VLSI@DEC-MARLBORO@sri-unix (06/28/82)
As I understand the current theory of planetary composition, it
has little to do with flares or instabilities in the sun. Rather, it's
based on the makeup of the original nebula that the sun and planets
condensed out of. As the nebula contracted there would be a temperature
gradient from the center to the outer fringes. The very center would
be too hot for any compounds whatsoever to solidify. As you went a little
farther out the refractory metals like tungsten would condense, because
the temperature (and pressure) would have dropped below their freezing
point. As you keep moving out more and more solids become possible.
By the time you get to the orbits of the gas giants, substances like
methane and ammonia will liqueify. Since there is a great deal more
carbon and nitrogen than there is tungsten and iron, the outer planets
wind up being a lot bigger. The gaseous methane and ammonia around the
inner planets gets blown away once the sun ignites.
Now, we know roughly what the original elemental composition was.
We know the boiling points of the common compounds and so can figure
out what the order of condensation should be. Given the mass of the
original nebula we can figure out how much energy is released in contraction.
If we know how much of this radiant energy is reabsorbed by the cloud, and
if we assume certain models for mixing, we can calculate what the
temperature and pressure gradients should be. This should tell us the chemical
compostion of the inner planets. J. S. Lewis of MIT predicts that:
"Mercury has a massive Fe-Ni alloy core surmounted by a small mantle of
Fe+2-free magnesium silicates. Refractory oxides are present but only traces
of alkali metals, sulfur, FeO, etc. Venus has Fe_ni core, a massive mantle
of Fe+2 free magnesium silicates, and a silicate rich crust similar to earth's.
Sulfur is probably absent. The earth has an inner core of Fe_ni and an outer
core of Fe-FeS melt. Cetain chalcophile elements are deficient in the mantle
and crust but enriched in the outer core. The mantle contains 10% FeO.
Deficiencies of S, K, Rb, and Cs in the crust and upper mantle are due to
their extraction into an FeS-rich melt. Mars is almost devoid of free iron,
may contain a core of FeS, and has a mantle rich in FeO. Hydrous minerals
were probably retained. The crust should be more iron-rich than the earth's."
The densities calculated agree with those observed, but I don't know
if info from the Viking, Voyager, and Venera probes confirm the theory.
The theory also explains the composition of meteorites, though I get the
impression that it doesn't explain it all that well. I'm kind
of awed that this can be done at all. I learned all this from a book called
"Frontiers of Astrophysics" edited by Eugene H. Averett.
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