More than 4.5 billion years ago, an ancient star exploded in
a supernova. The heavy elements it created were spread around the blast zone in
their pure form, as well as in cohesive little groups of atoms called molecules.
At first, this debris existed only as a vast cloud of gas and dust called a
nebula. Over time, though, gravity and the random motion of particles produced
clumping, which led to the formation of a new solar system. At the center of
the cloud, a new star began to coalesce. Meanwhile, elsewhere in the nebula,
other clumps of matter came together, condensing under their own weight and
combining with other clumps in their vicinity to form planets in orbit around
the new star. This is how our solar system began.
Lighter elements in the outer nebula condensed to form the
four gas giants - Jupiter, Saturn, Uranus, and Neptune. At the same time,
heavier elements, upon which the Sun's gravity had a more pronounced effect,
remained within the inner solar system. These came together to form the four
terrestrial planets - Mercury, Venus, Earth, and Mars.
Terrestrial planets tend to accumulate mass through collision. If two objects meet in a head-on collision, the result is usually destructive, but most collisions in space are not head-on. Sometimes the impacts are glancing, and sometimes one object merely overtakes another moving in the same general direction. In these situations, the two objects often merge and become a single larger object more capable of surviving future collisions.
About twenty-five million years after becoming a planet,
Earth went through a molten phase, becoming a giant ball of lava. For this
reason, no rocks from its planet-building phase survive on Earth, but similar
rocks do exist elsewhere in the solar system. The asteroid belt that lies
between the orbits of Mars and Jupiter is full of planetary material that never
coalesced because of Jupiter's strong gravitational pull. Every now and then,
one of these asteroids strikes Earth, giving scientists the chance to study
them. Composition analysis has revealed that these rocks contain the same
ninety elements commonly found on Earth, while radiometric dating indicates
that they are all about 4.5 billion years old. Nothing older has yet been found.
(‘The Bedside Baccalaureate’, edited by David Rubel)
