One of many new Cassini images received after yesterday’s successful Enceladus flyby. Image Credit: NASA/JPL/Space Science Institute - “W00043237.jpg“

Slow server speeds mark the arrival of new raw data on the Cassini-Huygens website taken during yesterday’s successful Enceladus flyby. In addition to the images, Cassini sampled the material in Enceladus’ water ice plumes erupting from the moon’s south pole. Images like the one above will be validated and calibrated while scientists pour over all the new data, hoping for insight into the process that leads to the geyser-like features.

Cassini took this image of Enceladus’ water ice plumes on November 27, 2005.  New pictures and other data from the most recent flyby on Wednesday, March 12, 2008 are expected on Thursday, March 13, 2008.  Image Credit: NASA/JPL/Space Science Institute - “Jet Blue“

NASA’s Cassini spacecraft has successfully completed the closest flyby of Saturn’s mysterious moon Enceladus yet. Coming as close as 52 kilometers (32.3 miles) from the surface of Enceladus, Cassini used its array of instruments to sample the water ice geysers that erupt from the moon’s south pole. The close, and somewhat risky, flyby maybe provide just the required data to begin understanding the source of these geysers. These data have started to arrive at the Earth and will be completely downloaded over the next several hours.

On Wednesday, March 12, 2008 around noon PST, Cassini flew away from Enceladus along the outer extent of the plume that surprised scientists just over two years ago. The surprise was due to the moon’s small size; small moons were generally thought to be inactive, as no internal forces would be present to drive any surface activities. Whatever is driving the spectacular activity on Enceladus remains a mystery.

Recently, two sources for the geysers have been proposed. The “dry” theory suggests that tidal forces along the “Tiger Stripes” fractures discovered at the moon’s south pole rub ice until it is heated sufficiently to escape. The “wet” theory suggest that these tidal forces instead heat a spot in the moon’s interior, resulting in enough heat to melt a lake or ocean of liquid water just below the surface. This pressurized water then escapes through the discovered fractures.

The latest flyby is intended to sample the geysers to determine which of these theories might be correct, or suggest alternative theories. An underground reservoir of liquid water and the forces required might create a potentially habitable location and an intriguing site to look for life. Cassini will look for the presence of sodium, among other elements and compounds, to support the “wet” theory.

Cassini is nearing the completion of its nominal four-year mission to explore Saturn, its rings, and its moons. The mission has been extended for two more years, allowing mission planners to take more risks. Scientists believe that the ice and other particles in the geyser are tiny enough to pose no harm to Cassini. If confirmed, this will allow scientists to fly Cassini closer yet in upcoming explorations of Enceladus. Raw data could start appearing on the Cassini website as early as 5:00 AM PST on Thursday and mission scientists hope to have a preliminary analysis of data completed by Thursday afternoon.

More Information

• Cassini-Huygens Website
  • Saturn’s Moons section
  • Enceladus Information
  • Press Release (NASA/JPL): “Cassini Spacecraft to Dive Into Water Plume of Saturn Moon“
  • Blog: Visiting Enceladus

In 2005 Saturn’s moon Enceladus was discovered to be an active world with water ice particle geysers at its south pole. The driver of this activity on a moon so small remains a mystery. One possible explanation has been suggested by Dr. Francis Nimmo, a planetary scientist from the University of California Santa Cruz. Nimmo visited the University of Arizona Lunar and Planetary Laboratory to present his model on Thursday, September 18, 2007 for the department’s weekly Planetary Sciences Colloquium. The talk was based on his recent publication [reprint PDF] “Shear heating as the origin of the plumes and heat flux on Enceladus” in Nature.

While the old view of the outer solar system as a cold and dead region devoid of most geological activity has eroded with surprising discoveries since the 1980’s, one bias persisted until 2005: only large moons in the outer solar system could potentially be active. Below a particular diameter, moons were not believed to be able to produce sufficient internal heating to drive surface activity. With a diameter of only 252 kilometers (157 miles), Enceladus was expected to have frozen solid long ago. The discover of geysers and a thin oxygen atmosphere at Enceladus proved that this was not the case.

