December 18, 2008 | David F. Coppedge

Cassini Celebrates Season of Change

It’s approaching equinox on Saturn.  Cassini is now well into its first extended mission, aptly dubbed the Equinox Mission, till Sept. 2010.  The Cassini Team just exhibited its snazzy new website.  It’s not all bells and whistles.  The science is ringing the phones off the hook.  Even without the pictures the following announcements could stop the presses.
    Once again, though, Saturn the Giant had to wait in the wings while a couple of moons stole the show.  Before yielding them the stage, Saturn wanted to announce that its ring spokes are back.  Exeunt.  Flourish.

  1. Titan:  Papers presented at last week’s AGU (American Geophysical Union) conference revealed several exciting new things about the second-largest moon in the solar system – the only one with a substantial atmosphere.
    1. Lake of the Clouds:  Midwesterners may be familiar with “lake effect clouds” around the Great Lakes.  Water vapor from lakes tends to condense over land downwind.  National Geographic News echoed reports by Cassini scientists that a similar effect may be occurring on Titan.  Evanescent methane clouds appear to form downwind of the lakes dotting the northern hemisphere.
    2. Volcano Not:  New radar images of the Ganesha Macula feature have disappointed volcanologists.  They hoped it would prove to be a cryovolcanic dome.  It wasn’t to be; the Planetary Society reported that the feature is irregular, with topographic highs and lows, unlike a dome.  Planetologists had hoped that eruptions from this and other candidate cryovolcanos would replenish the atmospheric methane.  A Cassini press release explained why this is significant: “Without replenishment, scientists say, Titan’s original atmospheric methane should have been exhausted long ago.”
    3. Volcanoes Yes?  BBC News quoted AGU scientists optimistic that cryovolcanos are common on Titan.  Two regions re-imaged over time show changes in brightness.  The Virtual and Infrared Mapping Spectrometer team figures that ice-slurry flows 200 meters thick might be carving some of the channels seen.  The tentative detection of ammonia is another clue.  Scientists expect that ammonia would have to be erupted from the interior.
          Again, the significance of this relates to dating the age of the atmosphere: “Scientists like the idea of cryovolcanism because it is one way to explain why so much methane is retained in Titan’s atmosphere,” the article said.  “Without some means of replenishment, the moon’s original methane content should have been destroyed long ago by the Sun’s ultraviolet light.”  When they say long ago, they really mean long ago.  Some estimates put an upper limit on the lifetime of current methane at 100 million years – one forty-fifth the assumed age of the moon.  All the methane should have been gone 4.4 billion years ago.  Why is there methane still present today?
          The article ended with skepticism about the ammonia.  Not every planetary scientist accepts the interpretation of active eruptions on Titan.  One skeptic is Emily Lakdawalla, the blogger at the Planetary Society.  She provided a first-hand report from the AGU and did not find the evidence convincing.  Her detailed blog report is worth visiting for the detailed radar and VIMS images of the surface.  One thing that stands out in her report is a series of great questions about Titan posed by Jonathan Lunine, a Cassini scientist who has studied this mysterious moon for over two decades.  Among the questions:

      Titan has methane in its atmosphere, which should quickly be destroyed by solar radiation.  So it’s got to be replenished somehow.  Is cryovolcanism responsible?  Or something else? Where has all the ethane gone?  Where there’s methane, there should be ethane, but there isn’t much in the atmosphere or on the surface, though it’s been detected in one southern lake….
      Is there active cryovolcanism or geysering on Titan?  Has there been significant cryovolcanism in the past?  This is an area of particularly active debate.
      What is the origin of Titan’s mountains?  Are they a sign of internal geologic activity, or not?
      Is the primary degradation process fluvial erosion or burial? Titan’s surface has few obvious craters, so something must be erasing them.  Are they being eroded or buried?

      Four years of Cassini orbits have failed to answer these questions.  It must be remembered that scientists in the 1990s expected Titan to be covered in a global ocean of liquid ethane over half a mile thick (02/15/2008, 07/31/2008), due to accumulating condensation products from the steady breakup of high-altitude methane by the solar wind.

  2. Enceladus:  Titan’s equal in scientific interest if not size is the little moon Enceladus.  The BBC News and Jet Propulsion Lab both reported more evidence of activity on the surface.  Careful perusal of patterns in surface cracks led Paul Helfenstein of the Cassini imaging team to theorize that Enceladus has spreading centers comparable to those at Earth’s mid-oceanic ridge – only these operate like a one-way conveyor belt.  One-way spreading is rare on Earth and not well understood, he commented.  “Enceladus has asymmetric spreading on steroids.  We are not certain about the geological mechanisms that control the spreading, but we see patterns of divergence and mountain-building similar to what we see on Earth, which suggests that subsurface heat and convection are involved.”  Earth, however is a lot bigger.  Enceladus is about the width of Arizona.
        In addition, the famous geysers emanating from the “tiger stripe” fissures at the south pole must be on the move.  There are no geological differences along the cracks at the spots where ice is shooting out now.  It may be that the orifices get plugged, only to reopen at other spots along the fissures.  Ice appears to have fallen symmetrically on both sides of the cracks along their entire length.  This suggests the geysers continually move up and down along the tiger stripes.
        A Cassini press release last month provided more evidence that the geysers are coming from a subsurface ocean of liquid water.  An amazing upshot of this activity is that it affects Saturn itself:

