THE SOLAR SYSTEM
THE RINGS OF NEPTUNEName Distance from centre Width, km
of Neptune, kmGalle 41,900 50
Le Verrier 53,200 50
‘Plateau’ 53,200–59,100 4000
— 62,000 30
Adams 62,900 50 Neptune’s two main
rings, about 53,000 km
(33,000 miles) and 63,000 km
(39,000 miles) from the
centre of the planet, were
backlit by the Sun as
Voyager 2 swept past.
Neptune’s rings appear
bright as microscopic ring
particles scatter sunlight
towards the camera.
Particle-size distribution
in Neptune’s rings is
quite different from that
in Uranus’ rings. Views of Neptune’s
weather,on opposite
hemispheres. Taken
on 11 August 1998 with
Hubble’s Wide Field
Planetary Camera 2, these
composite images show
Neptune’s blustery weather.
The predominant blue colour
is a result of the absorption
of red and infra-red light
by its methane atmosphere.
Clouds elevated above most
of the methane absorption
appear white, while the
very highest clouds tend
to be yellow-red. Neptune’s
powerful equatorial jet –
where winds blow at nearly
1500 km/h (900 mph) – is
centred on the dark blue
belt just south of Neptune’s
equator. Further south, the
green belt indicates a region
where the atmosphere
absorbs blue light.quite likely that the core is not sharply differentiated from
the ice components.
Almost all our detailed knowledge of Neptune
has been provided by one spacecraft, Voyager 2, which
flew past the planet on 25 August 1989 – at 4425 million
kilometres (2750 million miles) from the Earth. Voyager
passed over the darkened north pole at a relative
velocity of just over 17 kilometres (10 miles) per second;
at that time the southern hemisphere was having its
long ‘summer’.
Well before Voyager closed in, the images showed
that Neptune is a far more dynamic world than Uranus.
The most conspicuous feature on the blue surface was a
huge oval, the Great Dark Spot, at latitude 8 degrees 28
minutes south; it had a rotation period of over 18 hours, so
that it drifted westwards relative to the nearby clouds at
30 metres (100 feet) per second. It rotated in an anti-
clockwise direction, and showed more or less predictable
changes in shape and orientation. Above it lay wispy
clouds made of methane crystals (‘methane cirrus’) and
between these and the main cloud deck there was a
50-kilometre (31-mile) clear zone. Further south (latitude
42 degrees S) was a smaller, very variable feature with a
bright centre, which had a shorter rotation period and was
nicknamed the ‘Scooter’; still further south (latitude 55
degrees S) was a second dark spot.
Neptune is a windy place. At the equator the winds
blow westwards (retrograde) at up to 450 metres (1500
feet) per second; further south the winds slacken, and
beyond latitude 50 degrees they become eastwards, reach-
ing 300 metres (1000 feet) per second but decreasing
once more near the south pole. Temperature measurements
show that there are cold mid-latitude regions with a
warmer equator and pole.
The upper atmosphere is made up chiefly of hydrogen
(85 per cent), with a considerable amount of helium and a
little methane. There are various cloud layers, above
which lies the general methane haze.
Neptune is a source of radio waves, which was only to
be expected, but the magnetic field proved to be very sur-
prising. The magnetic axis makes an angle of 47 degrees
with the axis of rotation, so that in this respect Neptune
resembles Uranus more than Jupiter or Saturn; here also
the magnetic axis does not pass through the centre of the
globe, but is displaced by 10,000 kilometres (6200 miles).
The magnetic field itself is weaker than those of the
other giants. Aurorae were confirmed, though they are
of course brightest near the magnetic poles.
Voyager confirmed that Neptune has a ring system,
though it is much less evident than those of the othergiants. Altogether there seem to be five separate rings,
plus the so-called ‘plateau’, a diffuse band of material
made up of very small particles. There may also be ‘dust’
extending down almost to the cloud-tops.
The rings have been named in honour of astronomers
who were involved in Neptune’s discovery. The Adams
ring is the most pronounced and is ‘clumpy’ with three
brighter arcs which may be due to the gravitational pull
of Galatea, one of the newly discovered small satellites.
The ring at 62,000 kilometres (38,750 miles) is close to
Galatea’s orbit. The rings are dark and ghostly, and the
fainter sections were only just above the threshold of
visibility from Voyager.
Details on Neptune are now within the range of the
Hubble Space Telescope – and, to general surprise, the
images taken in August 1996 (and subsequently) showed
no trace of the Great Dark Spot. Smaller features were
seen, and there seems no escape from the conclusion that
the Great Dark Spot has disappeared. Other spots have
been recorded, and it is becoming clear that Neptune is
much more variable than had been expected.D108- 151 UNIVERSE UK 2003CB 7/4/03 5:16 pm Page 131