Of Jupiter’s Galilean moons, Io is the closest to the planet at an average distance of 422,000 km (262,219 miles), and is the moon with the highest density. It is also the solar system object with the least amount of water, and the only object other than the Earth that is known to be volcanically active. Below are some more interesting facts about Io:
Io may not have been discovered by Galileo
Although Galileo is generally credited with the discovery of the four Galilean moons (Europa, Ganymede, Callisto, Io), German astronomer Simon Marius claimed to have made a prior sighting of Io in his book Mundus Jovialis, which was published in 1614.
In his book, Marius claims to have made the sighting late on December 29th of 1609 according to the Julian calendar, which equates to January 8th, 1610, the date on which Galileo claimed to have discovered Io. However, since Galileo’s sighting was published before Marius’, Galileo is credited with the discovery.
Io was the first recorded moon after our own moon
Regardless of who discovered them, the Galilean moons were the first moons to be discovered that orbited a planet other than Earth. In fact, it was viewing these moons orbiting another planet through the newly invented telescope that helped disprove Aristotle’s theory that all things orbited the Earth, and so eventually led to a heliocentric, instead of geocentric view of the solar system.
Io is named after yet another nymph
As was Zeus’ wont, he had an extra-marital affair with the nymph Io, whom he callously turned into a cow when his wife, Hera, discovered the illicit dalliance.
The moon Io is an electrical generator
As Jupiters’ magnetosphere sweeps up around a ton of dust and gas from Io’s surface each second, the interaction between the swept-up material and Jupiter’s magnetic field creates an electrical current of up to 400,000 volts and 3 million amperes. The effect of this current is dramatic; it inflates Jupiter’s magnetosphere to more than double the size it would have been had Io not been so close.
The materials involved in the process consists primarily of both ionized and atomic sulphur, oxygen and chlorine, atomic sodium, atomic potassium, molecular sulphur dioxide and sulphur, and sodium chloride dust that eventually end up either in one of several neutral radiation belts around Jupiter, or being ejected from the Jovian system altogether.
Radiation on Io can kill an unprotected human in a day or so
Io is located in a plasma cloud, known as the Io Plasma Torus that is shaped like a doughnut. Io’s rotation within the plasma cloud strips ions from the moon, which has the effect of creating radiation that is 1,000 times stronger than is needed to kill an unprotected human being. The generated radiation reaches about 3,600 mSv (36 rem), per day, which is a lot higher than the 100 rem that would kill a person within a few weeks if that person had received a radiation dose of 100 rem over a short period.
Io has more than 400 active volcanoes
The number of active volcanoes on Io makes the moon the most active solar system body in terms of volcanic and geological activity, which incidentally, is responsible for spreading the sulfur and other compounds that give the moon its yellow color.
First detected by the Voyager 1 space craft, some volcanoes can blast material up to 300 km (190 miles) into the Ionian “sky”, as the result of the tidal flexing caused by the stretching and squeezing of the moon as it approaches and then recedes from Jupiter during its orbit. Interestingly, tidal flexing of the moon’s surface causes the surface to rise and fall by as much as 100 meters during a single orbit.
Some mountains on Io are higher than Mt Everest
Despite being as volcanic as it is, most of the 150 or so mountains on Io are of non-volcanic origin. In fact, the tallest non-volcanic mountain on Io, Boösaule Montes, rises to a height of 17,000 meters (10.6 miles) and is the highest known non-volcanic mountain in the solar system. The average height of mountains on Io is 6,000 meters (4 miles), while the average length of Ionian mountains is 157 km (98 miles).
Io is the densest moon in the solar system
With a density of 3.5275 g/cm3, Io has the highest density of all the known moons in the solar system, including our own Moon. In fact, its composition of silicate rock that envelops a molten core of iron or iron sulfide gives it a density that compares more favorably with the terrestrial planets, than with typical solar system moons.
Io is tidally locked to Jupiter
Like the other Galilean moons, Io is tidally locked to Jupiter, always keeping one face turned toward the planet. Io completes one orbit and one revolution around its own axis in just 42.5 hours, which is fast enough to observe the moon’s orbital motion in one night. Io is also locked in orbital resonances with other Galilean moons; completing four orbits for every orbit of Ganymede and two orbits for every orbit of Europa. This forced resonance with other moons is what maintains the slight eccentricity (0.0041) of Io’s orbit, which in turn, is the ultimate source of the tidal flexing and heating that causes the extreme volcanism on Io.
Io has almost no visible impact craters
Somewhat surprisingly, and despite its rocky nature, there are almost no impact craters visible on Io with which to estimate the moon’s age. The lack of craters turns out to be the result of the volcanic and geological activity that continuously renews or recycles the moon’s surface rather than circumstances that may have caused impactors to pass by the moon without hitting it.
However, because Io lacks craters, scientists have yet to reach a consensus on the age of the current surface of the moon, but despite this, there is no reason to believe that Io did not form around the same time as the other Galilean moons did, which was about 4.5 billion years ago.