Enceladus’ geysers do not appear to be a global phenomena, however. They are localized to the moon’s south pole, apparently along fractures referred to as “Tiger Stripes.” Taking into consideration the moon’s size, possible internal structures, orbital eccentricity, and other characteristics Nimmo has been working to develop a model that can explain the localized nature of this activity.

An important feature of his model is reorientation theory, where a mass such as a diapir rising to the surface from the mantle can reorient a body, driving the mass’ surface expression toward the poles or equator over time. The presence of such a mass would indicate some driving force such as convection. Applied to Enceladus, this could explain why the tiger stripes have reorientated to their present location at the south pole.

While this may explain the tectonic activity at the south pole, the presence of active water ice plumes requires further explanation. If the tiger stripes are strike-slip faults like the San Andreas Fault on the Earth, then rapid enough motion back and forth along the faults could lead to uplift and shear heating. This friction leads to sublimation of water ice, the primary component of the crust of Enceladus, with some of that vapor being released with heat along the tiger stripes in the form of the spectacular plumes photographed by the Cassini spacecraft.

After developing this model, which Nimmo believes is a better explanation than other theories that have been put forward, such as a near-surface ocean or clathrate decomposition, Nimmo and his colleagues explored possible predictions. While a near-surface ocean does not provide the water vapor to geysers in his model, the model still suggests the presence of an ocean underneath a solid ice crust of at least 5 kilometers (3 miles). The model suggests that this ocean may be transitory, with significant freezing out or remelting depending on the tidal dynamics that change as Enceladus’ orbit around Saturn changes over time.

Furthermore, the orientation of the tiger stripes should change over time, and their fault motions should result in hotter or colder relative temperatures depending on how fast they are moving. In the paper, the team of researchers specify two portions of stripes they predict will have the highest relative tempatures. Some researchers believe fossil tiger stripes may be present elsewhere on Enceladus. Further observations of Enceladus may allow them to test this prediction and confirm or deny the presence of fossil tiger stripes.

Nimmo acknowledged that at present this model of Enceladus is simplistic with several questions still outstanding. Why, for example, does heating appear only at the south pole and not also at the north pole, as predicted by some models of subsurface tidal heating? What prevents the ocean, if it exists, from freezing out completely? Enceladus remains a mystery and will continue to be an important study target for planetary scientists still marveling at a frigid but incredibly active outer solar system.

More Information

• Dr. Francis Nimmo at the University of California Santa Cruz
• Cassini-Huygens Website
  • Saturn’s Moons section
  • Enceladus Information

Cassini-Huygens mission scientists discovered last year that plumes of ice erupt from the surface of Saturn’s moon Enceladus, but the mechanism for the this process has not been fully explained. Now a review of competing theories and available data implicates the more unlikely source: pressurized liquid water pools or an ocean near the moon’s surface. How water can be liquid on an apparently frozen body in the cold of the distant solar system is just the latest mystery regarding Enceladus.

Several research papers regarding recent findings about Enceladus are available in the March 10, 2006 issue of Science. In a Research Article entitled “Cassini Observes the Active South Pole of Enceladus”, Dr. Carolyn Porco, Cassini imaging team leader at Space Science Institute in Boulder, Colorado, USA and other mission scientists suggest that “water vapor probably venting from subsurface reservoirs of liquid water” provides the source for the majestic surface plumes discovered last year. In turn, these plumes may continually replenish Saturn’s E-ring.

Enceladus become just the fourth object in the solar system suspected of containing liquid water reservoirs because of tantalizing new evidence. Only Earth is known for certain to have liquid water on and below its surface. Evidence for liquid water aquifers on Mars and an ocean deep beneath the ice on Jupiter’s moon Europa have already led to excitement within the scientific community over the past decade. Now that Enceladus has been added to list, the mystery of processes that lead to liquid water only deepens.

