Keeping Icy Moons Warm for Billions of Years
Each spacecraft that has explored the outer solar system has yielded surprises. It is common knowledge that Voyager scientists were blown away by the first views of active moons they expected to be cold and old. Recent discoveries have only intensified the surprises. Richard Kerr wrote recently in Science,1
Why is there geology on Saturn’s icy satellites? Where did these smallish moons get the energy to refresh their impact-battered surfaces with smoothed plains, ridges, and fissures? These questions have nagged at scientists since the Voyager flybys in the early 1980s, and the Cassini spacecraft’s recent discovery that Saturn’s Enceladus is spouting like an icy geyser has only compounded the problem. (Emphasis added in all quotes.)
(See 11/28/2005 entry about the discovery of eruptions on Enceladus, and 08/30/2005, 07/14/2005 and 03/04/2005 about its young surface and active south pole.) The temperature of a body depends primarily on four factors: its nearness to the sun (related to its composition), its mass (related to volume), the amount of tidal flexing imposed on it, and the amount of radioactive heating in its interior. Trouble is, small bodies short in all four quantities are looking pretty lively. Several small, icy moons at great distances from the sun show young surfaces and eruptive activity: these include Europa, Triton, and most recently Enceladus. Io, of course, has a great deal of volcanic activity which is only partly explained by tidal flexing. Titan is more massive than the other Saturnian moons, but its surface looks very young and active; it may have active cryovolcanos. And unlike all the other moons, it has a dense atmosphere that is quickly eroding. While many of the other moons appear quiescent, some, like Ariel, Miranda, Tethys, Iapetus, show evidence of recent surface activity.
Planetary scientists never question the age of these bodies. They unanimously assume that they are 4.6 billion years old – the consensus view of the age of our solar system. (This is sometimes stated as “geologic time”). Presumably, the planets and moons all formed near the beginning, 4.6 billion years ago, and have been cooling off ever since (but see 09/12/2005). The small bodies should cool much more rapidly than the planets, because as radius decreases, surface area decreases by the square, but volume by the cube. The smaller the body, therefore, the greater the surface area for the interior heat to leak out.
It’s interesting to watch how planetary scientists deal with surprises. It takes creative modeling to keep a moon active that should have frozen solid billions of years ago. Here’s what the planetary scientists have been up to:
Enceladus: Stoke the Furnace: Throw some radioactive aluminum-26 into the core furnace; maybe that will help. Kerr reported that the Cassini team tried this to keep Enceladus warm enough to spout. Others find this interesting, but are not convinced: “At each stage [of the calculations], there are several knobs you can twiddle,” said Francis Nimmo (UC Santa Cruz). “There are so many free parameters it’s hard to make a strong statement.” Why the other nearby moons, such as Mimas (same diameter) are not erupting is a problem, but Enceladus does appear to have higher density and therefore a larger core for storing the hot Al-26. Nevertheless, the buzz around JPL is that nobody really has a good answer yet. Enceladus is a problem moon that has scientists scratching their heads.
Update 03/09/2003: A JPL Press Release says there may even be liquid water erupting, like cold versions of the geysers of Yellowstone. “We realize that this is a radical conclusion,” said the imaging team lead. Science Dec. 10 had a special section on Enceladus with a dozen articles from the Cassini team exploring all aspects of the bizarre moon, from images to magnetic fields, from infrared and ultraviolet measurements to in situ particle measurements. “Finding such active geology on such a tiny moon is a big surprise,” said Joanne Baker in the introductory article. The only other active bodies in the solar system (Earth, Triton, Io) are larger than Enceladus. One model was offered to show how pockets of liquid water might form under the surface, but most scientists are saying this is a huge mystery.
- Iapetus: Slam on the Brakes: The big midriff bulge on Saturn’s yin-yang moon Iapetus presents a different problem. Scientists are dealing with this by having it start with a high spin rate with a good dose of aluminum-26 to keep it deformable. If tidal interactions with Saturn forced it to spin down rapidly, maybe the bulge was able to freeze in place. For more on Iapetus, see 01/07/2005.
