Satellites of Neptune
T
wo satellites of Neptune were known before the
Voyager fly-by: Triton and Nereid. Each was excep-
tional in its own way. Triton, discovered by Lassell a few
weeks after Neptune itself had been found, is large by
satellite standards but has retrograde motion; that is to
say, it moves round Neptune in a sense opposite to that
in which Neptune rotates. This makes it unique among
major satellites, since all other attendants with retrograde
motion (the four outermost members of Jupiter’s system,ATLAS OF THE UNIVERSE
and Phoebe in Saturn’s) are asteroidal. Nereid is only
240 kilometres (140 miles) across; and though it moves in
the direct sense its eccentric orbit is more like that of a
comet than a satellite; the distance from Neptune varies
by over 8 million kilometres (5 million miles), and the
revolution period is only one week short of an Earth year,
so that obviously the axial rotation is not synchronous.
Voyager discovered six new inner satellites, one of
which (Proteus) is actually larger than Nereid, but is vir-
tually unobservable from Earth because of its closeness to
Neptune. Proteus and one of the other new discoveries,
Larissa, were imaged from Voyager, and both turned out
to be dark and cratered; Proteus shows a major depression,
Pharos, in its southern hemisphere, with a rugged floor.
No doubt the other inner satellites are of the same type.
Nereid was in the wrong part of its orbit during the
Voyager pass, and only one very poor image was
obtained, but Triton more than made up for this omission.
Three new satellites – S/2002 N1, N2 and N3 – were
discovered in 2002 by a team of astronomers led by
M. Holman and J. J. Kavelaars, using the 4-metre Blanco
Telescope at Cerro Tololo and the 6-metre CFH Telescope
in Hawaii. They were missed by Voyager 2 because they
are very faint (mag. 25) and very distant from Neptune,
with highly inclined orbits. S/2002 N1 is in a retrograde
orbit; the other two are prograde. Their diameters are
probably between 30 and 40 km (19 and 25 miles).
Estimates of the diameter of Triton had been discor-
dant, and at one time it was even thought to be larger than
Mercury, with an atmosphere dense enough to support
clouds similar to those of Titan. Voyager proved other-
wise. Triton is smaller than the Moon, and is well over
twice as dense as water, so that its globe is made up of
more rock than ice. The surface temperature is around
236°C, so that Triton is the chilliest world so far
encountered by a spacecraft.
The escape velocity is 1.4 kilometres (0.9 miles) per
second, and this is enough for Triton to retain a very
tenuous atmosphere, made up chiefly of nitrogen with an
appreciable amount of methane. There is considerable
haze, seen by Voyager above the limb and which extends
to at least 6 kilometres (3.7 miles) above the surface; it is
probably composed of tiny particles of methane or nitro-
gen ice. Winds in the atmosphere average about 5 metres
(16 feet) per second in a westwards direction.
The surface of Triton is very varied, but there is a
general coating of water ice, overlaid by nitrogen and
methane ices. There are very few craters, but many flows
which are probably due to ammonia-water fluids; surface
relief is very muted, and certainly there are no mountains.
The most striking feature is the southern polar cap, which
is pink and makes Triton look quite different from any
other planet or satellite. The pink colour must be due to
nitrogen ice and snow. The long Tritonian season means
that the south pole has been in constant sunlight for over
a century now, and along the borders of the cap there
are signs of evaporation. North of the cap there is an
‘edge’ which looks darker and redder, perhaps because of
the action of ultra-violet light upon methane, and running
across this region is a slightly bluish layer, caused by the
scattering of incoming light by tiny crystals of methane.
The surface imaged from Voyager 2 is divided into
three main regions: Uhlanga Regio (polar), Monad Regio
(eastern equatorial) and Bubembe Regio (western equat-
orial). It is in Uhlanga that we find the remarkable nitro-
gen geysers. According to the most plausible explanation,
there is a layer of liquid nitrogen 20 or 30 metres (65 to
100 feet) below the surface. If for any reason this liquid Neptune’s small moon
Proteus was discovered in
June 1989, early enough for
Voyager personnel to target
it. The image was taken on 25
August 1989 from a distance
of 146,000 km (90,500 miles).
It has an average diameter ofover 400 km (250 miles). It is
dark (reflecting only 6 per
cent of light it receives) and
spectrally grey. Craters and
grooves can be seen, and
one major surface feature – a
depression in the southern
hemisphere, named Pharos. Tritonon 25 August- This view is about
500 kilometres (300 miles)
across. It encompasses two
depressions that have been
extensively modified by
flooding, melting, faulting,
and collapsing.
▲A detailed view of Triton
taken by Voyager 2 on 25
August 1989 from 40,000 km
(25,000 miles). The frame is
about 220 km (140 miles)
across and shows details as
small as 750 m (0.5 miles).
Most of the area is covered
by roughly circular
depressions separated by
rugged ridges. This terrain
covers Triton’s northern
hemisphere, and is unlike
anything seen elsewhere in
the Solar System.D108- 151 UNIVERSE UK 2003CB 7/4/03 5:16 pm Page 132