If pools or oceans of liquid water exist on Enceladus, where do they come from? A heating source is required to raise the temperature to above 273 degrees Kelvin (0 degrees Celsius.) Mission scientists suspect flexing of the moon caused by its orbit around Saturn or a gravitational tug-of-war between Titan and Saturn, along with radioactive decay within the moon’s interior lead to heating that locally heats crustal water ice. Under pressure, this liquid water eventually escapes to the surface as geysers near the moon’s south pole.

The region near Enceladus’ south pole is striking for its so-called Tiger Stripes, enormous gashes across the surface that a significantly more warm than the surrounding terrain. The lack of craters and the apparent youth of ice in the area all suggest recent resurfacing. The discovery of the ice plumes confirmed this observation.

Mission scientists will continue to sift through the data returned by Cassini to investigate Enceladus. Cassini is currently scheduled to return for a close flyby of Enceladus on Wednesday, March 12, 2008 after a mission focus on Titan over the next two years has concluded.

More Information

• Cassini-Huygens Website
  • NASA/JPL News Release, March 09, 2006 - “NASA’s Cassini Discovers Potential Liquid Water on Enceladus“
  • Saturn’s Moons section
  • Enceladus Information

Planetary scientists long assumed that the moons of the outer planets were cold, dead, and airless worlds, heavily cratered but otherwise little changed from their original formation. In 1979, Voyager 1, looking back just after its flyby of Jupiter, discovered volcanic activity on Jupiter’s moon Io. Over a year later, Voyager 1 discovered that the atmosphere of Saturn’s moon Titan, first detected in 1944 by Gerard Kuiper, was thicker than the Earth’s, orange, and opaque. When Voyager 2 passed by Neptune’s moon Triton in 1989, it snapped images of geysers erupting despite the extreme cold.

After Galileo toured the Jovian system during the 1990s and early 2000s, planetary scientists views had been irreversibly changed. Gone was a bias for current geological activity only on Earth, replaced with a stunned awareness of just how dynamic our solar system remains after its formation 4.5 billion years ago. By the time Cassini-Huygens entered the Saturnian System in June 2004, expectations were high for new discoveries of current activity to rival those made by the Galileo spacecraft. A year and a half later, these expectations have been exceeded.

With the advanced remote sensing capabilities offered by Cassini, and the successful landing of the Huygens probe, Titan was revealed to be a world more Earth-like than any other yet discovered. Nearly all geological activities present on the Earth appear to have counterparts on Titan. On Titan are low-lying boundaries that look like shorelines, river channels cut out by the recent activity of liquid methane, snow made of clumps of hydrocarbons, and sand dunes that stretch for hundreds of kilometers. Other features look like volcanoes, lakes, and craters (so few that just like the Earth the surface of Titan must be very young.)

The overwhelming realization that Titan was very much like a planet in its own right could not prepare scientists for what came next.

There was another bias at work, one that seemed more like common sense. All the current geological activity discovered in the solar system to date had been on the largest moons, moons nearly as large or much larger than our own. Of these moons - the giants Ganymede, Callisto, Titan, or Luna-comparable Io, Europa, and Triton – only the Earth and Callisto appear to have ancient surfaces unchanged by current geological activity. Smaller moons, and there are many of them, simply do not have the internal energy or structure to support activity. More recent missions had confirmed what was simply common sense. They were the cold, dead, and airless bodies originally assumed by planetary scientists. Even if they were just large enough to be round, the history of activity could be traced to their early formation and rare impacts on their surfaces.

The Saturn moon Enceladus was observed by Cassini as part of its tour of the system. The tiny moon is only 512 kilometers (318 miles) in diameter, with just over the mass expected by theory to lead to roundness instead of irregularity. The first images revealed that the surface was not heavily cratered or ancient as expected by scientists. Instead, the surface appeared to be very young, especially near the south pole.

Other instruments observed Enceladus during these early flybys. Strange surface features labeled the “Tiger Stripes” were warmer than the surrounding terrain. The ice here was incredibly young, perhaps as young as yesterday. Just as surprising was the localized presence of a thin water vapor atmosphere, around a body that was simply too small to hold one.

