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AS SEEN from Earth through a telescope, it’s a mere speck of light,
orbited by two even fainter specks. But now we know Neptune almost as well
as we understand the nearer planets – thanks to the doughty spacecraft Voyager
2. On the morning of 25 August, Voyager swept a mere 4900 kilometres from
the planet’s cloud tops, its closest encounter with a planet since leaving
the Earth in 1977.

Voyager’s last port of call was, ironically, highly reminiscent of its
home world – another Blue Planet. Neptune’s deep blue atmosphere is tinted
partly by scattering of light and partly because its methane gas absorbs
red light.

At about 20 Degree south on Neptune there is a large dark oval, almost
the size of planet Earth, and resembling the Great Red Spot on Jupiter.
Around the edge of Neptune’s ‘Great Dark Spot’, and stretching away like
bright banners, hang wispy white cirrus clouds of methane ice. Near the
south pole is a smaller dark spot, with a small patch of cirrus in its centre.
Between the two is a small irregularly shaped white patch that the Voyager
scientists call ‘the Scooter’ because it shoots around the planet much faster
than the other clouds.

All these cloud motions are relative, however, and meteorologists need
to know how fast the planet’s core rotates. At other planets, the Voyager
scientists have measured the rotation of the planet’s core by detecting
bursts of radio waves. These come from Neptune’s magnetic field, which is
generated well within the planet.

At Neptune, too, Voyager picked up regular radio bursts, which indicate
that the core turns once every 16 hours and 3 minutes. This means that almost
all the other features in the atmosphere are travelling extremely quickly,
with velocities up to 400 metres per second, in an easterly direction –
the opposite way to the planet’s rotation.

¿ìè¶ÌÊÓƵs had not expected so much weather and such complicated patterns,
because Neptune has comparatively little power to drive atmospheric circulation.
The total power that the planet receives from the Sun and from its interior
is only 5 per cent of that available to Jupiter. The atmospheric motions
on Neptune show that flows of gas can coast along at high speeds without
losing much energy through turbulence. According to Andy Ingersoll, the
chief meteorologist on the team, ‘one of the lessons from Voyager is that
it is possible to have quite complex patterns in frictionless flows’.

The cirrus clouds cast shadows on the lower layers of cloud and haze,
showing that they are extremely high up – between 50 and 100 kilometres
above the rest of the weather. Jim Pollack, an atmospheric scientist at
NASA, has proposed a ‘methane cycle’ on Neptune that can explain these high
cirrus clouds. Ultraviolet radiation from the Sun converts methane high
in the atmosphere into bigger hydrocarbons such as ethane and acetylene.
These drift downwards and condense into solid particles, which fall into
the warmer atmosphere below and undergo reactions that break them down to
methane again. The warm methane is buoyant, so it bubbles upwards. When
it reaches the cold region of the stratosphere, the methane condenses into
cirrus clouds again. The dark spots appear to channel methane up into the
stratosphere to continue the cycle.

Neptune’s magnetic field provided one of the biggest surprises of the
mission. It is weaker than those of the other giant planets, Jupiter, Saturn
and Uranus, and it is also weaker than the Earth’s field. More interesting
still, the magnetic field is tilted at 50 Degree to the axis of rotation
– as if the Earth’s north magnetic pole were not in Canada but in Los Angeles.
Uranus, too, has a highly inclined magnetic field. Astronomers had been
wondering if they found this high tilt because they witnessed Uranus’s magnetism
at a time when it was changing direction, just as rocks show that the Earth’s
magnetism has changed many times in the past. But the new observations of
Neptune suggest that a high tilt is normal for these worlds. What is even
stranger is that Neptune’s field does not originate at its centre, but comes
from a region four-fifths of the way out, towards the south magnetic pole.

As Voyager approached Neptune, it discovered six new moons, bringing
the total to eight. The largest of these, provisionally called 1989N1, is
200 kilometres across. This makes it Neptune’s second largest moon: the
smaller of Neptune’s two previously known moons, Nereid, has a diameter
of 170 kilometres. The Voyager team changed its original flight plan so
that it could photograph 1989N1 and the rather smaller 1989N₂.
These turn out to be dark worlds that are irregular in shape: 1989N1 looks
roughly triangular.

