We describe the main scientific goals to be addressed by future in situ exploration of Saturn

Atkinson, D. H.

Atreya, S.

Cavalié, T.

Coustenis, A.

Fletcher, L. N.

Gautier, D.

Guillot, T.

Lebreton, J.-P.

Lunine, J. I.

Marty, B.

Morse, A. D.

Mousis, O.

Reh, K. R.

Simon-Miller, A.

Spilker, T.

Waite, J. H.

Wurz, P.

English

Open Research Online (OU)

Open Research OnlineThe Open University’s repository of research publicationsand other research outputsScientific rationale of a Saturn probe missionConference or Workshop ItemHow to cite:Mousis, O.; Fletcher, L. N.; Lebreton, J.-P.; Wurz, P.; Cavalie´, T.; Coustenis, A.; Atkinson, D. H.; Atreya, S.;Gautier, D.; Guillot, T.; Lunine, J. I.; Marty, B.; Morse, A. D.; Reh, K. R.; Simon-Miller, A.; Spilker, T. and Waite,J. H. (2014). Scientific rationale of a Saturn probe mission. In: 45th Lunar and Planetary Science Conference, 17-21Mar 2014, The Woodlands, TX, USA.For guidance on citations see FAQs.c© [not recorded]Version: Accepted ManuscriptLink(s) to article on publisher’s website:http://www.hou.usra.edu//meetings/lpsc2014/Copyright and Moral Rights for the articles on this site are retained by the individual authors and/or other copyrightowners. For more information on Open Research Online’s data policy on reuse of materials please consult the policiespage.oro.open.ac.ukSCIENTIFIC RATIONALE OF A SATURN PROBE MISSION.  O. Mousis1, L. N. Fletcher2, J.-P. Lebreton3,4, P. Wurz5, T. Cavalié6, A. Coustenis4, D. H. Atkinson7,8, S. Atreya9, D. Gautier4, T. Guillot10, J. I. Lunine11, B. Marty12, A. D. Morse13, K. R. Reh8, A. Simon-Miller14, T. Spilker15, and J. H. Waite16, 1Université de Franche-Comté, Institut UTINAM, CNRS/INSU, UMR 6213, OSU THETA, France (olivier.mousis@obs-besancon.fr), 2Atmospheric, Oceanic & Planetary Physics, Department of Physics, University of Oxford, Clarendon Laboratory, UK, 3LPC2E, CNRS-Université d'Orléans, France, 4LESIA, Observatoire de Paris, CNRS, UPMC, Univ. Paris-Diderot, France, 5Physics Institute, University of Bern, Switzerland, 6Max-Planck-Institut fur Sonnensystem-forschung, Germany, 7Department of Electrical and Computer Engineering, University of Idaho, USA, 8Jet Propul-sion Laboratory, California Institute of Technology, USA, 9Department of Atmospheric, Oceanic, and Space Sci-ences, University of Michigan, USA, 10Observatoire de la Côte d'Azur, Laboratoire Lagrange, France, 11Center for Radiophysics and Space Research, Cornell University, USA, 12CRPG-CNRS, Nancy-Université, France, 13Planetary and Space Sciences, Department of Physics, The Open University, UK, 14NASA Goddard Space Flight Center, USA, 15Solar System Science & Exploration, USA, 16Southwest Research Institute (SwRI), USA.   Introduction: Remote sensing observations meet some limitations when used to study the bulk atmos-pheric composition of the giant planets of our solar system. A remarkable example of the unicity of in situ probe measurements is illustrated by the exploration of Jupiter, where key measurements such as noble gases abundances and the precise measurement of the helium mixing ratio have only been made available through in situ measurements by the Galileo probe. Here we de-scribe the main scientific goals to be addressed by fu-ture in situ exploration of Saturn.  Planet formation: To understand the formation of giant planets and the origin of our Solar System, statis-tical data obtained from the observation of exoplane-tary systems must be supplemented by direct meas-urements of the composition of the planets in our solar system. A giant planet’s bulk composition depends on the timing and location of planet formation, subsequent migration and the delivery mechanisms for the heavier elements.  By measuring a giant planet’s chemical in-ventory, and contrasting these with measurements of (i) other giant planets, (ii) primitive materials found in small bodies, and (iii) the composition of our parent star and the local interstellar medium, much can be revealed about the conditions at work during the for-mation of our planetary system [1].  To date, the Galileo probe at Jupiter (1995) remains our only data point for interpreting the bulk composi-tion of the giant planets. Galileo found that Jupiter exhibited an enrichment in C, N, S, Ar, Kr and Xe compared to the solar photospheric abundances, with some notable exceptions - water was found depleted, possibly due to meteorological processes at the probe entry site; and neon was also found depleted, possibly due to rain-out to deeper levels [2]. Explaining the high abundance of noble gases requires either condens-ing these elements directly at low-temperature in the form of amorphous ices [3], trapping them as clathrates [4-7] or photoevaporating the hydrogen and helium in the protoplanetary disk during the planet's formation [8]. The in situ Galileo measurements at Jupiter also include a highly precise determination of the planet's helium abundance, crucial for studies of the structure and evolution of the planet. Because of the lack of in situ measurements, Saturn noble gas abundances are unknown and their determi-nation is missing to properly understand its formation conditions. There is however some indication for a non-uniform enrichment in C, N and S. [5] suggests that observations are well fitted if the atmospheric C and N of the planet were initially mainly in reduced forms at 10 AU in the protosolar nebula. Alternatively, [6] finds that it is possible to account for these enrich-ments in a way consistent with those measured at Jupi-ter if the building blocks of the two planets shared a common origin. As in Jupiter, the missing piece of the puzzle remains the measurement of the