As "Voyager 1" approached the Jupiter system in March 1979, it acquired numerous images of the planet and its four largest satellites, including Io. One of the most distinctive features of these distant images of Io was a large, elliptical, footprint-shaped ring on the satellite's trailing hemisphere (the side facing away from the direction of motion in a synchronously-rotating satellite like Io). During the encounter itself on March 5, 1979, "Voyager 1" acquired high-resolution images of the footprint-shaped region. At the center of bow tie-shaped dark region in the middle of the ring was a depression partially filled with dark material, by in size. This depression, later found to be the source of the Pele volcano, is at the northern base of a rifted mountain later named Danube Planum. With the other dramatic evidence for volcanic activity on the surface of Io from this encounter, researchers hypothesized that Pele was likely a caldera.