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“

Cassini passed within 2,043 kilometers (1,270 miles) of the surface of Titan on Saturday, January 14, 2006 (Pacific Standard Time) in its eleventh targeted flyby (after a more distant flyby a day earlier.) The event heralds a new phase in NASA’s mission to the Saturnian system. For the next two and a half years, all close flybys by Cassini will be of Titan in an effort to answer the mysteries of this enigmatic moon. 13 targeted flybys are planned for this year.

Titan is the most Earth-like neighbor in our solar system with a similar predominance of nitrogen in its atmosphere, the presence of river bed and sand dunes, and other features that may be lakes, shorelines, and volcanoes. These similarities suggest that geological processes work very much the same on both Titan and the Earth, despite Titan’s rocks being made of water ice and its surface solvent being liquid methane compared to silicate rocks and liquid water on the Earth.

Until Cassini and the Huygens landing probe began exploring Titan in 2004, little was known about the moon because of its thick atmosphere. Huygens landed on the surface of Titan in January 2005 while Cassini repeatedly scanned the surface with its instruments using various wavelengths of light that can see through the haze. The flybys planned for this year will map Titan in unprecedented detail while various other experiments will try to explain phenomena like Titan’s magnetic field and interaction with the other members of the Saturnian system.

More Information

• Cassini-Huygens Website
  • Flyby 11
  • Flyby 11 Mission Description
  • Saturn’s Moons section
  • Titan Information

On December 26, 2005 Cassini returned to Titan for its ninth targeted flyby and the last of the year. The latest data includes information about Titan’s magnetic field and more images of albedo features and landmarks on the surface that have been labeled Aztlan, Quivira, Bazaruto. Elba Faculae, and Omacatl Macula. The raw images show the complex boundaries between light and dark areas that have reminded some scientists of shorelines.

More Information

• Cassini-Huygens Website
  • Flyby 9
  • Saturn’s Moons section
  • Titan Information

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

Titan, the largest of the Saturnian moons, with the thick planet-like atmosphere. The moon with the Earth-like surface, of deeply cut fluid channels, broad sea-like basins, pebbled channel beds, lakes, wind-driven sediments, and occasional craters. The alien moon with water ice as rock carved by periodically flowing methane streams and rivers, with hydrocarbons snowing from its nitrogen and methane atmosphere to collect downstream in basins, with methane clouds and perhaps rain, and with brights spots and warms spots that point to heating caused by unknown mechanisms.

Titan, the latest destination in the search for extraterrestrial life and a key to our own terrestrial history of life. Where the methane at the surface and in the atmosphere must be constantly replenished from an unknown vast reservoir. The moon that has planetary scientists scratching their heads and holding hours long meetings to discuss the latest findings painstakingly gathered by Cassini as it attempts to peer through the atmospheric shroud at various wavelengths.

Titan, a distant neighbor in our solar system that has captured the public imagination unlike any other object since Mars.

Titan, where the best is yet to come…

More Information

• Cassini-Huygens Website
  • Saturn’s Moons section
  • Titan 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

Scientists examining recent images of the surface of Titan have discovered a feature that resembles a lake. The feature is near persistent methane clouds and the smoothness of its boundaries suggests shorelines, but it could instead be a dry lake bed, a depression filled with solid hydrocarbons “snowing” out of the atmosphere, or a tectonic-related landform. Similar dark features in the region suggest common processes at work. Future observations by the Cassini space probe will attempt to detect reflections on the surfaces of these features that could determine whether or not they contain liquid.

The region in which the landform was discovered is near Titan’s southern pole (the red cross in the lower center of the image.) The bright white objects in the lower right quadrant of the image are methane clouds that persist through southern summer. NASA also released a time-lapse movie of the region showing movement of these clouds.

Images returned by the Cassini space probe have provided tantalizing evidence for active processes currently at work on the surface of Titan, making it the most Earth-like body in the solar system. Image resolution is hampered by Titan’s thick nitrogen-methane atmosphere, however, and scientists often combine several images of the same region to tease out greater detail. This adds lengthy procedures to an already lengthy process that turns raw images returned by Cassini into image products ready for scientific analysis.

More Information

• Press Release (Source: Jet Propulsion Laboratory) - “NASA’s Cassini Reveals Lake-like Feature on Titan”
  • Time-lapse movie - “PIA06241: Clouds in the Distance“
  • Image - “PIA06240: Land of Lakes?“
• Cassini-Huygens

Titan may harbor “ice volcanoes” and scientists have presented evidence for a likely candidate in the June 9 issue of Nature. Several images of the possible volcano were released to the Internet today by the Jet Propulsion Laboratory.

