HST images of lo taken in 1994 and 1995, 16 months apart, reveal a change dramatic than anything witnessed in the previous 15 years. A yellowish white circular feature 320 kilometers (200 miles) across developed where there previously a much smaller spot.
Io is one of the four large moons of Jupiter discovered by Galileo in 1610. It is about the same size as Earth's Moon and circles Jupiter once every 42 hours and 28 minutes. In March 1979, the interplanetary space probe Voyager 1 discovered large-scale volcanic activity on lo. Repeated eruptions and volcanic flows are continuously changing the surface, so its appearance varies noticeably from year to year. The vivid colored material and bright spots on lo are thought to be sulfur and chemical compounds of sulfur. The temperature on Io's surface is about -1 WC (-238'F), but the hot-spots where there is volcanic activity may be as warm as 1,000'C (1,800'F).
The HST picture on the left, taken in March 1994, shows only subtle changes on Io since it was last seen in close-up, by Voyager 2 in 1979. But by July 1995, the volcano Ra Patera. was surrounded by new material from a large volcanic explosion or fresh lava flows, seen in the image on the right. The new bright spot is much yellower than other bright regions of Io, perhaps because the material is so fresh.
Camera: WFPC2 in PC Mode.
Technical Information: Exposures through three color filters
(near-ultraviolet, violet and yellow) combined to create color images.
Credit: J. Spencer (Lowell Observatory), and NASA
