The Solar System

542 PART 4^ |^ THE SOLAR SYSTEM

as sunlight converted methane into organic compounds.

Triton’s south pole had been turned toward the sun for 30

years when Voyager 2 fl ew past, and deposits of nitrogen frost

in a large polar cap appeared to be vaporizing there and

refreezing in the darkness of the north pole (Figure 24-16a).

Th e cycle of nitrogen on Triton resembles in some ways the

cycle of carbon dioxide on Mars.

The Moons of Neptune

Before Voyager 2 visited Neptune, only two moons were known,
but Voyager 2 discovered six more small moons, and a few more
small moons have been found since, using Earth-based tele-
scopes. Neptune has at least 13 moons. Th e two largest moons,
Triton and Nereid, have been a puzzle for years because of their
peculiar orbits.
Triton has a nearly circular orbit, but it travels retrograde—
clockwise as seen from the north. Th is makes Triton the only large
satellite in the solar system with a backward orbit; all other retro-
grade satellites are very small. Th e other Neptunian satellite visible
from Earth, Nereid, moves in the prograde direction, but its orbit
is highly elliptical and very large (■ Figure 24-15). Nereid takes
359.4 days to orbit Neptune once. Many astronomers have specu-
lated that the orbits of the two moons are evidence of a violent
event long ago. An encounter with a massive planetesimal may have
disturbed the moons, or Triton itself may have been captured into
orbit during a close encounter with Neptune.
Th e six moons that Voyager 2 found orbit Neptune among
the rings. No new moons were found beyond the orbit of Triton,
and some astronomers have suggested that Triton, in its retro-
grade orbit, would have consumed any moons near it.
Th e Voyager 2 photographs show that Triton is highly com-
plex. Although it is only 2720 km in diameter (78 percent the
size of Earth’s moon), it is so cold (35 K, or −396°F) that it can
hold a thin atmosphere, 10^5 times less dense than Earth’s atmo-
sphere, composed of nitrogen and some methane. Although a
few wisps of haze can be seen in the photographs, the atmosphere
is transparent, and the surface is easily visible (■ Figure 24-16).
Th e surface of Triton is evidently composed of ices. Th e
surface ice is dominated by frozen nitrogen with some meth-
ane, carbon monoxide, and carbon dioxide. Th at is consistent
with the nitrogen-rich atmosphere. Some regions of the sur-
face that look dark may be slightly older terrain that darkened

■ Figure 24-15

Nereid has a large, eccentric orbit around Neptune, but Triton follows a
small, circular, retrograde orbit. At a distance of 30 AU from Earth, Triton is
never seen farther than 16 arc seconds from the center of Neptune.

Orbit of Triton

Neptune

Orbit of Nereid

■ Figure 24-16

(a) Triton’s south pole (bottom) had been in sunlight for 30 years when

Voyager 2 fl ew past in 1989. The frozen nitrogen in the polar cap appears

to be vaporizing, perhaps to refreeze in the darkness at the north pole.

The dark smudges are produced when liquid nitrogen in the crust vaporizes

and drives nitrogen geysers. (b) Roughly round basins on Triton may be old

impact basins fl ooded repeatedly by liquids from the interior. Notice the

small number of craters on Triton, a clue that it is a partially active world.

(NASA)

b

a

Visual-wavelength images