Philips Atlas of the Universe

Satellites of Saturn

S

aturn’s satellite family is quite different from that of

Jupiter. Jupiter has four large attendants and a dozen

small ones; Saturn has one really large satellite (Titan)

and seven which are medium-sized, together with the

remote Phoebe, which has retrograde motion and is almost

certainly a captured asteroid. Eight new satellites, all very

small, were found on the Voyager images.

Titan, with a diameter of over 5000 kilometres

(3200 miles), is the largest satellite in the Solar System

apart from Ganymede, and is actually larger than the plan-

et Mercury, though less massive. It is also unique among

satellites in having a dense atmosphere; all that Voyager

was able to see was the top part of a layer of orange

‘smog’. The atmosphere was found to be made up chiefly

of nitrogen, with a good deal of methane.

Of the icy satellites, Rhea and Iapetus are around

1500 kilometres (940 miles) in diameter, Dione and

Tethys around 1110 kilometres (690 miles), and

Enceladus, Hyperion and Mimas between 220 kilometres

and 320 kilometres (between 140 and 320 miles), though

Hyperion is decidedly irregular in shape (it has been

likened, rather unromantically, to a cosmic hamburger).

The globes appear to be made up of a mixture of rock and

ice, though Tethys in particular has a mean density only

just greater than that of water, so that rock may be a very

minor constituent. Dione is much denser and more mas-

sive, with craters and bright ice cliffs; it has been tectoni-

cally much more active than Tethys.

All the icy satellites have their own special points of

interest. They are not alike; for example Rhea and Mimas

are very heavily cratered, while Enceladus has a surface

which looks much younger, and Tethys shows a tremen-

dous trench which reaches three-quarters of the way round

the globe. Iapetus has one hemisphere which is bright and

one which is dark; evidently dark material has welled up

from below and covered the icy surface. This had been

suspected long before the Voyager missions, because

Iapetus is so variable in brightness. When west of the

planet, with its reflective area turned towards us, it is easy

to see with a small telescope; when east of Saturn, with its

blacker side displayed, owners of small telescopes will

find it very elusive.

The ‘new’ satellites found on the Voyager images are

also, presumably, icy and cratered. Pan actually moves

inside the Encke Division in Ring A, while Prometheus

and Pandora act as ‘shepherds’ to the F Ring. Epimetheus

and Janus seem to take part in what may be called a game

of musical chairs; their paths are almost the same, and

ATLAS OF THE UNIVERSE

SATELLITES OF SATURN

Name Distance from Orbital Orbital Orbital Diameter, Mean

Saturn, km period, days incl.,° ecc. km opp. mag.

Pan 133,600 0.57 0.0 0.0 19 19

Atlas 137,670 0.602 0.3 0.002 37 
 34 
 27 18.1

Prometheus 139,350 0.613 0.8 0.004 48 
 100 
 68 16.5

Pandora 141,700 0.629 0.1 0.004 110 
 88 
 62 16.3

Epimetheus 151,420 0.694 0.3 0.009 194 
 190 
 154 14.5

Janus 151,470 0.695 0.1 0.007 138 
 110 
 110 15.5

Mimas 185,540 0.942 1.52 0.020 194 
 190 
 154 12.9

Enceladus 238,040 1.370 0.07 0.004 421 
 395 
 395 11.8

Tethys 294,670 1.888 1.86 0.000 1046 10.3

Telesto 294,670 1.888 2 0 30 
 25 
 15 19.0

Calypso 294,670 1.888 2 0 30 
 16 
16 18.5

Dione 377,420 2.737 0.02 0.002 1120 10.4

Helene 377,420 2.737 0.2 0.005 35 18.5

Rhea 527,040 4.518 0.35 0.001 1528 9.7

Titan 1,221,860 15.495 0.33 0.029 5150 8.4

Hyperion 1,481,100 21.277 0.43 0.104 360 
 280 
 225 14.2

Iapetus 3,561,300 79.331 7.52 0.028 1436 10 (var.)

Phoebe 12,954,000 550.4 175 0.163 30 
 220 
 210 16.5

Twelve new small satellites were discovered in 2000 by B.J. Gladman and his team. Of these, two

move between the orbits of Iapetus and Phoebe, and the rest beyond Phoebe. All are below 50 km

in diameter, and five have retrograde motion, so that they are presumably asteroidal.

Hyperion

Rhea Dione

Iapetus

Phoebe

Planetary orbit Enceladus

Titan

Tethys

Mimas

Enceladus(Voyager 2).

Enceladus is quite different

from Mimas, with several

completely different types of

terrain. Craters exist in many

areas, but appear ‘sharp’ and

relatively young, while there

is also an extensive, almost

crater-free plain. In February

and March 2005 the Cassini

probe flew past Enceladus

and detected a very thin

water-vapour atmosphere.

As Enceladus could not retain

an atmosphere for long,

there must be a continual

source of replenishment by

gases escaping from the

satellite’s interior.

Satellite orbits of Saturn’s

nine larger satellites. In

addition there are at least

eight smaller moons, some

in very unusual orbits.

For instance, two tiny

satellites, Telesto and

 Mimas(Cassini). The

surface is dominated by

one very large crater, now

named Herschel, which

has a diameter of 130 km

(80 miles) – one-third that

of Mimas itself – with walls

which rise to 5 km (3 miles)

above the floor, the lowest

part of which is 10 km

(6 miles) deep and includes

a massive central mountain.

Calypso, move at the same

distance as Tethys, one

about 60° ahead of, and the

other 60° behind Tethys in

its orbit, in the same way

that the Trojan asteroids

travel in Jupiter’s orbit.

108-151 Atl of Univ Phil'05 7/6/05 12:20 pm Page 116