Geology Reference
In-Depth Information
Occasionally, during a particularly destructive collision, a meteorite happened to snatch
a piece of a planetesimal's core-mantle boundary, where chunks of silicate minerals and
iron-rich metals coexisted. The result is a beautiful pallasite—a stunning mixture of shiny
metal and golden crystals of olivine. Thin polished slabs of pallasite, with light reflecting
off the metal and passing through the olivine like stained glass, are among the most prized
specimens in the world of meteorite collecting.
As gravity clumped the early chondrites together—and as crushing pressure, scalding
temperature, corrosive water, and violent impacts reworked the growing planetesim-
als—more and more new minerals emerged. All together, more than 250 different minerals
have been found in all the varieties of meteorites—a twenty-fold increase over the dozen
presolar ur-minerals. These varied solids, which include the first fine-grained clays, sheet-
like mica, and semiprecious zircon, became the building blocks of Earth and other planets.
Bigger and bigger the planetesimals grew, as the largest swallowed the smaller. Eventually
a few dozen big balls of rock, each the size of a small planet, acted as giant vacuum clean-
ers,sweepingswathsoftheSolarSystemcleanofmostofitsdustandgasastheycoalesced
and settled into near-circular orbital paths. Where an object ultimately wound up depended
in large measure on its mass.
Assembling the Solar System
TheSun,whichhasthelion'sshareoftheSolarSystem'smass,dominateseverything.Ours
is not a particularly massive star system, and so the Sun is a modest sort of star—a good
thing for a nearby living planet. Paradoxically, the more massive a star, the shorter its life-
time. The greater interior temperatures and pressures of big stars push nuclear fusion reac-
tionsfasterandfaster.SoastartentimesthemassofourSunmightlastlessthanatenthas
long—several hundred million years at most, barely enough time for an orbiting planet to
get life started before the star explodes in a killer supernova. Conversely, a red dwarf star
a tenth the Sun's mass will last more than ten times longer—one hundred billion years or
more—though the energy output of such a weak star might not prove as life-sustaining as
our radiant yellow benefactor.
Our middling-size Sun has struck a happy medium: not too large and short-lived, not
too small and cool. And its projected nine or ten billion years of reliable hydrogen burning
mean there's been plenty of time for life to get going, and there's plenty more for it to con-
tinue to evolve. True, in another four or five billion years, the Sun will run out of hydrogen
in its core and will have to switch to helium burning. In the process, it will swell to a much
less benign red giant star more than one hundred times its present diameter, engulfing poor
littleMercury,firstscaldingandthenswallowingVenus,andmakingthingsprettyunpleas-
