A Grab Bag of distant Goodies
Io
As the innermost of the 4 large Galilean moons of Jupiter, Io also holds the distinction of being the most volcanically active body in the solar system. Our best images come from the Galileo mission and show a very pockmarked surface.On its way to Pluto, the New Horizon craft captured this 5 frame sequence of an eruption of one of these volcanoes.
Io's density is approximately 3.5g/cm3 which means it is a rocky satellite, rather than an icy one. The source of Io's internal heat is not the decay of radioactive isotopes as we have discussed for terrestrial planets. Io is heated by tidal heating, a process that we will discuss in more detail when we get to the icy satellites. As pointed out here:
As you might expect, there are a number of issues that we don't quite understand on Io. For example, the volcanoes are not where we expect them to be, given our understanding of tidal heating. It has recently been suggested that Io has an underground magma ocean which may explain the locations of the volcanoes. But we'll leave Io behind and move on to...Although Io always points the same side toward Jupiter in its orbit around the giant planet, the large moons Europa and Ganymede perturb Io's orbit into an irregularly elliptical one. Thus, in its widely varying distances from Jupiter, Io is subjected to tremendous tidal forces. These forces cause Io's surface to bulge up and down (or in and out) by as much as 100 m (330 feet)! Compare these tides on Io's solid surface to the tides on Earth's oceans. On Earth, in the place where tides are highest, the difference between low and high tides is only 18 m (60 feet), and this is for water, not solid ground!
This tidal pumping generates a tremendous amount of heat within Io, keeping much of its subsurface crust in liquid form seeking any available escape route to the surface to relieve the pressure. Thus, the surface of Io is constantly renewing itself, filling in any impact craters with molten lava lakes and spreading smooth new floodplains of liquid rock. The composition of this material is not yet entirely clear, but theories suggest that it is largely molten sulfur and its compounds (which would account for the variegated coloring) or silicate rock (which would better account for the apparent temperatures, which may be too hot to be sulfur). Sulfur dioxide is the primary constituent of a thin atmosphere on Io. It has no water to speak of, unlike the other, colder Galilean moons. Data from the Galileo spacecraft indicates that an iron core may form Io's center, thus giving Io its own magnetic field.
Mimas
Mimas, at 396 km in diameter is a fairly small moon of Saturn. We could argue that we place it in this list because it represents the smallest solar system body to have rounded itself into a spherical shape by self-gravitation. But we'd be lying. Mimas is here because it looks like this:And yes, even scientists at NASA watch Star Wars; hence this description here:
Its most distinguishing feature is a giant impact crater -- named Herschel after the moon's discoverer -- which stretches a third of the way across the face of the moon, making it look like the Death Star from "Star Wars." The Herschel Crater is 80 miles (130 km) across -- one third of the diameter of the moon itself -- with outer walls about 3 miles (5 km) high and a central peak 3.5 miles (6 km) high. The impact that blasted this crater out of Mimas probably came close to breaking the moon apart. Shock waves from the Herschel impact may have caused the fractures, also called chasmata, on the opposite side of Mimas.
Triton
Triton is the largest of Neptune's moons. It is unusual because it has a retrograde motion; it orbits Neptune in a direction opposite to those of most of the objects in the solar system (recall a distant module 1). Because it shares a number of characteristics with Pluto it has been proposed that Triton is a Kuiper belt object that has been captured by Neptune. The reason why it appears on this page is because it has a young surface, which looks like this:From this summary:
Triton has a diameter of 2,700 km (1,680 miles). Spacecraft images show the moon has a sparsely cratered surface with smooth volcanic plains, mounds and round pits formed by icy lava flows. Triton consists of a crust of frozen nitrogen over an icy mantle believed to cover a core of rock and metal. Triton has a density of about 2.050 g per cubic cm (The density of water is 1.0 g per cubic cm.) This is a higher density than that measured for almost any other satellite of an outer planet. (Europa and Io have higher densities.) This implies that Triton contains more rock in its interior than the icy satellites of Saturn and Uranus.
