Beginner's Guide to Io

by Jason Perry

So you came to this website not knowing anything about Io?  You looked at all the neat stuff but you don't understand most of it?  Well you have come to the right page!  This is the Beginner's Guide to Io where I answer readers questions about Io.  Since this is a brand new page I will be answering some questions that someone who might know nothing about Io might ask.  If you have a question about Io, email me at volcanopele@netzero.com .  I will send you an email answering your question and I will put your question and answer on this page.

Q: What is Io?

A: Io is a moon of Jupiter.  It orbits 262,000 miles away from Jupiter's cloudtops.  Io is 2250 miles in diameter making it a little larger than Earth's moon.

Q: What is so special about Io?

A: Io has active volcanism.  Io's surface is littered with volcanoes with sizes from a mile to the size of the state of Arizona.  The lava at these volcanoes streams for hundreds of miles from the volcanoes' calderas (volcanic crater) and are much hotter than lava on Earth.  In fact, the temperatures seen at some volcanoes are so hot that only the surface of the sun is hotter!  The active volcanoes also cause towering plumes of sulfur and sulfur dioxide dust, snow, and gas.  Some of these plumes can reach heights of 800 km and produce bright red rings on the surface like that seen at Pele and Tvashtar (the volcano seen above).  In addition to have tons of active volcanoes, Io has some of the tallest mountains in the solar system.  One mountain, Boosaule Montes, is 52,000 feet tall, 23,000 feet taller than Mount Everest.

Q: Where does all the heat come from?

A: Because of Io's size, the same radioactive decay that heats Earth's interior can not heat Io's interior.  So what does heat Io's interior.  It is believed that tidal heating heats Io's interior.  That's right, the same kinds of forces that causes Earth's tides drives Io's unbelieveable volcanism.  How does this work?  Io's orbit around Jupiter isn't completely circular.  Instead, Io's orbit is slightly eccentric which means that Jupiter's gravity acts differently on Io at different points on Io's orbit.  This produces friction within Io's interior.   In addition to Jupiter's varying gravitational pull, Io's sister moons, Europa and Ganymede, are in a orbital resonance with Io. This means that their gravity acts predictably on Io and causes Io's interior to be pulled like a rubber band producing massive tides.  In fact, every point on Io's surface rises 300 feet every Ionian day.   Notice that I said surface not ocean like on Earth.  All this friction has heated Io's interior to the point that the heat needs to escape to relieve the pressure inside Io.  The causes Ioquakes and volcanoes.

Q: How do the mountains form?

A: This is an intiguing question that geologist have been trying to figure out for 20 years.  The latest theory goes like this.  A mantle plume (a mass of hot rock in the mantle that rises) rises close to the lithosphere.  The pressure of the mantle plume causes cracks to form in the crust.  As a mantle plume continues to rise, a chunk of the crust is tilted upwards, some times vertical, into space.  An Ionian mountain is formed.  The same cracks that formed the mountain can also provide a conduit for lava.  These same mantle plumes also cause cracks to from on the mountains themselves, spliting them in two.

Q: Which volcano is that above?

A: That would be Tvashtar Catena.  In November 1999, Galileo witnessed a lava curtain (a chain of lava fountains like those seen at Kilauea) coming from a fissure at this volcano.  When the image above was taken in February 2000, the lava fountain had stopped and had left a line of black lava and a medium size lava flow flowing from it.  This volcano erupted again in December 2000, producing a Pele-type plume and a large red deposit surrounding the volcano. By the way, a catena is a chain of craters, in this case two volcanic craters.  The western one is covered in hot lava and is surrounded by new pyroclastic (ash) material.  The eastern one has many volcanoes within that one section.  One of them is the fissure where the lava curtain was.

Q: What is the surface made of?/What causes Io's strange colors?

A: Io's surface is made of sulfur, sulfur dioxide frost, and silicate lava flows and mountains.  The sulfur is the cause of Io's outragous coloration. When it is heated it forms different chains with other sulfur atoms and this produces various colors.  Other colors, like green, are produced when sulfur forms compounds with other elements like iron.

Q: What spacecraft have visited Io?

A: Io and Jupiter have been visited by Pioneer 10&11, Voyager 1&2, Ulysses, Galileo, and Cassini.  In the near future, Io will be visited by the Europa Orbiter.  Galileo has flown close to Io 4 times, once in late 1995, twice in late 1999, and once in early 2000.   Galileo's mission has just been extended allowing it to flyby Io three more times in late 2001 and early 2002. Cassini flew by Jupiter in late December 2000 producing low resolution (compared to Galileo) images of Io.

Q: Are there any impact craters on Io like the other moons of Jupiter or have they all been wiped clean by Io's volcanoes?

A: Yes there are impact craters on Io but they are VERY rare.  In all my time of looking at images of Io, I have only seen 1 bonafide impact crater on Io, and that was on a mountain. Check out my page on this crater for more info. Craters on Io are covered by lava almost as soon as they form.

Q: What is the topography like on Io?

A: Mostly extremely flat plains with the occasional mountain and volcanic caldera.  As I said in a previous answer, the tallest mountain is 16 km tall (Boosaule Mons).  The mountains are isolated, no large mountain-chains like along plate boundaries on Earth.  Also, many of the calderas have a depth in excess of 2 km.  Chaac Patera, a volcano in the antijovian hemisphere noted for its green floor, is 2.8 km deep.

Q: How are the volcanoes formed?

A: Many ways.  Some volcanoes are calderas, which are pits formed by a collapsing lava chamber.  Others are formed when due to lava taking advantage of a basin formed by an extensional fault or split-apart fault.  One volcano, Hi'iaka Patera, was formed by a strike-slip fault where the two sides of the fault slide past each other.

Q: What is the surface gravity on Io?

A: Gravity on Io is 18% of the gravity on the surface of Earth.