Geology Reference
In-Depth Information
elements that were formed in its
core are returned to interstellar
space and are available for inclusion
in new stars. In this way, the com-
position of the universe is gradually
enhanced by heavier elements.
After
Before
Our Solar System—Its
Origin and Evolution
Our solar system, which is part of
the Milky Way galaxy, consists of the
Sun, eight planets, one dwarf planet
(Pluto), 101 known moons or satel-
lites (although this number keeps
changing with the discovery of new
moons and satellites surround-
ing the Jovian planets), a tremen-
dous number of asteroids—most of
which orbit the Sun in a zone be-
tween Mars and Jupiter—and mil-
lions of comets and meteorites, as
well as interplanetary dust and gases
(
6 cm
12 cm
Figure 1.6 The Expanding Universe The motion of raisins in a rising loaf of raisin bread
illustrates the relationship that exists between distance and speed and is analogous to an
expanding universe. In this diagram, adjacent raisins are located 2 cm apart before the loaf rises.
After one hour, any raisin is now 4 cm away from its nearest neighbor and 8 cm away from the
next raisin over, and so on. Therefore, from the perspective of any raisin, its nearest neighbor has
moved away from it at a speed of 2 cm per hour, and the next raisin over has moved away from
it at a speed of 4 cm per hour. In the same way that raisins move apart in a rising loaf of bread,
galaxies are receding from each other at a rate proportional to the distance between them.
Figure 1.7). Any theory formu-
lated to explain the origin and evolution of our solar sys-
tem must therefore take into account its various features and
characteristics.
Many scientifi c theories for the origin of the solar system
have been proposed, modifi ed, and discarded since the French
scientist and philosopher René Descartes fi rst proposed, in
1644, that the solar system formed from a gigantic whirlpool
within a universal fl uid. Today, the solar nebula theory for
the origin of our solar system involves the condensation and
collapse of interstellar material in a spiral arm of the Milky
Way galaxy (
Arno Penzias and Robert Wilson of Bell Telephone
Laboratories made the second important observation
that provided evidence of the Big Bang in 1965. They dis-
covered that there is a pervasive background radiation of
2.7 Kelvin (K) above absolute zero (absolute zero equals
-273°C; 2.7 K = -270.3°C) everywhere in the universe. This
background radiation is thought to be the fading afterglow of
the Big Bang.
Currently, cosmologists cannot say what it was like at
time zero of the Big Bang because they do not understand the
physics of matter and energy under such extreme conditions.
However, it is thought that during the fi rst second following
the Big Bang, the four basic forces— gravity (the attraction of
one body toward another), electromagnetic force (combines
electricity and magnetism into one force and binds atoms into
molecules), strong nuclear force (binds protons and neutrons
together), and weak nuclear force (responsible for the break-
down of an atom's nucleus, producing radioactive decay)—
separated and the universe experienced enormous expansion.
By the end of the fi rst three minutes following the Big Bang,
the universe was cool enough that almost all nuclear reactions
had ceased, and by the time it was 30 minutes old nuclear re-
actions had completely ended and the universe's mass con-
sisted almost entirely of hydrogen and helium nuclei.
As the universe continued expanding and cooling, stars
and galaxies began to form and the chemical makeup of the
universe changed. Initially, the universe was 100% hydrogen
and helium, whereas today it is 98% hydrogen and helium
and 2% all other elements by weight. How did such a change
in the universe's composition occur? Throughout their life
cycle, stars undergo many nuclear reactions in which lighter
elements are converted into heavier elements by nuclear
fusion. When a star dies, often explosively, the heavier
Figure 1.8).
The collapse of this cloud of gases and small grains into
a counterclockwise-rotating disk concentrated about 90%
of the material in the central part of the disk and formed
an embryonic Sun, around which swirled a rotating cloud
of material called a solar nebula. Within this solar nebula
were localized eddies in which gases and solid particles con-
densed. During the condensation process, gaseous, liquid,
and solid particles began to accrete into ever-larger masses
called planetesimals, which collided and grew in size and
mass until they eventually became planets.
The composition and evolutionary history of the planets
are a consequence, in part, of their distance from the Sun (see
Geo-inSight on pages 14 and 15). The terrestrial planets
Mercury, Venus, Earth, and Mars—so named because they
are similar to terra , Latin for “earth,” are all small and com-
posed of rock and metallic elements that condensed at the
high temperatures of the inner nebula. The Jovian planets
Jupiter, Saturn, Uranus, and Neptune—so named because
they resemble Jupiter (the Roman god was also called Jove ),
all have small rocky cores compared to their overall size, and
are composed mostly of hydrogen, helium, ammonia, and
methane, which condense at low temperatures.
 
Search WWH ::




Custom Search