May 4, 2004 | David F. Coppedge

Io, Io, It’s Off to Work We Go

The innermost large moon of Jupiter, Io is the most volcanically active body in the solar system.

About the size of our moon but no more than a speck of light in small telescopes, Io caused a sensation when Galileo first glimpsed it and the other three major satellites of Jupiter in 1610. Back then, it upset tradition about the hierarchy of the heavens; today, it is upsetting tradition about the age and composition of planetary bodies. The volcanoes were first observed by the Voyagers in 1979, and have been monitored with earth instruments since then, but were most clearly and dramatically revealed by the Galileo spacecraft between 1995 and 2003. Now that its seven-year orbital tour of the Jupiter system is history, planetary scientists are trying to come to grips with the startling findings from all four large moons. The May issue of Icarus is devoted to the puzzles of Io, whose volcanos dwarf those on earth. “Io After Galileo” provides a status report, a state of the moon address, before it’s off to work they go for more data mining and problem solving.

Most of the articles are descriptive of the dramatic and colorful volcanos seen in the photographic images: Tupan Patera, a lava lake 47 miles across and half a mile deep; Tvashtar Catena, a chain of craters that displayed a 240-mile-high plume and 30-mile-long fire fountain; Thor, an eruption that reached 310 miles high; Amirami, the largest lava flow in the solar system; mountains towering up to 36,000 feet (Everest is 29,000); and much more.

The fact that such activity could exist on a small moon that should be mostly frozen by now is calling into question traditional theories about the dynamics of planetary interiors. Io’s lavas, for instance, are generally much hotter than the basaltic lavas on earth. It appears they contain heavy elements like iron and magnesium (called ultramafic lavas). Theory demands that the heavy elements sink into the interior; how can these heavy elements erupt out onto the surface? What drives the incessant heat flow that is as active at the poles as at the equator, and shows no cooling down during the night?

The first-order explanation is that Io is tidally pumped by its orbital resonance between Jupiter and Europa. Like a rubber ball repeatedly squeezed, Io’s tides generate heat and that heat has to come out. Volcanic activity was actually predicted on this principle shortly before Voyager 1 arrived. The problem is that there is more heat flow – by an order of magnitude – than most models of tidal flexing predict. Veeder, Matson, Johnson, Davies and Blaney1 have made the problem worse in their paper by recalculating the heat flow from thermal anomalies and adding in the extra amount detected from polar sources, arriving at a weighted average of 2.5 watts per square meter – “well above that predicted by most theories of tidal dissipation in Jupiter and Io.” Considering all the heat emitted by cooling lavas over the entire surface, Matson in an earlier paper had set an upper bound of 13.5 watts per square meter. This is nearly five times the heat coming out of Yellowstone’s thermal basins.

The final paper by Keszthelyi, Jaeger, Turtle, Milazzo and Radebaugh2 is entitled “A post-Galileo view of Io’s interior.” In proposing their “mushy magma ocean” model, in which the interior has no solid core but is mushy all way through, they seem to be meekly standing up with bull’s-eyes painted on their backs, waiting for the inevitable criticisms: how can the tall mountains exist? How does the model prevent runaway melting? How do you stop the magma from escaping too fast? How do you prevent differentiation? More complex models will be required, they meekly admit, and “Such future work may show that the mushy magma ocean model will need to be further refined, or even rejected.” They point to previous critiques: “ Stevenson (2002) predicts that a mush zone >20 km deep would be unstable over geologic timescales.

Another issue is that, if the temperature of the mantle were to change significantly on a time scale of less than 106 [one million] years, then our model for stresses in the lithosphere would be inaccurate (McKinnon et al., 2001).” Hey, it’s only a model, a “useful starting point for future discussions.” So Io, Io, it’s off to work we go.

1Glenn J. Veeder, Dennis L. Matson, Torrence V. Johnson, Ashley G. Davies and Diana L. Blaney, “The polar contribution to the heat flow of Io,” Icarus Volume 169, Issue 1, May 2004, Pages 264-270, doi:10.1016/j.icarus.2003.11.016.
2Laszlo Keszthelyi, Windy L. Jaeger, Elizabeth P. Turtle, Moses Milazzo and Jani Radebaugh, “A post-Galileo view of Io’s interior,” Icarus Volume 169, Issue 1, May 2004, Pages 271-286; doi:10.1016/j.icarus.2004.01.005.

One model they never seem to consider is that Io might not be as old as they assume. Did you catch the phrase “geologic timescales”? That’s code for 4.6 billion years. If the model does not fit “geologic timescales” then the model must be tweaked till it does. 4.6 billion years is the golden parameter, the figure that must not be altered, because Darwinian evolution depends on it.

Io might be considered just a special case if it were alone in displaying recent surface activity. Actually, most of the moons in the solar system possess young-looking features that defy long ages. Europa may be gushing out water even today, Ganymede indicates recent cryovolcanism against expectations and has a global magnetic field, and Callisto shows signs of erosion and has an induced magnetic field. Tidal flexing is not available to explain these features.

Same at Saturn: Enceladus shows widespread resurfacing and may have active water volcanos, Dione and Rhea show vast fields of surface frost, Iapetus is half-coated in dark material, and Titan has an atmosphere that is quickly eroding.

At Uranus, Ariel and Titania show resurfacing and Miranda is a mosaic of old-looking and young-looking features. Even as far out as Neptune, the coldest body in the solar system – Triton, at 300 below zero – has active nitrogen geysers and few craters, looking like much of its surface has been reworked recently.

Back at home, our own moon exhibits transient lunar phenomena, short-lived bright or gaseous emissions from an interior that should long ago have solidified if as old as claimed. Io is forcing planetary geologists to question their assumptions. Would that one of them would break rank and question the assumption of 4.6 billion years. But that would be aiding and abetting the enemy, the young-earth creationists. No respectable scientist would want to be caught dead in such a trespass, or risk offending the Darwin Party.

Check out this issue of Icarus. Look at the pictures and read the descriptions with a mind freed of evolutionary presuppositions. Where does the evidence lead?

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