Another of Voyager’s tasks was to look for rings. Astronomers on Earth
had suspected Neptune could have partial rings, or ‘arcs’, that had hidden
the light from distant stars as Neptune passed in front. Voyager has now
found three narrow rings around Neptune, with a broad sheet of dust stretching
outwards from one of them, and another broad ring close to the planet. According
to the leader of Voyager’s imaging team, Brad Smith, material in these rings
can account for all the observations made from Earth.

The most interesting of the rings is the outermost. Although material
in this ring stretches all the way around Neptune, most of its matter is
concentrated in three bright regions, which correspond to the ‘arcs’ suggested
by astronomers on Earth. The Voyager camera picked out at least half a dozen
‘moonlets’ in one of these arcs. Smith estimates that these moonlets are
each between 10 and 20 kilometres across.

But the high point of Neptune encounter – and one of the most exciting
moments of the whole Voyager mission – came with the close-up pictures of
Neptune’s largest moon, Triton. It is 2720 kilometres in diameter – slightly
smaller than our Moon – but is extremely bright. Its southern hemisphere
is entirely covered by pink ‘snow’ – a frozen mixture of nitrogen and methane,
coloured by organic compounds that result from radiation striking the methane
(This Week, 2 September).

Voyager found that Triton has an atmosphere – making it one of only
three moons in the Solar System with its own ‘air’ (along with Jupiter’s
Io and Saturn’s Titan). The atmosphere stretches some 800 kilometres upwards
from the surface. It consists mainly of nitrogen, which is ten thousand
times as abundant as the other principal ingredient, methane. The pressure
at the surface is 10 microbars, about ten millionths of the Earth’s pressure
at sea level. Despite the low pressure, Triton’s atmosphere can support
some thin clouds and haze layers around the moon.

But what makes Triton remarkable is the texture of its surface. There
are long double ridges, and polygonal patterns of ridges and grooves. None
of these features is more than a few hundred metres high, suggesting to
Voyager scientists that Triton’s surface contains a lot of frozen nitrogen
and methane that make a very soft and slushy mixture.

Before the encounter, some scientists had predicted that Voyager would
find pools of liquid nitrogen on Triton. In fact, it turns out to be too
cold. At 37 K, it is the chilliest place we know in the Solar System. But,
instead, Voyager has found evidence for nitrogen-powered volcanoes.

There are dark streaks across the polar cap that are almost certainly
organic matter spread by winds. But the atmospheric pressure is too low
for winds to lift particles. So Larry Soderblom, the chief geologist on
the Voyager team, has proposed that the organic matter has been blown upwards
in eruptions from volcanoes. He suggests that pockets of liquid nitrogen
could form below the surface, and then erupt. Sue Kieffer, of the US Geological
Survey, calculates that such volcanoes erupt gases which rise to 40 kilometres
at 250 metres per second.

No one knows if the volcanoes are still active. But parts of Triton’s
surface are comparatively young. Astronomers date a planet’s surface by
counting the number of craters on it: the older the surface, the larger
the number of impacts it has suffered, and therefore the more craters there
are. Soderblom finds that even the oldest parts of Triton are younger than
the surface of our Moon – indicating that all of Triton melted after its
formation, erasing the traces of its beginnings.

The youngest regions of Triton are not more than 500 million years old.
The calculation in these areas involves so few craters that Soderblom says
‘it allows the possibility that Triton is active through modern times –
it’s not necessarily active now, but it could be again in the future’.

These observations support the idea that Triton was one of the many
bodies that originally made up Uranus and Neptune, but was caught in orbit
rather than ending up inside the outer planet. Astronomers have already
proposed that Pluto is such a body and Triton now turns out to be very similar
in size and density to Pluto.

After Triton, Voyager has no more worlds to visit. But NASA is considering
one final image: a view of the whole Solar System from the outside. This
would be a fitting farewell to its home for a spacecraft that will now end
its almost flawless mission to the planets with an endless trek through
interstellar space towards the stars.

Further reading: Planets Beyond: Discovering the Outer Solar System;
by Mark Littman, Wiley. See Review, this issue.