Saturn's moon becomes a refractive lens

When a planet with an atmosphere passes in front of a star (an event known as an occultation), earthbound scientists can glean information about the planet's atmosphere by examining how the starlight dims and winks out, then reappears.

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When a planet with an atmosphere passes in front of a star (an event known as an occultation), earthbound scientists can glean information about the planet's atmosphere by examining how the starlight dims and winks out, then reappears. On Nov. 14, 2003, Titan, the only moon in the solar system with a substantial atmosphere, occulted two stars. The path of Titan's shadow from one of the stars, TYC-1343-1615-1, was predicted to move across southern Africa (see Fig. 1), while the shadow from the second, TYC-1343-1865-1, was calculated to move across Southwest Europe and the Caribbean. Astronomers, both at permanent observatories and small portable telescopes, were at the ready.

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FIGURE 1. The path of Titan's occultation of star TYC-1343-1615-1 was predicted to fall across southern Africa, with the central flash observable in a narrow swath within the shadow. The actual path may have differed slightly from what was predicted.
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Not only were they prepared to monitor the quenching and rekindling of starlight, they were also hoping to observe an even more interesting phenomenon: the appearance of a brief flash of light just as Titan's center coincided with the star's position. Caused by refraction by a narrow band of Titan's atmosphere, the flash would provide further atmospheric information not revealed by the mere passage of Titan's limb in front of the star.

The telescopes ranged from 1.4 m down to 20 cm in size, with many of them portable, according to Bruno Sicardy of the Paris Observatory (Meudon, France), who organized the observation of the occultation of TYC-1343-1615-1. The area on Earth in which the flash was predicted to be observable was a small swath down the center of the path of Titan's shadow. Small telescopes were deployed at La Reunion Island (Indian Ocean), South Africa, and Namibia.

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FIGURE 2. The occultation of TYC-1343-1615-1 by Titan produced a central flash caused by refractive focusing by a 250-km-high layer of Titan's atmosphere.
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The central flash was indeed spotted, and at more than one wavelength.1 At the Sutherland site of the South African Astronomical Observatory (SAAO), Ian Glass of the SAAO and Francesca Ferri of the University of Padova (Padova, Italy) observed the spike at 0.9 µm, as well as attenuation by Titan's limb, using a fast photometer attached to the facility's 1-m telescope and a 0.1-s sampling time (see Fig. 2). The central flash will provide information on atmospheric winds at approximately 250 km above Titan's surface, while the limb attenuation probes altitude to 500 km at a 1-km spatial resolution. Although the data is still being studied, preliminary results reveal a strong inversion layer near 500 km in altitude, where the temperature increases by about 20 K in less than 10 km, according to Sicardy.

The 1.4-m ƒ/10 altazimuth Cassegrain telescope of the Infrared Survey Facility, also at the SAAO's Sutherland site and operated by astronomers from Nagoya University (Nagoya, Japan), was used to detect the flash at 2.2 µm. A comparison of data at 0.9 and 2.2 µm will allow differential extinction of Titan's atmospheric aerosols to be derived and constraints on aerosol size to be deduced, says Sicardy.

REFERENCE:

  1. http://saturn.jpl.nasa.gov/news/features/titanOccult/index.cfm.

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