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From Titania to large trans-Neptunian objects : ground-based stellar occultations in the quest for the billionth of atmospheric pressure

1er février 2009

Titania is the largest Uranian moon. A team of astronomers of Paris Observatory, with more than a hundred amateur and professional astronomers in a campaign of an unprecedented scale, could establish from the ground, with an array of instruments of small diameter (of the order of 20-25 cm), the size of Titania at a better precision than Voyager in 1986. It could also determine an upper limit for an atmosphere. This limit is about one thousand times smaller than the pressure currently measured on Pluto. This is promising in view of the detection of volatile ices on several transneptunian objects, to constrain their physical characteristics, origin and evolution.

Recent analysis of a stellar occultation (Figure 1) allowed us to constrain the radius, oblateness and density at better accuracy than the Voyager-II flyby obtained in 1986. In particular, we determine the radius at a sub-kilometer precision of 788.4 ± 0.6 km. The spread of measurements between observers, both amateurs and professionals, is dominated by Titania’s topography, not by method uncertainties (Figure 2).

Figure 1 : Trajectoire de l’ombre de Titania sur Terre le 8 septembre 2001. Les étoiles indiquent la position géographique des stations ayant observé l’occultation stellaire sur trois continents, parmi lesquelles des observations visuelles conduites par des amateurs. Cliquer sur l’image pour l’agrandir

Near-IR spectroscopy has indicated the presence of water ice and carbon dioxide ice on the surface of Titania. Solar radiation may cause a tenuous, seasonal atmosphere induced by CO2 ice sublimation. We could determine a surface pressure upper limit of 10-20 nanobars (1) for such an atmosphere (Figure 3), as well as for other constituents like CH4 or N2, which could result of outgassing associated with internal heating and cryovolcanism (2), as on Enceladus (a Saturn’s satellite) or Triton (a Neptune’s satellite).

Figure 2 : Les différentes cordes d’occultation ont permis de déterminer le rayon de Titania, 788.4 ± 0.6 km, améliorant la valeur obtenue lors de la mission Voyager, 788.9 ± 1.8 km. La précision est limitée non par l’incertitude de mesure, mais par la topographie du satellite, qui montre des failles et des escarpements de l’ordre du kilomètre, conduisant à cet histogramme des valeurs obtenues par 27 stations. Titania se trouvait alors à 2,85 milliards de kilomètres de la Terre. En arrière-plan : Image de Titania obtenue à partir de Voyager-II en 1986. Cliquer sur l’image pour l’agrandir

Those very small values demonstrate the power of stellar occultations to put pressure upper limits at a distance of 19 UA (3) down to levels of about 10 nanobars, much more tenuous than current pressures on Pluto or Triton, by typical factors of 1000. This is promising in view of the detection of volatile ices on several transneptunian objects, at a distance of 40-70 AU. CH4 has been clearly detected on dwarf planets Eris, Makemake and Quaoar, while the presence of N2 on Eris is indirectly suggested. At those distances these ices are marginally stable over the age of the Solar System, and therefore constitute a significant reservoir for atmospheric formation.

Figure 3 : En présence d’une atmosphère même ténue, l’intensité lumineuse de l’étoile est sensible à la variation de réfraction, fonction du profil vertical de température et du constituant. L’effet géométrique de la réfraction du signal stellaire pour un niveau de pression donné est inversement proportionnel à la distance. Compte-tenu des limites supérieures obtenues pour Titania, situé à 19 UA, on s’attend à pouvoir contraindre lors d’une occultation la présence d’une atmosphère autour des grands OTN au niveau de quelques nanobars, soit un milliardième de la pression terrestre. Cliquer sur l’image pour l’agrandir

This research acknowledges Agence Nationale de la Recherche’s grant « Beyond Neptune » in 2009-2012 (Observatoire de Paris-LESIA, SARL Shelyak Instruments (France), Observatoire de Haute-Provence, National Tsing Hua University, Taiwan), and support from Observatoire de Paris and Programme national de planétologie (CNRS/INSU). (1) 1 nanobar= 1 billionth of the Earth’s surface atmospheric pressure = 0.1 milli-pascal (2) cryovolcanism (icy volcanism) : eruption of water and other liquid or vapor-phase volatiles, together with gas-driven solid fragments, onto the icy surface of a low-temperature planet or satellite due to internal heating (e.g. Triton, Encelade). (3) AU = astronomical unit = Sun-Earth distance, 149.6 millions km.

