Having looked at Titan and Europa yesterday, we can complete our outer planetary trifecta with a stop at Enceladus, lately of great interest as an active and possibly life-bearing moon. One hot spot detected there by Cassini is ejecting plumes of ice and vapor above the arid world in a cloud so fine that, according to William McKinnon (Washington University, St. Louis), the result is like a smoke made of ice, its particles about one-thousandth of a millimeter across. Enceladus is clearly a geologically active world, far from the inert desolation once expected. All of which makes the Saturnian moon intriguing in the extreme when you start wondering about the presence of water and the possibilities of life. But McKinnon is quick to dash that hope when asked bluntly whether there is evidence for a subterranean ocean: "I don't think so," McKinnon said. "The strongest piece of evidence against that is measurements made from Earth of the plume don't show any sodium. If the source of the...

The outer planets turn out to be far livelier places than we ever expected in those distant pre-Voyager days. Io set the tone, but look at all the activity we've found from Enceladus to Triton, and now we've got continuing Cassini revelations as well as new interpretations of what the Galileo probe found around Jupiter. Some of this is striking indeed, as witness a paper called to my attention by Larry Klaes and Adam Crowl discussing what may be happening on Europa in terms of energy. The apparent presence of that sub-ice ocean on Europa has made it of great interest for astrobiology. The problem has been the availability of energy. Christopher Chyba (Princeton University), working with Kevin Hand (Princeton) and Robert Carlson (Caltech) have used data from Galileo's Near-Infrared Mapping Spectrometer to determine that the Jovian moon could maintain an oxidized ocean. I should have the full text of this one shortly (right now I'm working solely from the abstract), but the gist of the...

We wrap up our look at the American Geophysical Union meeting last week in San Francisco with an update on Europa. An interesting point that William McKinnon (Washington University, St. Louis) made in a news briefing there has that what had been a belief that there is an ocean on Europa is now hardening into a certainty. The intriguing next step is to learn more about that ocean, and here things get tricky. Just how do you study an ocean on a distant world upon which you have yet to land? Fortunately, magnetic variations around Europa as well as observations of how the moon flexes and deforms as it orbits Jupiter can tell us much. Thus the need for a Europa orbiter, a mission that could measure gravity and magnetic fields as well as determining surface composition. Another area of interest for such a spacecraft: Are there recent eruptions from this geologically young world? The search for hot spots from such events could get interesting, to judge from what Cassini found on Enceladus....

The accepted take on the formation of Saturn's rings not so long ago was that they had emerged within the past 100 million years. The most likely driver: A comet that broke a larger moon into pieces, forming ring features seemingly consistent with what the Voyagers saw in the 1970s and the Hubble telescope has seen ever since. But leave it to Cassini to stir things up yet again with much more precise data suggesting that the rings did not form in a cataclysmic event but are continually recycled. Thus Larry Esposito (Colarado University, Boulder), who is also principal investigator for Cassini's Ultraviolet Imaging Spectrograph: "The evidence is consistent with the picture that Saturn has had rings all through its history. We see extensive, rapid recycling of ring material, in which moons are continually shattered into ring particles, which then gather together and re-form moons." The findings received less media attention than Voyager 2's crossing of the termination shock, discussed...

It seems extraordinary to speak of picking up pieces of an asteroid on the surface of the Earth, but the meteorites known as eucrites are confidently identified with Vesta, the brightest asteroid in the sky (and the only one visible with the naked eye). With the Dawn mission on its way to both Ceres and Vesta, we'll learn much more about the composition of both, but Vesta is coming into its own as a most unusual object that has contributed much to the surrounding system. For the 330-mile wide asteroid sports a huge gouge taken out of its south pole, apparently the result of a collision between protoplanetary objects. The hole, some eight miles deep, once contained a half million cubic miles of asteroid material that was subsequently blasted into interplanetary space, where interaction with Jupiter came into play. Gravitational tugging changes orbits, and some of these objects were put onto trajectories that brought them to Earth. Image: A 3-D computer model of the asteroid Vesta...

