Saturn’s Iapetus Takes Cassini’s Spotlight

Scientists are eagerly poised for Cassini’s long-awaited ultra-close flyby of Iapetus on September 10.  The previous visit in 2005 was over 77,000 miles away; this flyby will skim the surface from less than 1,000 miles.  Moreover, it will see a portion of the moon only vaguely imaged by Voyager and Cassini before.  Jet Propulsion Laboratory, which manages and operates the Cassini mission, posted a map, encounter description and links to other information about Iapetus in preparation for the event.
    Iapetus made the cover of Science News on August 18.¹  Ron Cowen, in “Idiosyncratic Iapetus,” surveyed competing theories about the moon’s formation – with particular regard to two major anomalies, its walnut shape, and the 8-mile-high ridge of mountains on its equator (up to 12 miles high in some places).  Did a ring pile up on the equator, as one Taiwanese theorist proposed last year, or did a fast braking action cause Iapetus to bleed molten material from an equatorial wound?
    The latter model was elucidated by lead Cassini scientists in Icarus this month.²  Dennis Matson, Julie Castillo-Rogez, Jonathan Lunine, Torrence Johnson and others, many familiar with Iapetus since Voyager 1 and 2 flew by Saturn in 1981, listed the physical constraints on any model proposed to explain the Iapetus anomalies.  The moon had to start with a fast spin and slow down rapidly due to tidal stress.  In addition, there had to be enough short-lived radioisotopes (aluminum-26 and iron-60) to keep the interior molten long enough for its walnut shape to become frozen.  Then, an equatorial crack had to open up to spill its guts into the mountain range seen today.  It’s “all in the timing” in this model, wrote Cowen.  He quoted veteran Saturnologist Joseph Burns admitting, “The model is pretty improbable since it requires special timings, but Iapetus itself is pretty improbable.”
    The two papers also speculated that Iapetus formation models might shed light on a bigger controversy: the origin of planets themselves.  The slow process of core accretion was long the only player in town.  “A decade ago,” Cowen wrote, “the leading model of planet formation would have had a hard time making a large planet in 5 million years or less.”  According to the authors of the Icarus paper, though, the formation of Iapetus must have occurred 2.5 to 5 million years after the solar system first began to form.  That leaves the newer “disk instability” cosmogony more suitable for the “quick creation” of Iapetus.  Indeed, solar system modelers are now having to worry about getting planets formed even faster (08/30/2007).  They have been realizing that incipient planetesimals would quickly be swept into the sun unless something protected them (see National Geographic).  It’s common now for planetologists to speak of gas giants forming in mere hundreds of years.
    Will the latest models be able to endure whatever data Cassini beams to Earth on Monday?  If the past is any guide, any puzzles solved will be superseded by bigger ones.  Voyager scientists were confident they would understand Iapetus’s dark material 25 years ago, but Cassini scientists today still do not have an accepted explanation.
    Incidentally, Voyagers 1 and 2 are celebrating their 30th anniversary in flight (see JPL feature), and Cassini is approaching its 10th anniversary away from Earth.  In addition, the Mars Exploration Rovers have bounced back from the brink of doom after last month’s global dust storm (see Science Daily); Spirit and Opportunity are now both operational 40 months beyond expectations.  Those JPL engineers sure make some hardy stuff.  The navigators, too, know how to give a thrill ride – for space lovers around the world.
    For Cassini’s catalog of images of Iapetus, go to the Cassini Multimedia Images page and select “Iapetus” from the pull-down menu at upper right.  Raw images from the flyby will become available sometime Monday or Tuesday.  For previous entries in these pages about Iapetus, visit 01/07/2005, 03/01/2006 and 07/18/2007.  The 02/06/2006 commentary discusses the use of models to reconstruct the past.

¹Ron Cowen, “Idiosyncratic Iapetus,” Science News, Week of Aug. 18, 2007; Vol. 172, No. 7 , p. 104.
²Castillo-Rogez, Matson, Sotin, Johnson, Lunine and Thomas, “Iapetus’ geophysics: Rotation rate, shape, and equatorial ridge,” Icarus, Volume 190, Issue 1, September 2007, Pages 179-202, doi:10.1016/j.icarus.2007.02.018.

Tune in again Tuesday.  Should be an interesting show.  Just remember that there is a big difference between observing data in the present and proposing models about what happened in an assumed remote past without human observers.  For every honest observer, regardless of beliefs, better data are always a blessing.