May 12, 2017 | David F. Coppedge

Cassini Diving Through Saturn’s Young Rings

There’s a gap between the D ring and the planet that is empty enough to allow the spacecraft to pass through, taking incredible pictures and data.

Artwork of Cassini ring dive (JPL)

As we reported a few days ago, Cassini is in the midst of its exciting “Grand Finale”— a series of daring maneuvers between the rings and Saturn. As of this writing, three of the ring dives (of 22) have been completed. Amazing photos and videos posted on the JPL Cassini Website are just a foretaste of unprecedented observations to come before the final plunge into Saturn on September 15.

Evidence continues to support the youth of the rings. A new paper in Icarus, co-authored by veteran ringmaster Jeff Cuzzi, shows that the rings cannot be nearly as old as Saturn:

We find that the overall pollution exposure time for the A and B rings and the Cassini Division ranges from ∼30–150 Myr, which is in line with the ∼15–90 Myr we previously derived for most regions in the C ring. These exposure times assume an initially nearly pure-ice ring that has been continuously contaminated by in-falling micrometeoroids since its formation, using the currently accepted value of the micrometeoroid flux (Grün et al., 1985; Cuzzi and Estrada, 1998; Kempf et al., 2013; Altobelli et al., 2015). Our results here, taken together with our previous findings for the C ring, further support the idea that Saturn’s rings may be ≲150 Myr old suggesting an origin scenario in which the rings are derived from the relatively recent breakup of an icy moon.

150 million years sounds old, but that’s only 3% of the consensus age for Saturn. Planetary scientists had long thought the rings were as old as Saturn till evidence from spacecraft showed otherwise.

In science, it is generally distasteful to posit ad hoc scenarios to support a theory. Getting a moon of just the right size to come toward Saturn at just the right speed to break up and form such a complex pattern of rings at just the right time so that we can observe them today sounds like special pleading (example, 10/07/2010).

When I was at JPL, I heard Jeff Cuzzi talk very frankly and honestly about Saturn’s rings being young. Another veteran ringmaster, Larry Esposito, openly told a critic of one of my ICR articles about the youth of Saturn’s rings why he wanted to keep them old through a “recycling” mechanism. “It is true that my major impetus for my recycling proposal is philosophical: I am not comfortable with the low probability of recent ring creation.” (email dated 11/06/2008).

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  • tjguy says:

    “It is true that my major impetus for my recycling proposal is philosophical: I am not comfortable with the low probability of recent ring creation.”

    At least he is honest about his bias and willing to openly admit it. I appreciate that! Most scientists do not even recognize their bias.

    It looks like there are different interpretations of the data that could be possible. How does a scientist go about deciding which interpretation is correct when experiments are out of the question.

    This is one example of how it is done!

  • Roccop777 says:

    Here the story is — a moon gets torn apart and distributed as fine gravel/dust in rings around Saturn. The current story about our earth moon — our earth was hit by a (or a few, at different times) massive Mars-sized object(s) which broke off a large piece(s) of our earth, but they didn’t crumble into fine dust rings around our earth (like Saturn), but rather the debris caused by this collision lumped together due to gravitation and formed our fairly smooth, spherical moon by accretion.
    This sounds like a major contradiction of scenarios, or am I overlooking something here? Is there a good explanation to explain these different results — or are these stories just contradicting, unverifable “just-so” stories?

    • One has to consider the Roche Limit and the type of material. There is an inconsistency, though, in the fact that particles tend to bounce, not cling. Saturn’s ring material may tend to flocculate to some extent, but having it grow into large bodies has not been demonstrated.

  • Buho says:

    Thank you for your personal insight, David! I agree with tjguy: Jeff’s honesty is refreshing.

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