Is there really an underground ocean on Enceladus? The Cassini spacecraft's striking images have created a cottage industry in speculation, with spectacular glimpses of erupting plumes composed of ice and water vapor. This week, however, we get two contrasting views on what all this means. In one, a paper in Nature by a European team led by Frank Postberg (Universität Heidelberg), studies of sodium salts in dust ejected by the Enceladus plumes reveal telltale signs of a salty ocean deep below the surface. Postberg was working with data from the Cosmic Dust Analyzer (CDA) instrument aboard Cassini, and the results imply a level of sodium chloride that may be as high as that found in Earth's oceans. The data come from ice grains in Saturn's E-ring, which is thought to consist largely of material from Enceladus. Thus we seem to be gathering direct evidence for the presence of the hypothesized ocean, which should be salty from long contact with the rocky core. But not so fast. The same...

With a dense atmosphere of nitrogen and methane, Titan is the only moon in our Solar System that shares Earth-like characteristics in climate. But Titan's climate, receiving one hundred times less sunlight at ten times Earth's distance from the Sun, operates at a much slower pace. The seasons on the distant moon last more than seven Earth years, and the motion of its clouds is slow and deliberate. We've had a good look via the Cassini spacecraft at the movement of those clouds, some two hundred of them being examined between July 2004 and December 2007 in a study of global circulation patterns. Summer changes to fall at the equinox in August of this year. We're at a time when the circulation models say clouds in the southern latitudes should have already disappeared, but it's clear from the Cassini imagery that many clouds remained as late as 2007. Image: This infrared image of Saturn's moon Titan shows a large burst of clouds in the moon's south polar region. These clouds form and...

It was Percival Lowell who suggested that anomalies in the orbit of Uranus might point to the existence of the body he called 'Planet X.' The discovery of Pluto in 1930 gave us confirmation of a planet beyond Neptune (since downgraded, of course), but the idea of other large bodies in the outer Solar System still has its appeal, and although we've found such interesting objects as Eris and Sedna, questions remain about what else might be found lurking at the fringes of the system. Theories of the Outer System Thus the active theorizing, which includes one study speculating on an Earth-sized planet at 100 to 170 AU, a body that would help to explain what we know about the architecture of the Kuiper Belt. Another investigation looked at a possible Mars-sized body at 60 AU, which would help us understand the distribution of various Trans-Neptunian Objects, a term that basically covers any object orbiting the Sun beyond Neptune. Other theories abound, one of which sees a giant planet...

by Larry Klaes Our fascination with ringed worlds continues to grow as we learn more about what circles the worlds of the outer system. If you're looking for what may be the most spectacular ring system imagined -- two ringed exoplanets locked in a tight gravitational embrace -- be sure to read Jack McDevitt's novel Chindi (Ace, 2003), and spend some time with his crew on the surface of the moon that orbits their center of mass. Meanwhile, join Tau Zero journalist Larry Klaes as he focuses on continuing revelations from Cassini about Saturn's rings and the moons that feed them. And join us in our celebration of the extended Cassini mission. Who knows what discoveries await? In the 1968 novel version of 2001: A Space Odyssey, author Arthur C. Clarke said that the magnificent rings of the gas giant planet Saturn were made by visiting advanced extraterrestrial intelligences who tore up some moons in the Saturn system in the process of making their incredible Star Gate. This artificial...

Every now and then a new space photo completely snares the attention. This one is a Hubble shot showing four of Saturn's moons moving in front of the planet. Note Titan at the top, while below it from left to right are Enceladus, Dione and (at extreme right) Mimas. To see the smaller moons, you'll want to click the image, which will take you to a zoomable view that captures these tiny satellites against the immensity behind them. Image: Saturn and four of its moons, as seen by Hubble's Wide Field Planetary Camera 2 on February 24, 2009, when Saturn was at a distance of roughly 1.25 billion kilometers from Earth. Hubble can see details as small as 300 km across on Saturn. The dark band running across the face of the planet slightly above the rings is the shadow of the rings cast on the planet. Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA). The transit of the four moons is an unusual event because the rings only become tilted edge-on to Earth every fourteen to fifteen...

With an atmospheric pressure one hundred thousand times less than that on Earth, Pluto becomes an even more intriguing object than usual when it moves closer to the Sun in its 248-year orbit. This period, occurring now, causes the temperature of the surface to increase, and that causes what had been frozen nitrogen (with trace amounts of methane and, probably, carbon dioxide) to sublimate into gas. Studying these matters with ESO's Very Large Telescope, astronomers have now found unexpectedly large amounts of methane in that atmosphere. Image: Artist's impression of how the surface of Pluto might look, according to one of the two models that a team of astronomers has developed to account for the observed properties of Pluto's atmosphere. The image shows patches of pure methane on the surface. At the distance of Pluto, the Sun appears about 1000 times fainter than on Earth. Credit: ESO/L Calçada. A second discovery: The atmosphere of the distant ice world is some forty degrees Celsius...

