Is there an atmosphere around Titania, satellite of Uranus ?

On September 8, 2001, around 2h UT, the Hipparcos star #106829 (alias SAO 164538), of visible magnitude 7.2, was occulted by Titania, the largest of Uranus’ moons. This event was discovered independently by Jean Meeus and Claudio Martinez (Argentina), late 1999 and beginning of 2001, respectively. It was widely announced by IOTA (International Occultation Timing Association) at the beginning of 2001 and triggered an intense preparation among related countries (Figure 1). Bruno Sicardy and Thomas Widemann, with François Colas, Jean Lecacheux, Sylvain Pau and William Thuillot, from Observatoire de Paris, took this exceptional occasion to search for an atmosphere around Titania, and report today the results of their search.

The unusual brightness of the star allowed many astronomers, amateurs or professionals, to observe this very rare event, considering the very small angular diameter of Titania from Earth (0.11 arcsec). A rough estimate yields a recurrence period of the event every few millennia ! The observations were undertaken either visually with small telescopes, or by the recording of several images per second of the event on videotape or CCD. The occultation was successfully followed from Western Europe, low above the horizon, in the Northern Atlantic, the Caribbean, and the north of South America, where the occultation was visible near zenith (Figure 1). titania_f3l.jpg Figure 1. Cast shadow on Earth of star Hipparcos #106829 occulted by Titania. Titania’s shadow was cast on Earth’s night side at a ground velocity of 21,3 km s-1, for a maximum duration of approximately 74 s. The phenomenon was observed at low height angle from France, UK, Italy, Spain, Morocco, Portugal, and closer to zenith from the Caribbean (Barbados, Tobago, Aruba) and from Northern South-America (Ecuador, Venezuela). Each dot on trajectory center indicates the shadow center transit time in UT by 1-minute intervals. The shadow width equals Titania physical diameter, or 1580 km. Click on the figure to enlarge. titania_f5l.jpg Figure 2. Titania’s disk superimposed to various occultation chords of star Hipparcos #106829 on the night of September 8, 2001. The ellipse portion in dark solid line is Titania’s equator, dividing the southern (left) and northern hemisphere (right). The small square near the limb on the left indicates Titania’s south pole. The star immersion occurred on the right, in the northern hemisphere (the uranian system was in retrograde motion at the time of occultation). The emersion occurred to the left, near the satellite ’s south pole. Each parallel track corresponds to a terrestrial observing location where an observer recorded successfully the phenomenon. Click on the figure to enlarge. Titania was discovered in 1787 by William Herschel. It was named after the Queen of Fairies, Oberon’s wife, in William Shakespeare’s play A Midsummer Night’s Dream, written in 1597 (see below a painting by Frank Cardogan Cowper representing Titania’s sleep). http://www.emory.edu/ENGLISH/classe... Painting from Frank Cadogan Cowper Click on the figure to enlarge. This satellite, composed of 40-50% ice, has a diameter of 1580 km and orbits Uranus’ equatorial plane in a period of 8.7 days, at a distance of 436,000 km. It was also discovered by William Herschel in 1781. On the southern hemisphere portion observed in January 1986 by Voyager II (Figure 3), the surface is a mixture of cratered terrain and systems of interconnected valleys hundreds of kilometers long. Like Uranus, Titania’s south pole remained in sunlight for several decades while the northern hemisphere was dark, as the uranian system reached winter solstice in 1985. Approaching spring equinox (2006), gradual heating of regions which stood long in obscurity may produce a tenuous atmosphere through sublimation. The occultation measurement gives an upper limit in pressure for such an atmosphere. titania_f7l.jpg Figure 3. Titania, observed by the Voyager II probe in January 1986. With a diameter of 1580 km, it is the larger uranian satellite. It represents 60% of Triton’s size (and 16% of its mass), or 70% of Pluto’s size (26% of its mass), two bodies with a tenuous nitrogen (N2) atmosphere). But being closer to the Sun than those two bodies, Titania is also warmer, T 60-65K, instead of 40K (Grundy et al. 1999, Icarus 142, 536). This prevents a permanent N2 atmosphere, which would escape at such temperature. A water vapor (H2O) atmosphere would be on the contrary precipitated in ice form, because Titania is too cold. A methane (CH4) atmosphere is possible theoretically, while no indication of CH4 ice in IR spectra of Titania’s surface has been obtained, putting into question the source of such an atmosphere. However, a transient atmosphere, composed of N2, CO or NH3 may be considered, in light of the fact that sunlight is currently warming the northern hemisphere, parts of which remained in darkness for more than 40 years. Data obtained on September 8 allowed to establish an upper limit of 0.03 microbar for an atmosphere in local thermodynamical equilibrium near the surface. Click on the figure to enlarge. Data collected by Observatoire de Paris members, in Aruba (T. Widemann) and Ecuador (B. Sicardy, F. Colas, S. Pau) present the best signal-to-noise ratio, thanks to small angular distance of object to zenith, and to quality of instrumentation (CCD cameras of IOTA association, set up in fast acquisition mode by W. Beisker). Their teams’ mobility on the ground allowed for the choice of an optimal observing site only hours before the event. A first analysis of those results allows to : determine Titania’s O-C (observed position - calculated position) to a