July 14, 2005 | David F. Coppedge

Cassini Skimmed Over Enceladus at Close Range

The Cassini spacecraft made its closest-yet flyby of Enceladus July 14, skimming just 109 miles above the surface.  This was the closest approach to any object thus far in the four-year mission.  It was nearly three times closer than the earlier record, the March 9 Enceladus flyby (see encounter map).
    Enceladus has long been one of the most intriguing moons in the solar system because of its extremely bright surface, and its large uncratered regions riddled with cracks and ridges (see photo gallery).  The BBC News summarized some of the puzzles regarding this Saturnian satellite.  Is it a source of E-ring particles?  Does it have ice volcanos and geysers?  Is there an ocean below the surface?  The biggest mystery is the source of energy to drive resurfacing processes.  Enceladus has a very nearly circular orbit and is not in any known tidal interactions with other moons, or with Saturn.  No ammonia has yet been detected in the spectrum (ammonia might allow a lower melting point for the water ice that makes up the bulk of the moon).  How, then, could melting occur, especially in recent times?
    A beautiful picture of the little moon Prometheus shepherding the F-ring was released by the Cassini team this week: click here, and also the first tantalizing look at a moon that looks like a sponge: Hyperion.
Update 07/15/2005: The raw images were posted at the Cassini website late Friday.  New and improved images of Rhea taken from 182,000 kilometers were posted first.  Later, the Enceladus data stream came in.  Wide-angle views revealed a bizarre set of tiger-like stripes near the south pole (a region never before imaged), and a huge canyon on the eastern limb (example).  Distinct boundaries between cratered and resurfaced regions were clearly visible.  Closer in (example), the stripes began to look like four nearly parallel canyons emanating from a rough highland across a smooth plain.  From 9,000 miles (example), they are seen to intersect earlier canyons in complex ways.  The two highest-resolution images near closest approach (narrow and wide angle), slightly smeared due to the speed of the spacecraft (over 18,000 mph) at such close range, reveal a somewhat fluffy-looking region of hills and valleys.  No clear signs of volcanos or geysers are apparent, either at highest resolution or looking back along the limb during the outbound leg (example).  To first order, the topographic features appear tectonic rather than fluvial.  Data from other instruments, such as the visual and infrared mapping spectrometer (VIMS), will assist scientists in interpreting the images.  Here is the BBC News post-encounter report, and check out the photo gallery on the Imaging Team website.  The Planetary Society also did a full page spread on the flyby sequence and posted sample images.
    The Cassini team has pulled off another resounding success in a string of spectacular encounters during the spacecraft’s first year in orbit (see 07/01/2004 story).  The rapid-fire sequence of images of Enceladus today, along with those from March and February this year, have brought sudden fulfillment to years of yearning since Cassini was first designed in the early 1980s, with dreams of Enceladus at close range part of the plans.  Interpreting the new images is sure to keep planetary geologists busy for years.
Note: dark lines cutting across the right side of some raw images are due to the lossless compression algorithm used.  The software estimates the compression ratio in advance because of the constraints of time during the encounter.  Interpolation can reconstruct the missing pixels.
Update 07/20/2005: New Scientist quoted some scientists puzzling over the bouldery surface seen at highest resolution: “That’s a surface texture I have never seen anywhere else in the solar system,” said one; said another, “This is just strange.  In fact, I have a really hard time understanding what I’m seeing.”  The boulders seem to avoid rather than filling in the cracks, and there are no small craters.  Neither is there hoped-for evidence of liquid flow on the surface, and Enceladus, being six times smaller than Europa, seems too small to maintain a subsurface ocean.  “Trying to figure out what is going on is going to take a lot longer than a weekend of swapped emails,” said one member of the imaging team.
Update 07/26/2005: a JPL press release says that the south pole looks younger than the rest of the moon.  The region has no impact craters and has been carved into hills of house-sized boulders by unique tectonic features.  This was unexpected; the south polar region appears to be distinct from other parts of the moon, and is being called one of the youngest surfaces in the solar system.  “Young terrain requires a means to generate the heat needed to modify the surface,” the report says.  More interesting facts about Enceladus may come to light when data from the other instruments are combined with the visible-light images.  JPL also provided a dramatic zoom-in movie (also on the imaging team site) showing where the highest resolution image was taken in context to the moon as a whole.
Update 07/29/2005: Another press release from Jet Propulsion Laboratory has announced the discovery of “ice volcanism” near the south pole.  The heavily cratered north pole is very different from the south, where the temperature is significantly warmer, and there are no impact craters.  Scientists believe water ice is erupting from cracks that crisscross the south pole like tiger stripes.  If so, Enceladus becomes the smallest moon to exhibit cryovolcanism.  The previously-reported atmosphere turns out to be localized around the south pole, where it leaks away and is continuously replenished.  This eruptive activity, however, is not the source of the E-ring particles.  Cassini’s cosmic dust analyzer has confirmed, as suspected, that micrometeor impacts against the moon eject particles that become distributed into the broad, diffuse E-ring.  To find such activity at one of the poles was a big surprise; one scientist said, “This is as astonishing as if we’d flown past Earth and found that Antarctica was warmer than the Sahara.”