With the Kepler launch scheduled for no earlier than Friday, I'm keeping one eye on the mission site while I develop today's material. Kepler launches aboard a Delta II, but engineers are now having to check common hardware between that rocket and the Taurus XL launch vehicle that failed to get NASA's Orbiting Carbon Observatory into orbit last week. Thus the March 5 launch date slips to March 6, which itself is still tentative. Meanwhile, an unusually interesting story in Nature also has my attention, dealing not with exoplanets but with the early Solar System and what may have been a period of planet migration that caused heavy asteroidal bombardment of the inner planets. This one comes out of the University of Arizona, where scientists have been looking at the distribution of asteroids with diameters greater than fifty kilometers. UA's David Minton and Renu Malhotra ran simulations beginning with a uniform asteroid belt to see how the present-day gaps in the belt may have arisen...

You wouldn't think the thickness of ice on a distant moon of Jupiter could emerge as something of a political hot-button, but that seems to be what has happened in the ongoing investigation of Europa. Thick ice or thin? The question is more complicated than it looks, because by 'thin' ice we don't mean just a few inches, but perhaps ten kilometers, perhaps five. The key question is not a specific measurement, but whether the ice is thin enough to allow the surface and the global ocean beneath to be connected, in the form of occasional cracks, melt-throughs or other events. Much hinges on the answer. As Richard Greenberg shows in Unmasking Europa: The Search for Life on Jupiter's Ocean Moon (Springer, 2008), the small world quickly fell under the scrutiny of scientists with a geological bent after first Voyager and then Galileo imagery became available. The latter was a problem, for the failure of the spacecraft's high-gain antenna meant the total number of images was sharply reduced,...

Comet C/1919 Q2 Metcalf catches the attention. The intriguing object was discovered in August of 1919 and remained visible until early 1920, but no subsequent observations have been made. In 1973, Allan Cook discovered that the Omicron Draconids meteor stream seemed to be following the orbit of the earlier comet. Suspicion is strong that the comet broke up and that the Omicron Draconids are simply the result of that event, a manifestation of cometary debris. All of which makes the fireball that streaked through European skies last July a bit more interesting than your average bolide. A new paper will suggest that the boulder that caused it -- probably a meter across and massing 1.8 tons -- was a chunk of the original comet, a boulder that broke apart from the original ice and rock nucleus as C/1919 Q2 Metcalf disintegrated. That would mean we have comet fragments out there waiting to be discovered. Josep M. Trigo-Rodríguez (Institute of Space Sciences, CSIC-IEEC, Spain) explains: "If...

Asteroid (234) Barbara is an unusual object, a denizen of the main belt that may be a binary. The European Southern Observatory's Very Large Telescope Interferometer is able to piece together two bodies, with diameters of 37 and 21 kilometers respectively, separated by a bit over 20 kilometers. But as seen from Earth, the objects seem to overlap, so we don't know whether this is a true binary or an asteroid in the shape of a giant peanut. The former would be more interesting, for if we can calculate the orbits of these objects and combine them with diameter measurements, we'll learn about their density. This is why Sebastiano Ligori (INAF-Torino, Italy) calls Barbara "...a high priority target for further observations." Ligori is one of the researchers who used the combined light from two of the Very Large Telescope's 8.2-meter instruments to make these interferometric studies, creating a view as sharp as a single telescope whose diameter is as large as the separation between the...

Rain seems to have been plentiful at Titan's south pole. A new analysis of Cassini imagery compares the region in recent times with what it was about a year earlier, noting new features in areas many scientists believe to be lakes of liquid hydrocarbons. Adding to the conjecture is the fact that extensive cloud systems covered the region during this period, evidence for a large rainstorm amid changing seasons. All this comes from the almost global surface map Cassini's Imaging Science Subsystem has been acquiring since April of 2004. Have a look at some of this imagery, and keep an eye in particular on Ontario Lacus, at the bottom of each image, noting the difference in brightness. Image (click to enlarge): The images on the left (unlabeled at top and labeled at bottom) were acquired July 3, 2004. Those on the right were taken June 6, 2005. In the 2005 images, new dark areas are visible and have been circled in the labeled version. The very bright features are clouds in the lower...

