Encyclopedia of the Solar System 2nd ed

488 Encyclopedia of the Solar System

FIGURE 6 Triton approach

sequence, overlaid with a

latitude-longitude grid. Details on

Triton’s surface unfold dramatically

as the resolution changes from

about 60 km/pixel at a distance of 5

million km for the image in the

upper left to about 5 km/pixel at a

distance of 0.5 million km for the

image in the lower right. Mainly

looking at the southern

hemisphere, Triton rotates

retrograde (counterclockwise) over

an observational period of 4.3 days.

(Courtesy of Alfred McEwen,

University of Arizona.)

indicates a prolonged history of melting and separation of
icy phases. In the model, ice, structurally represented by the
most abundant solar system ice (H 2 O) forms a deep mantle
around a rock+metal core. A metallic (Fe, Ni, and proba-
bly S) inner core is also shown. The proportions of rock and
metal in the core are fixed to solar composition (carbona-
ceous chondrite) values, because relatively involatile rock
and metal should have been completely condensed in the

TABLE 1 Properties of Triton

Radius,R 1352.5 km

Mass,M 2.140× 1022 kg

Surface gravity,g 0.78 m sec–^2

Mean density,ρ 2065 kg m–^3

Percent rock+metal by

mass

65–70%

Distance from Neptune 354.8× 103 km=14.33 RN

Distance from Sun 30.058 AU

Orbit Period 5.877 days

Orbit Period around Sun 164.8 yr

Eccentricity 0.0000(16)

Inclination (present) 156.8◦

Geometric albedo (average) 0.70

Bond Albedo (average) 0.85

Surface Temperature 38 K (1989), 39 K (2003)

Surface Composition N 2 ,H 2 O, CO 2 , CO, CH 4 ,

C 2 H 6 ices

Surface Atmospheric

Pressure

14 μbar (1989), 19μbar

(2003)

Atmospheric Composition N 2 , minor CH 4

Tropopause Height 8 km

outer solar system. Melting and separation of metal from

rock are justified by theoretical arguments for intense tidal

heating in Triton’s past, and by the example of Ganymede,

where theGalileoorbiter’s discovery of a dipole magnetic

field demands that such an inner metallic core exists.

Whether Triton is also a magnetized body depends on

when its tidal heating ended, butVoyager 2passed too far

away to tell. Triton is, however, a sufficiently rock-rich body

that solid-state convection in its icy mantle should be oc-

curring today, powered by the heat released by the decay of

U, Th, and^40 K in its rocky core. Its icy mantle should also

be warm enough to mobilize lower-melting-point ices such

FIGURE 7 Internal structure model for present-day Triton.