May 4, 2007 | David F. Coppedge

Tweaking Mercury to Keep it Old

Mercury has a magnetic field.  That’s odd.  It shouldn’t.  If it were 4.6 billion years old, the little planet should be solid stiff by now.  Planetary scientists have published a new model of its interior with the required molten outer core that allows a dynamo to generate the observed magnetic field.  What’s interesting are the initial conditions required to get the interior to stay molten this long.
    Sean Solomon in Science1 describes the reaction of scientists 30 years ago when Mariner 10 detected Mercury’s magnetic field:

Some 30 years ago the planetary science community was surprised when the Mariner 10 spacecraft flew by the planet Mercury and detected an internal magnetic field.  Earth’s internal field is produced by a magnetic dynamo sustained by convective motions in the planet’s molten, iron-rich outer core.  Although Mercury’s high bulk density indicates that its dominantly iron central core is the largest by fractional mass among the planets, the detection of its magnetic field was surprising because Venus has no field and Mars and the Moon show evidence only for ancient global fields.  With a mass about 5% that of Earth, Mercury had been expected to have cooled internally to the point where either the core had solidified or core convection no longer occurs.  A necessary condition for Mercury’s magnetic field to arise from an active Earth-like dynamo is that at least the outer shell of its core be molten.  On page 710 of this issue, Margot et al.  report new observations of variations in Mercury’s spin rate made with Earth-based radar, providing strong evidence that this condition is met.

The paper by Margot et al.2 does, in fact, present the physics that allows Mercury to generate a magnetic field.  With a little libration, and a little reorganization of the melt layers, they can match the observed field.  That, however, is only part of the problem.  Keeping the inside of Mercury hot enough for 4.6 billion years is the next challenge.  Previously, planetary scientists thought that Mercury only swept up material from the inner solar disk when it formed.  That paradigm has had to soften under heat and pressure:

The presence of a liquid core in Mercury has important implications for theories of planet formation and evolution.  Thermal models predict a frozen core unless a sufficient amount of sulfur (at least 0.1% weight fraction) depresses the melting temperature of the core material.  But chemical condensation models indicate that sulfur cannot condense in the primordial solar nebula at the heliocentric distance of Mercury.  Hence, the need for a sufficient amount of accreted sulfur to keep the core molten over the age of the solar system implies that Mercury was accreted from planetesimals that originated over a wide range of heliocentric distances.

This solution indicates that the age of Mercury was never called into question.  Since the scientists already know how old it is, something else had to give.  The free parameter became the assumption about the solar nebula.  Without explaining how, they assumed sulfur throughout the primordial disk – out to billions of miles – became incorporated into the planet Mercury, so that the interior could stay molten.  The magnetic field, in fact, became evidence that this had indeed occurred: “Hence, the need for a sufficient amount of accreted sulfur to keep the core molten over the age of the solar system implies that Mercury was accreted from planetesimals that originated over a wide range of heliocentric distances.”
    This story was announced on a JPL press release as solving a “30 year mystery.”  It was picked up by New Scientist, National Geographic, Science Daily and other news outlets.  The explanation was echoed in each case, with no one questioning the date of Mercury or the method of adjusting parameters to keep it safely old without growing cold.3
    A NASA spacecraft named MESSENGER is on the way to Mercury, for arrival next January.  Planetologists hope it will be able to take more precise measurements of the magnetic and gravitational fields to deduce the structure of its interior dynamo.

1Sean C. Solomon, “Hot News on Mercury’s Core,” Science, 4 May 2007: Vol. 316. no. 5825, pp. 702-703, DOI: 10.1126/science.1142328.
2Margot, Peale, Jurgens, Slade and Holin, “Large Longitude Libration of Mercury Reveals a Molten Core,” Science
3According to one of Murphy’s Laws, “Every solution breeds new problems.”  The authors did not begin to speculate on what altering the amount of mixing in the solar disk does to other aspects of planetary evolution theory.  The old paradigm explained the differences between the rocky planets and gas giants by the composition and temperature of disk material: e.g., beyond the “frost line,” only gas giants could form, while rocky planets closer to the sun, unable to hold onto their volatiles, condensed primarily from heavy elements like rock and metal.  Requiring this much mixing of material from the far reaches of the disk would seem to have dramatic repercussions on both the orbits, compositions, and interactions of the planetesimals – and the comets, asteroids and other solar system objects that resulted.  None of the articles discussed those ramifications of the explanation offered in this paper.

Do you see the pattern?  Certain “facts” in science, like evolution and billions of years, are locked in a vault and never open to scrutiny.  No reporter is ever allowed to question the word of the wizards on those subjects.  Everything else in the universe, and even in multiple imaginary universes, is freely open to doubt, but never those.  There’s something rotten in the state of Men–dark.

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Categories: Physics, Solar System

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