Back in the days when the Solar System seemed a simpler place, asteroids were thought to be chunks of rocks whose features could be explained by impacts with other such objects. Comets were altogether different, laden with icy material that erupted when heated by the Sun. It was a straightforward picture, at a time when the system had nine planets, icy 'dwarf planets' were not yet in vogue, and distinctions between orbiting objects were clearly drawn. Today we work with a more complicated scenario, one in which some objects once thought of as asteroids develop comet-like features that can last for months. Thus the interest in the asteroid called (596) Scheila, which late last year developed plumes after brightening unexpectedly in December. Orbiting the Sun every five years, Scheila has a diameter of about 110 kilometers, and evidence from both the Hubble Space Telescope and the Swift satellite now indicates that the unusual activity here was caused by a collision with a smaller...

Last week we looked at the possibility of using a planet's aurora as an exoplanet detection tool, speculating that the LOFAR radio telescope in Europe might be able to detect such an emission, and I reminisced about listening for emissions from Jupiter on an old shortwave receiver. Jonathan Nichols' work at the University of Leicester makes the case for exoplanet detections, and recent news from analysis of Cassini data indicates that planetary aurorae can do more than just flag the presence of a planet. In some cases, they can provide information about that planet's moons, as in the case of Saturn, where careful analysis may offer us new insights into Enceladus. For it turns out that the electrical connection between Saturn and Enceladus is rather robust. Moreover, it was a connection that scientists had anticipated, given that in the Jovian system, Io creates a glowing auroral footprint near Jupiter's north and south poles. Why not expect something similar in the case of Enceladus,...

The study of carbon monoxide found in the atmosphere of Pluto -- a strong signal rendered in data from the James Clerk Maxwell Telescope in Hawaii -- gives us insight into the significant changes happening to the dwarf planet on its 248-year orbital path around the Sun. Pluto is one of a kind, offering us a cold planetary atmosphere that shows marked changes over time, an atmosphere that may at times freeze out and settle to the surface. No other dwarf planet is known to have an atmosphere, and Pluto's is now known to vary in pressure and probably in composition. Jane Greaves (University of St. Andrews) led the team doing the new Pluto work, and she's the first to admit that what the data revealed was a surprise. Pluto reached perihelion in 1989, its closest approach to the Sun, so astronomers assumed that as it began to recede, the atmosphere would contract. But as measured by the occultation of background stars, the atmospheric pressure and size actually increased between 1988 and...

These days funding for missions to some of the most interesting places in the Solar System is much in question. But sooner or later we're going into the outer system to investigate the possibilities for life on worlds like Europa, Enceladus or Titan. The case for Europa seems particularly compelling, but we have to be careful about our assumptions. When the Europa Orbiter Science Definition Team developed a strategy for Europan exploration in 1999, it was generally believed that any Europan ocean would be covered by a thick and impermeable layer of ice. Life, then, might exist around deep sub-oceanic volcanic vents if it existed at all. Thus the strategy for Europan exploration that evolved: Three missions, beginning with an orbiter, followed up by a lander and, finally, a third mission that would drill down through the presumably many kilometers of surface ice to explore whatever lay beneath. Even in more financially optimistic times, that strategy didn't get us into Europa's ocean...

Back in December, scientists from the Cassini team presented evidence for ice volcanoes on Titan, looking at a region called Sotra Facula, which bears some resemblance to volcanoes on Earth like Mt. Etna in Italy and Laki in Iceland. An ice volcano, also known as a cryovolcano, would draw on geological activity beneath the surface that warms and melts parts of the interior and sends icy materials through a surface opening. Sotra Facula's two 1000-meter peaks combine what appear to be deep volcanic craters with finger-like flows of material, a kind of surface sculpting that could explain some of the processes occurring on other ice-rich moons. But work like this is part of an ongoing dialogue testing various hypotheses, and the latest round takes us in a sharply different direction. In a new paper in Icarus, Jeff Moore (NASA Ames) and Robert Pappalardo (JPL) argue that Titan is in fact much less geologically active than some have thought. A cool and dormant interior would be incapable...

Send a spacecraft into the outer Solar System and unexpected things can happen. We're all anticipating the arrival of the New Horizons probe at Pluto/Charon in 2015, but the work the spacecraft has done along the way has recently been highlighted again. Moving toward Jupiter in 2007, New Horizons was programmed to image Jupiter's ring system in the hope of catching details about an odd effect. Back in the 1990s, the Galileo probe had shown unusual patterns in the Jovian ring, and the New Horizons imagery was able to spot not only the patterns Galileo had seen but two new ripple patterns as well, evidence of recent events in the Jovian system. Image: The New Horizons spacecraft took the best images of Jupiter's charcoal-black rings as it approached and then looked back at Jupiter in February 2007. The top image was taken on approach, showing three well-defined lanes of gravel- to boulder-sized material composing the bulk of the rings, as well as lesser amounts of material between the...

Studying the heliosphere and its interactions with the interstellar medium isn't easy, which is one among many reasons we follow the fortunes of the Voyager probes with such continuing fascination. They're pushing up against the boundary between the Sun's local 'bubble' and deep space beyond, where charged particles from the solar wind are no longer a factor and the deeper rhythms of the galaxy take hold. Now our other probe of this exotic region is back in the news in a new paper. IBEX (the Interstellar Boundary Explorer) is telling us much about how our system interacts with the interstellar medium and the effects of the galactic magnetic field upon the heliosphere. IBEX has provoked much discussion in these pages -- I was amazed to see I had written fully thirteen articles on the mission in the last six years, going back to pre-launch speculations. The mission caught my eye because it was the first ever sent with the express purpose of studying the outer edges of the Solar System....