oxygen abun-dance. Precisely measuring in situ the He/H2 ratio in Saturn is also needed for properly modeling its interior and thermal evolution. To address the origin of Saturn, the key measurements that should be targetted from an in situ  probe are the following:  • The atmospheric fraction of He/H2 with a 2% ac-curacy on the measurement; • The elemental enrichments in cosmogenically abundant species C, N, S and O. C/H, N/H, S/H, and O/H should be sampled with an accuracy bet-ter than +/– 10%. • The elemental enrichments in minor species deliv-ered by vertical mixing (e.g., P, As, Ge) from the deeper troposphere. P/H, As/H and Ge/H should be sampled with an accuracy better than +/– 10%. • The isotopic ratios in hydrogen (D/H), oxygen (18O/16O, 17O/16O), carbon (13C/12C) and nitrogen (15N/14N), to determine the key reservoirs for these species (e.g., delivery as N2 or NH3 vastly alters 1261.pdf45th Lunar and Planetary Science Conference (2014)the 15N/14N ratio in the giant planet's envelope). The isotope ratios 13C/12C, 18O/16O and 17O/16O should be sampled with an accuracy better than +/– 1%. D/H, 15N/14N should be analyzed in the main host molecules with an accuracy of the order of  +/– 5%. • The abundances and isotopic ratios for the chemi-cally inert noble gases He, Ne, Xe, Kr and Ar, provide excellent tracers for the materials in the subreservoirs existing in the protosolar nebula. The isotopic ratios for He, Ne, Xe, Kr and Ar should be measured with an accuracy better than +/– 1%.  Planetary Atmospheric Processes: Saturn’s complex and cloud-dominated weather-layer is our principle gateway to the processes at work within the deep inte-rior of this giant planet. We must extrapolate from this thin, dynamic region over many orders of magnitude in pressure, temperature and density to infer the planetary properties deep below the clouds [1]. Remote sensing provides insights into the complexity of the transitional zone between the external environment and the fluid interior, but there is much that we still do not under-stand. In situ measurements are the only method providing ground-truth to connect the remote sensing inferences with physical reality, and yet this has only been achieved twice in the history of outer solar sys-tem exploration, via the Galileo probe for Jupiter and the Huygens probe for Titan. In situ studies provide access to atmospheric re-gions that are beyond the reach of remote sensing, en-abling us to study the dynamical, chemical and aero-sol-forming processes at work from the thermosphere to the troposphere below the cloud decks. Two crucial questions in this theme remain i) the nature of the pro-cesses at work in planetary atmospheres, shaping the dynamics and circulation from the thermosphere to the deep troposphere (e.g., [9]) , and ii) the chemical prop-erties and conditions for cloud formation as a function of depth and temperature in planetary atmospheres (e.g., [10]). To address these important points, key measurements include:  • Atmospheric temperature and pressure throughout the descent to study (i) stability regimes as a func-tion of depth through transition zones (e.g., the adiative-convective boundary); (ii) atmospheric drag and accelerations; and (iii) the influence of wave perturbations and cloud formation on the vertical temperature profile (e.g., [11]); • Determination of the vertical variation of horizon-tal winds using Doppler measurements of the probe's carrier frequency (driven by an ultra-stable oscillator) during the descent [12].  This includes a study of the depth of the zonal wind fields, as well as the first measurements of middle atmospheric winds; • Vertical profiling of a host of atmospheric species via mass spectrometry [13], including atmospheric volatiles (H2O, H2S and NH3 in their saturated and sub-cloud regions); disequilibrium species (e.g., PH3, AsH3, GeH4, CO) convected upwards from the deeper atmosphere; photochemical species (e.g., hydrocarbons and HCN in the troposphere and stratosphere; hydrazine and diphosphine in the upper troposphere) and exogenic inputs (e.g., oxy-genated species in the upper atmosphere) (e.g., [14]); • Measurements of the vertical structure and proper-ties of Saturn's cloud and haze layers [10]; includ-ing determinations of the particle optical proper-ties, size distributions, number and mass densities, opacity, shapes and, potentially, their composition.  References: [1] Mousis O. et al. (2014) PSS, submit-ted. [2] Niemann H. B. et al. (1998) JGR, 103, 22831-22846. [3] Owen T. et al. (1999) Nature, 402, 269-270. [4] Gautier D. et al. (2001) ApJ, 550, L227-L230. [5] Hersant F. et al. (2008) PSS, 56, 1103-1111. [6] Mousis O. et al. (2009) ApJ, 696, 1348-1354. [7] Mousis et al. (2012) ApJ, 751, L7. [8] Guillot T. and Hueso R. (2006) MNRAS, 367, L47-L51. [9] Del Gen-io et al., (2009) Saturn after Cassini-Huygens, Ch. 6, pp. 113-159. [10] West et al., (2009), Saturn after Cas-sini-Huygens, Ch. 7, pp. 161-179. [11] Seiff et al., (1998), JGR, 103, 22857-22890 [12] Atkinson et al., JGR, 103, 22911-22928 (1998). [13] Wong et al., (2004), Icarus 171, 153-170. [14] Fouchet et al., Sat-urn after Cassini-Huygens, Ch. 5, pp. 83-112 (2009).  Acknowledgements: OM acknowledges support from CNES.  LNF is supported by a Royal Society Research Fellowship. 1261.pdf45th Lunar and Planetary Science Conference (2014)

Scientific rationale of a Saturn probe mission

Wurz, Peter

Waite, J.H.

Bern Open Repository and Information System (BORIS)

Scientific Rationale of a Saturn Probe Mission

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Scientific rationale of a Saturn probe mission

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