Pedagoguery

At one time, volcanism was considered to be unique to Earth. We now know that that is far from the case, and in fact volcanism exists, or has existed, on many solid bodies in the Solar System. The character of volcanism on each planet and moon can be very different, however.

We will start with the innermost planet, Mercury. The story of Mercury's volcanism is still being investigated, now that the Messenger spacecraft is now orbiting Mercury. It appears that volcanism on Mercury is of two general categories: explosive and effusive. Explosive volcanism happens when there are gasses mixed in with the lava. When the gasses reach the surface, they quickly expand out into the vacuum of space. Effusive eruptions have little gas and spread out quietly, creating lava plains or shield volcanoes. Most of the evidence we have seen so far shows us explosive eruptions on Mercury, but both have been seen. However, Mercury is volcanically dead now, due to its relatively small size.

Venus is a rather unique case. The topographical data returned by Magellan seemed to indicate a catastrophic resurfacing of Venus between 300 million and 1 billion years ago, but that it had been volcanically quiet since then. However, newer data from Europe's Venus Express orbiter indicated that several areas showed significant volcanism no more recently than 250,000 years ago. While not incompatible with the catastrophic resurfacing hypothesis, it does lead us to the conclusion that we do not yet have a clear picture of Venus' past.

We are, of course, most familiar with volcanism on Earth. Earth is unique in that most of our volcanism is driven by plate tectonics, which is something that no other planet or moon in our Solar System has. Our moon, however, greatly resembles Mercury in its volcanism, although more of the Moon's volcanism is of the effusive variety. The lunar maria, are all effusive eruptions of very fluid lava. Like Mercury, the Moon is volcanically dead.

Mars has become famous for its volcanoes, particularly for the largest volcano in the Solar System: Olympus Mons. Mars produced such large volcanoes because it did not have plate tectonics. Olympus Mons grew on the site of an upwelling of magma in the mantle. A similar upwelling on Earth has produced the Hawaiian Island because the overlying plate moved relative to the hot spot. On Mars, the surface was stationary relative to the hot spot, which allowed Olympus Mons to eventually rise to a height of 24 kilometers and a breadth of 600 kilometers. There is extensive evidence of volcanism on Mars outside the massive shield volcanoes, but despite the fact that Mars is now the most observed planet other than Earth, there is no sign of current activity. The one potential sign is the presence of methane in Mars' atmosphere. It is a minute amount, only a few parts per billion, but methane is not stable in the environment of Mars, due to chemical processes that destroy it, so any methane has a lifespan of only a few hundred years – an eyeblink in planetary lifetimes. So, the methane must be renewed, and there are only two ways for that to happen. The first would be a biological process, while the second would be volcanism of some sort.

Next, we move out to the most volcanically active body in the Solar System: Io. Io is only 5% larger than our Moon, so under normal circumstances, it should be volcanically dead. However, its circumstances are anything but normal. Orbiting fairly close to the largest planet in our Solar System, and having orbital resonances with two other large moons, Io's interior is in a constant state of being stretched and compacted. The result is a great deal of frictional heating. Unlike the inner planets, where the volcanism is mostly of silicate or basaltic rock, Io's volcanism is composed of sulfuric compounds. Between the Voyager, Galileo, and New Horizons missions, more than 200 active volcanoes have been observed. Most of them are caldera-like depressions, the largest of which, Loki, is 200 kilometers across. The sulfuric compounds are very fluid, so they don't build up volcanic mountains like on Earth or Mars. Despite this, volcanic plumes can rise several hundred kilometers above the surface, and send some ejecta into orbit around Jupiter. In fact, there is a torus of material in Io's orbit that is the result of these volcanoes, so while some material escapes, other material falls back onto the moon as it orbits Jupiter.

Moving further out, we come to one of the most surprising volcanic bodies in the Solar System: Enceladus. This moon of Saturn is relatively puny at only about 500 kilometers in diameter. It also orbits relatively far out from Saturn, and has an orbital resonance with the moon Dione. This combination is believed to be the source of the heating that fuels the cryovolcanism of the moon. In the southern hemisphere are features called “tiger stripes” that vent water vapor, methane, and carbon dioxide. It is this volcanism that creates Saturn's E ring – a diffuse ring of material centered on Enceladus' orbit.

Our final stop is in some ways even more surprising. It is Neptune's moon Triton. Triton has been observed close up only once, by Voyager 2 in 1989. Triton is a large moon that may have been a Kuyper Belt object captured by Neptune, since it has a very close-in, retrograde orbit (it orbits in the opposite direction of Neptune's spin). Triton has a very young surface, providing evidence of volcanism. Like Enceladus, it is also very cold, due to its distance from the Sun. Voyager 2 observed geyser-like eruptions of nitrogen gas, and there are some features that could be rivers of methane. Most of the volcanism appears to be at the subsolar point, indicating that solar radiation is a factor, despite the great distance of Triton from the Sun. It is thought that there is a layer of transparent nitrogen ice overlying a darker substrate, which could create enough of a “greenhouse effect” to melt or vaporize enough subsurface nitrogen until enough pressure developed to erupt through the crust. It would only take a temperature rise of 4K, which is certainly possible.

Next time, the Late Heavy Bombardment.