Spacecraft encouters Enceladus plume

Enceladus has an exotic watery plume spewing out of it. Cassini passed Enceladus first on March 27, coming within about 46 miles of the moon's surface. The encounter was primarily designed for Cassini's ion and neutral mass spectrometer, which tasted the composition of Enceladus' south polar plume. Other instruments, including the Cassini plasma spectrometer and composite infrared spectrometer, also took measurements. Before the closest approach of this encounter, Cassini's cameras imaged the plume, which is comprised of jets of water ice and vapor, and organic compounds emanating from the south polar region. Later, the cameras captured a nine-frame mosaic of the surface of the moon's leading hemisphere as the spacecraft left the moon. After the Enceladus encounter, Cassini passed the small moon Janus with a closest approach distance of 27,000 miles (44,000 kilometers). The planet was in the background in some of these views. Enceladus is a relatively small satellite, with a mean diameter of 505 kilometers, only one-seventh the diameter of Earth's own Moon. In diameter Enceladus is small enough to fit within the length of the island of Great Britain. Janus is extensively cratered with several craters larger than 30 km but few linear features. From its very low density and relatively high albedo, it seems likely that Janus is a very porous and icy rubble pile. The moon is also highly non-spherical. Early on March 28, the spacecraft flew by Dione at a distance of 27,000 miles (44,000 kilometers) and collected, among other observations, a nine-frame mosaic depicting the side of the moon that faces away from Saturn in its orbit. At 1122 km in diameter, Dione is the 15th largest moon in the Solar System, and is more massive than all known moons smaller than itself combined.[10] It is composed primarily of water ice, but as the third densest of Saturn's moons (after Enceladus and Titan).