Study of the formation conditions of the first solids in protoplanetary disks and their effect on th Closing date: 2012-06-10
Contact: Jean-Marc Petit
The planetesimals, from which the planets and small bodies of the solar system
are fromed, have a composition that strongly depends on the carbon and oxygen
abundances in the protoplanetary disk, on the reduced state of carbon
(i.e. regions rich in CO compared to those rich in CH4), and on the fraction of
carbon trapped in organic solids. One must thus develop a model linking the
composition of planetesimals created in the disk to its gas phase and its
thermodynamic evolution.
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Scientific Rationale:
Spectroscopic data from transiting exoplanets allow nowaday to study their atmospheric composition. In the Solar System, gas and icy giants are known to be significantly enriched in heavy elements and volatile species with respect to the Sun. In the case of Jupiter, in situ measurements from the Galileo probe have shown that its atmosphere is enriched in both condensable elements and noble gases (Mahaffy et al. 2000 ; Wong et al. 2004). Spectroscopic observations, in particular from the Cassini spacecraft, have measured similar enrichements in several species in Saturn’s atmosphere (Fletcher et al. 2009a, 2009b). These enrichements are thought to result from the accretion and devolatilization of ice-rich planetesimals during the formation of planetary envelopes. Actually, these enrichements are reproduced from the determination of the volatile phase compostion trapped in planetesimals and the total mass of heavy elements predicted to be in the envelopes by internal structure models (Mousis et al. 2009a). This method was used in the case of the hot Jupiter HD189733b in order to explain how a low metalicity with respect to its host star could be compatible with existing giant planet formation models (Mousis et al. 2009b, 2011). This kind of models also permits to establish a link between the planet's composition and that of the protoplanetary disk from which it formed (Madhusudhan et al. 2011; Mousis et al. 2012).

Thesis objectives:
One of the first goals of this thesis consists in proposing a consistent model describing the chemical composition of planetesimals formed beyond the snow-line as a function of the protoplanetary disk metallicity. The planetesimals composition is supposed to strongly depends on the carbon and oxygen abundances in the disk, on the reduced state of carbon (i.e. regions rich in CO compared to those rich in CH4), and on the fraction of carbon trapped in organic solids. All these parameters strongly influence the fractions of silicates, metals and volatile elements existing in the solid phase. One must thus develop a model linking the composition of planetesimals created in the disk to its gas phase and its thermodynamic evolution. To generate the thermodynamic conditions of the various environments considered in this study, the student will use the EvAD1 model of turbulent accretion disk developed in Besançon. The large variations of metallicity measured in some transiting exoplanets suggest that the accretion conditions of these giant planets may differ widely from those expected for Jupiter and Saturn. The second part of this thesis will consist in simulating the accretion conditions of planetesimals onto the giant planets during their formation via N-body dynamical simulations. This methods allows to better understand the required initial conditions for exoplanets in order to explain their current metallicity. All this work will be conducted in the framework of i) the preparation of the analysis the data from the JUNO mission, currently en route to Jupiter and ii) the interpretation of the forthcoming observations from the future James Webb Space Telescope (JWST).

Expected skills:
The proposed study requires an important modelling effort. Good knowledge and understanding in computer science and numerical analysis (FORTRAN programming, UNIX shell scripting, …) are required. The successful applicant will have solid knowledge in thermodynamics and astrophysics. The work implies strong international collaborations ; fluency in English is needed. Prior knowledge of French, or a demonstrated will to learn it would be a definite plus.

Bibliography:
Madhusudhan, N., Mousis, O., Johnson, T. V., Lunine, J. I. 2011. Carbon-rich Giant Planets: Atmospheric Chemistry, Thermal Inversions, Spectra, and Formation conditions. The Astrophysical Journal, in press (ArXiv e-prints arXiv:1109.3183).

Mahaffy, P. R., Niemann, H. B., Alpert, A., Atreya, S. K., Demick, J., Donahue, T. M., Harpold, D. N., Owen, T. C. 2000. Noble gas abundance and isotope ratios in the atmosphere of Jupiter from the Galileo Probe Mass Spectrometer. Journal of Geophysical Research 105, 15061-15072.

Mousis, O., Madhusudhan, N., Lunine, J. I., Johnson, T. V. 2012. Could Jupiter be a carbon-rich planet ? The Astrophysical Journal Letters, submitted.

Mousis, O., and 15 colleagues 2011. On the Volatile Enrichments and Heavy Element Content in HD189733b. The Astrophysical Journal 727, 77.

Mousis, O., Lunine, J. I., Tinetti, G., Griffith, C. A., Showman, A. P., Alibert, Y., Beaulieu, J.-P., The Holmes Collaboration 2009b. Elemental abundances and minimum mass of heavy elements in the envelope of HD 189733b. Astronomy and Astrophysics 507, 1671-1674.

Mousis, O., Marboeuf, U., Lunine, J. I., Alibert, Y., Fletcher, L. N., Orton, G. S., Pauzat, F., Ellinger, Y. 2009a. Determination of the Minimum Masses of Heavy Elements in the Envelopes of Jupiter and Saturn. The Astrophysical Jou