Ganymede Age Threatened by Magnetism

first_imgThe biggest moon in the solar system is Ganymede, the third large moon out from Jupiter.  Larger than Mercury, Ganymede has a heterogeneous surface of dark and light areas (picture), grooved terrain, abrupt changes of landforms, and bright splashes where impacts have scarred its icy surface (gallery).  What goes on inside, though, is more surprising: it has an intrinsic magnetic field.  Researchers could only make it last for the assumed age of the solar system by appealing to “special conditions” that are not necessarily compatible with theories of its formation.    Bland, Showman and Tobie, writing in Icarus Dec 2008,1 realized that an intrinsic magnetic field requires a liquid core in which convection can occur to drive a dynamo.  A liquid core requires heat.  If they could find ways to stop runaway cooling inside the moon, maybe it would stay hot enough to maintain the magnetic field for 4.5 billion years.    They tried all kinds of things to keep the core hot.  They modeled Ganymede’s orbit passing through a resonance that would increase tidal pumping.  They varied the silicate rheology.  They altered Jupiter’s tidal dissipation factor.  They played with the size of the ice shell.  They imagined partial melting in the silicate mantle.  Nothing worked.  “We find that, contrary to expectations, there are no physically plausible scenarios in which tidal heating in the silicates is sufficient to cause the thermal runaway necessary to prevent core cooling.”    The only other possibility was if the amount of sulfur in the core was very low (less than 3%) or very high (greater than 21%).  Neither of those options was palatable, but they were stuck: “we must appeal to the special conditions described above to explain the presence of the field.”  At the end of the paper they tossed out one other possibility: late differentiation.  If the core didn’t form until 1 billion years ago (about 1/5 the assumed age of the moon), then convection might last for a billion years.  Either way requires invoking special conditions:We have shown that production of Ganymede’s magnetic field by secular cooling and chemical convection requires that a very specific set of conditions be met: the mass fraction of sulfur in the core must be low (or alternatively very high), the core must have formed hot, and the silicate mantle must be able to cool rapidly (i.e.  it must have a viscosity consistent with wet olivine).  If any of these criterion are not met magnetic field production fails.  These results contrast with previous workers who find that compositional convection can drive a core dynamo under a broad range of conditions.2Speaking of Mercury, which is slightly smaller than Ganymede, Dr. D. Russell Humphreys celebrated a confirmed prediction in the current Journal of Creation:Mercury’s magnetic field matches the measurements from the MESSENGER spacecraft (07/09/2008).  He adds this to his list of predictive successes for the magnetic fields of the outer planets.  Humphreys’ model assumes that magnetic fields are young – thousands of years old, not billions.1.  Bland, Showman and Tobie, “The production of Ganymede’s magnetic field,” Icarus 198 (Dec 2008), pp. 384�399, doi:10.1016/j.icarus.2008.07.011.2.  reference cited by Bland et al: S.A. Hauck, J.M. Aurnou and A.J. Dombard, “Sulfur’s impact on core evolution and magnetic field generation on Ganymede,” J. Geophys. Res. 111 (2006) 10.1029/2005JE002557 E09008.And so another phenomenon doesn’t fit the consensus view of the age of the solar system.  Creationists who think they are escaping difficulties by accepting the consensus age are merely trading one set of problems for another.  Every view has problems; face it.  But don’t think the long-age evolutionary naturalistic view is simple and straightforward.  When they pile on miracles needed to get their physicalist scenarios to work, then the appeal to miracles becomes academic: do you want purposeful miracles, or miracles of chance?  Like ketchup with fries, miracles go better with design.    This paper does not support a view that Ganymede is a few thousand years old, of course, but neither does it rule it out.  What it does, though, is put plausible upper limits on the age of Ganymede, beyond which appeals to highly contrived special conditions are required.    Reconstructing the core history of Ganymede can only be done with scientific models.  Being simulations with simplifying assumptions, models can only be judged by subjective criteria of plausibility.  If you think that it is plausible to insert special conditions to form Mercury, Venus, Earth, the moon, Mars, Jupiter, Io, Europa, Ganymede, Callisto (which has an induced magnetic field), Jupiter’s gossamer rings, Saturn, the F-ring, the A-ring, the C-ring, the D-ring, Enceladus, Rhea, Titan, Hyperion, Iapetus, Uranus, the rings of Uranus, Miranda, Neptune, the rings of Neptune, Triton, comets, binary asteroids and the Sun-Earth diameter, then be our guest.  Just admit that you are, in effect, applying your own intelligent design to imaginary models of reality, not to reality itself.(Visited 15 times, 1 visits today)FacebookTwitterPinterestSave分享0last_img

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