Io – Shrouded in sulphur
If you’ve never seen or heard about Io, you’re in for a treat: this small moon is afire with volcanism, spewing out lava, dust and sulphurous gas pretty much constantly. When the Galileo spacecraft visited it between 1999 and 2002, it witnessed eruptive plumes up to 500km high, lava flowing over areas of up to 620km2 and hot spots on the surface of over 1500°C.
So why so volcanic? Io is about the same size as the Moon and has a broadly similar composition, so if it relied on the same finite internal heat sources it would be as geologically dead as our own satellite. Instead, it’s the most volcanically-active body in the solar system. The reason? Jupiter.
As the closest large moon to the great gas giant, Io experiences a huge tidal pull. And because Io always presents the same face to Jupiter but the other moons force it to have an elliptical orbit, the magnitude of that pull varies around its orbit. The result: the whole moon flexes, releasing massive amounts of heat energy. That energy needs to escape somehow and because conduction through a silicate crust is too slow, it finds release through continuous volcanism.
The volcanoes on Io have pretty low relief, probably because of the fluidity of the lava flows which make them. But though this moon doesn’t have any spectacular edifices like Olympus Mons, it does have something equally impressive: huge lava lakes.
The classic example is Loki, a 220km diameter patera. As on Venus and Mars, paterae are very low volcanoes with outsized calderas. In this case, the caldera is more than just a subsided area in the center of the volcano: it’s 390 km long and 55 wide, smooth, dark and very, very hot – a lava lake. Its thermal signature can be detected from Earth, so we know it stays hot at all times, but sometimes it undergoes thermal ‘brightenings’ when it gives out a truly massive amount of heat. At its peak, it gives out 20% of Io’s total tidal heat flow1. Because these brightenings don’t seem to lead to any significant changes to the look of the caldera floor, it’s thought they represent overturns, when the dense cooled crust on its surface gets too thick and the whole lot turns over, reforming a crust immediately afterwards2.
A cloak of sulphur
The most eye-catching aspect of Io’s surface is its colour. It’s wildly multi-coloured: mostly yellow, but ranging from orange to red, black to white. The black is silicate and the white is SO2 ice, but the others are different forms (allotropes) of sulphur made up of chains of S atoms of different lengths.
The most obvious conclusion, looking at the planet from the distance, is that the volcanism is sulphurous, rather than the silicate volcanism we’ve seen so far on other bodies. But there’s a lot of evidence against that. Many of the eruptions are too hot, for a start: sulphur would boil away at those temperatures. Taking the temperature, spectral information about their composition and the extreme fluidity of the lava flows together, it’s most likely they’re komatiite. This low viscosity high-Mg lava flowed on the surface of the Earth way back in the Archaean, when our planet was hotter and there was a greater degree of melting of our mantle. It’s exactly what you’d expect on a rocky moon that is being intensely heated by Jupiter’s tidal forces.
Komatiite isn’t sulphur-rich, so where does the sulphur come from? A major source is the huge plumes erupting from the volcanoes: the S and SO2 gases in these condense and settle to the surface, making the striking rings around vents like Mount Pele. As the sulphur undergoes radiation damage, longer chains of S are broken up into the S3 and S4 allotropes, leading to much of Io’s surface becoming a yellow to orange colour.
As well as the plumes, there are some bright lava flows which are also likely to be sulphur. It’s thought that these are outflows from sulphur aquifers, pockets of molten sulphur not far below the moon’s surface. These form when sulphur at the surface gets progressively buried by more plume fallout and lava flows until it’s deep enough to be melted, especially in areas where silicate magma is rising up through the crust nearby and heating it. When an aquifer gets breached, the sulphur rushes to the surface and spills out, making the flows.
Ultimately, all this sulphur is likely to have been accumulated in the near-surface due to the constant volcanism. On Earth, sulphur dioxide is one of the volatiles which come up in eruptions, along with with gases like H2O and CO2. On Io, those lighter volatiles have been constantly stripped off Io’s thin atmosphere over billions of years by interaction with Jupiter’s magnetosphere, but heavier sulphur volatiles have fallen back to the surface. Because of this, Io is now pretty much anhydrous, but has a lot of sulphur at the surface.
It’s interesting to realise that this is just the state of differentiation Io has reached at this moment in history: earlier in its life, all those other gases would still have been coming up in the eruptions and the sulphur wouldn’t have had a chance to accumulate. We’re lucky to be viewing it at this time in history, when it’s developed into a unique and exotic place.