Geology Reference
In-Depth Information
outer four planets—Jupiter, Saturn, Uranus, and Neptune—are gas giants made primarily
ofhydrogenandhelium.Theseimmensesphereshavenosolidsurfaces,justanever-thick-
ening atmosphere the deeper you go. This dichotomy of worlds suggests that early in the
historyoftheSolarSystem,withinafewthousandyearsoftheSun'sbirth,anintensesolar
wind blew leftover hydrogen and helium far out to the colder realms. Sufficiently far from
the radiant Sun, these volatile gases could cool, condense, and gather into spheres of their
own. By contrast, the coarser, mineral-rich grains of dust that remained closer to the hot
central star quickly clumped together to form the rocky inner planets.
Details of the violent processes that formed Earth and the other inner planets are beauti-
fullypreservedintheamazinglydiversevarietiesofmeteorites.It'sabitunsettlingtothink
that our home is constantly being peppered by stones falling from the sky. In fact, the sci-
entific community didn't pay them much heed until about two hundred years ago, though
there was certainly no shortage of colorful meteorite anecdotes in folklore (including sev-
eral tales involving unfortunate French peasants). Even when scholars began to describe
meteorite falls more formally, little in the way of reproducible scientific evidence could be
mustered to document them, much less explain their provenance. The American statesman
and naturalist Thomas Jefferson, upon reading the technical report from Yale University of
an observed meteorite impact in Weston, Connecticut, quipped: “I find it easier to believe
that two Yankee professors would lie than that stones would fall from heaven.”
Twocenturiesandtensofthousandsofmeteoritefindslater,theirveracityisnolongerin
question. As meteorite experts cover more ground, and as avid collectors vie for the rarest
types, museum and private collections around the world have swelled. For a time, these re-
positories were skewed in favor of distinctive iron meteorites, whose black crusts, weirdly
sculpted shapes, and unusually high density made them stand out against everyday rocks.
But the 1969 discovery of thousands of meteorites lying on pristine Antarctic ice fields
changed that perception.
Meteorites are telltale clues to our planet's origins. The most common and ancient ones,
the 4.566-billion-year-old chondrites, date from the time just before the planets and moons
of the Solar System formed, when the Sun's nuclear reactor first turned on and intense ra-
diant energy broiled the encircling nebula. The blast furnace effect melted the dusty disk
intoclotsofsmall,stickyrockdroplets,calledchondrules,aftertheancientGreekwordfor
“grain.” Ranging from the size of BBs to that of small peas, these products of the Sun's
refining fire were melted multiple times, in repeated pulses of radiation that transformed
the regions closest to the Sun. Clusters of these ancient chondrules, cemented together by
finer-grained presolar dust and mineral fragments, compose the primitive chondrites that
havelandedontheEarthbythemillions.Chondritesprovideourbestviewofthebrieftime
just after the Sun was born but before the planets were formed.
