Outline

1. Formation of icy planetesimals in the primitive nebula: implications for the composition of gas giant planets, satellites, and cometsOlivier MousisUniversité de Franche-ComtéObservatoire de Besançon Institut Universitaire de France March 5, 2010 – Johns Hopkins University – NASA Astrobiology Institute – Space Telescope Science Institute

2. Outline • Introduction : Formation of ices in the primitive nebula - Structures of clathrates - Condensation sequence of ices and formation of icy planetesimals • Application to the atmospheres of giant planets • Enrichments in volatiles in the atmospheres of Jupiter and Saturn • The enigmatic case of HD 189733b • Constraints on the formation of regular icy satellites from thermochemical models of circumplanetary disks • Formation of the Jovian satellites

3. Outline • Formation of Titan and Enceladus • Constraints on the composition of small bodies of the solar system from the modeling of thermodynamic conditions in the solar nebula - Origin of volatiles in the Main Belt - Photophoresis as a source of hot minerals in comets • Constraints on the origin of comets from their D/H ratios

4. Evolution of the gas-phase in the primitive nebula • Current scenarios of the formation of the solar nebula consider that ices and gases present in the presolar cloud fell onto the disk during the collapse of the cloud • Most of these ices vaporized, either during the shock when entering into the disk or in the early nebula • H2O, CO, CO2, CH4, N2, and NH3 were thus initially in gaseous form in the nebula, within 30 AU (Chick & Cassen 1997) • Calculations of the temporal evolution of the CO:CH4, CO2:CH4 et N2:NH3 ratios throughout the solar nebula show that they remain constant in the vapor phase (Mousis et al. 2002, Mousis & Alibert 2006)

5. Two different reservoirs of ices in the nebula • Ices located at distances higher than 30 AU remained pristines (amorphous ices) • Ices transported at distances lower than 30 AU vaporized and condensed again in crystalline form (Chick & Cassen 1997)

6. Structures of clathrates X-5.75 H2O X-5.66 H2O

7. Structure of clathrates as a function of the guest molecule’s size

8. Clathrates on Earth

9. Composition of ices produced in the solar nebula: a recipe • What relative quantities of different species are included in icy • planetesimals? • Ingredients • Thermodynamic conditions in the protoplanetary disc (P, T, Σ) • Initial gas-phase molecular composition • Stability curves of the different ices • General assumptions • Focus on regions beyond the snow line • Limited number of species to H2O, CO, CO2, CH3OH, PH3, CH4, N2, NH3, H2S, Ar, Kr and Xe

10. Formation of icy planetesimals in the primordial nebula assuming a solar abundance for all elements CO:CO2:CH3OH:CH4 = 70:10:2:1 and N2:NH3 = 1:1 in the gas phase disk condensation /trapping grains accretion Formation of clathrates and pure condensates in the outer solar nebula planetesimals

11. Formation of icy planetesimals in the primordial nebula assuming a total clathration of volatiles CO2:CO:CH4 = 30:10:01 and N2:NH3 = 1:1 in the gas phase Total clathration scenario: requires an oversolar oxygen abundance (about twice the solar value) Ices are stable in domains located below their stability curves

12. Composition of ices formed in the primordial nebula • Composition of ices remains constant irrespective of: • the formation distance to the star • the input parameters of the disk • model (Mdisk, alpha, accretion • rate …) Composition of icy planetesimals as a function of the CO2/CO ratio in the gas phase of the disk (Marboeuf et al. 2008) Composition of icy planetesimals as a function of the C/O ratio in the gas phase of the disk (Marboeuf et al. 2008)

13. Volatiles enrichments in Jupiter Owen et al. (1999)

14. Carbon enrichment in Saturn +1.0 -0.8 +1.7 -1.4 +0.8 -0.2 +1.4 -0.3 +2.4 -1.9 +4.1 -3.3

15. Observed and calculated enrichments in volatiles in Jupiter and Saturn 100% clathration efficiency, CO2:CO:CH3OH:CH4 = 70/10/2/1 and N2/NH3 = 1/1 in the nebula gas phase Mousis et al. (2009a)

16. Formation of noble gas-poor planetesimals in presence of H3+ in the solar nebula gas phase Formation of X-H3+ complexes in the nebula gas phase (X = Kr and Xe) that impede the formation of X-6H2O clathrates or pure X condensates Mousis, Pauzat, Ellinger & Ceccarelli (2008)

17. Minimum masses of heavy elements required in the atmospheres of Jupiter and Saturn to fit the observed enrichments Mousis et al. (2009a)

18. The enigmatic case of HD189733b HD 189733b is a transiting hot Jupiter ( M = 1.15 ± 0.04 MJupiter ) orbiting a bright (V = 7.7) and close (d = 19 pc) K2V stellar primary at the distance of 0.03 AU (Bouchy et al. 2005). H2O, CO, CO2 and CH4have been measured to be in subsolar abundances in the atmosphere of HD 189733b whereas internal structure models predict 35 ± 15 EM of heavy elements in the planet (Guillot 2008). • The C and O atmospheric abundances may not be representative • of the envelope composition. Two possibilities: • Differential settling resulting from the combination of gravity • and irradiation effects (Baraffe et al. 2010) • Presence of soot in the envelope of HD 189733b (Zahnle et al. • 2010; Mousis et al. 2010, in prep) Volatile enrichment in HD 189733b as a function of the mass of accreted ices (Mousis et al. 2009b)