Reference

  • Titania’s Radius and an Upper Limit on its Atmosphere from the September 8, 2001 Stellar Occultation T. Widemann, B. Sicardy, R. Dusser, C. Martinez, W. Beisker, E. Bredner, D. Dunham, P. Maley, E. Lellouch, J.-E. Arlot, J. Berthier, F. Colas, W.B. Hubbard, R. Hill, J. Lecacheux, J.-F. Lecampion, S. Pau, M. Rapaport, F. Roques, W. Thuillot, C.R. Hills, A.J. Elliott, R. Miles, T. Platt, C. Cremaschini, P. Dubreuil, C. Cavadore, C. Demeautis, P. Henriquet, O. Labrevoir, G. Rau, J.-F. Coliac, J. Piraux, Ch. Marlot, C. Marlot, F. Gorry, C. Sire, B. Bayle, E. Simian, A.M. Blommers, J. Fulgence, C. Leyrat, C. Sauzeaud, B. Stephanus, T. Rafaelli, C. Buil, R. Delmas, V. Desnoux, C. Jasinski, A. Klotz, D. Marchais, M. Rieugnié, G. Bouderand, J.-P. Cazard, C. Lambin, P.O. Pujat, F. Schwartz, P. Burlot, P. Langlais, S. Rivaud, E. Brochard, Ph. Dupouy, M. Lavayssière, O. Chaptal, K. Daiffallah, C. Clarasso-Llauger, J. Aloy Doménech, M. Gabaldá-Sánchez, X. Otazu-Porter, D. Fernández, E. Masana, A. Ardanuy, R. Casas, J.A. Ros, F. Casarramona, C. Schnabel, A. Roca, C. Labordena, O. Canales-Moreno, V. Ferrer, L. Rivas, J.L. Ortiz, J. Fernández-Arozena, L.L. Martín-Rodríguez, A. Cidadão, P. Coelho, P. Figuereido, R. Gonçalves, C. Marciano, R. Nunes, P. Ré, C. Saraiva, F. Tonel, J. Clérigo, C. Oliveira, C. Reis, B.M. Ewen-Smith, S. Ward, D. Ford, J. Gonçalves, J. Porto, J. Laurindo Sobrinho, F. Teodoro de Gois, M. Joaquim, J. Afonso da Silva Mendes, E. van Ballegoij, R. Jones, H. Callender, W. Sutherland, S. Bumgarner, M. Imbert, B. Mitchell, J. Lockhart, W. Barrow, D. Cornwall, A. Arnal, G. Eleizalde, A. Valencia,V. Ladino, T. Lizardo, C. Guillén, G. Sánchez, A. Peña, S. Radaelli, J. Santiago, K. Vieira, H. Mendt, P. Rosenzweig, O. Naranjo, O. Contreras, F. Díaz, E. Guzmán, F. Moreno, L. Omar Porras, E. Recalde, M. Mascaró, C. Birnbaum, R. Cósias, E. López, E. Pallo, R. Percz, D. Pulupa, X. Simbaña, A. Yajamín, P. Rodas, H. Denzau, M. Kretlow, P. Valdés Sada, R. Hernández, A. Hernández, B. Wilson, E. Castro, J.M. Winkel 2009, Icarus 199, Vol. 2, pp. 458-476 (February 2009).

Contact

  • Thomas Widemann
    Observatoire de Paris, LESIA, et Université de Versailles Saint-Quentin
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