page_title: Io (moon)
section_title: Subsequent observations
hierarchical_section_title: Io (moon) / Observational history / Subsequent observations
caption_attribution_description: English: This montage compares similar sides of Io photographed by the Galileo spacecraft in October 1999 (left) and the New Horizons spacecraft on February 27, 2007. The New Horizons image was taken with its Long Range Reconnaissance Imager (LORRI) from a range of 2.7 million kilometers (1.7 million miles). Most features on Io have changed little in the seven-plus years between these images, despite continued intense volcanic activity. The largest visible feature is the dark oval composed of deposits from the Pele volcano, nearly 1,200 kilometers (750 miles) across its longest dimension. At high northern latitudes, the volcano Dazhbog is prominent as a dark spot in the New Horizons image, near the edge of the disk at the 11 o'clock position. This volcano is much less conspicuous in the Galileo image. This darkening happened after this 1999 Galileo image but before Galileo took its last images of Io in 2001. A more recent change, discovered by New Horizons, can be seen in the southern hemisphere (circled). A new volcanic eruption near 55 degrees south, 290 degrees west has created a roughly circular deposit nearly 500 kilometers (300 miles) in diameter that was not seen by Galileo. Other New Horizons images show that the plume that created this deposit is still active. The New Horizons image is centered at Io coordinates 8 degrees south, 269 degrees west.
context_page_description: Io, or Jupiter I, is the innermost and third-largest of the four Galilean moons of the planet Jupiter. It is the fourth-largest moon in the Solar System, has the highest density of all of them, and has the lowest amount of water of any known astronomical object in the Solar System. It was discovered in 1610 by Galileo Galilei and was named after the mythological character Io, a priestess of Hera who became one of Zeus's lovers.
  With over 400 active volcanoes, Io is the most geologically active object in the Solar System. This extreme geologic activity is the result of tidal heating from friction generated within Io's interior as it is pulled between Jupiter and the other Galilean satellites—Europa, Ganymede and Callisto. Several volcanoes produce plumes of sulfur and sulfur dioxide that climb as high as 500 km above the surface. Io's surface is also dotted with more than 100 mountains that have been uplifted by extensive compression at the base of Io's silicate crust. Some of these peaks are taller than Mount Everest, the highest point on Earth's surface.
context_section_description: Following Galileo's planned destruction in Jupiter's atmosphere in September 2003, new observations of Io's volcanism came from Earth-based telescopes. In particular, adaptive optics imaging from the Keck telescope in Hawaii and imaging from the Hubble telescope have allowed astronomers to monitor Io's active volcanoes. This imaging has allowed scientists to monitor volcanic activity on Io, even without a spacecraft in the Jovian system.
caption: Changes in surface features in the eight years between Galileo and New Horizons observations
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