Saturn's satellite Enceladus is shaping up as the star of the Cassini mission. Observations last year suggested that plumes of water vapor and dust tower above the moon's southern hemisphere, feeding a tenuous atmosphere and producing one of Saturn's rings. Now Cassini's camera has caught the plumes in the act. They appear to vent out of “tiger stripes”—parallel cracks that glow conspicuously in infrared images, a sign of escaping heat. Enceladus thus becomes the fourth body in the solar system (after Earth, Jupiter's moon Io and Neptune's moon Triton) with known active volcanism. What drives its geologic activity and frames its fearful north-south asymmetry, no one dares to say. The satellite is so small, just 500 kilometers across, that heat from its distant deeps should have leaked out long ago, and tidal forces do not seem up to the task. Additional details may have to wait for the next Cassini close fly-by, in March 2008.
This article was originally published with the title "Tiger, Tiger, Burning Bright" in Scientific American 294, 2, 28 (February 2006)