Thirty years ago it was all but impossible to tease the presence of Charon out of the Pluto images available to astronomers. Today we're using ground-based telescopes like the twin Keck instruments on Mauna Kea (Hawaii) to see the far tinier Nix and Hydra, the minute satellites discovered by the Hubble Space Telescope in 2005. Making Nix and Hydra visible results in Pluto and Charon, far brighter objects, appearing as a bright blob in the image at left. The streaks are the result of Pluto's motion against background stars during the exposure. Image (above): Nix and Hydra. This image combines all 16 exposures taken at Keck, with the contrast adjusted to show Pluto's new satellites Nix (left) and Hydra (right) as the small dots in the upper right. Both Nix and Hydra are about 5000 times fainter than Pluto, thus both Pluto and Charon are washed out in the image. The Pluto system moved with respect to the background stars during the one hour of observations, leaving the stars trailed....

Xanadu seems to be a misty, drizzly place. So say new images showing a persistent light rain of methane over the western foothills of this, the major continent of Titan. Titan's day is sixteen Earth days long, so if we say the drizzle or mist dissipates after about 10:30 AM local time, we're saying that it lasts until three Earth days after sunrise. As much as the Sun ever rises on this frigid, cloud-bound world. The work, conducted using data from the Keck Observatory (Hawaii) and Very Large Telescope (Chile), was presented today at the Division for Planetary Sciences meeting in Orlando (FL). The findings mark the first direct observation of methane rain, although precipitation has been presumed to occur for some time now. Features near Titan's poles have been interpreted as lakes of liquid hydrocarbons, presumably replenished by just such precipitation. Image: VLT and Keck near-infrared images of Titan's surface and lower troposphere can be subtracted to reveal widespread...

Ever since the launch of New Horizons in January of 2006 (can it really be that long?), the prospects of doing good science as the spacecraft whipped through the Jupiter system have tantalized and intrigued us. Eight spacecraft have now visited Jupiter, but New Horizons found things no previous mission had witnessed, including the evolution of a volcanic plume, lightning near the planet's poles, a tighter look at the Jovian rings and a trip down the unexplored length of the planet's magnetic tail. In addition to providing the gravity boost that will get New Horizons to Pluto faster, the Jupiter encounter was a valuable chance to shake out the spacecraft's instruments in preparation for the later encounter. And to hear principal investigator Alan Stern tell it, nothing could have gone better: "The Jupiter encounter was successful beyond our wildest dreams. Not only did it prove out our spacecraft and put it on course to reach Pluto in 2015, it was a chance for us to take sophisticated...

by Larry Klaes Tau Zero's Larry Klaes returns with more details on a novel form of propulsion that just might, in the long term, have interstellar implications. One of the most vital - and difficult - parts of a spacecraft is the type of propulsion it requires to move about in space. Most current forms of space propulsion, such as chemical fueled rockets, are both expensively heavy and explosively dangerous. Dr. Mason Peck and his team at Cornell University may have found a solution to this problem by utilizing the natural magnetic fields generated by our planet Earth and other worlds in space. "If our research is successful, we will have devised a new way of propelling spacecraft," declares Peck, who is an assistant professor of mechanical and aerospace engineering at Cornell, and the director of the Space Systems Design Studio. "We think of it as doing more with less. Instead of using rocket fuel, which is expensive, heavy, and often toxic, this technique allows spacecraft to...

Chance favors the prepared mind. So goes the old saying, never vindicated as clearly as in the encounter between Comet McNaught and the spacecraft called Ulysses. Thomas Zurbuchen (University of Michigan) notes that having a spacecraft on a mission to study the polar regions of the Sun pass through a cometary tail is chancy enough -- he likens it to putting your hand in Lake Michigan and pulling out a fish -- but having Ulysses already equipped with the needed instruments to study the solar wind means we had an unexpected chance to study the interactions between cold cometary materials from the Solar System's infancy and hot solar plasmas. Talk about being in the right place at the right time... Complex chemistry emerged from the serendipitous encounter, with O3+ oxygen ions showing the effects of cometary materials on the outgoing stream of solar wind ions. O3+ ions are oxygen atoms with a positive charge, resulting from the presence of five electrons instead of eight. Solar wind...

