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


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: Path of Titania's shadow on Earth on September 8, 2001. Green stars indicate stations which could observe or record the event on three continents, among which visual observations by amateur astronomers.
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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: Various occultation chords of the best observing stations were used for Titania's radius determination of 788.4 ± 0.6 km, improving the Voyager estimate of 788.9 ± 1.8 km. The accuracy is not limited by the measure uncertainties, but by the satellite's topography, characterized by scarps and faults of kilometric scale, yielding to this histogram of radius values retrieved by 27 stations. Distance between the Earth and Titania was 2.85 billion kilometers. Background : Titania's image obtained from Voyager-II in 1986.
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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: In the presence of even a tenuous atmosphere, stellar intensity is sensitive to refraction variations, a function of vertical temperature profile and the nature of the constituent. As pressure levels detected during refractive occultations are inversely proportional to distance, the upper limits obtained on Titania, at a distance of 19 AU, open promising perspectives to constrain the presence of an atmosphere of a large TNO at a few nanobars level during an occultation, or a billionth of the Earth's pressure.
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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)