[Illustration: Between the two encounters, the volcanic eruption at Loki (P₂) changed character. The single plume emanating from the western end of an apparent dark fissure seen in March was joined by a second fountain of similar size about 100 kilometers to the east. The plume also increased in height, from about 120 kilometers in the Voyager 1 image (left) to 175 kilometers in the Voyager 2 image (right). [260-662A and B]] [Illustration: The detailed structure near the volcano Loki is like nothing seen elsewhere on Io. [260-642B]] [Illustration: When this Voyager 1 picture was taken, the main eruptive activity (P₂) came from the lower left of the dark linear feature (perhaps a rift) in the center. Below it is the “lava lake,” a U-shaped dark area about 200 kilometers across. In this specially processed image, detail can be seen in the dark surface of this feature, possibly due to “icebergs” of solid sulfur in a liquid sulfur lake.] [Illustration: (Bottom) The IRIS on Voyager 1 found this “lava lake” to be the hottest region on Io, with a temperature about 150° C higher than that of the surrounding area.] [Illustration: One model for the structure of Io indicates that an ocean of liquid sulfur with a solid sulfur crust covers most of the satellite. Heat escapes from the interior in the form of lava, which erupts beneath the sulfur ocean. Secondary eruptions in the sulfur ocean heat liquid sulfur dioxide, which is mixed with solid sulfur in the crust. The rapid expansion of sulfur dioxide gas then produces the great eruptive plumes, which consist of a mixture of solid sulfur, sulfur dioxide gas, and sulfur dioxide snow.] Solid sulfur + solid SO₂ Solid sulfur + liquid SO₂ Molten sulfur Solid silicate The Voyager observations appear to confirm the theoretical calculations. The tidal heat source has presumably been acting since Io was formed more than 4 billion years ago. With a totally molten interior and continuing large-scale volcanism, Io has had an opportunity to thoroughly sort out its composition. In the process it would have lost all the volatile gases such as water and carbon dioxide, explaining why Io now has no appreciable atmosphere in spite of the outpouring of material from the interior. In addition, most of the sulfur from the interior could have risen to the surface, where it would be constantly recycled through volcanic activity.