The Chicago Tribune offered up a Christmas day story on ENDURANCE, the NASA robot recently sent out for a shakedown mission in Lake Bonney, Antarctica. The lake is locked down under fifteen feet of ice, a place that could prefigure what we find under the ice on Europa. ENDURANCE stands for Environmentally Non-Disturbing Under-ice Robotic ANtarctiC Explorer, a vehicle created by Texas-based Stone Aerospace that is the successor to the Deep Phreatic Thermal Explorer (DEPTHX), which explored Mexican geothermal sinkholes in early 2007. The Lake Bonney expedition is covering its story via blog entries accessible here, the most recent being a note from December 21, dealing with navigation in an environment rich in icebergs up against the face of a glacier. You may remember from an earlier story that ENDURANCE spent several days in the water at Lake Mendota, on the University of Wisconsin's campus, last winter, with the new work pushing it into much harsher conditions. And history buffs...

Europa is interesting enough without throwing in a new theory about energy sources. But Robert Tyler (University of Washington) has been studying the possibilities in Europan tides, using computer simulations that offer a different way of getting energy out of this icy world. We've speculated that Europa experiences enough tidal flex from Jupiter to create possible energy sources for life. What Tyler is saying is that the moon may experience not just internal pressures but large waves pushing through the submerged ocean. These waves, of course, could be a way of distributing heat and dissipating tidal energies. This being the case, the assumption that energy may come from flexing at the core, as well as pressures on the oceanic ice sheets, has to be supplanted by a different view: "If my work is correct then the heat source for Europa's ocean is the ocean itself rather than what's above or below it," Tyler says. "And we must form a new vision of the ocean habitat that involves strong...

I first encountered the surface of Jupiter decades ago, in a study hall in John Burroughs School in St. Louis, Missouri. It was a warm spring day and I was theoretically trying to bone up for a math test two periods hence. But deciding to squeeze in a little reading before I hit the algebra, I read the paragraphs that follow and spent little of the next two hours thinking about anything else: The wind came whipping out of eastern darkness, driving a lash of ammonia dust before it. In minutes, Edward Anglesey was blinded. He clawed all four feet into the broken shards which were soil, hunched down and groped for his little smelter. The wind was an idiot bassoon in his skull. Something whipped across his back, drawing blood, a tree yanked up by the roots and spat a hundred miles. Lightning cracked, immensely far overhead where clouds boiled with night. As if to reply, thunder toned in the ice mountains and a red gout of flame jumped and a hillside came booming down, spilling itself...

The phrase 'liquid water' is enough to quicken the pulse of the steeliest-eyed astrobiologist. We've long defined the concept of a habitable zone -- that zone around a star in which life might flourish -- by the presence of liquid water at the surface. But as we start pondering liquid water beneath the ices of outer satellites like Europa, we extend our investigations in exciting new ways. No wonder the new evidence of liquid water inside Enceladus received such attention in the mainstream media before the terrible news from Mumbai took center stage. Image: In this artist's concept, the Cassini spacecraft makes a close pass by Saturn's inner moon Enceladus to study plumes from geysers that erupt from giant fissures in the moon's southern polar region. Credit: Karl Kofoed (Drexel Hill, Pennsylvania). On one level, liquid water on such cold, distant worlds is exciting because of the possibility of finding life that has arisen completely independent of what happened on Earth. At...

The pace of change being what it is, adjusting our time frames can be a difficult task. That's particularly true in the planning of space missions, where the gap between what we seem able to do and the actual window for doing it can become as large as it is frustrating. NASA and the European Space Agency, for example, will make a choice in 2009 between a Jupiter/Europa mission and a project called the Titan and Saturn System Mission (TSSM). Whichever is chosen, the projected launch date is at least twelve years away, with arrival expected no earlier than 2030. The lengthy interval is the inevitable result of the complexity of mission planning and the realities of orbital mechanics. We're always in a hurry about space missions because we're so anxious for new information, but absent propulsion breakthroughs, we're still tied to multi-decade planning cycles. Even so, as we investigate ways to fly missions faster, the prospect of what we might do with the Titan and Saturn mission looms...

As if we didn't have enough trouble piecing together how planets form, we're now learning that objects much smaller than planets -- the planetesimals that collide and agglomerate to form planet-sized objects -- can be the subject of melting. The work, led by Benjamin Weiss at MIT, suggests that objects on the scale of 160 kilometers across were large enough to melt almost completely, a counter-intuitive notion that would explain the magnetism found in certain meteorites, which until now has remained a mystery. Weiss' team studied the record of this magnetic field as preserved in three angrite meteorites from the early Solar System. Such study is known as paleomagnetism, examining the record of magnetic fields as preserved in various magnetic minerals, and the angrites involved are thought to record the earliest stages of planet formation. The record of their magnetism extends beyond the lifetime of the early circumstellar disk, leading Weiss to conclude that the fields were produced...