What is P s e u d o c o l o r ?

When we want to measure very small changes in brightness, it helps to portray a black-and-white ("grayscale") image in color. One method is to assign each pixel a color based on its brightness level.

The picture below of Io was taken through the clear filter while the moon was eclipsed by Jupiter's shadow. The small bright spots are volcanic eruptions. The brightest is Pillan, and beside it is the Pele volcano. If you look closely you can also make out a diffuse glow surrounding the satellite, concentrated near some of the volcanic vents. This is the faint aurora of Io's tenuous atmosphere.

  • Io in Jupiter's shadow

    It is easier to see Io's aurora when we look at the pseudocolored image on the right. You can estimate its brightness with the help of a scale:

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