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Buratti BJ, Thomas PC, Roussos E, Howett C, Seiß M, Hendrix AR, Helfenstein P, Brown RH, Clark RN, Denk T, Filacchione G, Hoffmann H, Jones GH, Khawaja N, Kollmann P, Krupp N, Lunine J, Momary TW, Paranicas C, Postberg F, Sachse M, Spahn F, Spencer J, Srama R, Albin T, Baines KH, Ciarniello M, Economou T, Hsu HW, Kempf S, Krimigis SM, Mitchell D, Moragas-Klostermeyer G, Nicholson PD, Porco CC, Rosenberg H, Simolka J, Soderblom LA. Close Cassini flybys of Saturn’s ring moons Pan, Daphnis, Atlas, Pandora, and Epimetheus. Science 2019; 364:science.aat2349. [DOI: 10.1126/science.aat2349] [Citation(s) in RCA: 16] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/06/2018] [Accepted: 03/12/2019] [Indexed: 11/02/2022]
Abstract
Saturn’s main ring system is associated with a set of small moons that either are embedded within it or interact with the rings to alter their shape and composition. Five close flybys of the moons Pan, Daphnis, Atlas, Pandora, and Epimetheus were performed between December 2016 and April 2017 during the ring-grazing orbits of the Cassini mission. Data on the moons’ morphology, structure, particle environment, and composition were returned, along with images in the ultraviolet and thermal infrared. We find that the optical properties of the moons’ surfaces are determined by two competing processes: contamination by a red material formed in Saturn’s main ring system and accretion of bright icy particles or water vapor from volcanic plumes originating on the moon Enceladus.
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Affiliation(s)
- B. J. Buratti
- Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
| | - P. C. Thomas
- Cornell Center for Astrophysics and Planetary Science, Cornell University, Ithaca, NY 14853, USA
| | - E. Roussos
- Max Planck Institute for Solar System Research, 37077 Göttingen, Germany
| | - C. Howett
- Southwest Research Institute, Boulder, CO 80302, USA
| | - M. Seiß
- Department of Physics and Astronomy, University of Potsdam, 14476 Potsdam, Germany
| | | | - P. Helfenstein
- Cornell Center for Astrophysics and Planetary Science, Cornell University, Ithaca, NY 14853, USA
| | - R. H. Brown
- Lunar and Planetary Lab, University of Arizona, Tucson, AZ 85721, USA
| | - R. N. Clark
- Planetary Sciences Institute, Tucson, AZ 85719, USA
| | - T. Denk
- Institute of Geological Sciences, Freie Universität Berlin, 12249 Berlin, Germany
| | | | - H. Hoffmann
- Department of Physics and Astronomy, University of Potsdam, 14476 Potsdam, Germany
| | | | - N. Khawaja
- Institute of Geological Sciences, Freie Universität Berlin, 12249 Berlin, Germany
- Institute of Earth Sciences, University of Heidelberg, 69120 Heidelberg, Germany
| | - P. Kollmann
- Institute of Earth Sciences, University of Heidelberg, 69120 Heidelberg, Germany
| | - N. Krupp
- Max Planck Institute for Solar System Research, 37077 Göttingen, Germany
| | - J. Lunine
- Cornell Center for Astrophysics and Planetary Science, Cornell University, Ithaca, NY 14853, USA
| | - T. W. Momary
- Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
| | - C. Paranicas
- Applied Physics Laboratory, Johns Hopkins University, Laurel, MD 20723, USA
| | - F. Postberg
- Institute of Geological Sciences, Freie Universität Berlin, 12249 Berlin, Germany
- Institute of Earth Sciences, University of Heidelberg, 69120 Heidelberg, Germany
| | - M. Sachse
- Department of Physics and Astronomy, University of Potsdam, 14476 Potsdam, Germany
| | - F. Spahn
- Department of Physics and Astronomy, University of Potsdam, 14476 Potsdam, Germany
| | - J. Spencer
- Southwest Research Institute, Boulder, CO 80302, USA
| | - R. Srama
- University of Stuttgart, 70569 Stuttgart, Germany
| | - T. Albin
- University of Stuttgart, 70569 Stuttgart, Germany
| | - K. H. Baines
- Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
| | | | - T. Economou
- Enrico Fermi Institute, University of Chicago, Chicago, IL 60637, USA
| | - H.-W. Hsu
- Physics Department, University of Colorado, Boulder, CO 80303, USA
| | - S. Kempf
- Physics Department, University of Colorado, Boulder, CO 80303, USA
| | - S. M. Krimigis
- Applied Physics Laboratory, Johns Hopkins University, Laurel, MD 20723, USA
| | - D. Mitchell
- Applied Physics Laboratory, Johns Hopkins University, Laurel, MD 20723, USA
| | | | - P. D. Nicholson
- Cornell Center for Astrophysics and Planetary Science, Cornell University, Ithaca, NY 14853, USA
| | - C. C. Porco
- Space Sciences Institute, Boulder, CO 80301, USA, and Department of Astronomy, University of California, Berkeley, CA 94720, USA
| | - H. Rosenberg
- Institute of Geological Sciences, Freie Universität Berlin, 12249 Berlin, Germany
| | - J. Simolka
- University of Stuttgart, 70569 Stuttgart, Germany
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Woodfield EE, Horne RB, Glauert SA, Menietti JD, Shprits YY, Kurth WS. Formation of electron radiation belts at Saturn by Z-mode wave acceleration. Nat Commun 2018; 9:5062. [PMID: 30498204 PMCID: PMC6265320 DOI: 10.1038/s41467-018-07549-4] [Citation(s) in RCA: 21] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/20/2017] [Accepted: 11/07/2018] [Indexed: 11/16/2022] Open
Abstract
At Saturn electrons are trapped in the planet's magnetic field and accelerated to relativistic energies to form the radiation belts, but how this dramatic increase in electron energy occurs is still unknown. Until now the mechanism of radial diffusion has been assumed but we show here that in-situ acceleration through wave particle interactions, which initial studies dismissed as ineffectual at Saturn, is in fact a vital part of the energetic particle dynamics there. We present evidence from numerical simulations based on Cassini spacecraft data that a particular plasma wave, known as Z-mode, accelerates electrons to MeV energies inside 4 RS (1 RS = 60,330 km) through a Doppler shifted cyclotron resonant interaction. Our results show that the Z-mode waves observed are not oblique as previously assumed and are much better accelerators than O-mode waves, resulting in an electron energy spectrum that closely approaches observed values without any transport effects included.
