O. Mousis (UTINAM, Université de Franche-Comté, France) L. N. Fletcher (Oxford U., UK), N. André (IRAP, France), M. Blanc (IRAP, France), A. Coustenis (LESIA, France), D. Gautier (LESIA, France), W. D. Geppert (Stockhholm U., Sweden), T. Guillot (OCA, France), P. Irwin (Oxford U., UK), J.-P. Lebreton (LPC2E, LESIA, France), B. Marty (CRPG, France), A. Morse (OU, UK), A. Sanchez-Lavega (U. Pais Vasco, Spain), F.-X. Schmider (OCA, France), J. H. Waite (SWRI, USA), P. Wurz (Bern U., Switzerland) Concept of a Saturn probe mission for the future L2/L3 ESA call 1

Motivation and background  Giant planets must have played a significant role in shaping the architecture of our planetary system and the evolution of the smaller, inner worlds.  Remote-sensing observations have always been the favored approach of astronomers for studying the giants of our Solar System. However, the efficiency of this technique has some limitations when used to study the bulk atmospheric composition.  A remarkable example of these restrictions is illustrated by the exploration of Jupiter, where key measurements such as the determination of the noble gases and helium abundances have only been made in situ by the Galileo probe.  The Galileo probe provided a giant step forward our understanding of Jupiter, but one can wonder if these measurements are really representative or not of the whole set of giant planets of the solar system. Here we focus on the description of a Saturn probe scenario because such a mission will appear the next natural step beyond Galileo’s in situ exploration of Jupiter, and the Cassini spacecraft’s orbital reconnaissance of Saturn. 2

Theme A: formation history of our solar system investigated by measuring bulk elemental enrichments and isotopic ratios; Theme B: atmospheric processes (dynamics, waves, circulation, chemistry and clouds) from the upper atmosphere to below the cloud tops. 3 Primary science themes

Theme A: planet formation and the origin of the solar system 4

In situ measurements by the Galileo probe of the volatile enrichments in Jupiter Owen et al. (1999)

6 Noble gases in the outer solar system Titan is impoverished in primordial noble gases. No heavy noble gases other than argon were detected by the GCMS aboard the Huygens probe during its descent to Titan’s surface in 2005 January (Niemann et al. 2005, 2010 ) Despite many attempts, no firm detection of noble gases in comets (Rosetta will make soon in situ measurements) The noble gas abundances in Saturn, Uranus and Neptune are unknown (require in situ measurements)

Formation of ices in the solar nebula CO:CO2:CH3OH:CH4 = 70:10:2:1 and N 2 :NH 3 = 1:1 in the gas phase disk grains planetesimals condensation /trapping condensation /trapping accretion Formation of clathrates and pure condensates in the outer solar nebula Ices are stable in domains located below their stability curves

Composition of planetesimals in the solar nebula as a function of the oxygen abundance in the disk and their formation temperature C/O = 0.5 (solar case) C/O = 1 (carbon-rich case)

The heavy element content of gas giants Three steps in the formation of gas giants: 1.Core accretion 2.Gas accretion until M gas ~ M core 3.Runaway accretion of gas + gas-coupled solids => M gas >> M core See Pollack et al. (1996). Stage 1 Stage 2 Stage 3

Matching the volatile enrichments in Jupiter from different disk’s gas phase compositions Red bars correspond to observations. Green and blue bars correspond to calculations based on the assumption that C/O = 0.5 (solar case) and C/O = 1 (C-rich case) in the disk. Conclusion: Jupiter could be carbon rich!!!! Testing this hypothesis requires the measurement of O in Jupiter by Juno Mousis et al. (2012)

Theme A: planet formation and the origin of the solar system Measurement requirements: The atmospheric fraction of He/H 2 with a 2% precision on the measurement; The elemental enrichments in cosmogenically abundant species C, H, O, N and S C/H, N/H, S/H and O/H should be sampled with a precision better than +/- 10%. The isotopic ratios in hydrogen (D/H), oxygen ( 18 O, 17 O and 16 O), carbon ( 13 C/ 12 C) and nitrogen ( 15 N/ 14 N), to determine the key reservoirs for these species (e.g., delivery as N 2 or NH 3 vastly alters the 15 N/ 14 N ratio). 13 C/ 12 C, 18 O/ 16 O and 17 O/ 16 O should be sampled with a precision better than +/- 1%. D/H, 15 N/ 14 N should be analyzed in the main host molecules with a precision order of +/- 5%. The abundances and isotopic ratios for the chemically inert noble gases He, Ne, Xe, Kr and Ar, provide excellent tracers for the materials in the sub-reservoirs existing in the proto-solar nebula. The isotopic ratios for He, Ne, Xe, Kr and Ar should be measured with a precision better than +/- 1%. The elemental enrichments in minor species delivered by vertical mixing (e.g., P, As, Ge) from the deeper troposphere. P/H, As/H and Ge/H should be sampled with a precision better than +/- 10%. 11