All of these discoveries were leading somewhere. In images captured by Cassini in November 2005, revealed in a press release on December 6, 2005, were the smoking guns: the plumes of Enceladus. Stretching higher into space than the moon is wide, the plumes erupt with fine particles of water ice from the Tiger Stripes region of Enceladus. Some of these fall onto the surface of the moon, keeping it young, but the rest contribute something surprising to the Saturnian system: Saturn’s own E-ring.

Enceladus has become the first tiny moon to join the Earth, Io, and Triton as worlds known to have current and active volcanic activity. The exact process that drives this activity on Enceladus is still unclear. Scientists believe the tug of war between Titan and Saturn with Enceladus in the middle induce internal friction below the moon’s surface. They are unsure if this results in a partially melted mantle near Enceladus’ south pole, and whether or not this material erupts as liquid water that quickly freezes, or as already frozen water ice particles. What they can be certain of is that their preconceived notions about small moons were wrong.

Expecting one active moon, Cassini mission scientists discovered a second. New raw images returned by Cassini just last week reveal this activity with breathtaking beauty. The high haze layers of Titan seen through Saturn’s rings is a study in detail. And then there is tiny Enceladus, dwarfed by its parent planet and some of its sibling moons, but magnificent with is brilliant and angry plumes in the sunlight. There is certainly scientific data in these images to be categorized and analyzed by scientists when the raw images are cleaned up, but there is also artistic value to be enjoyed.

Walt Whitman wrote about a scientific lecture in a poem entitled “When I Heard the Learn’d Astronomer.” During the lecture, full of facts and data, Whitman writes:

“How soon, unaccountable, I became tired and sick;
Till rising and gliding out, I wander’d off by myself,
In the mystical moist night-air, and from time to time,
Look’d up in perfect silence at the stars.”

There is a great deal that can, has, and will be said about these and other images from Cassini. First, however, these images may invoke in some Walt Whitman’s perfect silence, not through annoyance or boredom, but instead through great awe and wonder. This is a silence that means one cannot, for the moment, speak. One instead simply appreciates.

More Information

• Cassini-Huygens Website
  • NASA/JPL/Space Science Institute Press Release - “NASA’s Cassini Images Reveal Spectacular Evidence of an Active Moon“
  • Saturn’s Moons section
  • Enceladus Information
  • Titan Information
• Poetry Archive - “When I Heard the Learn’d Astronomer“

Humanity improves the vision it turns on the universe in two ways: seeing farther than before and resolving greater detail. 2005 was a year of much more detail, of blurry bodies resolving into dynamic worlds and undiscovered objects in our own backyard suddenly coming into view. Augmented by robotic surrogates, adaptive optics, new remote sensing capabilities, and intelligent data-mining agents, here are the discoveries made or announced in 2005 that transformed our view of our solar system.

Spirit and Opportunity on Mars

Never before have robots on the surface of another world traveled so far or functioned for so long. Despite signs of old age, the Mars Exploration Rovers Spirit and Opportunity continue to explore Mars.

Spirit climbed a mountain, observed dust devils, and returned panoramas of Gusev Crater from its high vantage point. Opportunity spotted a meteorite, survived getting stuck in a dune, and returned images of a variety of outcrops on its way to new craters for exploration.

The 10th Planet

It was not a hoax, a conspiracy theory, or pseudoscience, but a soap opera of events that led to the announcement earlier this year of the tenth planet in our solar system, 2003 UB313. After being scooped by another team on an transneptunian object slightly smaller than Pluto, Michael E. Brown of the California Institute of Technology, Chad Trujillo of the Gemini Observatory, and David Rabinowitz of Yale University were forced to confirm that discovery and then announce one of their own: Planet X.

The planetary body larger than Pluto and much further away from the sun was actually discovered during a reanalysis of data from October 21, 2003. Dr. Brown and his team then went back through even older observations to see if the object had shown up before but simply been missed. Sure enough, they came across an observation of 2003 UB313 made in 1989.

