Astronomy Picture of the Day

Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer.

2022 December 11
The featured image shows Jupiter's moon Io which is
bright yellow from sulfur and covered with volcanoes and 
volcanic floes.
Please see the explanation for more detailed information.

Io in True Colour
Image Credit: NASA, JPL, Galileo Project

Explanation: The strangest moon in the Solar System is bright yellow. The featured picture, an attempt to show how Io would appear in the "true colours" perceptible to the average human eye, was taken in 1999 July by the Galileo spacecraft that orbited Jupiter from 1995 to 2003. Io's colours derive from sulfur and molten silicate rock. The unusual surface of Io is kept very young by its system of active volcanoes. The intense tidal gravity of Jupiter stretches Io and damps wobbles caused by Jupiter's other Galilean moons. The resulting friction greatly heats Io's interior, causing molten rock to explode through the surface. Io's volcanoes are so active that they are effectively turning the whole moon inside out. Some of Io's volcanic lava is so hot it glows in the dark.

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