484 Encyclopedia of the Solar System
FIGURE 1 Digital photomosaic of Triton, centered on the
Neptune-facing hemisphere at 15◦N, 15◦E. The latitude of the
subsolar point at the time of theVoyagerencounter was –45◦,so
the north polar region was in darkness. Triton’s surface is covered
with deposits of solid nitrogen with small admixtures of radiation
reddened and darkened methane; the bluish tinge is
characteristic of fresh frosts. Because Triton’s spin is tidally
locked to Neptune, the eastern hemisphere is also the leading
hemisphere in its orbit. (Courtesy of the NASA Planetary Data
System Photojournal.)
FIGURE 2 Orbits of Neptune’s family of satellites, except
distant irregulars. Shown is a perspective view along a line of
sight inclined 18◦to Neptune’s equatorial plane. The innermost
satellites are all relatively small and were not discovered until
Voyager 2passed through the Neptune system. They orbit in
Neptune’s equatorial plane, while much more massive Triton
circles outside them in an inclined, retrograde orbit. All the
satellites have virtually circular orbits except for Nereid. The
apparent crossing of Nereid’s and Triton’s orbits is an artifact of
the projection. [From J.S. Kargel (1997).In“Encyclopedia of
Planetary Sciences” ( J.H. Shirley and R.W. Fairbridge, eds.).
Chapman & Hall, London.]
the Solar System. At least three major terrain types can be
distinguished: smooth, walled, and terraced plains; an enig-
matic “cantaloupe” terrain; and a hemispheric-scale polar
deposit or cap. The polar cap is thought to be predomi-
nantly solid nitrogen. Other ices that have been identified
on Triton are, in approximate order of abundance, H 2 O,
CO 2 , CO, CH 4 , and C 2 H 6. The cap is a site of present-day
geological activity, in particular the eruption of plumes or
geysers of gas and fine particles.
In the following sections, Triton will be described in
greater detail with emphasis on its geology, the interaction
of its icy surface and atmosphere (including the plumes),
and its probable origin and violent early evolution.2. Discovery and Orbit
Acting on the mathematical prediction of Urbain Le Verrier,
the planet Neptune was first identified at the Berlin Obser-
vatory on September 23, 1846. It was announced in England
on October 1. On that day, Sir John Herschel, son of the
discoverer of Uranus, wrote to William Lassell, asking him
to look for any satellites of the new planet “with all pos-
sible expedition,” using his own 24-inch reflector. Lassell
was a brewer by profession, but he made his own tele-
scopes and was a keen visual observer. Herschel was no
doubt seeking to ease some of the sting of Neptune’s be-
ing found by continental astronomers, given that he was
aware of the independent prediction of Neptune’s position
by John Couch Adams and the unsuccessful search for the
planet from English soil. Lassell wasted no time, making his
first observations on October 2, and on October 10, 1846,
he discovered Triton.
By 1930 it was established that Triton was a most un-
usual moon. Orbiting at 14.3 Neptune radii, or RN(using
the modern value of 24,760 km for Neptune’s equatorial ra-
dius), the orbit was circular inasmuch as this could be mea-
sured, but distinctly retrograde compared with Neptune’s
prograde spin or sense of orbital motion. It was alsoalone.
No new satellites would be found for over 100 years. The
early contrast with the regular satellite systems of Jupiter,
Saturn, and Uranus could hardly have been greater. [See
Neptune.]
The year 1930 also marked the discovery of the dwarf
planet Pluto. It was soon determined that Pluto actually
crosses inside the orbit of Neptune for about 20 years of
its 248 year orbital revolution. Although Pluto’s orbit is also
substantially inclined so that it does not actually intersect
Neptune’s, British astronomer R.A. Lyttleton argued that
differential precession of the orbits could cause them to
intersect, either in the future or in the past. In 1936 he
published a paper that theoretically explored the possibility
that such an orbital configuration once did exist, and that
Pluto was in reality an escaped satellite of Neptune. Al-
though intriguing, planetary scientists now reject this early