100-C 25-C 50-C 75-C C+M 50-C+M C+Y 50-C+Y M+Y 50-M+Y 100-M 25-M 50-M 75-M 100-Y 25-Y 50-Y 75-Y 100-K 25-K 25-19-19 50-K50-40-40 75-K 75-64-64CHAPTER 26
Triton
William B. McKinnon
Washington University
St. Louis, MissouriRandolph L. Kirk
U.S. Geological Survey
Flagstaff, Arizona- Introduction 4. Voyager 2 Encounter 7. Atmosphere and Surface
- Discovery and Orbit 5. General Characteristics 8. Origin and Evolution
- Pre-Voyager Astronomy 6. Geology Bibliography
1. Introduction
Triton is the major moon of the planet Neptune. It is also one
of the most remarkable bodies in the solar system (Fig. 1).
Its orbit is unusual, circular and close to Neptune, but highly
inclined to the planet’s equator (by 157◦). Furthermore,
Triton’s sense of motion is retrograde, meaning it moves
in the opposite direction to Neptune’s spin (Fig. 2). Triton’s
history therefore must have been quite different from those
of “regular” satellites, such as the moons of Jupiter, which
orbit in a prograde sense in their primary’s equatorial plane.
The modern consensus is that Triton originally formed in
solar orbit and was subsequently captured by Neptune’s
gravity.
Like nearly all solar system satellites, tides have slowed
Triton’s spin period to be coincident with its orbital pe-
riod and shifted its spin axis to be perpendicular to its or-
bital plane. Consequently, one hemisphere of Triton per-
manently faces Neptune. The combination of Neptune’s
axial tilt (29.6◦) and Triton’s inclined orbit gives Triton a
complicated and extreme seasonal cycle. In the distant ge-
ological past, tides associated with Triton’s capture may have
strongly heated and transformed its interior.
Although discovered soon after Neptune, little was
learned about Triton until the modern telescopic era, and
even so, most of the information we have was acquired
during theVoyager 2encounter with Neptune in 1989.
Triton is a relatively large moon (1352 km in radius), larger
than all of the middle-sized satellites of Saturn and Uranus
(200 to 800 km in radius), but not quite as large as the
biggest icy satellites—the Galilean satellites and Titan (1570
to 2630 km in radius). It is a relatively dense world (close
to2gcm–^3 ), rock-rich, but with a substantial proportion
of water and other ices. Ices comprise its reddish visible
surface (Fig. 1), and the freshness of the ices cause Triton
to be one of the most reflective bodies in the solar system
(its total, or Bond albedo, is≈0.85). This, combined with
the satellite’s distance from the Sun (30 AU), make Triton’s
surface a very cold place (≈ 38 ◦K). Yet, despite these frigid
surface conditions,Voyager 2discovered a thin atmosphere
of nitrogen surrounding the satellite (14μbar surface pres-
sure, where 1 bar is the approximate surface pressure of
Earth’s atmosphere). Triton’s atmosphere is dense enough
to support clouds and hazes and to transport particles across
Triton’s surface. It is also changing; since 1989 it has been
warming and increasing in total mass and pressure.
As with all solid planets and satellites, Triton’s history is
written into the geological record of its surface. Triton, how-
ever, is a geologically young body. Most of the approximately
40% of the satellite’s surface that was imaged byVoyager
at sufficient resolution tells us that it is sparsely cratered.
No heavily cratered terrains survive from early solar system
times, an absence that may reflect an epoch of severe tidal
heating. The geologic terrains that do survive are unique in