Some Historical Background and General Data

Discovered in 1655 by the Dutch astronomer Christiaan Huygens, the largest moon ofSaturn was much later named "Titan" by John Herschel in the nineteenth century. Observations of limb darkening around the disk of Titan by the Spanish astronomer José Comas-Sola in 1908 suggested the presence of an appreciable atmosphere around Titan. This hypothesis was taken seriously into consideration by the British astronomer James Jeans who calculated that a planetary body like Titan can keep a substantial part of its atmosphere (most ofthe constituents with a molecular mass exceeding about 16 Dalton) in spite ofescape processes due to its relatively small size (diameter 5150 km), ifits temperature is low enough (less than —100 K). The presence of an atmosphere around Titan was definitively confirmed in 1944, with the detection the absorption bands of gaseous methane (CH4) by the American astronomer Gerard Kuiper. He thus demonstrated that this atmosphere includes methane as one of its constituents and even succeeded in deriving from his observations that the partial pressure ofmethane at Titan's surface should be about 0.1 bar (100 hPa). The detection ofmethane in Titan's atmosphere was then confirmed by many observations from ground based telescopes and by the several space missions that flew by Saturn.

With a radius of 2575 km, Titan (Fig. 3.5) is the second largest satellite of the Solar System after Ganymede (R = 2631 km). Like Saturn, its period of rotation around the Sun is about 30 years and its mean distance to the Sun is about 9.5 astronomical units (AU) corresponding to a received solar flux of about 1% of the flux at the level of the Earth orbit. If considered as a black body, the mean temperature of Titan is about 82 K. Titan is in fact the only satellite of the solar system to have a noticeable atmosphere, as well as the only planetary body with an atmosphere close to that of the Earth. Indeed, Titan's atmosphere is mainly made of dinitrogen (N2) and its surface pressure is 1.5 bar (1500 hPa). As mentioned above, the atmosphere also includes a few percent of methane. The surface temperature is about 94 K, higher by about 12 K than the black body temperature because ofa noticeable greenhouse effect (20 K, mainly produced by atmospheric methane and dihydrogen), in spite of the anti-green house effect (—8 K) produced by the haze particles. In fact, owing to its low surface temperature Titan's atmosphere near the surface is about 5 times denser than the Earth's atmosphere.

Titan spins around Saturn with a period of 16 Earth-days, with synchronous rotation. Thus the Titan solid surface rotates slowly; however its atmosphere presents a super-rotation caused by strong zonal winds. With a distance from Saturn equal to about 20 Saturnian radii, Titan is far enough from the giant planet to avoid interactions with the rings but still close enough to allow interaction ofits atmosphere with the electrons ofthe magnetosphere ofSaturn which thus play a role in its chemical evolution.

Figure 3.5. Mosaic of Titan's surface from images taken by the narrow angle camera of the Cassini spacecraft. The large bright white area is named Xanadu. The resolution is about 1.3 km/pixel. Credit: NASA/JPL/Space Science Institute.
Telescopes Mastery

Telescopes Mastery

Through this ebook, you are going to learn what you will need to know all about the telescopes that can provide a fun and rewarding hobby for you and your family!

Get My Free Ebook

Post a comment