19. Scenarios of formation of regular satellite systems • Formation in an accretion disk resulting from the planet’s contraction up to a smaller scale than the orbits of the regular satellites. Subnebula fed in gas and dust by the nebula (gas-starved disk model proposed by Canup & Ward 2002) Most common hypothesis to explain theformation of satellites around Jupiter and Saturn • Disk generated by a collision with an impactor owning a mass similar to that of Earth formation of the Uranian satellites (Mousis 2004) The study of the thermodynamic conditions within the subdisks provides strong physical and chemical constraints on the formation of the regular satellites

20. An evolutionary turbulent model of the Jovian subnebula Temperature profile in the Jovian subdisk The evolution of the Jovian subnebula is entirely ruled by the last phase of the giant planet’s formation: • At early epochs, the subnebula is warm and dense. It is fed by gas and gas- coupled solids coming from the solar nebula • At later epochs, the solar nebula is dissipated and the subnebula is no more fed. The subdisk progressively empties with time. Alibert, Mousis & Benz (2005)

21. Homogeneous chemistry in the Jovian subnebula Evolution of the condensation front of ices as a function of time in the Jovian subnebula Chemical conversion times (years) CO/CH4 ratios at equilibrium in the subnebula (Mousis & Alibert 2006)

22. Heterogeneous chemistry in the Jovian subnebula (Fischer-Tropsch catalysis) Time required for the conversions of CO into CH4 and of CO2 into CH4 at T = 550 K as a function of the total gas pressure (Mousis, Alibert, Sekine, Sugita & Matsui 2006) • Presence of a catalytically active zone in the Jovian subnebula • However the produced species are not trapped in planetesimals because • tfall << tformation of ices

23. Two extreme formation scenarios for the Galilean satellites Scenario 1:formation in a late subnebula. Solids are produced in the nebula and preserved during their migration/accretion within the subdisk. The composition of ices does not change Scenario 2:formation from solids produced in an early subnebula. Homogeneous and heterogeneous gas-phase chemistries inefficient. In both cases, the composition of ices incorporated in the satellites remains similar to that of ices produced in the solar nebula

24. In these conditions, what observational test would allow to constrain the right scenario ? Answer: the measurement of D:H ratio in water ice!!!! • Solids produced in the nebula: D:H ratio close to the one observed in comets • Solids produced in the subnebula: D:H ratio substantially lower than the cometary value Deuterium enrichment profiles in water (compared to the solar value) calculated at the epochs of the formation of ice in the primitive nebula (Mousis 2004).

25. Characteristics of Titan • Atmosphere dominated by N2 and CH4 CO/CH4 << 1 --> What happened to CO? • N2 would result from the NH3 photolysis or shock chemistry (Atreya et al. 1978; McKay et al. 1988). So, N2 is probably not primordial!! • High CH3D/CH4 ratio (Orton 1992; Bézard et al. 2007): atmospheric methane probably formed in the solar nebula and NOT via hydrothermal reactions in the interior of Titan (Mousis et al. 2009c) or via gas phase chemistry in Saturn’s subnebula (Mousis et al. 2002) • Deficiency of Titan’s atmosphere in primordial noble gases. Only low amounts of 36Ar have been detected (Niemann et al. 2005) • D/H ratio found to be cometary in Enceladus: the building blocks of Titan and Enceladus were formed in the nebula (Waite et al. 2009).

26. Why CH4 is expected to come from the primitive nebula? • ISM ices that fell onto the solar nebula are highly enriched in deuterium • The isotopic exchange between HD and CH3D is described by the following reversible equation in the gas phase dominated by H2: CH4 + HD = CH3D + H2 • The efficiency of the isotopic exchange increases at high temperature. The exchange stops once the methane is condensed or forms a clathrate hydrate.

27. Deuterium enrichment profile in CH4 in the nebula D/H in Titan’s methane is measured to be 5-7 times the protosolar value (Bézard et al. 2007). Mousis et al. (2002)

28. If CH4 was produced in Saturn’s subnebula (scenario of Prinn & Fegley 1981)… Isotopic exchange in the subnebula of Prinn & Fegley (1981) • In this scenario, CH4 results from the gas phase conversion of CO in a warm and dense subnebula. • The initial D-enrichment of CH4 is then of the same order that the protosolar value. Mousis et al. 2002 The isotopic exchange in the subnebula cannot explain the value observed in Titan

29. Multiple guest trapping in clathrates produced in the primitive nebula Multiple guest trapping probably occured at the time of clathrate formation in the nebula: Xe and Kr have been trapped in ices above 50 K in the nebula!!