Jupiter's protective role for Earth has long been assumed, the theory being that the giant planet deflects asteroids and comets away from the inner Solar System. But studies on the subject are sparse, and focus on long period comets in extremely elliptical orbits. What about short period comets, and in particular the Jupiter Family of Comets (JFC)? These are comets thought to originate in the Kuiper Belt whose orbits are now controlled by Jupiter. Comet 81P/Wild 2, encountered by Stardust, was one of these, as was Comet Shoemaker Levy-9. We all saw in 1994 what a planetary impact from a comet could do when Shoemaker Levy-9 struck Jupiter. And a new study suggests that Jupiter's presence offers Earth no real protection from such objects. A research team at the UK's Open University set up computer models that examined the Jupiter Family, defined for these purposes as comets whose semi-major axes are smaller than Jupiter's and whose orbital period is less than twenty years. Some studies...

We're used to astronomical phenomena being relatively stable over the course of a human lifetime, which is why unexpected activity catches the eye. Think of the surprise over Io's volcanoes when they were first revealed by Voyager. And now we learn that the ring system around Uranus has changed significantly since Voyager 2 photographed it some twenty-one years ago. The inner rings have become more prominent, showing dusty material moving in otherwise empty regions of the rings. So reports Imke de Pater (UC-Berkeley), who led the team of astronomers investigating these changes, and presented the results at the European Planetary Science Congress today in Potsdam. Says de Pater of the activity: "People tend to think of the rings as unchanging, but our observations show that not to be the case. There are a lot of forces acting on small dust grains, so it is not that crazy to find that the arrangement of rings has changed." Assisting the investigation is the edge-on view we now have of...

Have scientists found a new category of asteroid? The evidence for basalt of a hitherto unseen composition on two small objects in the outer asteroid belt points to the possibility. An igneous rock, basalt would indicate that the asteroids were once part of a larger body, one that underwent some form of internal heating. The problem is that basalt is unusual for this part of the asteroid belt, nor is it clear whether the two fragments under study came from the same parent body. Add to that an unusual reflectance spectrum and the picture gets interesting indeed. The asteroids in question are (7472) Kumakiri and (10537) 1991 RY16, and therein lies a tale. The two were chosen from a group of six candidate asteroids thought to be classified as V-type, a name deriving from Vesta, the second largest of the asteroids. Not long ago it was thought that all basaltic V-type asteroids were simply fragments of Vesta, but in the past few years several V-type objects not belonging to this family...

With the European Planetary Science Congress now in session in Potsdam, we should have several interesting presentations to discuss this week, the first of which involves Titan. Huygens and Cassini have shown us a frigid, rocky surface under a surreal orange cloud of hydrocarbons, a place where liquid methane seems to flow in lakebeds and rivers. Methane in these circumstances plays much the same role that water plays here on Earth. Image: The surface of Titan as seen by the Huygens probe. Credit: ESA/NASA/JPL/University of Arizona. But what's under that interesting surface? The permafrost crust seems to sit on a soggy layer of ammonia, methane and water, beneath which an icy layer should surround a rocky core. It's that icy layer that interests Vasili Dimitrov (Tel-Aviv University), who is discussing its nature at the Potsdam conference. Dr. Dimitrov is hoping to put limits on the methane reserve available within Titan, which will tell us, among other things, whether that thick...

