In addition to the ring particles, there are ten SATELLITES of SATURN . The diameters of the satellites are difficult to measure from the Earth. Turbulence in the Earth’s atmosphere blurs the image by at least 0.1 to 0.2 arc sec. This causes an appreciable error for an object whose apparent size may be only 0.3 second of arc, as is the case for Rhea. An opportunity to obtain more accurate values occurred on 29 March 1974 when the dark limb of the Moon occulted Saturn and its satellites. This was the first time that such an occultation of Saturn was observed by photoelectric devices, which enabled the time for each satellite to pass behind the lunar limb to be measured accurately. Observations were made of lapetus, Rhea, Tethys, Dione and Titan and the derived values of the radii .In the case of Titan the strong signal showed that this satellite is definitely highly limb-darkened and has a radius of 2900km. This value is substantially larger than the previously adopted value of 2500km. Titan is now known to be the largest satellite in the Solar System although its mass is less than that of Ganymede.

Masses of most of the satellites have been obtained by observing their mutual perturbations. Most of these are uncertain and consequently even moderately reliable densities are only available for three satellites: Tethys, Dione and Titan. The ten satellites are rather substantial bodies for the most part. All of them are larger than the outer irregular satellites of Jupiter and at least five are bigger than the largest asteroid. Titan can be seen in a 5-cm telescope and Rhea, Dione and Tethys are visible in an 8-cm instrument.

TITAN is certainly the most interesting of Saturn’s satellites as it is the largest and has an extensive atmosphere. Methane (CH4) was detected there by Gerard Kuiper in 1944 and is the only constituent of the atmosphere to have been definitely identified. In 1972 L. Trafton claimed to have detected molecular hydrogen, and there are unidentified features in the spectrum which suggest the existence of at least one other gas on Titan. Ethane (C2H6) seems to be the most likely but ethylene (C2H4) or an isotope of methane have also been suggested. The interpretation of the spectrum is further complicated by the probable existence of an opaque cloud deck in the satellite’s atmosphere. Titan’s albedo decreases rapidly through the blue, violet and near ultraviolet, behavior that is entirely opposite to what happens with a gas, and virtually the only explanation is the presence of an aerosol.

Before 1974 the mean density of Titan was believed to be 2100kgm-3. When the radius was remeasured and found to be larger than previously thought, the density dropped to 1400 kg m-3. The radius obtained from the occultation data refers strictly to the cloud tops and not to the real surface. The height of the clouds is not known but D.Hunten has estimated about 150km. This value decreases the radius to 2750km and increases the density to 1600 kg m-3. Whichever value is correct, Titan has a very low density and, like Ganymede and Callisto must be composed largely of low-density material such as solid ammonia, water and methane.

Another interesting satellite is IAPETUS, which is much brighter when to the west of Saturn than when it is to the east. This variation in brightness, of nearly two magnitudes, was immediately noticed by J.D.Cassini when he discovered lapetus, and suggested, ‘but it seems, that one part of his surface is not so capable of reflecting to us the light of the Sun which maketh it visible, as the other part is.’ This has indeed proved to be the case, because lapetus keeps one face turned towards Saturn as it revolves. It has a bright hemisphere, which reflects about 50 per cent of the incident sun¬light and a dark hemisphere which reflects only 10 per cent. The best explanation of the cause is that lapetus had a thin ice crust which has been eroded from only one hemisphere by bombarding meteoroids.

The inner eight satellites move in regular orbits almost exactly in the plane of Saturn’s equator. They would seem to be true satellites formed together with Saturn, although the orbits of Titan and Hyperion are somewhat eccentric. Further out is lapetus with an. orbit inclined by 15° to Saturn’s equator. Astronomers are un¬certain whether lapetus is a true satellite or a captured asteroid. The most distant satellite, Phoebe, is almost four times further from Saturn than is lapetus. It moves in a noticeably elliptical and inclined orbit in a retrograde sense and is most probably a captured asteroid.

Comments are closed.