30. Formation sequence of ices in the feeding zone of Saturn and the origin of Titan A formation of Titan in the 25-50 K temperature range is required to explain at least partly its atmospheric composition. In these conditions, building blocks of Titan are Ar- and CO-poor However, major issue: Kr and Xe are trapped in the building blocks of Titan!!! Mousis et al. (2009d)

31. Efficient trapping of Kr and Xe in clathrates formed on the surface of Titan Titan’s atmosphere is assumed to be initially composed of N2, CH4, C2H6 and noble gas (Xe or Kr) Thomas, Mousis, Ballenegger & Picaud (2007), A&A, 474, L17 Fraction of xenon and krypton in multiple guest clathrates formed on the surface of Titan is orders of magnitude higher than the atmospheric fraction Clathrates on the surface act as sinks of Xe and Kr!!!

32. Formation of Titan: summary Conditions required to explain the current composition of Titan’s atmosphere: • Volatiles were trapped in Saturn’s feeding zone in the form of hydrates, clathrates or pure condensates • Partial vaporization of solids accreted by Titan at about 50 K in Saturn’s subnebula: loss of CO and Ar; preservation of primordial CH4, NH3, CO2, Kr and Xe Two possibilities to explain the deficiency of Xe and Kr in Titan: • Efficient trapping of Xe and Kr in clathrates formed on the surface of Titan • Formation of Xe-H3+ and Kr-H3+ complexes in the solar nebula that impede the condensation of Xe and Kr or their incorporation in the forming clathrates

33. Characteristics of Enceladus Plume composition as derived from INMS measurements during the 9 October 2008 Enceladus flyby H2O, CO2, H2S, NH3 are probably primordial (Matson et al. 2007). Measurement of the D/H ratio in H2O: D/H = 2.9x10-4 a value close to the one measured in comets -> the building blocks of Enceladus were formed in the primordial nebula!!! H2 and CO are probably produced by the dissociation of H2O and CO2 through hypervelocity impact on, and reaction with, the walls of the INMS antechamber. However, the origin of CH4 in the interior of Enceladus remains uncertain: it can be produced via in hydrothermal reactions the body (Matson et al. 2007) or originate from the Solar nebula. Waite et al. (2009)

34. Formation of Titan and Enceladus in Saturn’s subnebula Alibert & Mousis (2007) The formation zone of Enceladus within Saturn’s subnebula was probably warmer than the formation zone of Titan!!

35. Formation of Enceladus in Saturn’s subnebula • Two possible scenarios: • The methane of Enceladus originates from the solar nebula: formation in the 25-50 K temperature range. • Xe/H2O = 7x10-7 • Kr/H2O = 7x10-6 • 2. The methane of Enceladus • was produced in the • satellite’s interior: • formation in the 50-70 K • temperature range. • Kr/H2O < 10-10 • 1x10-7 < Xe/H2O < 7x10-7

36. Origin of volatiles in the Main Belt Presence of ices within Ceres and outer belt asteroids would be likely!! Density of asteroids owning a given mix of ices + rocks as a function of porosity Trajectories of solid particles in the primitive nebula, for different starting locations and starting epochs. The trajectories are drawn until particles cross the snow line. Mousis et al. (2008)

37. Photophoresis and the high temperature minerals found in comets • Crystalline silicates found in a growing number of comets (Crovisier et al. 2000; Sitko et al. 2004; Wooden et al. 2004) • Grains resembling Calcium-Aluminium-Inclusions (CAIs) have been identified in the samples returned from Comet 81P/Wild 2 by the Stardust mission (Brownlee et al. 2006) All these minerals were formed at moderately to extremely high temperatures in the Solar Nebula (reaching 1400-1500 K for CAIs!!!!) What are the existing scenarios that may explain the origin of these minerals in the context of cometary formation?

38. Scenarios existing in the literature • Shock waves in the outer Solar nebula could anneal the amorphous silicates to cristallinity in situ prior to their incorporation in comets (Harker & Desch 2002) No observational evidences for the existence of such processes!! • Formation of crystalline silicates in the hot inner region of the solar nebula and rapid diffusive transport to the comet formation zone (Bockelée-Morvan et al. 2002) Implies a homogeneous distribution of grains within the Solar nebula which may not be consistent with the radial variations of the properties of minor planets, and large scale differences between solar system bodies (Gradie & Tedesco 1982; Brownlee et al. 2006)

39. Role of photophoresis in the primordial nebula

40. How photophoresis works? Only two conditions are needed: Gas Light

41. Ring formation in a disk Photophoresis + Rad. Press. Star Photophoresis + Rad. Press. Aggregate Migration Migration Residual gravity Residual gravity A ring forms here

42. Migration of aggregates in the nebula • Here, we consider: • - Gas opacity • Real velocity of particles • during their migration • Drag of aggregates due to • the accretion of gas onto • the proto-Sun Two cases must be envisaged as a function of the importance of the smallest particles opacity!!! Pathways followed by particles of sizes 10-5 to 10-1 m as a function of time and the size of the gap in the nebula Mousis et al. (2007), A&A 466, L9

43. A strong constraint on the origin of cometary reservoirs: the D:H ratio!! Deuterium enrichment profile f in water (compared to solar) in the primitive nebula. The value of f is fixed once water is condensed. Examples illustrating how formation regions of comets affect the distributions of deuterium enrichment Horner, Mousis & Hersant( 2008)