Titan: Hide the Goods: Planetary scientists were surprised, and perhaps disappointed, to find no liquid oceans of methane on the surface of Titan. The Huygens Probe landed on a dry lake bed, and the Cassini orbiter has failed to detect liquid on the surface. At current erosion rates, the atmospheric methane would be depleted in 10 to 100 million years – just 2% its assumed age. Clearly, scientists who want to keep Titan old need a source of methane to replenish the atmosphere. A new theory was just published in a letter to Nature this week.2 Jonathan Lunine, who has puzzled over Titan for over 20 years, has moved the methane reservoir underground:
Saturn’s largest satellite, Titan, has a massive nitrogen atmosphere containing up to 5 per cent methane near its surface. Photochemistry in the stratosphere would remove the present-day atmospheric methane in a few tens of millions of years. Before the Cassini-Huygens mission arrived at Saturn, widespread liquid methane or mixed hydrocarbon seas hundreds of metres in thickness were proposed as reservoirs from which methane could be resupplied to the atmosphere over geologic time. Titan fly-by observations and ground-based observations rule out the presence of extensive bodies of liquid hydrocarbons at present, which means that methane must be derived from another source over Titan’s history. Here we show that episodic outgassing of methane stored as clathrate hydrates within an icy shell above an ammonia-enriched water ocean is the most likely explanation for Titan’s atmospheric methane. The other possible explanations all fail because they cannot explain the absence of surface liquid reservoirs and/or the low dissipative state of the interior. On the basis of our models, we predict that future fly-bys should reveal the existence of both a subsurface water ocean and a rocky core, and should detect more cryovolcanic edifices.
(See also: ESA report and Science Now. For earlier stories on Titan, see 12/05/2005, 06/09/2005, 05/18/2005, 04/08/2005 and 03/11/2005). Cassini just made its 13th pass over Titan on Monday, and has many more passes this year, so we shall have to wait and see. For dramatic images of the most recent flyby, showing sharp boundaries between dark and light areas, see the Cassini Titan-11 flyby page and raw images: here is a good sample.
While modelers have many dials and switches to fiddle with, one factor may be complicating the matter. Kubo et al. did experiments with a high-pressure form of ice known as Ice II and found that it deforms, or “creeps” much faster than previously thought – by up to two orders of magnitude, depending on the grain size. Their paper in Science3 was joined by a commentary from Peter Sammonds,4 who agreed that “This realization could change our understanding of the dynamics and evolution of these planetary bodies” What this implies specifically was not made clear. Perhaps it means that an icy moon’s interior would reach equilibrium in less time. You figure it out:
Kubo et al. argue that grain size-sensitive creep of Ice I and Ice II plausibly dominates the evolution and dynamics of the interiors of the medium to large icy moons of the outer solar system. Ice II is considerably more viscous than Ice I. The transition from Ice I to Ice II, which occurs at depth, is accompanied by an increase in viscosity of four orders of magnitude. If grain size-sensitive creep does not operate, then the increase in viscosity would be six orders of magnitude. So if grain size-sensitive creep is not taken into account as a deformation mechanism, estimates for viscosities of the interiors of the icy moons are off by about two orders of magnitude. Such a difference would have profound implications for interpreting their evolution and dynamics.
(See also Lawrence Livermore press release.) Whatever this means, modelers apparently didn’t set the knob right on this parameter before now.
1Richard Kerr, “How Saturn’s Icy Moons Get a (Geologic) Life,” Science, 6 January 2006: Vol. 311. no. 5757, p. 29, DOI: 10.1126/science.311.5757.29.
2Gabriel Tobie, Jonathan Lunine and Christophe Sotin, “Episodic outgassing as the origin of atmospheric methane on Titan,” Nature 440, 61-64 (2 March 2006) | doi:10.1038/nature04497.
3Kubo et al., “Grain Size-Sensitive Creep in Ice II,” Science, 3 March 2006: Vol. 311. no. 5765, pp. 1267 – 1269, DOI: 10.1126/science.1121296.
4Peter Sammonds, “Creep and Flow on the Icy Moons of the Outer Planets,” Science,
No sooner did the word “water” appear in news reports about Enceladus, when reporters started talking about “life.” The press release twists the evidence for a young Enceladus into evidence for old evolution: “Scientists still have many questions. Why is Enceladus currently so active? Are other sites on Enceladus active? Might this activity have been continuous enough over the moon’s history for life to have had a chance to take hold in the moon’s interior?” Geological activity is not necessarily related to biological activity. Enceladus is not a case for OOL, but for YEC. Who are these spin doctors that write press blurbs like this gem from a Cassini press agent: “A masterpiece of deep time and wrenching gravity, the tortured surface of Saturn’s moon Enceladus and its fascinating ongoing geologic activity tell the story of the ancient and present struggles of one tiny world.” Get real.
Does a model correspond to reality? This is an interesting question in philosophy of science. Some things are too difficult to observe and explain. A model can provide a “cartoon” of the problem to help make it manageable (or provide comic relief). Based on the model, scientists make predictions, or open the model to falsification. Confidence in the model grows if it passes these tests. Unfortunately, the more switches, dials and free parameters in the model, the more the model becomes immune to falsification, and the more other models might make similar predictions. Consequently, it becomes increasingly difficult to know if the model really connects with the real world, or is just a convenient fiction. What we wonder is why there is a padlock on the rheostat labeled, “age of the solar system.” Rumor has it the Darwin Party put it there.