The Sun, the Earth, and the Evolution of the Earth’s Atmosphere - Air Pollution and Global Warming: History, Science, and Solutions - page 34

vaporizing. Meteorite bombardment was intense for
about 500 million years. Although the solar nebula has
since cooled and most of it has been converted to solar
or planetary material or has been swept away from the
solar system, some planetary growth continues today,
as leftover asteroids and meteorites occasionally strike
the planets.
Meteorites are classified as iron, stony, or stony iron.
Iron meteorites contain 90 to 95 percent iron, with
nickel making up most of the rest. They originate from
the exploded core of an earlier planet or asteroid. Stony
meteorites by far comprise the largest number of mete-
orites and originate from the outer crust of an exploded
planet or asteroid. Table 2.2 shows the average com-
position of stony meteorites, the total Earth, and the
Earth's continental and oceanic crusts. The table indi-
cates that stony meteorite composition is relatively sim-
ilar to that of the total Earth, supporting the theory that
stony meteorites played a role in the Earth's formation.
Stony iron meteorites comprise only

Figure 2.6. The asteroid Ida and its moon, Dactyl,
taken by the Galileo spacecraft as it passed within
10,878 km of the asteroid on August 28, 1993.
Available from National Space Science Data Center,
http://nssdc.gsfc.nasa.gov.

2 percent of
all meteorites and contain roughly equal amounts of
iron-nickel and other material. They originate from the
middle of exploded planets or asteroids.
Meteorites and asteroids consist partly of rock-
forming elements (e.g., Mg, Si, Fe, Al, Ca, Na, Ni,
Cr, Mn) that condensed from the gas phase to the liq-
uid phase before solidifying in the cooling solar nebula.
They also consist of noncondensable elements (e.g.,
H, He, O, C, Ne, N, S, Ar, P), which could not condense
in elemental form. How did noncondensable elements
enter meteorites and asteroids, particularly as they were
too light to attract to these bodies gravitationally?
One theory is that noncondensable elements may have

∼

in the solar nebula, condensed into small solid grains
as the nebula cooled. The grains grew by collision to
centimeter-sized particles. Additional grains accreted
onto the particles, resulting in planetesimals ,which are
small-body precursors to planet formation. Accretion of
grains and particles onto planetesimals resulted in the
formation of asteroids (Figure 2.6), which are rocky
bodies 1 to 1,000 km in size that orbit the sun. Today,
asteroids orbit primarily in the asteroid belt between
Mars and Jupiter. Asteroids collided to form the planets.
The growth of planets was aided by the bombardment
of meteorites ,which are 1-mm- to 1-km-wide solid
minerals or rocks that reach the planet's surface without

Table 2.2. Mass percentages of major elements in stony meteorites, total
Earth, and Earth's continental and oceanic crusts

Earth's Earth's

Stony Total continental oceanic

Element meteorites Earth crust crust

Oxygen (O) 33.24 29.50 46.6 45.4

Iron (Fe) 27.24 34.60 5.0 6.4

Silicon (Si) 17.10 15.20 27.2 22.8

Magnesium (Mg) 14.29 12.70 2.1 4.1

Sulfur (S) 1.93 1.93 0.026 0.026

Nickel (Ni) 1.64 2.39 0.075 0.075

Calcium (Ca) 1.27 1.13 3.6 8.8

Aluminum (Al) 1.22 1.09 8.1 8.7

Sodium (Na) 0.64 0.57 2.8 1.9

Source: Adapted from Cattermole and Moore (1985).