Theme B: planetary atmospheric processes In situ studies provide access to atmospheric regions that are beyond the reach of remote sensing, enabling us to study the dynamical, chemical and aerosol-forming processes at work from the thermosphere to the troposphere below the cloud decks. 12

Theme B: planetary atmospheric processes Two crucial questions have to be addressed: What processes are at work in planetary atmospheres, shaping the dynamics and circulation from the thermosphere to the deep troposphere? What are the properties and conditions for cloud formation as a function of depth and temperature in planetary atmospheres? Measurement requirements: Determine the thermal and density profile from thermosphere to troposphere, and the balance between different energy sources controlling atmospheric dynamics and structure; Measure the strength of the winds as a function of altitude and the importance of wave perturbations on atmospheric structure; Sample and determine the properties of cloud and haze layers as a function of depth (e.g., methane and hazes on ice giants; NH 3, NH 4 SH and N- and P-bearing hazes on Saturn); Measure the vertical profiles of chemical products, disequilibrium species and ions to understand vertical mixing and atmospheric chemistry. 13

Ancillary science themes A Saturn probe mission presents a substantial opportunity for “secondary” science:  Doppler seismology to probe the existence of a planetary core. Seismology requires continuous observations of spatially resolved global modes for period of weeks, or months. A spacecraft approaching a planet would provide a long enough observing period, with resolution sufficient for sounding precisely the central part of the planet, particularly the core.  In situ sampling of Saturn’s lightning radio emissions and Schumann resonance. The in situ exploration of Saturn’s weather layer will provide new insights into the cloud-forming processes below the levels normally visible to remote sensing. The measurement of the Schumann resonance could constrain the water abundance in Saturn (Simões et al. 2012).  Ring science. Voyager images and recent Keck observations reveal that Saturn has a major interaction between its atmosphere and ring-dominated inner magnetosphere. This “ring- rain” (O’Donoghue et al. 2013) must act to alter the chemistry and temperature of Saturn's atmosphere. In situ measurements of this material flux are absolutely essential in order to understand how the atmosphere responds to its planetary ring system raining into it. 14

Ancillary science themes  Magnetic dynamo, magnetosphere and radiation environment 15 Accurate measurements of the magnetic field very close to Saturn's atmosphere and thus unfiltered by the ring is a key to both the Composition and conductivity of Saturn's interior. Taking advantage of an instrumented carrier for a Saturn probe would enable us to obtain in situ magnetospheric observations in the innermost unexplored regions of Saturn's magnetosphere. These unique observations would complement the ones obtained by Cassini at the end of its mission.

Mission architectures The primary science objectives can be addressed by an atmospheric entry probe that would descend under parachute, and start to perform in situ measurements in the stratosphere after parachute deployment and down to a minimum of 10 bars. Concept would derive from the KRONOS mission proposal (Marty et al. 2009), in which the probe had a mass of 340kg. A spacecraft carrier would be required in order to bring the probe to its target planet. The range of possible carriers include: - Option 1: the carrier would detach prior to probe entry, follow the probe path and destroy itself when entering the atmosphere. if the carrier is instrumented properly, would allow performing approach science and in situ pre-entry science. - Option 2: a fly-by carrier would release the probe several months prior to probe entry and deflect its trajectory to be used for both probe data relay and for performing flyby science. This Architecture would provide the capability to perform approach science (for months) and flyby science (for a few days). This option would also reduce risk for data retransmission. - Option 3: an orbiter, which would provide a similar configuration to that of the Galileo orbiter/probe mission. 16

Mission architectures  Power generation: all mission architectures could be designed on batteries and solar power, pending LILT qualification extension to 9 AU conditions. The probe will be powered with primary batteries as were the Galileo and Huygens probes. The carrier could be equipped with a combination of solar panels, rechargeable batteries (option 1) and possibly a set of primary batteries for the phase that will require a high power demand. Nuclear power would be considered for the carrier only if available solar power technology would not work.  International collaborations: One of the key probe technologies is the heat shield material for the entry probe. Two possibilities: developing this new technology within Europe or establishing collaboration with an international partner that already has this technology available. In particular, recent NASA studies have been made concerning the thermal protection requirements for a Saturn entry probe. International collaboration may also be looked at for other mission elements. 17

Measurement requirements 18 Probe science In situ pre entry science (option 1) Carrier/flyby science (option 2)

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