The story did not end there. Allegations of fraud were made against the other team, who may have used Dr. Brown’s own work as their own. Meanwhile, the debate over the definition of “planet” was reignited, with Dr. Brown strongly defending the use of that label for his discovery. For many planetary scientists, Pluto is not a planet, but simply a large member of the Kuiper Belt, a region of small objects outside the orbit of Neptune. Dr. Brown and others argue that Pluto should retain its classification as a planet simply for cultural reasons. And if Pluto remains a planet, then any object discovered to be larger than Pluto and orbiting the Sun should also be classified as a planet. Thus, Dr. Brown concluded, 2003 UB313 must be considered the 10th planet.

2003 UB313 has a surface of methane ice, just like Pluto. With the recent discovery of a companion moon scientists hope to nail down the mass and size of both objects. A decision over the classification of 2003 UB313 is still forthcoming.

The Plumes of Enceladus

A tiny moon orbiting Saturn in the frigid outer solar system should be silent and long dead. Most of Saturn’s moons are in fact just that. A series of discoveries by the Cassini spacecraft in 2005 limited this to a generalization forever by returning spectacular images of ice plumes erupting from the surface of Enceladus. The mystery, of course, is where the energy comes from to drive this activity. There is heat inside of Enceladus, heat that makes the tiger stripes near its south pole warmer than the rest of the moon, heat that causes material to vaporize or erupt from this region, resulting in kilometers-high plumes that help support a tentative but oxygen-rich local atmosphere and provide the material to constantly replenish one of Saturn’s rings.

Pulling Back the Shroud of Titan

Cassini provided humans their first glimpse of the surface of Titan late last year. The view left scientists scratching their heads. Then, on January 14, 2005 ESA’s Huygens probe descended through the thick orange smog of Titan’s atmosphere to reveal terrains that were surprisingly Earth-like, with river channels and shore lines suggesting large volumes of liquid at work. When Huygens landed it continued capturing images from the surface, including an orange-hued view of its surroundings.

Not only does the surface of Titan show the signs of active reworking by liquid, but the atmosphere is full of methane, a relatively unstable gas that would not show up in the atmosphere if it were not constantly replenished. What Huygens did not provide was images of standing liquid on the surface, long suspected as the methane reservoir. After Huygens landed it began to settle into the soil and recorded a rise of methane, presumably liquid methane that was vaporized by the heat of entry. The pebbles surrounding the landing site were well-rounded, a sign of fluvial processes here on the Earth. The highlands, where the channels start, were light, while the channel beds and sea-like lowlands were stained dark. This comes from hydrocarbons that snow from the atmosphere and are carried downstream.

Where, then, is the liquid? Huygens had stopped broadcasting from the surface of Titan, but Cassini continues to encounter Titan, with the capability to pry beneath the thick atmosphere by using various remote sensing wavelengths including radar. During one flyby, Cassini captured an image of what appears to be a lake. During another, a volcano. Persistent methane clouds have been detected.

Scientists hypothesize that Huygens landed during a dry season, or perhaps during low tide. Titan might experience monsoonal seasons with periodic torrential liquid methane rains followed by little activity. The surface is obviously quite young, but many more observations by Cassini will be necessary before scientists feel confident in their understanding of the processes at work.

All but one of Cassini’s moon flybys over the next two and a half years will be of Titan. These flybys will be at altitudes of 2000 kilometers (1300 miles) or lower to provide even more detailed data about the surface of Titan.

Deep Impact

We bombed the hell out of a comet and learned that what we thought we knew about these objects was wrong.

On July 03, 2005 Deep Impact encountered Comet Tempel 1. When the event was over, Comet Tempel 1 had a new crater and a rising plume of debris from colliding with the Deep Impact impactor. The parent probe captured images and other data of the impact that are still be analyzed.

What we learned:

• Comets vary greatly among each other in their surface terrain.
• These surface terrains can be quite complex.
• Some comets are loosely packed, held together by gravity.
• Comets may be compositionally quite complex.

