Neptune
on Triton's horizon (Click on the image for a larger
view) |
The
largest of Neptune's eight known satellites, Triton is different
from all other icy satellites Voyager has studied. About
three-quarters the size of Earth's Moon, Triton circles
Neptune in a tilted, circular, retrograde orbit (opposite
to the direction of the planet's rotation), completing an
orbit in 5.875 days at an average distance of 330,000 kilometers
(205,000 miles) above Neptune's cloud tops. Triton shows
evidence of a remarkable geologic history, and Voyager 2
images show active geyser-like eruptions spewing invisible
nitrogen gas and dark dust particles several kilometers
into space.
Triton
has a diameter of about 2,705 kilometers (1,680 miles) and
a mean density of about 2.066 grams per cubic centimeter
(the density of water is 1.0 gram per cubic centimeter).
This means Triton contains more rock in its interior than
the icy satellites of Saturn and Uranus do.
The
relatively high density and the retrograde orbit offer strong
evidence that Triton did not originate near Neptune, but
is a captured object. If that is the case, tidal heating
could have melted Triton in its originally eccentric orbit,
and the satellite might even have been liquid for as long
as one billion years after its capture by Neptune.
While
scientists are unsure of the details of Triton's history,
icy volcanism is undoubtedly an important ingredient.
To
understand what is happening on Triton, one must ask, "How
cold is cold? How soft is soft? How young is young?"
Water ice, whose melting point is 0 degrees Celsius (32
degrees Fahrenheit), deforms more easily and rapidly on
Earth than rock does, but becomes almost as rigid as rock
at the extremely low temperatures found on Triton, more
than 4.5 billion kilometers from the Sun. Most of the geologic
structures on Triton's surface are likely formed of water
ice, because nitrogen and methane ice are too soft to support
much of their own weight.
On
the other hand, nitrogen and methane, which form a thin
veneer on Triton, turn from ice to gas at less than 100
degrees above absolute zero. Most of the geologically recent
eruptions at those low cryogenic temperatures are due to
the nitrogen and methane on Triton.
Evidence
that such eruptions occur was found in Voyager images of
several geyser-like volcanic vents that were apparently
spewing nitrogen gas laced with extremely fine, dark particles.
The particles are carried to altitudes of 2 to 8 kilometers
(1 to 5 miles) and then blown downwind before being deposited
on Triton's surface.
An
extremely thin atmosphere extends as much as 800 kilometers
(500 miles) above Triton's surface. Tiny nitrogen ice particles
may form thin clouds a few kilometers above the surface.
Triton is very bright, reflecting 60 to 95 percent of the
sunlight that strikes it (by comparison, Earth's Moon reflects
11 percent).
The
atmospheric pressure at Triton's surface is about 14 microbars,
a mere 1/70,000th the surface pressure on Earth. Temperature
at the surface is about 38 kelvins (-391 degrees F), the
coldest surface of any body yet visited in the solar system.
At 800 kilometers (500 miles) above the surface, the temperature
is 95 kelvins (-290 degrees F).
Despite
remarkable differences between Triton and the other icy
satellites in the solar system, photographs reveal terrain
that is reminiscent of Ariel (a satellite of Uranus), Enceladus
(a satellite of Saturn), and Europa, Ganymede and Io (satellites
of Jupiter). Even a few reminders of Mars, such as polar
caps and wind streaks, can be seen on Triton's surface.
Triton
appears to have the same general size, density, temperature
and chemical composition as Pluto (the only outer planet
not yet visited by any spacecraft), and will probably be our
best model of Pluto for a long time to come.
