462 PART 4^ |^ THE SOLAR SYSTEM
called the large-impact hypothesis (p. 454). The moon appears to have
formed from a ring of debris ejected into space when a large planetesimal
struck the proto-Earth after it had differentiated. This would explain, for
example, the moon’s low density and lack of volatiles.
▶ The moon rocks show that the moon formed in a molten state.
▶ The moon differentiated but contains little iron. Its low-density crust
was heavily cratered and shattered to great depth.
▶ Lava, fl owing up from below the crust, fi lled the lowlands to form
the smooth maria plains. The maria formed after the end of the heavy
bombardment and contain few craters.
▶ The near side of the moon has a thin crust, possibly due to tidal forces.
The back side has a thicker crust and little lava fl ooding.
▶ Because the moon is small, it has lost its internal heat and is geo-
logically dead. The only slow surface evolution occurring now is the
blasting of micrometeorites.
▶ Mercury is tidally locked to the sun and rotates 1.5 times per orbit
in a resonance (p. 457) relationship between its rotation and its
revolution.
▶ Mercury is a small world that has been unable to retain a true atmo-
sphere and has lost most of its internal heat. It is cratered and geologi-
cally inactive.
▶ As the metallic core cooled and contracted, the brittle crust broke
to form lobate scarps (p. 457), like wrinkles in the skin of a drying
apple.
▶ The Caloris Basin is a large, multiringed basin on Mercury that has been
partially fl ooded by lava fl ows.▶ (^) The intercrater plains (p. 459) on Mercury appear to be later lava
fl ows that covered older craters and then accumulated more craters. The
smooth plains (p. 459) seem to be more recent lava fl ows that contain
few craters. All of these plains are a similar shade of gray, so dark lava
fl ows are not visible on Mercury as they are in the moon’s maria.
▶ (^) Mercury formed in the inner solar system and contains a larger propor-
tion of dense metals. It is possible that a large impact shattered and
drove off some of the planet’s crust. This could explain why it has a
larger metallic core even than would be predicted by the condensation
sequence.
▶ (^) It is not clear how much heat remains in Mercury. It is not geologically
active, and it does not have a strong magnetic fi eld. Nevertheless it has
a weak magnetic fi eld, and radar observations of its rotation suggest
that the outer layers of its large iron core remain molten.
▶ (^) Mercury was heavily cratered during the heavy bombardment, but
lava fl ows covered some of those craters, and new craters formed the
intercrater plains. Fractures produced by the Caloris impact may have
triggered later lava fl ows that formed the smooth plains.
▶ (^) The MESSENGER spacecraft will go into orbit around Mercury in 2011
and provide more extensive photography of its surface and detailed
measurements of its physical properties.
Review Questions
- How could you fi nd the relative ages of the moon’s maria and
highlands? - How can you tell that Copernicus is a young crater?
- Why did the fi rst Apollo missions land on the maria? Why were the
other areas of more scientifi c interest? - Why do astronomers suppose that the moon formed with a molten
surface?
Summary
▶ The moon is tidally coupled (p. 443) to Earth and rotates on its axis
once each orbit, keeping the same side facing Earth.
▶ The moon is small and has only one-sixth the gravity of Earth. It has
such a low escape velocity that it is unable to retain an atmosphere.
Observers on Earth see sharp shadows on the moon’s surface, espe-
cially near the terminator (p. 443), and stars disappear behind the
limb (p. 444) of the moon without dimming. Both observations are
evidence of the lack of an atmosphere. Astronauts visiting the moon
verifi ed that the moon has no measurable atmosphere.
▶ Large, smooth, dark plains on the moon called maria (singular, mare)
(p. 444) are old lava fl ows that fi ll the lowlands. Evidence of lava is
seen as sinuous rilles (p. 444) that once carried fl owing lava, faults
where the lava plain cracked, and wrinkles in the surface.
▶ When a meteorite strikes the moon, it digs a large round impact crater
and throws out debris that falls back as ejecta (p. 446) and can form
rays (p. 446) and secondary craters (p. 446).
▶ The largest impacts on the moon dug huge multiringed basins
(p. 447).
▶ The highlands on the moon are saturated with craters. Rare meteorite
impacts continue to form new craters, and micrometeorites (p. 447)
constantly grind the surface down to dust, but most of the craters on
the moon are very old.
▶ Astronomers can fi nd relative ages (p. 445) for lunar features by
looking to see which lies on top and which lies below. Absolute ages
(p. 445) in years can be found by counting craters on surfaces and
comparing the results with an estimate of the rate of crater formation
calibrated using radioactive ages of lunar rock samples.
▶ Most of the craters on the moon were formed during the heavy bom-
bardment at the end of planet building about 4 billion years ago. No
crater is known with certainty to have been formed on the moon in
historic times.
▶ Between 1969 and 1972, 12 astronauts set foot on the moon and
returned specimens to Earth.
▶ The moon rocks are all igneous, showing that they solidifi ed from
molten rock. Some are vesicular (p. 450) basalts, showing that
they formed in lava fl ows on the surface. Light-colored anorthosite
(p. 451) is part of the old crust and helps make the highlands
brighter than the maria in the lowlands. Many of the rocks are breccias
(p. 451), which shows that much of the lunar crust was fractured by
meteorite impacts.
▶ The surface of the moon is covered by rock crushed and powdered by
meteorite impacts to form a soil called regolith (p. 451).
▶ (^) There is evidence from lunar crater counts and rock sample ages of an
episode of increased impact rate, labeled the late heavy bombard-
ment (p. 452), about 4 billion years ago near the end of the heavy
bombardment era. Many of the giant impact basins containing mare
were formed during this time. That event may have been caused by
Jovian planets migrating and scattering remnant planetesimals across
the solar system.
▶ (^) The Imbrium basin was formed about 4 billion years ago by the impact
of a planetesimal at least 60 km (40 mi) in diameter. Seismic waves
traveling through the moon were focused to the far side and produced
jumbled terrain (p. 453). Later fl ooding has nearly buried the original
basin.
▶ (^) The fi ssion, condensation, and capture hypotheses for the origin of the
moon have all been abandoned. The commonly accepted explanation is