The Solar System

CHAPTER 25 | METEORITES, ASTEROIDS, AND COMETS 573

SCIENTIFIC ARGUMENT

How do comets help explain the formation of the planets?

This argument must refer to the solar nebula hypothesis. The plan-

etesimals that formed in the inner solar nebula were warm and

could not incorporate much ice. The asteroids are understood to

be the last remains of such rocky bodies. On the other hand, plan-

etesimals in the outer solar system contained large amounts of

ices. Many of them were destroyed by being accreted together to

make the Jovian planets, but some survived intact. The icy bodies

of the Oort cloud and the Kuiper belt may be the solar system’s

last surviving icy planetesimals. When those icy objects have their

orbits perturbed by the gravity of the planets or passing stars,

some are redirected into the inner solar system where you see them

as comets. The gases released by comets indicate that they are

rich in volatile materials such as water and carbon dioxide. These

are the ices you would expect to find in the icy planetesimals.

Comets also contain dust with rocklike chemical composition, and

the planetesimals must have included large amounts of such dust

frozen into the ices when they formed. Thus, the nuclei of comets

seem to be frozen samples of the original outer solar nebula.

Nearly all of the mass of a comet is in the nucleus, but the light

you see comes from the coma and the tail. Build a new argument to

discuss observations. What do spectra of comets tell you about

the process that converts dirty ice into a comet?

■ Figure 25-18

(a) Kuiper belt objects are small bodies with dark surfaces that are very
hard to detect from Earth. In your imagination, you can see them as icy,
cratered worlds orbiting far from the sun. (b) The New Horizons spacecraft
is planned to fl y past at least one Kuiper belt object after it passes Pluto.
Dust, mostly from comets, produces the horizontal glow centered on the sun
in this artist’s conception. (Image: NASA and G. Bernstein; Art: Johns Hopkins
University Applied Physics Lab/Southwest Research Institute)

Visual-wavelength image

a

b

Kuiper Belt object 2000 FV53

about 120 km in diameter, moves

against stars in the background.

Asteroid and Comet

Impacts

Meteorite impacts on homes of the type described at the start

of this chapter are not common. Most meteors are small particles

ranging from a few centimeters down to microscopic dust.

Astronomers estimate that Earth gains about 40,000 tons of mass

per year from meteorites of all sizes. (Th at may seem to you like

a lot, but it is less than a hundred-thousandth of a trillionth of

Earth’s total mass.) Statistical calculations indicate that a meteor-

ite large enough to cause some damage, like the one that hit Mrs.

Hodges and her house in 1954, strikes a building somewhere in

the world about once every 16 months.

Objects a few tens of meters or less in diameter are likely to

fragment and explode in Earth’s atmosphere without reaching

the surface, but the shock waves from their explosions could still

cause serious damage on Earth’s surface. Declassifi ed data from

military satellites show that Earth is hit about once a week by

meter-size asteroids. What happens when even larger solar sys-

tem objects collide with Earth?

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