Tiger, Tiger, Burning Bright

Abstract

The Cassini spacecraft has been touring Saturn's neighborhood since it entered orbit there on 1 July 2004. As well as interrogating the giant planet and its filigree rings, understanding the formation of Saturn's moons is one of the key goals of the mission. Saturn has 35 named moons in its family, as well as a plethora of smaller objects hidden in the rings. The variety of moons is staggering. Some consist of exposed rock; others are enveloped in thick ice. Some are pock-marked with billion-year-old craters; others varnished with fresh snow. Cassini's busy schedule included more than 20 encounters with Saturn's moons in the past year, offering but a sampling of these different worlds. One of the most curious of all Saturn's moons is Enceladus. The sixth largest saturnian satellite—although at a mere 504 km across, its width is smaller than that of Arizona or Spain—Enceladus rides in the middle of Saturn's E ring, a wide and diffuse blue ring of fine particles. Because this ring lacks the bigger shards characteristic of ancient satellite smash-ups, astronomers have long suspected that the E-ring particles emanate from Enceladus itself. Earlier Earth-based and Voyager space probe images revealed that Enceladus' surface is icy and complex. Old cratered terrains butt up against newly resurfaced smooth ice flows. So Cassini scientists hoped to see signs of recent activity and possibly ice volcanism. As the papers in this special issue show, their anticipation was rewarded. In three flybys, in February, March, and July 2005, Cassini trained its instruments on Enceladus. The first two cruises descended to about 1000 and 500 km above the moon's equator. Onboard cameras snapped images of lines of folded mountain ridges and cracked white ice plains streaked with dark green organic material. The magnetometer saw signs of ions leaking out from Enceladus'atmosphere, and it localized, in the second flyby, a strong outflow from the south pole. The trajectory of the third flyby on 14 July 2005 was then adjusted to fly through the emergent gas just 168 km above the south pole. The flybys show that Enceladus' south polar landscape is still active today and is being resurfaced by cryovolcanism and fresh snowfall. Tidal forces have twisted and buckled the surface ice, producing long ridges and fractures. At infrared wavelengths, the south pole actually glows. An underground heat source lies beneath a surface grid of “tiger stripes”: a parallel set of linear trenches stained with dark organic material. From these warm vents, water vapor, ice, and dust particles are lofted in a spectacular plume, like spray from a Yellowstone geyser. Ions are driven further into the atmosphere and out into the magnetosphere of Saturn itself and the E ring. The rate at which water is being blown out is enough to replenish not only the E ring but also oxygen throughout the whole saturnian system. So is the mystery of Enceladus solved? Not at all. Finding such active geology on such a tiny moon is a big surprise. Neptune's Triton and Jupiter's Io have ice and sulfur volcanoes, respectively, but are larger bodies; Europa has experienced plate tectonics but no current activity is seen there. Yet tiny Enceladus produces a plume large enough to drench the whole Saturn system. The origin of Enceladus' internal heating is also still a major puzzle. To produce the plume, water needs to be boiled off or sublimated. Ammonia could act as antifreeze but was not detected by Cassini. Does liquid water, or locked-up ammonia, lie beneath the surface? We may need to wait for future missions to this enigmatic moon to find out for sure, but Enceladus is definitely on the map.