Io is close to our moon in size, but unlike our tranquil moon, it is covered with exploding volcanoes. Applying thermodynamic relationships commonly used to study magma inside Earth, the scientists were able to decipher clues to characteristics of Io's mantle from their analysis of the gases ejected from Pele. Previous data taken by Voyager and HST of volcanoes on Io revealed only the evidence of sulfur dioxide. However, recent reanalysis of 1996 data and examination of new 1999 data by planetary investigators presented sulfur in additional chemical incarnations in the atmosphere around Pele. Oxidation occurred when iron reacted with water, playing out like a jealous game among lovers.
Water is a royal coupling of hydrogen and oxygen.
You could say that the lonely iron metal was jealous of the water's beauty and eventually managed to seduce oxygen away from the hydrogen. Like a partner in a successful marriage who evolves while bonding into a team, the iron changed from its lonely metallic state into iron oxide. Even though water is not found on Io today, Zolotov and Fegley suggest that it could have existed there earlier in its history and contributed to the oxidation of iron inside the moon like it did in the Earth. And water, which is gone now, also could have caused the high oxidation state of Io. Another jovian moon that could have an interior much like the differentiated insides of Io and Earth is Europa.