Deep Impact detected the presence of water vapor and carbon dioxide gas after impact, while the Spitzer Space Telescope detected “clays; iron-containing compounds; carbonates, the minerals in seashells; crystallized silicates, such as the green olivine minerals found on beaches and in the gemstone peridot; and polycyclic aromatic hydrocarbons, carbon-containing compounds found in car exhaust and on burnt toast” according to September 07, 2005 press release from NASA and the Jet Propulsion Laboratory.

The parent probe continues to function and was placed into a new orbit that will allow mission scientists to return to Comet Tempel 1 or encounter a different comet in a few years.

More Information

• Spirit and Opportunity on Mars
  • Mars Exploration Rovers
• The Tenth Planet
  • Science Paper (PDF) - “DISCOVERY OF A PLANETARY-SIZED OBJECT IN THE SCATTERED KUIPER BELT” M. E. Brown, C. A. Trujillo, and D. L. Rabinowitz
  • NASA Press Release, July 29, 2005 - “Planetary Scientists Discover Tenth Planet“
  • California Institute of Technology, Division of Geological and Planetary Sciences - Dr. Michael E. Brown
• The Plumes of Enceladus
  • Cassini-Huygens Website
    • Saturn’s Moons section
    • Enceladus Information
• Pulling Back the Shroud of Titan
  • Cassini-Huygens Website
    • Saturn’s Moons section
    • Titan Information
• Deep Impact
  • Press Release – NASA/JPL-Caltech - “How to Make Comet Soup“
  • Deep Impact - NASA
  • Deep Impact - Jet Propulsion Laboratory

Cassini has returned spectacular images of huge plumes of water ice particles emanating from Enceladus, confirming that this tiny moon of Saturn is an active and watery world. Several plumes of various sizes can be seen clearly along the limb of the moon backlit by the Sun. Spraying out into space, these plumes may provide the source material for Saturn’s E ring.

The plumes coincide nicely with the fractured “tiger stripe” region surrounding Enceladus’ south pole. Earlier this year the tiger stripes were found to be warmer than surrounding terrain and the most recently resurfaced area on the moon. In addition to being the probably source for Saturn’s E ring, the activity here also generates a localized thin atmosphere.

The exact mechanism for this activity is still not known. One theory posits that the plumes result from geysers of liquid water created by internal heating that violently escapes to the moon’s surface. Another theory suggests that the plumes are caused by the vaporization of surface ice over internal warm spots. A gravitational tug of war on Encealdus between Titan and Saturn may provide the needed energy to keep Enceladus’s interior warm.

This latest discovery caps an already incredible year of results from the Cassini-Huygens mission to the Saturnian System, including the first known spiral arm of ice and dust particles around a planet, a thin oxygen-rich atmosphere around the rings, and liquid-cut channels and possible lakes of liquid methane on Titan. NASA and the European Space Agency (ESA) plan to present on Wednesday, November 30, 2005 “the latest Huygens probe and Mars Express orbiter results during back-to-back briefings from the agency’s Paris headquarters.” The briefings will be aired live on NASA TV.

More Information

• Cassini-Huygens Website
  • Saturn’s Moons section
  • Enceladus Information

Out of nowhere emerged a Saturn moon to rival Titan in mystery and activity. Here, suddenly, was an ice world not dead but alive and active, spewing out water ice and oxygen from a youthful surface, creating only the second moon atmosphere known in the Saturnian system. Because Enceladus is so small, the atmosphere is localized rather than global, and it must be replenished constantly to compensate for gas and dust loss to the space. In the process, Enceladus may just help build one of the rings of Saturn.

Near the south pole of the moon lies a roughly parallel group of fractures called the “tiger stripes.” The ice here is brand new, perhaps as young as yesterday. The tiger stripes are the warmest region on Enceladus. At 91 and 89 degrees Kelvin (or minus 296 and minus 299 degrees Fahrenheit), this certainly isn’t paradise, but compared to the surrounding temperatures that range from 74 to 81 degrees Kelvin (or minus 326 to minus 313 degrees Fahrenheit), this is downright balmy. To bring out the differences in temperatures, the pictures shown on this page are in false-color.

