Exp Astron (2009) 23:947–976 DOI 10.1007/s10686-008-9094-9 ORIGINAL ARTICLE Kronos: exploring the depths of Saturn with probes and remote sensing through an international mission B. Marty & T. Guillot & A. Coustenis & the Kronos consortium & N. Achilleos & Y. Alibert & S. Asmar & D. Atkinson & S. Atreya & G. Babasides & K. Baines & T. Balint & D. Banfield & S. Barber & B. Bézard & G. L. Bjoraker & M. Blanc & S. Bolton & N. Chanover & S. Charnoz & E. Chassefière & J. E. Colwell & E. Deangelis & M. Dougherty & P. Drossart & F. M. Flasar & T. Fouchet & R. Frampton & I. Franchi & D. Gautier & L. Gurvits & R. Hueso & B. Kazeminejad & T. Krimigis & A. Jambon & G. Jones & Y. Langevin & M. Leese & E. Lellouch & J. Lunine & A. Milillo & P. Mahaffy & B. Mauk & A. Morse & M. Moreira & X. Moussas & C. Murray & I. Mueller-Wodarg & T. C. Owen & S. Pogrebenko & R. Prangé & P. Read & A. Sanchez-Lavega & P. Sarda & D. Stam & G. Tinetti & P. Zarka & J. Zarnecki Received: 7 December 2007 /Accepted: 2 April 2008 /Published online: 13 May 2008 # Springer Science + Business Media B.V. 2008 Abstract Kronos is a mission aimed to measure in situ the chemical and isotopic compositions of the Saturnian atmosphere with two probes and also by remote sensing, in order to understand the origin, formation, and evolution of giant planets in general, including extrasolar planets. The abundances of noble gases, hydrogen, B. Marty (*) CRPG, Nancy-Université, CNRS, BP 20, 54501 Vandoeuvre, Cedex, France e-mail: [email protected] T. Guillot Observatoire de la Côte d’Azur, BP 4229, 06304 Nice Cedex 04, France e-mail: [email protected] A. Coustenis : B. Bézard : P. Drossart : T. Fouchet : D. Gautier : E. Lellouch : R. Prangé : P. Zarka Laboratoire d’Etudes Spatiales et d’Instrumentation en Astrophysique (LESIA), Observatoire de Paris-Meudon, 5, place Jules Janssen, 92195 Meudon Cedex, France A. Coustenis e-mail: [email protected] 948 Exp Astron (2009) 23:947–976 carbon, nitrogen, oxygen, sulfur and their compounds, as well as of the D/H, 4He/3He, 22Ne/21Ne/20Ne, 36Ar/38Ar, 13C/12C, 15N/14N, 18O/(17O)/16O, 136Xe/134Xe/132Xe/130Xe/129Xe isotopic ratios will be measured by mass spectrometry on two probes entering the atmosphere of Saturn at two different locations near mid-latitudes, down to a pressure of 10 Bar. The global composition of Saturn will be investigated through these measurements, together with microwave radiometry determination of H2O and NH3 and their 3D variations. The dynamics of Saturn’s atmosphere will be investigated from: (1) measurements of pressure, temperature, vertical distribution of clouds and wind speed along the probes’ descent trajectories, and (2) determination of deep winds, differential rotation and convection with combined probe, gravity and radiometric measurements. Besides these primary goals, Kronos will also measure the intensities and characteristics of Saturn’s N. Achilleos Atmospheric Physics Laboratory, Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK Y. Alibert Inst Phys, University of Bern, CH-3012 Bern, Switzerland S. Asmar Jet Propulsion Laboratory, Pasadena, CA 91109, USA D. Atkinson Department of Electrical and Computer Engineering, University of Idaho, Moscow, ID 83844-1023, USA S. Atreya Department of Atmosphere Ocean and Space Science, University of Michigan, Ann Arbor, MI 48109, USA G. Babasides : X. Moussas Space Group, Laboratory of Astrophysics, Faculty of Physics, National and Kapodistrian University of Athens, Panepistimiopolis, 15783 Zographos, Athens, Greece K. Baines : T. Balint Jet Propulsion Laboratory, 4800 Oak Grove Blvd, Paadena, CA 91109-8099, USA D. Banfield Department of Astronomy, Cornell University, Ithaca, NY 14853, USA S. Barber : I. Franchi : M. Leese : A. Morse : J. Zarnecki Open University, Walton Hall, Milton Keynes MK7 6AA, UK G. L. Bjoraker : F. M. Flasar : P. Mahaffy NASA, Goddard Space Flight Ctr Code 693, Greenbelt, MD 20771, USA M. Blanc Centre d’Etudes Spatiales des Rayonnements (CESR), Toulouse, France S. Bolton Southwest Research Institute, San Antonio, TX, USA N. Chanover New Mexico State University, Las Cruces, NM 88003, USA Exp Astron (2009) 23:947–976 949 magnetic field inside the D ring as well as Saturn’s gravitational field, in order to constrain the abundance of heavy elements in Saturn’s interior and in its central core. Depending on the preferred architecture (flyby versus orbiter), Kronos will be in a position to measure the properties of Saturn’s innermost magnetosphere and to investigate the ring structure in order to understand how these tiny structures could have formed and survived up to the present times. Keywords Saturn . Atmosphere . Probes . Cosmic vision S. Charnoz AIM, Université Paris 7/CEA/CNRS, 91191 Gif sur Yvette, France E. Chassefière Service d’Aéronomie du CNRS/IPSL, 91371 Verrières-le-Buisson, France J. E. Colwell Department of Physics, University Cent Florida, Orlando, FL 32816, USA E. Deangelis : A. Milillo NAF/Instituto di Fisica dello Spazio Interplanetario, via del Fosso del Cavaliere 100, 00133, Rome, Italy M. Dougherty Imperial