By now, our outer Solar System probes have brought us so many surprises that finding yet another one should be passé. But it never is, and imagine the wonders we'll find, for example, when New Horizons arrives at Pluto/Charon and moves on to the Kuiper Belt beyond. Now Cassini has once again made news with the discovery of what may be described as an 'ice volcano.' The beauty of the concept is that it may explain the presence of atmospheric methane on Titan. "Before Cassini-Huygens, the most widely accepted explanation for the presence of methane in Titan's atmosphere was the presence of a methane-rich hydrocarbon ocean," said Dr. Christophe Sotin, distinguished visiting scientist at NASA's Jet Propulsion Laboratory, Pasadena, Calif. That otherworldly ocean was the most exotic of constructs, creating images of the Huygens lander bobbing about amidst frigid swells as its batteries failed. What the probe parachuted into was quite different. In a way, it's a shame to lose the...

Deep Impact's arrival at Comet Tempel 1 should be spectacular. Due to reach the comet on July 4 of this year, the two-part probe will launch a 360 kg impactor designed to produce a crater on the comet's nucleus and a plume of gas and dust. Getting inside a comet is a key mission goal: the ejected material should tell us much about the early days of the Solar System. Right now astronomers are engaged in making baseline observations through Earth-based telescopes to characterize Tempel 1 as completely as possible. By examining its albedo (reflectivity), rotation period and size, the mission team will be able to differentiate between impact effects and the natural activity of the comet during the Deep Impact encounter. The photograph on the right (click to enlarge) shows an image of Tempel 1 together with a number of visual artifacts. What has happened here is that images were taken through various filters, one after the other. The motion of the comet against background objects accounts...