All About Space Astronomer Book - 2014 UK

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and 8 million miles from Saturn; it goes round the planet in 79 days and its

brightness is variable. When west of Saturn it is easily seen but when east

of Saturn it drops below 12th magnitude. Like almost all major satellites,

Iapetus has captured rotation, like our Moon, so the orbital and rotation

periods are equal, Iapetus keeping the same face toward Saturn all the time.

Part of the surface is bright as ice and another part is almost pure black. This

baffled astronomers, but now we realize that the clue to the black and white

surface lies much further out, with another satellite, Phoebe, discovered

by W.H. Pickering in 1898. It is responsible for a very tenuous ring quite

beyond the range of telescopes of amateur size, and was detected initially

by its infrared radiation. It appears that Phoebe itself moves round Saturn

in a retrograde or wrong-way direction like a car going the wrong way

round a roundabout. What happens is that particles are knocked off Phoebe

by meteoroids, and this dust spirals inwards, some of it landing upon the

leading hemisphere of Iapetus and darkening it. This is why Iapetus has its

unique yin-yang aspect.

But the mysteries do not end here. Running along its equator Iapetus has

a huge mountain ridge of unexplained origin. Iapetus is indeed an interesting

satellite, and we await new data from future probes.

Imaging Saturn’s Moons

High-frame-rate cameras on small instruments can also be used to image

the planet and its moons, again using the techniques outlined for Jupiter.

Interactions between Saturn’s brighter moons and the planet are less

common than in the Jovian system because Saturn’s rotational axis is tilted

by 26.7°. From Earth we only get to see these events when Saturn is close to

an equinox. This occurs approximately 14–15 years apart and at such times

the planet’s wonderful ring system appears edge-on to us.

Most of the larger moons are quite tricky to image as they cross Saturn’s

disc, and their shadows, although quite dark, are much less distinctive than

those cast by the Galilean satellites on to the cloud tops of Jupiter.

One notable exception is when Saturn’s largest moon Titan casts its

shadow on Saturn. The large dark spot that is formed when this happens is

very much noticeable.

With the naked eye, Saturn appears as a brightish, slightly yellow star.

Only when a telescope is used can its full beauty be appreciated. The planets

[26] The Rings of Uranus, seen by the Hubble Space Telescope (NASA). [27] Neptune seen through the Hubble Space Telescope showing its Great Dark Spot.

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