We always knew the surface of Enceladus was cold, but those tantalizing plumes breaking out of the Saturnian moon's south polar region gave hope of warmer things within. Liquid water fits with one model, pockets of which could account for the occasional geysers of ice crystals mixing with methane, nitrogen and carbon dioxide that Cassini has measured. Fill Enceladus with an internal ocean and the possibility of some kind of biology becomes an attractive study. But an alternate take on the plumes has been on view for some time now. Coming out of the University of Illinois, it's based on the idea that stiff compounds of ice called clathrates may cover Enceladus to a considerable depth. Whereas the warm interior model could produce such geysers, coined 'Cold Faithful' out of analogy to Yellowstone National Park's Old Faithful geyser, so could the clathrate model. But the latter, dubbed 'Frigid Faithful,' could operate far below the freezing point of water, with obvious implications for...

Saturn's moon Iapetus has given us something never before available: A history of its rotation and the effects of that rotation on its development. No other moon in the Solar System is quite like this one, for Iapetus maintains the shape it had when it was only a few hundred million years old. Cassini showed us that shape in a 2005 flyby, revealing a bulge at the moon's midsection, and a chain of mountains along its equator. How did the bulge form? The notion, presented in a recent paper published online in Icarus, is that Iapetus' walnut shape points to a much faster spin rate than we see today and a far warmer interior. The size of the bulge implies a rotation as fast as five hours per revolution, stretching the moon into its current oblate shape. By the time the rotation slowed, the outer shell had frozen and the excess material began to pile up in the mountain chain visible today at the equator. Image: The most unique, and perhaps most remarkable feature discovered on Iapetus in...

As if New Horizons didn't already have its work cut out for it, now we have the possibility of seeing a frigid geyser going off on Pluto's companion Charon when the probe arrives in 2015. The process is called cryovolcanism, the movement of liquid water onto the surface where it freezes into ice crystals. New high-resolution spectra obtained at the Gemini Observatory (Mauna Kea) show ammonia hydrates and water crystals spread patch-like across the surface of the distant world. The suggestion is that liquid water mixed with ammonia is pushing out from deep within Charon, leading to an interesting conclusion. Thus graduate student Jason Cook (Arizona State), who led the team surveying Charon's surface: "Charon's surface is almost entirely water ice. So it must have a vast amount of water under the surface, and much of that should be frozen as well. Only deep inside Charon could water be a liquid. Yet, there is fresh ice on the surface, meaning that some liquid water must somehow reach...

A human presence in space is one day going to mean something more than putting a crew into low Earth orbit or even going to the Moon. But longer journeys -- to Mars, to Jupiter's moons and beyond -- count among their many challenges the problem of radiation. To solve it, we'll have to start closer to home, puzzling out our own local radiation hazards from the Van Allen belts, those regions of high-energy electrons and ions caught within the magnetic field of Earth. Because electromagnetic waves can accelerate electrons, causing so-called 'enhancement events' or surges that are up to a thousand times more dense than the norm. The danger to spacecraft electronics can be acute. A powerful solar storm in 2003, for example, caused instrument damage to several spacecraft and may have been the cause of the loss of two Japanese satellites. We're learning that we need radiation-hardened systems that can withstand such battering. The 2003 event -- actually two storms that occurred back to back...

We're getting a closer look at Saturn's moon Hyperion, the result of data analysis following Cassini's flyby in September of 2005. Using near-infrared and ultraviolet spectroscopy, researchers have been able to analyze the moon's surface composition, with results suggestive of water and carbon dioxide ices as well as an analysis of dark material indicating hydrocarbons. That's a mix of materials not unlike what we've found in comets and probably similar to what we'll detect in Kuiper Belt objects. Here's Dale Cruikshank (NASA Ames), lead author on the paper: "Of special interest is the presence on Hyperion of hydrocarbons -- combinations of carbon and hydrogen atoms that are found in comets, meteorites, and the dust in our galaxy. These molecules, when embedded in ice and exposed to ultraviolet light, form new molecules of biological significance. This doesn't mean that we have found life, but it is a further indication that the basic chemistry needed for life is widespread in the...