The surface of Io displays a wide variety of colors—pale
shades of red, yellow, orange, and brown. Mixtures of
sulfur, sulfur dioxide frost, and sulfurous salts of sodium
and potassium on the moon’s surface are the likely cause
of the colors. Bright patches may be fields of sulfur dioxide
snow. Liquid sulfur dioxide flows beneath Io’s surface,
held at high pressure by the weight of overlying material.
Like water from a spring, this pressurized sulfur dioxide
is pushed out though fractures in the crust, producing
sprays of sulfur dioxide snow crystals that travel for up to
hundreds of kilometers before settling back to the moon’s
surface. A similar process takes place in a carbon dioxide
fire extinguisher. These fire extinguishers contain liquid
carbon dioxide at high pressure that immediately turns to
“dry ice” snow as it leaves the nozzle.