rings

Publication date: 17/06/2019

Press, Research

Related themes : Origins

During the “grand finale”, Cassini succeeded in passing between Saturn and its rings, before plunging into the heart of the planet. This unique configuration enabled exceptional observations of the ring system.
An international team has compiled all the high-resolution images, as well as spectral and thermal profiles, in order to better constrain the physical properties of these rings. Researchers from two French laboratories, the Laboratoire de Planétologie et Géodynamique (LPG, CNRS/Univ. Angers/Univ. Nantes), and the Institut de Physique du Globe de Paris (IPGP/Université de Paris, CNRS) were involved in this international study, published on June 13th 2019 in Science.

This international team describes the results of the analyses carried out by four instruments on the Cassini probe that closely studied Saturn’s main rings. The images collected show in particular the close interaction between small embedded satellites and the rings (named from A to G, in the order in which they were discovered), which they help to sculpt from the inside. These tiny moons act like protoplanets forming within the disc of matter. They thus provide a glimpse into the formation of our Solar System, through its accretion disc evolving under the influence of the masses that make it up.

These new observations give scientists a better appreciation of these rings, with each detail revealing new complexities.

The false-color image on the right reveals the spectral variability of Saturn's A, B and C rings, as seen by the Cassini probe's VIMS visible and near-infrared imaging spectrometer. Credits: NASA/JPL-Caltech/University of Arizona/CNRS/LPG - Saturn image credit: NASA/JPL-Caltech/Space Science Institute/G. Ugarkovic

“At the LPG, we concentrated on analysing the data from the VIMS hyperspectral imager, which was used in a very special mode to take maximum advantage of the low-level flybys carried out by Cassini,” explains Stéphane Le Mouélic, a CNRS researcher at the LPG (CNRS/Université de Nantes/Université d’Angers), who also worked with Sébastien Rodriguez, a researcher at the IPGP and lecturer at the Université Paris Diderot. “The result is a series of observations a few dozen pixels wide and several hundred pixels long, crossing all the rings and highlighting their spectral diversity. The rings, which are very finely structured, appear to be dominated by the infrared signature of water ice, with a more or less significant component of contaminants that may correspond to nanophase metallic iron particles. No traces of ammonia, methane or organic molecules were detected.

Cécile Ferrari, a researcher at the IPGP, professor at Paris Diderot University and co-investigator of the CIRS instrument, contributed to the analysis and interpretation of the observations made by the CIRS infrared spectrometer, which determined the temperature of the rings throughout the mission. “The temperature of the rings traces the dynamic behaviour of particles in the equatorial plane of the planet and also their internal structure (porosity, for example). We had already discovered that in the densest rings A and B, the more optically dense the ring (capable of absorbing incident sunlight), the greater the temperature gradient between its sunlit and non-sunlit sides”, which can be interpreted as being due to a greater thickness at this point. This behaviour was confirmed by the very high-resolution data obtained during the latest grazing orbits. “However, these latest observations have enabled us to obtain very good resolution on the narrower dynamic structures (in radial extent) of the A ring or the plateaus of the C ring, and have shown us that in the A ring we can observe a different behaviour, which can no doubt be explained by more pronounced vertical dynamics of the particles and more efficient heat transfer. In the C ring, despite the better resolution obtained on the plateaux, the noise remains high and it is difficult to establish a clear correlation between their optical depth and a stronger thermal gradient”.

Some questions have therefore been answered thanks to the probe’s dive into the heart of the rings, but many mysteries remain as to the dynamics of these structures and the scientists involved are now focusing on modelling the evolution of these rings.
The Cassini-Huygens mission is a cooperative project between NASA, the European Space Agency (ESA) and the Italian Space Agency. The Jet Propulsion Laboratory (NASA, Caltech) designed, developed and assembled the Cassini orbiter. The radio antenna was built by JPL and the Italian Space Agency, in collaboration with team members from the United States and several European countries.

Ref : M. S. Tiscareno, P. D. Nicholson, J. N. Cuzzi, L. J. Spilker, C. D. Murray, M. M. Hedman, J. E. Colwell, J. A. Burns, S. M. Brooks, R. N. Clark, N. J. Cooper, E. Deau, C. Ferrari, G. Filacchione, R. G. Jerousek, S. Le Mouélic, R. Morishima, S. Pilorz, S. Rodriguez, M. R. Showalter, S. V. Badman, E. J. Baker, B. J. Buratti, K. H. Baines, C. Sotin, Close-range remote sensing of Saturn’s rings during Cassini’s ring-grazing orbits and grand finale, Science, June 2019

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