Triton:

A New Version of Ice Geology

Craig Levin

Copyright (c)1989

Although we did not get as good a view of Triton as we did of the moons of Saturn or Jupiter, the pictures that the world has received have shown that the surfaces of the ice worlds of the outer solar system cannot be pigeonholed and categorized, but studied as subjects in their own right. However, before 1986, certain types of ice-based surfaces had been noticed on the moons of Saturn and Jupiter; and it is with a short explanation of these that I begin this small article on Triton.

The major moons of Jupiter that are composed of ice are Callisto, Ganymede, and Europa. Callisto has a ``classic'' small-body surface -- it is covered with craters of various sizes, at least at first glance. At a closer look, however, an observer can find numerous faults and ridges on that battered world. Ganymede is cratered as well, but it shows much evidence of internal activity -- the large, light colored upwellings that cover the moon. Europa is perhaps one of the few worlds without craters; instead, its molten (read fluid H$_{2$O) interior erases evidence of cratering by periodic fissure eruptions from the numerous linear features.

Saturn's ice moons, however, are much larger in number. Luckily, though, some of them are fundamentally the similar -- variations on a fugue, if you will. For example, Rhea and Dione both have streaked surfaces -- most probably the result of fluid water upwelling from the fractures of the crust. Meanwhile, Enceladus shows many prominent ice ridges, a result of more energetic ice volcanism. Also, its white surface bespeaks a geology that recycles the surface, rather than letting the meteorite impacts darken the surface. Tethys and Mimas show many small fault criss-crossing the surface. Finally, Iapetus has confused everyone by its extreme hemispherical differences -- one icy light like the rest of the moons of Saturn, and one black as charcoal. Possibly there was an upwelling of carbonaceous ice from the deep core of Iapetus. Titan, while quite definitely large enough to have a very interesting surface, is shrouded in clouds; so it is rather regrettably obvious that I must skip this planet-sized moon.

Uranus's quadruplets look nothing like each other, aside from the cratering that all bodies in the solar system have suffered. Oberon, the outermost, shows some evidence of crater-filling volcanism, but none for fissure eruptions or faulting. Titania has the start of faulting and fissure eruptions, in addition to some crater-filling. Umbriel shows no evidence at all for an active geology. Ariel has a planet-wide system of fissure eruptions and faults. Finally and most extremely, Miranda shows almost all types of volcanism and tectonism -- faults, fissure eruptions of both pure and carbonaceous ice, and compressional faults. About the only type of volcanism that it fails to have is what we associate with when we hear the word ``volcano'' -- mountainous volcanism.

Finally, we come to Triton. As you may have expected, Triton has a geological ``style'' all its own. Using the pictures from the November Scientific American, I have noticed some features that could be caused by the moon's internal geological processes. The first, going from south to north, are the plumes covering the polar cap. These are probably ``lava'' (most likely carbonaceous ice) from local volcanos. The second is the polar cap itself, caused by volcanic outgassing. The third is the system of faults in the non-capped areas, probably the result of expansion of the core as the frozen crust trapped the heat. The fourth are the open, unmarked areas; caused by ice ``lava flows'' flooding lowlands and freezing. The fifth is the ``cantaloupe'' terrain that seems to cover the rest of the moon; this complex of hills and valleys could be the result of local volcanism. Much of the terrain is uncratered, which means that the moon is still active.

As far as is known, none of the ice worlds look completely alike. All have their own idiosyncrasies. The ``pearls-on-a-string'' theory of ice moon geology is utterly dead. Another matter left for consideration is that these moons are imperfectly mapped -- some only have less than a hemisphere mapped. Perhaps the best thing for the study of ice moons are going to be the Galileo and Cassini rocket probes, as they will map the ice moons of Jupiter and Saturn completely.

List of References

Briggs, G.A., & F.W. Taylor, _The Cambridge Photographic Atlas of the Planets_, New York: Cambridge University Press, 1988

Hartman, William K., & Ron Miller, _The Grand Tour_, New York: Workman Publishing Co., Inc., 1981

Kinoshita, June, ``Neptune'', Scientific American, November, 1989

Craig Levin began to get involved in astronomy when, in second grade, he received H.A. Rey's ``Find the Constellations'' as a birthday present. He learned those few constellations visible from Chicago, and his interest remained at a low level until Halley's Comet pulled him out of his freshman high school doldrums. That January, he received his first telescope and started up again. As a high school junior, he had his first article published in the now-defunct Small Scope Observers' Association's newsletter, and by his senior year in high school was helping to establish the ``Astronomical Newsletter,'' a now-defunct magazine based in Atlanta. At present, he is a physics major at Bradley University who intends to turn his first love, planetology, into his profession.

He can be reached at bradley!bucc2!moonman@a.cs.uiuc.edu