|
Upper limits for a TitaniaÕs
atmosphere and for a large TNOÕs atmosphere from ground-based stellar
occultations
Summary. - Titania is the largest uranian
moon (d = 1576.8 ± 1.2 km). Recent analysis of a stellar occultation allowed
us to constrain diameter, oblateness and density at better accuracy than
Voyager-2. Near-IR spectroscopy has indicated the presence of water ice and
carbon dioxide ice on the surface of Titania. While H2O ice is
clearly involatile, CO2 ice stability against sublimation over a
seasonal cycle of Titania can be considered (Grundy et al., 2006). The analysis
allowed to set surface pressure upper limits of 10-20 nbar for a CO2,
CH4 or N2 atmosphere. At maximum temperature given by
instantaneous equilibrium with solar input, a CO2
sublimation-induced atmosphere is still a factor of 3-6 lower than the upper
limit provided by our analysis. We also give an upper limit for N2
and CH4 content in the same pressure range. Although unlikely they
could be temporarilypresent as possible products of outgassing associated
with internal heating and cryovolcanism, as on Enceladus or Triton (Widemann
et al., 2009). Although the non-detection of an
atmosphere of Titania is not surprising, we demonstrate the power of stellar
occultations to put pressure upper limits at a ditance of 19 UA down to
levels of about 10 nbar, 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. 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 thoses ices are marginally stable over the age of the Solar System
(Schaller and Brown, 2007). On a longer term, as pressure levels detected
during refractive occultations are inversely proportional to distance, the
upper limits obtained on Titania open promising perspectives to constrain
atmospheres of large TNOs at a few nbar level, or a billionth of the Earth's
surface pressure. RŽsumŽ. - Titania est le plus gros
satellite d'Uranus (d = 1576.8 ± 1.2 km). L'analyse rŽcente d'une occultation
stellaire nous a permis de contraindre le diamtre, l'aplatissement et la
densitŽ ˆ une prŽcision supŽrieure ˆ celle de Voyager-2. La spectroscopie
proche-IR indique la prŽsence de glace d'eau et de glace de CO2. H2O
est non volatile, mais CO2 pourrait tre sujette ˆ sublimation
saisonnire, induite par le rayonnement solaire (Grundy et al., 2006). Cette
recherche nous a permis de mesurer une limite supŽrieure de 10-20 nbar de
pression pour une atmosphre constituŽe de CO2 ˆ l'Žquilibre
thermique. Les conditions d'Žquilibre radiatif indiquent au maximum un
facteur 3-6 infŽrieur ˆ cette limite de dŽtection. Nous obtenons une limite supŽrieure pour un contenu
en N2 ou CH4 dans la mme gamme de pression, ces gaz
pouvant rŽsulter d'une activitŽ de dŽgazage par chauffage interne et
cryovolcanisme, suivant l'exemple d'Encelade ou Triton (Widemann et al.,
2008). La mŽthode d'occultations dŽmontre
ainsi sa facultŽ de contraindre la pression au sol d'un objet ˆ 19 UA ˆ des
niveaux de l'ordre de 10 nbar, soit un facteur 1000 infŽrieur aux pressions
actuelles mesurŽes sur Triton et Pluton. C'est un rŽsultat trs prometteur
pour la recherche d'atmosphres sur les gros TNOs distants de 40 ˆ 70 UA, o
l'on a identifiŽ des surfaces glacŽes potentiellement volatiles: le mŽthane
CH4 sur Eris, Makemake et Quaoar, l'azote N2
suggŽrŽ sur Eris. A ces distances les glaces sont pratiquement stables ˆ
l'Žchelle de l'‰ge du systme solaire (Schaller et Brown, 2007). L'effet gŽomŽtrique de la rŽfraction
diffŽrentielle pour un niveau de pression donnŽ Žtant inversement
proportionnel ˆ la distance, on s'attend ˆ pouvoir contraindre la prŽsence
d'une atmosphre autour des grands TNOs au niveau de quelques nanobars, soit
un milliardime de la pression terrestre.
Grundy et al. 2006, Icarus 184,
543-555 Schaller, E.L. and Brown, M.E.
2007, Astrophys. J. 659, L61-L64 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. RieugniŽ,
G. Bouderand, J.-P. Cazard, C. Lambin, P.O. Pujat, F. Schwartz, P. Burlot, P.
Langlais, S. Rivaud, E. Brochard, Ph. Dupouy, M. Lavayssire, O. Chaptal, K.
Daiffallah, C. Clarasso-Llauger, J. Aloy DomŽnech, M. Gabald‡-S‡nchez, X.
Otazu-Porter, D. Fern‡ndez, 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. Fern‡ndez-Arozena, L.L.
Mart’n-Rodr’guez, A. Cidad‹o, P.
Coelho, P. Figuereido, R. Gonalves, C. Marciano, R. Nunes, P. RŽ, C.
Saraiva, F. Tonel, J. ClŽrigo,
C. Oliveira, C. Reis, B.M. Ewen-Smith, S. Ward, D. Ford, J. Gonalves, 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. GuillŽn,
G. S‡nchez, A. Pe–a, S. Radaelli, J. Santiago, K. Vieira, H. Mendt, P.
Rosenzweig, O. Naranjo, O. Contreras, F. D’az, E. Guzm‡n, F. Moreno, L. Omar
Porras, E. Recalde, M. Mascar—, C. Birnbaum, R. C—sias, E. L—pez, E. Pallo,
R. Percz, D. Pulupa, X. Simba–a, A. Yajam’n, P. Rodas, H. Denzau, M. Kretlow,
P. ValdŽs Sada, R. Hern‡ndez, A. Hern‡ndez, B. Wilson, E. Castro, J.M. Winkel
2009, Titania's Radius and an Upper Limit on its Atmosphere from the
September 8, 2001 Stellar Occultation, Icarus 199, Vol. 2, pp. 458-476
(February 2009).
Titania occultation
(T. Widemann obs.) – 20-cm (12-in) telescope, 8 sept. 2001, Aruba
island (Caribbean) |
|
|
