The Solar System

CHAPTER 25 | METEORITES, ASTEROIDS, AND COMETS 567

nuclei appear to have densities between 0.1 and 0.25 g/cm^3 ,

much less than the density of ice. Also, as you will learn later in

this chapter, comets subjected to tidal stresses from Jupiter or

the sun come apart very easily. Comet nuclei have been

described as dirty snowballs or icy mudballs, but that seems to

be incorrect; their shapes, low densities, and lack of material

strength suggest that comets are not solid objects. Th e evidence

leads astronomers to conclude that most comet nuclei must be

fl uff y mixtures of ices and dust with signifi cant amounts of

empty space. On the other hand, images of the nucleus of

Comet Wild 2 revealed cliff s, pinnacles, and other features that

show the material has enough strength to stand against the

weak gravity of the comet.

Photographs of comet comae (Figure 25-13) often show jets

springing from the nucleus into the coma and being swept back

by the pressures of sunlight and the solar wind to form the tail.

Studies of the motions of these jets as the nucleus rotates reveal

that they originate from small active regions that may be similar

ammonia, and so on. Th ese are the kinds of compounds that
would have condensed in cold regions of the solar nebula. Th is
convinces astronomers that comets are ancient samples of the
gases and dust from which the outer planets formed. As the ices
absorb energy from sunlight, they sublime—change from a solid
directly into a gas. Th e gases break down and combine chemi-
cally, producing other substances found in comet spectra. For
example, vast clouds of hydrogen gas observed around the heads
of comets are understood to derive from the breakup of ice
molecules.
Five spacecraft fl ew past the nucleus of Comet Halley when
it visited the inner solar system in 1985 and 1986. Other space-
craft fl ew past the nuclei of Comet Borrelly in 2001, Comet
Wild 2 (pronounced vildt-two) in 2004, and Comet Tempel 1 in

2005. Images show that all these comet nuclei are 1 to 10 km
      across, similar in size to many asteroids, and irregular in shape
      (■ Figure 25-13). In general, these nuclei are darker than a lump
      of coal, which suggests composition similar to carbon-rich carbo-
      naceous chondrite meteorites described earlier in this chapter.
      Th e mass and density of comet nuclei can be calculated
      from their gravitational infl uence on passing spacecraft. Comet ■^ Figure 25-13
      Visual-wavelength images made by spacecraft and by the Hubble Space
      Telescope show how the nuclei of comets produce jets of gases from regions
      where sunlight vaporizes ices. (Halley nucleus: © 1986 Max-Planck Institute; Halley
      coma: Steven Larson; Comets Borrelly, Hale–Bopp, and Wild 2: NASA)

The nucleus of

Comet Wild 2

is highly

irregular.

Jets vent

from active

regions

Debris ejected

from the nucleus

Nucleus

5 km

5 km

Active regions on the nucleus

of Comet Borrelly emit jets

when they rotate into sunlight.

Jet

Jet

Enhanced visual images

Hubble Space Telescope

image of coma of Comet

Hale–Bopp.

Jets from the nucleus of

Comet Halley form a pinwheel

in the coma because of the

rotation of the nucleus.

10 km

The nucleus of Comet Halley

is irregular and emitting jets

from active regions.

b

c

d

e

a