Ice volcanoes may explain the high level of methane present in Titan’s atmosphere. Methane generally breaks down over a relatively short period of time. Scientists presume some active process keeps high levels of methane in the atmosphere but are unsure which process is at work. Oceans or lakes of liquid methane have previously been proposed as an explanation but neither the Cassini space probe nor the Huygens lander has detected them. The alternative? Ice volcanoes, also known as cryovolcanoes.

Images taken by Cassini using various wavelengths of light suggest a circular surface feature northeast of the Huygens probe landing site. Various compositional layers superimposed on each other may indicate different flows from the volcano. However, any “lava” deposits would be made of methane-ice mixtures, perhaps with other hydrocarbons and ammonia mixed in. Internal heating may provide enough melting to allow eruption of this material onto the moon’s surface.

Future Cassini flybys should help clarify the situation. Cassini is currently focusing on Saturn’s rings, but will return to closer inspection of Saturn’s moons in July.

More Information

• Jet Propulsion Laboratory news release:
  • “Scientists Discover Possible Titan Volcano“
• Cassini-Huygens Website
  • Saturn’s Moons section

The Cassini space probe has discovered a variety of complex hydrocarbons and carbon-nitrogen compounds in the upper atmosphere of Titan. Described by the Jet Propulsion Laboratory as an “organic ‘factory’ of hydrocarbons” the upper atmosphere seems to be an unlikely spot for such high levels of these compounds, since they should condense and rain out on the moon’s surface. The mechanism by which these compounds are created likely includes ultraviolet radiation from the sun, although the exact details are not well understood.

The graph below shows the presence of compounds containing 1, 2, 3, 4, 5, 6, and 7 carbons in their molecular structure, some of which are hydrocarbons (carbon combined with hydrogen) and others that are carbon-nitrogen compounds. Titan’s atmosphere is composed mostly of nitrogen and a significant amount of methane (CH4).

This discovery is not that same as discovering life, but complex hydrocarbons are known precursors to the building blocks that somehow lead to life. Scientists hope data about organic chemistry on Titan will give them a better understand of how life may have started on our own planet.

The latest flyby of Titan by Cassini occurred on March 31, 2005. NASA has not yet released processed images and scientific feedback, but the raw images have been available for a couple days now. When looking at these raw images of Titan, there is something profoundly strange yet incredibly familiar about them. The only reasonable response is awe. What are the images showing us? The image to the left reveals more detail of a large black shape with sharp boundaries. Is it a sea of methane made dark by hydrocarbon sediments? Is it a huge expanse of methane, hydrocarbon, and water ice mud? Why does it look so familiar?

You do not need to be a scientist to understand that there is something very interesting happening on Titan. Recent attendance by the public at lectures about data returned by the Cassini-Huygens space probes indicates the extent of this interest. Surprisingly enough, planetary scientists do not seem to be any better equipped than the layperson to explain what is going on.

Cassini will make an even closer flyby of Titan on April 16, 2005 before orbital mechanics result in a four month gap between flybys.

Imagine a frigid world where complex hydrocarbon particles clump together in the thick smog-like atmosphere and fall like black snow onto a light water ice surface. Occasional methane rains and methane springs that emerge from the side of hills wash some of the particles off higher ground and concentrate them in river channels. These rivers flow through the hills to lower-lying areas. Some of the liquid methane with its hydrocarbon particle load collects into pools and lakes. Eventually, the liquid evaporates or soaks into the ground, until the pools are dry, waiting for the next rainfall.

Titan is that world, revealed this morning by scientists involved with the Huygens space probe mission. After last week’s successful landing on the exotic Saturnian moon, data from the probe’s mass spectrometer, imaging system and other sensors confirmed the presence of liquid methane on or near the surface of Titan. Because Huygens detected methane just a few centimeters below the surface, scientists believe that it may have rained recently in the area, perhaps in the few days or weeks before the landing.

Marty Tomasko, Principle Investigator for the Descent Imager-Spectral Radiometer (DISR) on board Huygens, stated that Titan was a world of “earth-like processes” using “exotic materials.” Instead of water, methane is the primary liquid at work on the surface of the moon, and rather than silicate rocks, the rocks of Titan are made of water ice. They interact to create breathtaking vistas such as those near the Huygens landing site: river channels flowing down ridges to temporary lowland lakes with distinct shorelines.

Because no current pools of liquid were detected on the surface, the region photographed by Huygens may be a relatively more arid region of Titan, though little is known about the rest of the moon to substantiate this theory. The region may resemble the deserts of Arizona, where liquid water plays an important role in shaping the landscape despite the aridness, resulting in dry river beds between episodes of seasonal flooding.