The planets
artefacts during the processing of high-frame-rate captures. Here, the
seeing plays an important role too, certain types of fast-moving seeing
appearing to "drag" some of these features to the side, making them look
indistinct and blurred. The "gap" between the two brightest rings, A and B,
known as the Cassini division, is especially prone to this effect.
If the seeing is very good, one difficult-to-image target, highly sought
after by Saturnian imagers, is the 200-mile-wide Encke Division or Gap.
This lies close to the outer edge of the A ring and requires a large scope
and good, still atmospheric conditions to capture well. As was the case
with Jupiter, low-power afocal shots can produce results, although the
lower light levels mean you’ll need to use longer exposures or higher
ISO settings.
Saturn's Moons
Next let us turn to Saturn’s family of satellites, which is of special interest.
At the end of the 20th century, nine Saturnian satellites were known. Many
have been added since though most are small – probably captured asteroids.
Of the classic satellites the largest by far is Titan, discovered in 1665 by the
Dutch astronomer Christiaan Huygens. Titan is of planetary size and is larger
than Mercury though less massive; of all the satellites only Ganymede in
Jupiter’s family is larger. However, Titan is unique in possessing a dense
atmosphere made up chiefly of nitrogen.
There is a sort of curious fact here. Retention of an atmosphere depends
upon two things: first of all the escape velocity of the central body and
secondly the temperature. Our Moon has an escape velocity of 1.5 miles
per second, and is unable to retain a substantial atmosphere. On Titan, the
escape velocity is much the same but the temperature is much lower, being
so much further from the Sun. The higher the temperature, the quicker
the various atoms and molecules move around, so that those in Titan’s
atmosphere are more leisurely than those of the Moon. This means Titan
has no problem in hanging on to a thick atmosphere. None of the four large
Jovian satellites has had equal success.
We know a great deal about Titan thanks to Cassini, which carried a much
smaller probe, Huygens, which landed gently on the surface of the satellite.
This is one of the most amazing feats of space research so far: bringing a
probe down gently on the surface of a satellite nearly 900 million miles away.
After landing, Huygens was able to go on transmitting data for some hours
and the results were fascinating. The landing site was “mushy”, but there
were lakes and seas within range, not of water but of methane. Titan is the
only world apart from Earth where we have definitely detected liquid oceans.
However, going for a bath in them is not to be recommended because they
are, to put it mildly, distinctly chilly!
23 2425The rest of Titan has been well mapped by Cassini and been found to
be a strange place. The methane clouds in the sky never clear, so there is
probably a constant methane drizzle, making Titan a rather gloomy world.
There is no such thing as a sunny day there.
Fascinating though Titan is, it is surpassed by one of the much smaller
satellites, Enceladus.
The two classic satellites closer to Saturn, Mimas and Enceladus, were
discovered in 1789 by William Herschel with the aid of his new large
telescope. Both are less than 400 miles across. Mimas is icy and inert, but
it's Enceladus that is the real problem. Close-range views have shown
there is a very tenuous atmosphere, and that vents near the poles are
streaming out water. In fact it has water geysers. This means there must
be an underground sea to provide the material. Enceladus, less than 400
miles across, is in theory far too small to show any trace of activity, let alone
constant fountains.No one has yet come up with a satisfactory answer. The
fountains ought not to be there, but they are. It is doubtful if we will learn
much more before we have new probes, possibly manned. A very long-term
project indeed.
Beyond Enceladus there are three more icy satellites – Rhea, Dione and
Tethys, all discovered by Giovanni Cassini. Beyond the orbit of Titan is a small
irregular moon, Hyperion, a severe test for owners of small or moderate
telescopes. But then another surprise, Iapetus, a thousand miles across