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| Io |
Io is one of the most curious objects in our solar system. The innermost of Jupiter’s big moons has plenty of features that set it apart from anything else that we have ever seen, including volcanoes, aurora and a sulphur atmosphere.
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| Jupiter |
Jupiter has 79 moons that we know of so far. There are four large moons, known as the Galilean moons, named after Galileo who discovered them in 1610. From innermost to outermost, these moons are Io, Europa, Ganymede and Callisto. Beyond them are the irregular moons of Jupiter, all of which are much further out than the previous moons.
Io orbits very closely to Jupiter, only 3,50,000km above Jupiter’s cloud tops. This means that from Io’s surface, Jupiter would appear 39 times bigger in the sky than our moon. Io orbits Jupiter in only 42.5 hours compared to our moons monthly orbit. Its orbit is actually in sync with two of the other Galilean moons. It orbits twice for every orbit of Europa, and four times for every orbit of Ganymede. This is what we call an orbital resonance.
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| Volcano |
Orbital resonances greatly enhance the mutual gravitational influence of the moons, which means the gravitational forces from the other moons cause the orbit of Io to have a little more eccentricity than it otherwise would have. This is likely the primary heat source for all its geological activity, as Jupiter’s gravity pulls and tugs on Io, causing tidal heating.
At some points in its orbit, the tidal bulge on Io is thought to be up to 100m. This affect is similar to what we see on Earth, with the oceans tides being caused by the moon, although on Earth the effect is much more minimal, the tides only usually shifting about 2m from high to low.
Io is getting 300% more tidal force exerted on it in comparison to our moon on us because of its close proximity to the biggest planet in the solar system, Jupiter, and the other big moons in the system don’t allow the moon’s orbit to be less eccentric, meaning Io isn’t going to get any respite anytime soon.
A day on Io is the same as its orbital rotation, which means that Io is tidally locked to Jupiter. Just like we can only see one face of our moon from Earth, only one face of Io can ever be seen from Jupiter.
Io is a pretty big moon, although it is the second smallest out of the Galilean moons. It is comparable in size to Earth’s moon and shares a similar density, meaning it has a similar amount of gravity.
Interestingly, it has the highest density of any other moon in the solar system, one of its many unique features.
Another is that is it composed of mainly silicate rock and iron, similar to the terrestrial planets and our moon, in comparison to most other big moons in the solar system which are made of water ice and silicates. Io in fact has the least amount of water of any known body in the Solar System. Its core is likely is be made of iron or iron sulphides, surrounded by a silicate rich mantle and crust. The core is not thought to be convecting though, as no magnetosphere has been detected around the moon. The mantle is thought to be liquid near the crust, and is at least 50km thick. This is where all the volcanism originates. Which brings us to perhaps the most interesting part about Io, the hundreds of huge volcanoes all over its surface.
Before the 1970s, we didn’t know much about Io at all, although telescopes were starting to pick up hints that the moon was devoid of water, and that it may have a surface of sulphur.
The first mission to see Io in any kind of detail was Pioneer 11. Pioneer 11 detected that Io was made of silicate rock and not water ice, and that it has a thin atmosphere. The next missions to Jupiter were the Voyager I and II missions in 1979. Voyager I flew by at a distance of only 20,000km, and was able to take some impressive close ups of Io’s surface. What it saw was a remarkable landscape, full of vibrant colours and a total absence of impact craters.
It found mountains taller than Everest, as well as volcanic pits hundreds of kilometres wide, and what looked to be lava flows. Most notable however, was the presence of plumes coming from the surface. This proved that Io is volcanically active, and it is still the first and only place this has been visibly seen beyond Earth.
Voyager I also confirmed that the surface of Io is covered in different sulphur frosts. It found that it is these sulphur compounds that dominate the atmosphere.
Voyager II also saw Io in July of 1979, but was much further away at 1,000,000 km, although it still saw 7 of the nine plumes Voyager I saw in March, which meant those volcanoes had likely remained active throughout those 4 months.
The really interesting images came about with the Galileo spacecraft that arrived at Jupiter in 1995. The spacecraft wasn’t especially designed to only study Io, but it was able to acquire some of the highest resolution images we now have of its surface. What it was able to see though were plumes from many volcanoes, as well as confirming the volcanoes were erupting sulphur and silicate magmas, similar to what we have on earth, except the magma on Io is also rich in magnesium. The surface of Io is spectacularly colourful. The yellow plains are composed of mainly sulphur; the white areas are mainly fresh sulphur dioxide frosts. Towards the poles, the sulphur is damaged by radiation, which can be seen as the poles appear redder than the rest of the planet.
In other places, the colour of red are the deposits left by volcanic plumes that reached hundreds of kilometres above Io. The most obvious deposit is from the volcano Pele, sadly an inactive volcano when Galileo was around.
Voyager I was able to see a massive plume when it passed by, in this image the plume is 300km tall and 1,200km wide, in other words, roughly the size of Alaska. Loki, the largest volcanic depression on Io, 200km in diameter. On average, Loki produces 25% of the average heat output of Io, but sometimes the crust on the lava lake sinks back into the lava, causing Loki to produce 10 times more heat than normal.
This can especially be seen in one of Io’s other big volcanoes, Tvashtar. Normally this area looks like this, but here the crust is seen falling into the lava lake. In this image where there is just white, the radiant energy from the lava curtain was so intense that the camera only registered white.
In 2007, New Horizons used Jupiter as a gravity assist on its way to Pluto. It also used the opportunity to test its equipment. It focused its lens on Io during its flyby and what it saw was amazing. Tvashtar was in full eruption, and the plume could be seen hundreds of kilometres above Io’s surface. Even though the volcanoes tend to be flat, it also has some extremely tall mountains, the highest one reaching 18km tall.
Another of the unique aspects of Io is its interaction with the magnetic field of Jupiter. Jupiter has an extremely large and strong magnetic field, and Io orbits within some of the strongest sections. The unusual thing about this interaction is that when particles from some of Io’s thin atmosphere and eruptions are lost to space, these particles float in orbit around Jupiter in what is known as a neutral cloud. This cloud can extend far beyond and behind the orbit of Io.
But also surrounding Jupiter is something known as a plasma torus, a doughnut of ionised particles that follows the rotation of Jupiter’s magnetic field. The plasma torus rotates a lot faster than Io orbits, at 70km/s compared to Io’s 17km/s orbital velocity. Io orbits right through the middle of it, with the particles from the torus bombarding the particles in the neutral cloud, exciting them to higher energies. These newly ionised particles feed into the torus, attracted by the magnetic field lines of the magnetosphere. These particles are lost from the neutral cloud into the plasma torus at a rate of about 1 tonne of matter per second, which greatly increases the size of Jupiter’s magnetic field. In fact, if it was visible, Jupiter’s magnetosphere would be about the same size as the moon in the sky. Jupiter's magnetic field lines, which Io crosses, couple Io's atmosphere and neutral cloud to Jupiter's polar upper atmosphere by generating an electric current known as the Io flux tube.
A flux tube is basically a concentration of magnetic field lines. The sun has these between sunspots, and it is very visible on the Sun because of the charged plasma that flows between them.
Io’s flux tube causes an aurora trail around Jupiter’s poles. This point here is the flux tube from Io striking the upper atmosphere of Jupiter. Aurorae are also visible on Io, although they are not just limited to the poles. The different colours represent the different particles being ionised, green is sodium, red is oxygen, and blue from sulphur.