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Affiliation(s)
- E E Woodfield
- British Antarctic Survey, High Cross, Madingley Road, Cambridge, CB3 0ET, UK.
| | - R B Horne
- British Antarctic Survey, High Cross, Madingley Road, Cambridge, CB3 0ET, UK
| | - S A Glauert
- British Antarctic Survey, High Cross, Madingley Road, Cambridge, CB3 0ET, UK
| | - J D Menietti
- Department of Physics and Astronomy, University of Iowa, Iowa City, IA, 52242, USA
| | - Y Y Shprits
- Helmholtz Centre Potsdam, GFZ German Research Centre for Geosciences, Potsdam, 14473, Germany
- Institute for Physics and Astronomy, Universität Potsdam, 14469, Potsdam, Germany
- Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, CA, 90095, USA
| | - W S Kurth
- Department of Physics and Astronomy, University of Iowa, Iowa City, IA, 52242, USA
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5
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Kollmann P, Roussos E, Paranicas C, Woodfield EE, Mauk BH, Clark G, Smith DC, Vandegriff J. Electron Acceleration to MeV Energies at Jupiter and Saturn. JOURNAL OF GEOPHYSICAL RESEARCH. SPACE PHYSICS 2018; 123:9110-9129. [PMID: 30775196 PMCID: PMC6360449 DOI: 10.1029/2018ja025665] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 05/09/2018] [Revised: 08/16/2018] [Accepted: 09/24/2018] [Indexed: 06/09/2023]
Abstract
The radiation belts and magnetospheres of Jupiter and Saturn show significant intensities of relativistic electrons with energies up to tens of megaelectronvolts (MeV). To date, the question on how the electrons reach such high energies is not fully answered. This is largely due to the lack of high-quality electron spectra in the MeV energy range that models could be fit to. We reprocess data throughout the Galileo orbiter mission in order to derive Jupiter's electron spectra up to tens of MeV. In the case of Saturn, the spectra from the Cassini orbiter are readily available and we provide a systematic analysis aiming to study their acceleration mechanisms. Our analysis focuses on the magnetospheres of these planets, at distances of L > 20 and L > 4 for Jupiter and Saturn, respectively, where electron intensities are not yet at radiation belt levels. We find no support that MeV electrons are dominantly accelerated by wave-particle interactions in the magnetospheres of both planets at these distances. Instead, electron acceleration is consistent with adiabatic transport. While this is a common assumption, confirmation of this fact is important since many studies on sources, losses, and transport of energetic particles rely on it. Adiabatic heating can be driven through various radial transport mechanisms, for example, injections driven by the interchange instability or radial diffusion. We cannot distinguish these processes at Saturn with our technique. For Jupiter, we suggest that the dominating acceleration process is radial diffusion because injections are never observed at MeV energies.
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Affiliation(s)
- P. Kollmann
- The Johns Hopkins University, Applied Physics LaboratoryLaurelMDUSA
| | - E. Roussos
- Max Planck Institute for Solar System ResearchGóttingenGermany
| | - C. Paranicas
- The Johns Hopkins University, Applied Physics LaboratoryLaurelMDUSA
| | | | - B. H. Mauk
- The Johns Hopkins University, Applied Physics LaboratoryLaurelMDUSA
| | - G. Clark
- The Johns Hopkins University, Applied Physics LaboratoryLaurelMDUSA
| | - D. C. Smith
- The Johns Hopkins University, Applied Physics LaboratoryLaurelMDUSA
| | - J. Vandegriff
- The Johns Hopkins University, Applied Physics LaboratoryLaurelMDUSA
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6
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Roussos E, Kollmann P, Krupp N, Kotova A, Regoli L, Paranicas C, Mitchell DG, Krimigis SM, Hamilton D, Brandt P, Carbary J, Christon S, Dialynas K, Dandouras I, Hill ME, Ip WH, Jones GH, Livi S, Mauk BH, Palmaerts B, Roelof EC, Rymer A, Sergis N, Smith HT. A radiation belt of energetic protons located between Saturn and its rings. Science 2018; 362:362/6410/eaat1962. [DOI: 10.1126/science.aat1962] [Citation(s) in RCA: 22] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/04/2018] [Accepted: 09/05/2018] [Indexed: 11/03/2022]
Abstract
Saturn has a sufficiently strong dipole magnetic field to trap high-energy charged particles and form radiation belts, which have been observed outside its rings. Whether stable radiation belts exist near the planet and inward of the rings was previously unknown. The Cassini spacecraft’s Magnetosphere Imaging Instrument obtained measurements of a radiation belt that lies just above Saturn’s dense atmosphere and is decoupled from the rest of the magnetosphere by the planet’s A- to C-rings. The belt extends across the D-ring and comprises protons produced through cosmic ray albedo neutron decay and multiple charge-exchange reactions. These protons are lost to atmospheric neutrals and D-ring dust. Strong proton depletions that map onto features on the D-ring indicate a highly structured and diverse dust environment near Saturn.
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