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ISS007-E-10807 (21 July 2003) --- This view of Earth's horizon as the sunsets over the Pacific Ocean was taken by an Expedition 7 crewmember onboard the International Space Station (ISS). Anvil tops of thunderclouds are also visible. Credit: Earth Science and Remote Sensing Unit, NASA Johnson Space Center

Image Credit: ISS007-E-10807 (21 July 2003) – Earth Science and Remote Sensing Unit, NASA Johnson Space Center

Cassini Confirms Active Enceladus



The tiny moon orbiting Saturn named Enceladus has become the third body in the solar system known to have current volcanic activity driven by internal heating, after Earth and Jupiter’s moon Io. The surprising discovery was made after the Cassini spacecraft performed a close flyby of Enceladus on July 14, 2005 and returned the closest images yet of the moon. Data obtained by other instruments revealed that the moon’s south pole is warmer than its equator. Warm fractures and a cloud of water vapor over the region point to active processes replenishing Enceladus’ atmosphere and providing material for Saturn’s E-ring, the largest ring in the system.

Planetary geologists knew something strange was going on when the first images of Enceladus revealed a heavily fractured world with large regions devoid of impact craters. In April 2005 NASA announced that Cassini had discovered an atmosphere around the moon. Because Enceladus is so tiny, any atmospheric gases should readily escape the moon’s gravitational pull. Detection of a substantial atmosphere indicates that some process must be replenishing it.

A massive cloud of gases being vented from the south pole, the tortured appearance of the landscape, and the warmer temperatures there than even the equator of Enceladus all point to active internal heating, perhaps caused by the gravitational tug-and-pull between Enceladus and other objects in system. Enceladus’ south pole contains more fractures than any other region on the moon and house-sized boulders litter the terrain. Fractures called “tiger stripes” because of their regular and roughly parallel pattern appear to be most warmed by the internal heating. Evaporation of water caused by the warming of ice may result in a locally denser atmosphere.

Planetary scientists have long expected the outer moons to be inactive worlds with heavily cratered and ancient surfaces. The discovery of active tectonics at work on Enceladus will completely rewritten our understanding of the outer solar system. Enceladus is the second active world to be discovered orbiting Saturn. Although not yet confirmed, there has been some evidence for volcanic activity on Saturn’s largest moon, Titan. Several interesting features on the surface of Titan have appeared in images, but because the moon is surrounded by a thick nitrogen/methane atmosphere, scientists have been struggling to understand what they are seeing through the haze. The atmosphere around Enceladus is much thinner, allowing scientists to proceed more quickly with their analysis of data from the moon.

More surprising discoveries are certain as Cassini continues its four-year tour of the Saturnian system. The next close flyby will be of Titan on August 22, 2005. Cassini will not return for a close flyby of Enceladus until March 12, 2008.

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