    Enceladus’ output of ice and vapor dramatically impacts the entire Saturnian system by supplying the ring system with fresh material and loading ionized gas from water vapor into Saturn’s magnetosphere.
        “The ions added to the magnetosphere are spun up from Enceladus’ orbital speed to the rotational speed of Saturn,” said Cassini magnetometer science team member Christopher Russell of the University of California, Los Angeles.  “The more material is added by the plume, the harder this is for Saturn to do, and the longer it takes to accelerate the new material.”

    It’s a case of the tail wagging the dog.  This tiny moon puts out enough material to create a vast ring around Saturn, and its ions tug at the planet’s magnetic field and affect its rotation.  Such a thing was hardly imaginable before Cassini found it to be so.
        Where is the heat coming from?  Small bodies lose their original heat much more quickly due to the increased ratio of surface area to volume.  One participant in the Unmanned Spaceflight forum, where planetary scientists and their friends hang out, joked, “I wonder if there’s any way that Enceladus could have been impacted by an extrasolar AL26 [Aluminum-26, a short-lived radioactive isotope] mass sometime within the last several thousand years, which could be powering a short-term period of activity within the moon.”  Talk about an ad-hoc hypothesis.  Others referred to a recent paper in Nature that calculated internal tidal forces in a liquid ocean like that on Europa could be much greater than thought.  That does not explain, however, why other moons of comparable or larger size in the Saturn system are inactive.
        Whatever has been happening on Enceladus, it appears to have been going on for some time and moving around.  The moon is riddled with cracks and ridges from top to bottom – some of which may be relics of geyser activity in the past.  The E-ring produced by today’s geysers would vanish within decades without constant replenishment.  Cassini will surely be watching for variations in activity at every opportunity during the Equinox Mission.  If lucky, Cassini scientists may be able to keep watching till the next Saturn solstice – in 2017.

Pictures!  These science stories were accompanied by some of the most stunning images of the mission.  One particular Enceladus portrait is both beautiful and intriguing (for full resolution, go to the Planetary Photojournal).  Patterns of cracks seem to show episodic reorientations of geologic activity.  The lack of craters on many parts of this bright, active moon is surprising: “remarkable tectonic activity for a relatively small world,” the caption exclaimed.  That big crack in the north is over half a mile deep (another angle).  The image is a mosaic from 28 exposures.  Large icy grains have been artificially colored blue-green for analysis.  Browsing the image at high resolution gives one a personal flyover of an exotic world.
    Fly in even closer with the image called Tiger Stripes Magnified.  This is a mosaic of the highest-resolution images from the recent south-pole flybys, overlain on a lower-resolution image of the southern hemisphere.  A labeled version shows locations of known eruption plumes (circles) and the footprints of “skeet shoot” images taken August 10 (green) and October 31 (brown).  The footprints show that the surface of this little moon is cracked and folded down to the smallest scales.  It is also littered with boulders of ice the size of houses.  If these boulders were ejected by the geysers, they speak of remarkable force down deep.  Studying this mosaic at high res is an emotional experience.  It is the nearest we will probably get in this lifetime to staring down the throat of active geysers on an ice-blanketed world 800 million miles away.
    The Cassini Imaging Team website has additional images, maps, and diagrams for further study.  There’s even a rotating globe showing the angles of the geysers, and Paul Helfenstein’s movie of the spreading centers, showing his interpretation of how separated features appear to fit together when you move pieces around.  An updated polar mosaic of the entire southern hemisphere pulls all the data into a flat map with all the resolution one could want; even at quarter size, the view overflows the screen.  Finally, for those who like graphs and scientific detail with their pictures, the Nov. 25 CHARM presentation (Cassini-Huygens Analysis and Results from the Mission has a lot of interesting data (and pictures) in a Powerpoint/PDF presentation about Enceladus with the latest from the recent flybys.

Before Cassini got to Saturn, planetary scientists had an inkling that Enceladus would be a star of the show.  They were certainly not disappointed.  In fact, none of the targets – icy moons, Saturn, rings magnetic field, and Titan – have been anything less than astonishing.
    Cracks similar to those on Enceladus, with ice piled along the flanks, are visible on Jupiter’s moon Europa, another body thought to have a subsurface ocean of liquid water.  Could there be similar mechanisms at work?  The analysis will take years.  So far, it looks like Titan and Enceladus are giving pains to believers in billions of years.  For relief, try the fountain of youth.

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Categories: Solar System

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