Triton 491
FIGURE 10 Young volcanic region on Triton. Towards the bottom and right, smooth undulating
flows apparently emanate from complex caldera-like depressions and linear alignments of
volcanic pits and vents, burying preexisting topography. At the upper left, terraced plains
surround an exceptionally level plain, Ruach Planitia. This region, 675 km across, is very sparsely
cratered. (Courtesy of NASA/Paul Schenk, Lunar and Planetary Institute.)
create some complex forms, but calderas are generally com-
posed of quasicircular elements. The two paterae (from the
Latin for saucer) in Figure 10 are clearly of the caldera type
in which renewed volcanism has occurred, because both are
partially buried by younger icy lavas.
The compositions of the icy lavas are, strictly speaking,
unknown.Voyager 2carried no remote sensing instruments
designed to determine compositions. The icy plain-forming
lavas shown in Fig. 10 were clearly viscous enough to form
thick enough deposits to bury preexisting topography of a
few hundred meters elevation.
The favored composition for viscous lavas on icy satellites
has long been ammonia-water. As outlined by pioneering
planetary chemist J.S. Lewis, ammonia (NH 3 ) is the chem-
ically stable form of nitrogen in a low-temperature gas of
solar composition, and when condensed forms various hy-
drates with water ice, all of which have low melting points.
Triton would not have accreted much ammonia if it formed
in solar orbit, because N 2 would have been the dominant
original form of nitrogen in the outer solar nebula for the
same reasons CO and organic material were favored over
CH 4 (see Section 3.3), but it still would have acquired some
NH 3 based on cometary compositions (up to a percent or
two compared with water). A water-rich NH 3 –H 2 O mix-
ture (0 to 33 mole% NH 3 ) would be composed of frozen
H 2 O and ammonia dihydrate (NH 3 ·2H 2 O), which yields a
lowest-melting-point (or eutectic) melt at∼177 K at pres-
sures typical of Triton’s mantle. This melt (or cryolava) is
ammonia-rich (about 32%) and has a viscosity similar to
some types of basaltic magma.