March 1, 2010 | David F. Coppedge

Swinging at Saturn’s Moons: Keep Your Eye on the Ball

Cassini flew by Saturn’s moon Rhea March 2 at just 100 km.  Dr. Paul Schenk, one of the planetary scientists, said on his blog Stereo Moons, “it should be axiomatic by now that the closer you look at a planetary object the more surprises you see.”  Keep your eye on the ball.
    One surprise from 2008 Cassini scientists want to check more closely is evidence for a ring around Rhea (03/10/2008).  Schenk, whose talents include generating 3-D flyovers of surfaces from image data, posted a “ringside seat” flyover of Rhea Feb. 25 on YouTube from the 2008 flyby.  It has a striking characteristic that may provide more smoking-gun evidence for a ring: bluish patches along the equator.  Schenk believes these are marks of impacts of low-orbiting ring particles on the surface.  The discussion going on at Unmanned Spaceflight, a blog frequented by planetary scientists and knowledgeable amateurs, doubted that the streaks could be ancient, because they would have been erased by now.  The images of Rhea arrived at the Imaging Team website the next day.  Unmanned Spaceflight prosumers started stitching and processing the images immediately.  It will take time to analyze the data from the radar, optical and remote sensing cameras, and particles-and-fields instruments.
    Gushin’ geysers; Enceladus is bustin’ out all over its tiger stripes.  Space.com, Science Daily and National Geographic were among the news feeds highlighting amazing views of the plumes emanating from the small moon of Saturn (see Imaging Team gallery).  The images, taken last Nov. 21, were released Feb. 23 with enhanced measurements of heat taken by the composite infrared spectrometer (CIRS).  The photos show that the plumes vary with time.  The jets can erupt along the whole lengths of the stripes.  A new detailed map of one of the stripes “illustrates the link between the geologically youthful surface fractures and the anomalously warm temperatures that have been recorded in the south polar region.”  Eleven more flybys of this moon are scheduled for the seven-year second extended mission.
    Titan continues to astonish planetary scientists.  Two papers appeared in last month’s Icarus on this giant gas-shrouded moon.  The one with the most extensive list of authors evaluated what is known about Titan’s geologic processes.1  “The paucity of impact craters implies that Titan’s surface is geologically young, having a crater retention age between 0.2 and 1 Gyr” (200,000 to 1 billion years), the paper claimed.  But “There is also no evidence of impact craters superposing any cryovolcanic feature,” which implies the volcano candidates are “relatively young” also.  The dunes and channels are among the youngest features on Titan.  In conclusion, “The majority of Titan’s surface is young, that is, less than a billion years, even assuming that all the crateriform structures discussed are due to impact.”  What would cause global resurfacing activity for 80% of the assumed lifetime of this moon was left unexplained.  “It is not yet clear what the role of cryovolcanism has been on crater obliteration, as so far cryovolcanic processes are not seen to be as widespread as erosional processes.”  The mountains appear the oldest, but are only relatively older – unmodified by dunes, channel erosion, and craters.  “It is clear, however, that the patches of hummocky and mountainous terrain are scattered all over the surface and that nowhere do they appear uneroded or stratigraphically younger than another local terrain type.”  And the volcanoes are young, too: “Cryovolcanism may be a relatively young process or possibly ongoing…,” the article said: “The large flow fields mapped so far do not show any evidence of fluvial erosion, perhaps implying that they are quite young.”  To keep this big moon as old as the assumed age of the solar system (4.5 billion years), the authors had to suggest episodic activity.  Despite the belief in long ages, “young” was a frequent word in this paper.
    The conclusion stated, “Titan’s surface is overall very young, given the small number of impact craters and the clear evidence of lacustrine, fluvial, and aeolian processes on the surface.”  And Titan is still active: “It is likely that both aeolian deposition and fluvial activity are still ongoing.” In addition, “it is possible that some cryovolcanism may still be happening on the surface.”  The paper made a passing reference to the methane–ethane problem (01/17/2002, 03/11/2005, 10/18/2006, 02/15/2008, 12/18/2008): “the observed lake inventory is inconsistent with photolysis throughout Titan’s history.”  To keep the methane budget from being depleted over 4.5 billion years, they said it “seems likely” that there has been “episodic injection, by cryovolcanism, of methane from the interior” into the atmosphere, “Although there is no direct evidence of such events….”.  The other Titan paper in Icarus investigated the longevity of methane in proposed eruptive events.2  Their model of outgassing “would be sufficient to maintain the presence of methane in Titan’s atmosphere” the paper claimed, but for far less than the amount of time needed: “for several tens of thousands of years after a large cryovolcanic event.”
    If Saturn were alone in having active moons, it might be considered an anomaly.  But Jupiter has Io, the most volcanically active body in the solar system, and Europa, a smooth moon with lines and cracks that “remaining active or being periodically active as Europa’s decoupled icy shell rotates with respect to its interior,” according to Patterson and Head in Icarus.3  They noted that some of the cracks appear “relatively young” although they did not speculate on absolute ages of features.  Then, two planets out from Saturn, there is Triton.  Another Icarus paper said,4Triton is a spectacularly dynamic world” with a “geologically young surface” – despite being the coldest moon in the solar system.  Observations over the last decade show seasonal variations in volatiles from ices in different geological regimes.  That seems to be happening on Pluto, too.  The BBC News reported observations of seasonal changes on the surface of this body, formerly called a planet but now looking more like Triton, Sedna and other Trans-Neptunian Objects.  If volatile ices are moving about on these small worlds, some of it must be escaping to space.  Maybe that’s why Space.com called Pluto “still a big mystery” 80 years after its discovery.


1.  Lopes et al, “Distribution and interplay of geologic processes on Titan from Cassini radar data,” Icarus, Volume 205, Issue 2, February 2010, Pages 540-558.
2.  Choukroun, Grasset, Tobie and Sotin, “Stability of methane clathrate hydrates under pressure: Influence on outgassing processes of methane on Titan,” Icarus, Volume 205, Issue 2, February 2010, Pages 581-593.
3.  Patterson and Head, “Segmented lineaments on Europa: Implications for the formation of ridge complexes and bright bands,” Icarus, Volume 205, Issue 2, February 2010, Pages 528-539.
4.  Grundy, Young, Stansberry, Buie, Olken and Young, “Near-infrared spectral monitoring of Triton with IRTF/SpeX II: Spatial distribution and evolution of ices,” Icarus, Volume 205, Issue 2, February 2010, Pages 594-604.

If these bodies are not really as old as claimed, one would expect the old-age consensus to encounter frequent anomalies.  Since the surprise density exceeds the prediction of consensus planetary science, it should lead rational planetologists to re-open the assumption of billions of years.  Why do they never go down that path?

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