Just what is occurring beneath the surface of Enceladus here is not known, though the gravitational tug of war between Saturn and Titan on Enceladus may play a role. As usual, where there is present day activity and the right conditions there is speculation about lifeforms. Heat from below, liquid water or soft(er) ice around you, and oxygen above (albeit at atmospheric pressures barely above that of a vacuum) - all the ingredients for those especially hardy alien lifeforms. The search in on.

More Information

• Cassini-Huygens Website
  • Saturn’s Moons section
  • Enceladus Information

Table of Contents

• Introduction
• Pan
• Atlas
• Prometheus
• Pandora
• Janus
• Epimetheus
• Mimas
• Pallene and Methone
• Enceladus
• Telesto
• Tethys
• Calypso
• Helene
• Dione
• Polydeuces
• Rhea
• Titan
• Hyperion
• Iapetus
• Phoebe

The Earth has it, Io has it, and now a third world can be added to list. I’m talking about active volcanism. The Cassini spacecraft finds active icy volcanism reshaping the surface of Saturn’s tiny moon Enceladus and spewing water vapor and ice particles into Saturn’s E-ring. What a day for planetary science!

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The tiny moon orbiting Saturn named Enceladus has become the third body in the solar system known to have current volcanic activity driven by internal heating, after Earth and Jupiter’s moon Io. The surprising discovery was made after the Cassini spacecraft performed a close flyby of Enceladus on July 14, 2005 and returned the closest images yet of the moon. Data obtained by other instruments revealed that the moon’s south pole is warmer than its equator. Warm fractures and a cloud of water vapor over the region point to active processes replenishing Enceladus’ atmosphere and providing material for Saturn’s E-ring, the largest ring in the system.

Planetary geologists knew something strange was going on when the first images of Enceladus revealed a heavily fractured world with large regions devoid of impact craters. In April 2005 NASA announced that Cassini had discovered an atmosphere around the moon. Because Enceladus is so tiny, any atmospheric gases should readily escape the moon’s gravitational pull. Detection of a substantial atmosphere indicates that some process must be replenishing it.

A massive cloud of gases being vented from the south pole, the tortured appearance of the landscape, and the warmer temperatures there than even the equator of Enceladus all point to active internal heating, perhaps caused by the gravitational tug-and-pull between Enceladus and other objects in system. Enceladus’ south pole contains more fractures than any other region on the moon and house-sized boulders litter the terrain. Fractures called “tiger stripes” because of their regular and roughly parallel pattern appear to be most warmed by the internal heating. Evaporation of water caused by the warming of ice may result in a locally denser atmosphere.

Planetary scientists have long expected the outer moons to be inactive worlds with heavily cratered and ancient surfaces. The discovery of active tectonics at work on Enceladus will completely rewritten our understanding of the outer solar system. Enceladus is the second active world to be discovered orbiting Saturn. Although not yet confirmed, there has been some evidence for volcanic activity on Saturn’s largest moon, Titan. Several interesting features on the surface of Titan have appeared in images, but because the moon is surrounded by a thick nitrogen/methane atmosphere, scientists have been struggling to understand what they are seeing through the haze. The atmosphere around Enceladus is much thinner, allowing scientists to proceed more quickly with their analysis of data from the moon.

More surprising discoveries are certain as Cassini continues its four-year tour of the Saturnian system. The next close flyby will be of Titan on August 22, 2005. Cassini will not return for a close flyby of Enceladus until March 12, 2008.

More Information

• NASA Press Release, July 29, 2005 - “Cassini Finds an Active, Watery World at Saturn’s Enceladus“
• NASA Press Release, July 26, 2005 - “Cassini Finds Recent and Unusual Geology on Enceladus“
• July 14, 2005 Enceladus Flyby
• Cassini-Huygens Mission
  • Saturn’s Moons section
  • Raw Images