Geology Reference
In-Depth Information
objectsafewmilesorsoindiameterallvyingforspaceinthesamenarrowringaroundthe
young Sun.
Larger and larger they grew: to the size of Rhode Island, then Ohio, Texas, Alaska. As
thousands of such planetesimals underwent this chaotic process of accretion, they diver-
sified in new ways. As they grew to fifty miles or more in diameter, two coequal sources
of heat compounded. The gravitational potential energy of many small objects smashing
together was matched in intensity by the nuclear energy of fast-decaying radioactive ele-
ments like hafnium and plutonium. The minerals making up these planetesimals were thus
transformed by heat, while their interiors melted outright, differentiating into an egglike
arrangement of distinctive mineral zones: a dense metal-rich core (analogous to the egg's
yolk), the magnesium silicate mantle (the egg white), and the thin brittle crust (the shell).
The largest planetesimals were altered by internal heating, by reactions with water, and by
the intense shock of frequent collisions in the crowded solar suburbs. Perhaps three hun-
dreddifferentmineral speciesaroseasaconsequence ofsuchdynamicplanet-forming pro-
cesses. Those three hundred minerals are the raw materials from which every rocky planet
must form, and all of them are still found today in the diverse suites of meteorites that fall
to Earth.
From time to time, when two big planetesimals smashed together with sufficient force,
theywereblastedtosmithereens.(ThisviolentprocesscontinuestothisdayintheAsteroid
Belt beyond Mars, thanks to the gravitational disruptions from the giant planet Jupiter.)
Consequently, most of the diverse achondrite meteorites we find today represent different
parts of destroyed miniplanets. Analyzing achondrites is thus a bit like a messy anatomy
lesson from an exploded cadaver. It takes time, patience, and a lot of bits and pieces to get
a clear picture of the original body.
The dense metallic cores of planetesimals, which wound up as a distinctive class of iron
meteorites,aretheeasiesttointerpret.Thoughoncethoughttobethemostcommontypeof
meteorites, the unbiased Antarctic sampling reveals that irons represent a modest 5 percent
of all falls. Planetesimal cores must have been correspondingly small.
The contrasting silicate-rich mantles of planetesimals are represented in a host of exotic
meteorite types: howardites, eucrites, diogenites, ureilites, acapulcoites, lodranites, and
more—each of distinctive composition, texture, and mineralogy, and most named for the
locality where the earliest known example was recovered. Some of these meteorites are
close analogues to rock types found on Earth today. Eucrites represent a rather typical kind
of basalt—the rock type that spews out of the Mid-Atlantic Ridge and blankets the ocean
floor.Diogenites,composedprimarilyofmagnesiumsilicateminerals,appeartobetheres-
ult of crystal settling in a large underground magma reservoir. As the magma cooled, crys-
tals more dense than their surrounding hot liquid grew and then sank to the bottom to form
a concentrated mass, just as they do in magma chambers deep inside Earth today.