Scientists find the temperatures difficult to explain if sunlight is the only heat source.  More likely, a portion of the polar region, including the “tiger stripe” fractures, is warmed by heat escaping from the interior.  Evaporation of this warm ice at several locations within the region could explain the density of the water vapor cloud detected by other instruments.  How a 500-kilometer (310-mile) diameter moon can generate this much internal heat and why it is concentrated at the south pole is still a mystery.

    Another Cassini Press Release from July 29 provides more of the pieces to the cryovolcanism puzzle.  The atmosphere earlier detected by the magnetometer turns out to be lopsided; eruptions of material are not coming from the northern hemisphere, but only from the south polar region.  The UV spectrometer also detected this asymmetry during an occultation.  One of the infrared instruments showed the south to be warmer than the north, and the visible-light cameras observed many tectonic features near the south pole.  The cooperative activity of multiple instruments onboard Cassini each contributed to the interpretation that cryovolcanism is occurring.  Why, and how, this should happen on a tiny moon subjected to insufficient tidal stresses, and with insufficient radioactive heating in its interior, is unexplained.
    Since this moon is turning out to be one of the big attractions of the mission, Enceladus will probably be a front-runner for additional close flybys during any approved extended mission.  The next extremely close flyby is scheduled for March 12, 2008 at only 97 km (58 miles).  Several others before then are in the 80,000 km range.

Enceladus illustrates several processes going on at Saturn that appear unlikely to be sustainable for billions of years (see 03/10/2005 and 03/04/2005 entries).  The BBC news article, for instance, mentioned that E-ring particles can only survive for hundreds of years, not billions.  This means that to maintain belief that the ring is ancient, planetary scientists must find that Enceladus has been continually replenishing the particles for over four billion years.  Recall, however, that a huge explosion and loss of mass from the E-ring was observed in early 2004 (see 07/02/2004 entry).  It is unlikely that event was atypical.  Such destructive processes do not add to the ring; they erode it even faster.
    Regardless of one’s interpretation of the images, one thing all can agree on: the Cassini team deserves congratulations from around the world.  To be able to glimpse this kind of detail on bodies that are mere specks from Earth telescopes, to be able to navigate a ship 880 million miles away with such precision, and to be able to send streams of ones and zeroes through empty space and reconstruct them into photographs on Earth, is truly astonishing.  With the extraordinary becoming so commonplace in our technological civilization, we tend to forget that we are living in a historic period of discovery.  Early astronomers could never have imagined what we saw today.  Had they been told it would one day become possible, how they would have longed for a chance to share this experience.  Catch the drama of what is happening.

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