College London, South Kensington Campus, London SW7 2AZ, UK I. Mueller-Wodarg Imperial College Sci Technol and Med, Space and Atmosphere Phs grp, University of London, London SW7 2BW, UK R. Frampton Boeing NASA Systems, MC H012-C349, 5301 Bolsa Ave, Huntington Beach, CA 92647-2099, USA L. Gurvits : S. Pogrebenko Joint Institute for VLBI in Europe, P.O. Box 2, 7990 AA Dwingeloo, The Netherlands R. Hueso : A. Sanchez-Lavega Departamento de Fisica Aplicada I, E.T.S. Ingenieros, Universidad del Pais Vasco, Alameda Urquijo s/n, 48013 Bilbao, Spain B. Kazeminejad Deutsches Zentrum für Luft-und Raumfahrt (DLR), German Space Operations Center (GSOC), 82234 Wessling, Germany T. Krimigis : B. Mauk Appl Phys Lab, Johns Hopkins University, Laurel, MD 20723, USA A. Jambon MAGIE UMR 7047, Université Pierre et Marie Curie, 4 place Jussieu, 75252 Paris Cedex 05, France G. Jones Max Plank Inst. Gravitat Phys, Albert Einstein Inst, Katlenburg-Lindau, Germany Y. Langevin Institut d’Astrophysique Spatiale Bat. 121, 91405 Orsay Campus, France J. Lunine Department of Planetary Science, University of Arizona, Tucson, AZ 85721, USA 950 Exp Astron (2009) 23:947–976 1 Introduction Giant planets are mostly made of the gas that was present in the protosolar disk before the terrestrial planets accreted (Fig. 1). Their comparative study is thus essential to understand planet formation in general and the origin of the Solar System. Saturn in particular appears to be a natural target for near-future exploration, after the fine characterization of Jupiter by Galileo and Juno and before future ambitious missions to Uranus and Neptune. Saturn, the ring planet, is mysterious in many aspects, and plays a key role to understanding planet formation, the evolution of solar and extrasolar giant planets, planetary meteorology, magnetospheric interactions, dynamo generation and the physics of planetary rings. Saturn, like Jupiter, has an atmosphere that appears to be enriched in elements other than hydrogen and helium with respect to the solar composition. This enrichment may be the result of planetary precursors formed at low temperatures, or of a progressive enrichment of the protosolar disk, with profound consequences for understanding the formation of the Solar System. The different formation scenarios that result can be disentangled by a study of the atmospheric composition of Saturn in noble gases and major volatile (H, C, N, S, O) elemental and isotopic compositions, which requires in situ measurements Table 1. Noble gases are particularly relevant in this study, because: (1) they are chemically inert and their abundances are determined by physical processes such as phase partitioning, and (2) their isotopic compositions present large-scale inhomogeneities between the original protosolar nebula composition and sub- reservoirs of the solar system that make them isotopic tracers as well. Indeed, analyses of meteorites point to the existence of a primordial reservoir of volatiles M. Moreira Laboratoire de Géochimie et Cosmochimie (UMR 7579 CNRS), Institut de Physique du Globe de Paris, Université Paris, 7, 4 place Jussieu, 75252 Paris cedex 05, France C. Murray Queen Mary & Westfield College, University of London, London E1 4NS( UK T. C. Owen Institute of Astronomy, University of Hawaii, Honolulu, HI 96822, USA P. Read Clarendon Laboratory, University of Oxford, Oxford OX1 3PU( UK P. Sarda Groupe géochimie des Gaz Rares, Département des Sciences de la Terre, Université Paris Sud, UMR CNRS 8148 (IDES), 81405 Orsay Cedex, France D. Stam Astronomical Institute “Anton Pannekoek” Kruislaan 403, 1098 SJ Amsterdam, The Netherlands G. Tinetti Institut d’Astrophysique de Paris, CNRS, Université Pierre et Marie Curie, 75014 Paris, France Present address: P. Sarda Laboratoire de Sciences de la Terre, Ecole Normale Supérieure de Lyon cedex 07, France Exp Astron (2009) 23:947–976 951 Fig. 1 Interiors of Jupiter and Saturn highlighting the impor- tance of hydrogen and helium for the structure of these two planets (yellow indicates that hydrogen is in molecular form, red that it is metallic, and the central dense core is shown in blue) with an isotopic composition different from that of the Sun. The discovery of this reservoir through its isotopic signature in Saturn’s atmosphere would have a direct impact on the primordial history of the Solar System and on the study of meteorites and comets. Generally, the elemental and isotopic determinations of Saturn’s atmospheric composition would permit to explore sources of matter and processes of formation for the giant planets. Table 1 Composition measurements in Saturn’s deep atmosphere and their consequences Species Consequence He Determine extent of helium sedimentation in Saturn’s interior. Crucial for accurate understanding of the thermal evolutions of Saturn and Jupiter Ne Test prediction of Ne capture in He droplets. Refine H–He phase separation diagram CH4 Fine determination crucial to understand the formation of the planet NH3 NH4SH Key to decide between models of planetesimal delivery and planet formation.