Geology Reference
In-Depth Information
produce new stars and slowly alter the composition of the cosmos. Countless billions of
stars have emerged in countless billions of galaxies.
Cosmic Clues
Once upon a time, five billion years ago, our future real estate in the galactic suburbs lay
halfway out from the Milky Way's center, at the uninhabited edge of a star-studded spiral
arm. Little was to be found in that unassuming neighborhood, apart from a great nebula of
gas and icy dust stretching light-years across the dark void. Nine parts in ten of that cloud
were hydrogen atoms; nine parts in ten of what remained were helium atoms. Ice and dust,
rich in small organic molecules and microscopic mineral grains, accounted for the remain-
ing 1 percent.
Anebularcloudinspacecanlastmanymillionsofyearsbeforeatrigger—ashockwave
from a nearby exploding star, for example—begins its collapse into a new star system. Al-
most 4.6 billion years ago, such a trigger initiated our Solar System. Ever so slowly, over
thecourseofamillion years,theswirling massofpresolar gasanddustwasdrawninward.
Like a twirling ice-skater, the big cloud rotated faster and faster as gravity pulled its wispy
arms to the center. As it collapsed and spun faster, the cloud became denser and flattened
into a disk with a growing central bulge—the nascent Sun. Larger and larger grew that
greedy hydrogen-rich central ball, which ultimately swallowed 99.9 percent of the cloud's
mass. As it grew, internal pressures and temperatures rose to the fusion point, igniting the
Sun.
CluestowhathappenednextarepreservedintherecordofourSolarSystem—itsplanets
and moons, its comets and asteroids, and its abundant and varied meteorites. One striking
feature is that all the planets and moons orbit the Sun on the same plane, and in the same
direction. What's more, the Sun and most of the planets rotate on their axes in more or less
that same plane and direction. Nothing in the laws of motion requires this commonality
of spin; planets and moons could orbit and rotate any which way—north to south, east to
west, top to bottom, bottom to top—and still obey the law of gravity. If planets and moons
werecapturedfromdistant,randomsources,onemightexpectsuchahodgepodge.Theob-
served orbital near-uniformity in our Solar System, by contrast, suggests that planets and
moons all coalesced from the same flat rotating disk of dust and gas at more or less the
same time. All of these grand objects preserve the same sense of rotation—the shared an-
gular momentum of the entire Solar System—from the time of the original swirling cloud.
A second clue to the Solar System's origins is found in the distinctive distribution of
its eight major planets. The four planets closest to the Sun—Mercury, Venus, Earth, and
Mars—are relatively small rocky worlds composed mostly of silicon, oxygen, magnesium,
and iron. Dense rocks, like black volcanic basalt, dominate their surfaces. By contrast, the
