TABLE 2.1 Physical Properties of Pluto, Charon, and Triton

Object

Distance from Sun (AU)

Radius (km)

Mass (1021 kg)

Density (g cm-3)

Estimated Percent Rock (by mass)

Pluto

29.7–49.5

1168±33a

13.14±0.18a

1.79–2.17a, b, c

50–70d

Charon

29.7–49.5

626±24a

1.62±0.09a

1.33–1.87a, b, c

<60d

Triton

29.86–30.28

1352.6±2.4e

21.398±0.053e

2.053–2.076e

65–72e

SOURCES:

a G.W. Null and W.M. Owen, Jr., “Charon/Pluto Mars Ratio Obtained with HST CCD Observations in 1991 and 1993,” Astronomical Journal 111:1368, 1996.

b L.A. Young et al., “The Charon-Pluto Mars Ratio from MKO Astrometry,” Icarus 108:186, 1994.

c J.A. Foust et al., “Determination of the Charon-Pluto Mars Ratio from Center-of-Light Astrometry,” Icarus 126:362, 1997.

d W.B. McKinnon, D.P. Simonelli, and G. Schubert, “Composition, Internal Structure, and Thermal Evolution of Pluto and Charon,” Pluto and Charon, S.A. Stern and D.J. Tholen, eds., University of Arizona Press, Tucson, Arizona, 1997, p. 347.

e W.B. McKinnon, J.I. Lunine, and D. Banfield, “Origin and Evolution of Triton,” Neptune and Triton, D.P. Cruikshank, ed., University of Arizona Press, Tucson, Arizona, 1995, p. 807.

Triton's Surface.

About 30% of the surface of Triton was imaged by Voyager 2 at resolutions ranging from about a kilometer to tens of kilometers (see Plate 1). The geology of the imaged portion is among the most complex and varied of any of the solar system's icy satellites.4 There is no evidence for preserved ancient heavily cratered terrain, which implies that Triton was internally active for at least several hundred million years following its formation. The total crater population is low and indicates an average age for Triton's surface on the order of several hundred million years. This is a relatively short time and suggests that Triton has been internally active in the recent past and may still be active.

Most of the surface of Triton appears to be composed of materials that have erupted from the interior, and many of the landforms present are clearly of cryovolcanic (i.e., ice eruption) origin. This observation supports the interpretation of continued internal activity. A crude stratigraphy has been defined. The enigmatic “cantaloupe terrain,” a complex landscape of pits, ridges, and troughs, is the oldest part of the preserved surface. Superimposed on this terrain are smooth materials in the valley floors. The youngest materials, of presumed endogenic origin, make up walled plains. The bright surficial materials, generally considered to represent ephemeral frosts condensed from the atmosphere, are not thick enough to obscure the underlying geological material units. The dark materials are thought to be carbonaceous materials, possibly vented from the interior.

The degree of differentiation of the interior of Triton is a major issue. A generally accepted model for the history of Triton includes its capture from solar orbit by Neptune.5 The initial capture may have involved gas drag with the protoneptunian nebula (if captured early) or collision with one or more satellites (if captured after satellite formation).6 In either case, the subsequent circularization of Triton's orbit would cause significant tidal heating, melting, and differentiation. Alternatively, it is possible that Triton's youthful geology may have been powered by radiogenic heating alone if low-melting-point ices (e.g., ammonia) were sufficiently abundant to lower the viscosity of the upper layers and allow the crust to creep. A greater understanding of Triton's surface geology and interior structure could provide evidence of Triton's thermal history. It would be interesting to compare Triton with Pluto, which is not believed to have been heated tidally but has probably undergone a major collision with Charon.

Triton is one of three places (with Earth and Io) in the solar system where active eruptions have been seen. Voyager images revealed dark eruptive plumes—geysers—which were highly collimated and roughly 1 km in diameter, rising vertically from the surface.7 At an altitude of ~8 km, the ascending material was abruptly sheared



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