Our own Moon is one that appears to have been active in the past but has quietened down to become almost entirely dormant, retaining little to none of the volcanic activity that once sculpted its surface. Indeed only a few planets. Earth included, have changeable environments at all, making the discovery of Io all the more exciting Where once our Solar System was regarded as an ever-present museum of the past, moons such as this one have proven that it is still a lively and effervescent place. So what is it that makes Io so amazing?
To date, Io has more than 400 known active volcanoes, making it the most volcanically active object in the Solar System, even more so than Earth. Dozens of vents are strewn across its surface leaking gas into the atmosphere, while at its poles and even occasionally close to the moon's equator vast icy plains can be found. The remnants of Io's volcanic past and present are as clear as day, with large volcanic rings the size of California encircling either dormant or active volcanoes.
While data from the Voyager probes, and later the Galileo spacecraft, has shown us volcanic plumes erupting from the surface of the moon, we are also able to discern some of Io's erupting monsters from observations on Earth. Some volcanoes have even been active for over two decades, meaning that the driving force below Io's surface is even more violent and ferocious than once thought.
The reason for Io's outbursts of activity is that it is being battered and bruised by Jupiter and its other moons. Io sits at a distance of 420,000 kilometres (260,000 miles) from Jupiter, which might sound quite far away but consider that our own Moon sits 385,000 kilometres (240,000 miles) away from us and that Jupiter is almost 318 times more massive than Earth, while Io is almost exactly the same size as our Moon. For this reason it's obvious that while the Earth exerts a small but noticeable force on the Moon, causing it to become gravitationally locked to our planet only a billion or so years ago, Jupiter is exerting a huge force on Io. This moon, which itself is gravitationally locked to Jupiter, is being constantly pushed and pulled by the huge gas giant, which in turn is churning its insides. Add into this that the other three Galilean moons, of which Io is the second smallest but the closest to Jupiter, also exert a gravitational influence on the poor moon, and you might start feeling sorry for this troubled space rock. The influence of the other moons means that Io's orbit around Jupiter is eccentric, with a difference of 3,400 kilometres (2,100 miles) between its closest and furthest points. So, even though the same face always points towards Jupiter, Io experiences huge changes in gravitational force from this big bully of a planet.
In fact, the force on Io is so intense that its solid surface acts in a similar manner to oceans on Earth. It bulges up and down by as much as 100 metres (330 feet) in places, compared to 18 metres (60 feet) for the highest tides on Earth. Bear in mind that Io's tides are made of solid rock and those on Earth are made of water, and you might realise just how tough a time this moon is having.
These tidal forces create a huge amount of heat in Io's interior, and therefore the majority of its subsurface crust is a liquid. This liquid is under intense pressure and looks for any escape route possible out onto Io's surface, be it in the form of a volcano, geyser or vent. This makes the surface a constantly changing place, with plumes of sulphur dioxide snow ejecting all over it. Any meteorites that hit the moon instantly see their impact craters filled with lakes of molten lava from the interior. The composition of this lava is still somewhat of a mystery but the two main theories suggest that it is either made from various compounds of molten sulphur or silicate rock. The former would account for the odd colouring of the moon, while the latter would better explain the hot temperatures under the surface where it may be too hot for sulphur to exist.
As more missions are sent to the Jupiter system we will learn more about this fascinating and mysterious moon. NASA's solar-powered spacecraft Juno, which launched in August 2011, will enter orbit around the largest planet in our Solar System in July 2016. This will be followed by the ESA's Jupiter Icy Moon Explorer, which is set to launch in 2022 and arrive at Jupiter in 2030.