Geoscience Reference
In-Depth Information
Planet-making in the Dry Zone
Inside the snow line, and near to the young Sun, is the zone where less
volatile minerals condense out: the zone of rock and metal, where the
Earth orbits.
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There is now a race against time. The raw dusty mater-
ials that build the planets are being swept outwards even as, whirling
and jostling, they aggregate into larger bodies, less susceptible to the
gusts of the solar wind. It is a chaotic process—but one that is clearly
efficient (for we are here on Earth to ponder it). Smaller and slower-
moving
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particles could simply 'stick' together in loose aggregates,
while larger ones could smash into each other and break up. Through
all of this process of building and destruction, though, dust grew into
larger aggregates that in turn clumped together into planetesimals
some few hundred metres to a few kilometres across, and these by
collision grew rapidly into 'planetary embryos' perhaps as big as Mars.
The final encounters between these produced the rocky planets that
we know today. Most of this process probably took place in only a few
million years.
The materials that condensed out of the fast-clearing disc of gas
and dust give some clue to the kind of temperatures at which this
happened. At very high temperatures (above 1,600 degrees Celsius)
minerals rich in elements such as aluminium, titanium, and zirco-
nium can condense into particles from the hot gas; particles of such
high-temperature minerals are indeed found in the oldest meteor-
ites known. As temperatures drop, farther from the young Sun,
minerals including silicon and phosphorus condense. Farther out
still, there are those with arsenic, copper, and silver—then, with ele-
ments such as sodium and potassium, then lead and zinc. Finally,
there are the most volatile elements: carbon, nitrogen, and hydro-
gen. When the last of these is combined with oxygen, the resultant
water vapour condenses into ice at the solar system's 'snow line', at
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