Mike Kretlow

Mainz-Kastel, Germany

On November 14th two bright stars will be occulted by Titan, the largest moon of Saturn and the second largest moon in the solar system. Titan (equatorial radius 2575 km) is even larger than Mercury and Pluto. It has a planet-like atmosphere denser than those of the Earth (about 1.5 bar surface pressure). The atmosphere is mainly made up of molecular nitrogen, a few percent methane (which dominates the upper atmosphere), other organic compounds and water. These hydrocarbon rich elements are the primary structures for amino acids necessary for the formation of life. Titan's environment may be similar to that of the Earth's before life began releasing oxygen into the atmosphere. Because of the low surface temperature (about -180°C) it is still an open question whether liquids exists on the surface (probably ethane or an ethane/methane mixture).

In June 2004 the Cassini spacecraft will arrive at Saturn and later the European Huygens probe will enter Titan’s atmosphere. Thus, these important occultation events will be the last chance to make measurements on Titan’s atmosphere before the Huygens probe will touch down. It might help to refine and optimize the Huygens experiments.

In 1989 the occultation of 28 Sgr by Titan was successfully observed in Europe, Israel and Uzbekistan. These observations (including a couple of stations in the central flash region) provided us with important information about the structure and dynamics of the upper atmosphere (Sicardy et al., 1999).

The table below gives the basic observational data on the upcoming occultations by Titan. Refinements of these predictions are expected and will be published on mailing lists and dedicated websites like www.iota-es.de and http://despa.obspm.fr/~sicardy/titan/titan.html where you also find maps of the visibility areas etc.

Date and time[UT]   Star                    Magnitude          Area of visibility                           Central flash area
14th Nov 2003 @
0h 11min UT           TYC 1343-1615-1 B=9.9   V=8.6    Southern Africa,Indian Ocean        Southern Namibia,
                                                                                                                                  Northern South Africa,
                                                                                                                                  Central Madagascar
14th Nov 2003 @
6h 57min UT           TYC 1343-1865-1 B=11.9  V=10.7 Western Europe, North and           Southernmost Spain,
                                                                                Central America                           Morocco, Florida, Texas, Mexico

Titan has V=8.4mag. For the second occultation only infrared observations will be possible in Western Europe because of the bright twilight.
 

Later in November, on the 29th, Neptune’s largest moon Triton (radius about 1350 km) will occult the star GSC 6348-476  =  USNO-A2.0 0675-35937787 (B=13.5, R=12.7). Triton has an extremely thin atmosphere. Nitrogen ice particles might form thin clouds a few kilometres above the surface. The atmospheric pressure at Triton's surface is about 15 microbars. According to current predictions this event will be visible in the region of the Pacific Ocean (Northern Australia, New Guinea, Indonesia, Hawaii, and Western USA at low altitude). The visibility area, the fainter target star and the 34% moon in only 8.5 deg distance will make this occultation much harder to observe.
References
Sicardy et al.: The Structure of Titan’s Stratosphere from the 28 Sgr Occultation, Icarus 142, 357-390 (1999).
 
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