Triton's thin atmosphere is composed mainly of nitrogen with small amounts of methane. This atmosphere most likely originates from Triton's volcanic activity, which is driven by seasonal heating by the Sun. Triton, Io and Venus are the only bodies in the solar system besides Earth that are known to be volcanically active at the present time.
Triton is one of the coolest objects in our solar system. It is so cold that most of Triton's nitrogen is condensed as frost, giving its surface an icy sheen that reflects 70 percent of the sunlight that hits it.
NASA's Voyager 2 -- the only spacecraft to fly past Neptune and Triton -- found surface temperatures of -235°C (-391°F). During its 1989 flyby, Voyager 2 also found Triton has active geysers, making it one of the few geologically active moons in our solar system.
The really cool part (pun intended) is that Voyager 2 found active geysers, which means we have footage of that activity. It is thought that sun light heated up subsurface reservoirs of nitrogen, with the result of a nitrogen geyser eruption. This kind of activity is refereed to as cryovolcanism and this footage from 1989 is it:
It may not look like much but keep in mind that it is from a couple of frames, taken during a single fly-by, more than a quarter of a century ago. After we have seen nitrogen eruptions there is only one place we can visit that is even cooler.
Pluto
A future iteration of this course will no doubt incorporate more scientific discoveries from the New Horizons mission. But for now we want to comment on just one recently explained feature of Pluto's surface. Pluto's surface is composed of 98% nitrogen ice. So it should not be surprising that the surface process involves nitrogen. Let us first get an overview of the different types of nitrogen terrain as is shown in this Pluto fly-over.
The area of interest is referred to as "Cellular Nitrogen Ice plains" in the video. In detail, they look like this:
Using computer modelling, scientists have determined that the strange cell shape is the result of convection of the nitrogen ice, driven by small internal planetary heat sources. As this press release states:
“For the first time, we can determine what these strange welts on the icy surface of Pluto really are,” said William B. McKinnon, from Washington University in St. Louis, who led the study and is a co-investigator on the New Horizons science team. “We found evidence that even on a distant cold planet billions of miles from Earth, there is sufficient energy for vigorous geological activity, as long as you have ‘the right stuff,’ meaning something as soft and pliable as solid nitrogen.”
McKinnon and colleagues believe the pattern of these cells stems from the slow thermal convection of the nitrogen-dominated ices that fill Sputnik Planum. A reservoir that’s likely several miles deep in some places, the solid nitrogen is warmed by Pluto’s modest internal heat, becomes buoyant and rises up in great blobs – like a lava lamp – before cooling off and sinking again to renew the cycle.
The computer models show that ice need only be a few miles deep for this process to occur, and that the convection cells are very broad. The models also show that these blobs of overturning solid nitrogen can slowly evolve and merge over millions of years. Ridges that mark where cooled nitrogen ice sinks back down can be pinched off and abandoned, resulting in Y- or X-shaped features in junctions where three or four convection cells once met.
“Sputnik Planum is one of the most amazing geological discoveries in 50-plus years of planetary exploration, and the finding by McKinnon and others on our science team that this vast area—bigger than Texas and Oklahoma combined – is created by current day ice convection is among the most spectacular of the New Horizons mission,” said New Horizons Principal Investigator Alan Stern, of the Southwest Research Institute, Boulder, Colorado.
These convective surface motions average only a few centimeters a year – about as fast as your fingernails grow – which means cells recycle their surfaces every 500,000 years or so. While slow on human clocks, it’s a fast clip on geological timescales.
“This activity probably helps support Pluto’s atmosphere by continually refreshing the surface of ‘the heart,’” McKinnon said. “It wouldn’t surprise us to see this process on other dwarf planets in the Kuiper Belt. Hopefully, we’ll get a chance to find out someday with future exploration missions there.”
Summary
We have briefly examined a variety of distant satellites and a dwarf planet. Io illustrates that there are other ways of generating volcanic activity. And we saw that the same material, nitrogen, can act as an erupting fluid or as a convecting ice. Changing temperatures affect the modes by which substances behave. And we learned that without the George Lucas' "Star Wars" universe, Mimas would be just another small satellite.
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