This picture of Io was taken as Voyager 2 closes in on the Jovian system.

This picture of Io was taken as Voyager 2 closes in on the Jovian system. Scientists are studying these distant views of Io for evidences of changes since Voyager 1 observations in March of 79. Voyager 1 discovered that Io, the innermost of the Galilean satellites, is the most volcanically active body yet seen in the solar system, surpassing even earth. In this picture, the first volcano discovered by Voyager 1 is again visible in the lower left portion of the disk as a dark oval with a dark spot in the center. In March, this volcano appeared as a heart-shaped marking, not a symmetrical oval. Scientists believe that the non-symmetric markings earlier resulted from a constriction in the mouth of the volcanic vent causing erupting material to extrude preferentially in certain directions. Apparently, the volcanic eruptive activity, which sends material to altitudes of 280 kilometers (175 miles) or more above this volcano, has changed the vent geometry or dislodged an obstruction. Such changes in the form of eruptive fountains are common in terrestial volcanos, although on a much smaller scale than on Io.