Last updated on April 20, 2014 at 5:20 EDT
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April 22, 2005
Io has a diameter of 3660 km and orbits Jupiter at a mean distance of 422,000 km - one revolution takes 42.5 hours. Like the Earth's moon, it always turns the same side towards the planet. As shown by the Voyager spacecraft in 1979, its surface is covered by active volcanoes and lava fields - it is in fact the most volcanic place known in the solar system.

Due to this activity, Io's surface is continuously reshaped. The features now seen are all correspondingly young, with a mean age of the order of 1 million years only. The variations in appearance and colour are due to different volcanic deposits of sulphur compounds. The cause of all this activity is Jupiter's strong gravitational pull that leads to enormous stresses inside Io and related heating of the entire moon.

This image is a composite of two images at different exposures. The first image was obtained through a near-infrared, narrow optical filter (Brackett-gamma at wavelength 2.166 µm). The second was obtained at longer wavelength (3.8 µm). A latitute-longitude grid has been superposed, with the most prominent features identified by name, including some of the large volcanoes and sulphurus plains on this very active moon.