CHAPTER 21 | THE MOON AND MERCURY: COMPARING AIRLESS WORLDS 455
Th e large-impact hypothesis is consistent with the known evidence
and is now considered likely to be correct.
Th is hypothesis would explain other things. Th e collision
must have occurred at a steep angle to eject enough matter to
make the moon, so the objects could not have collided head-on.
A glancing collision would have spun the material rapidly enough
to explain the observed angular momentum in the Earth–moon
system. And, as mentioned above, if the two colliding planetesi-
mals had already diff erentiated, the ejected material would be
mostly iron-poor mantle and crust. Calculations indicate that the
iron core of the impacting body would have fallen into the larger
body that became Earth. Th is would explain why the moon is so
poor in iron and why the abundances of other elements are so
similar to those in Earth’s mantle. Finally, the collision-heated
material that eventually became the moon would have remained
in a disk long enough for water and other volatile elements, which
the moon lacks, to be outgassed and lost to space.
Th e moon is evidently the result of a giant impact. Until
recently, astronomers have been reluctant to consider such cata-
strophic events, but a number of lines of evidence suggest that other
planets also may have been aff ected by giant impacts. Consequently,
the third theme identifi ed in the introduction to this chapter, giant
impacts, has the potential to help you understand other worlds.
Catastrophic events are rare, but they can occur.
SCIENTIFIC ARGUMENT
If the moon was intensely cratered by the heavy bombardment
and then formed great lava plains, why didn’t the same thing
happen on Earth?
Is this argument obvious? It is still worth reviewing as a way to
test your understanding. In fact, the same thing did happen on
Earth. Although the moon has more craters than Earth, the moon
and Earth are the same age, and both were battered by meteorites
during the heavy bombardment. Some of those impacts on Earth
must have been large and dug giant multiringed basins. Lava fl ows
must have welled up through Earth’s crust and fl ooded the low-
lands to form great lava plains much like the lunar maria.
Earth, however, is a larger world and has more internal heat,
which escapes more slowly than the moon’s heat did. The moon
is now geologically dead, but Earth is very active, with heat fl ow-
ing outward from the interior to drive plate tectonics. The moving
plates long ago erased all evidence of the cratering and lava fl ows
dating from Earth’s youth.
Comparative planetology is a powerful tool in that it allows you
to see similar processes occurring under different circumstances.
For example, expand your argument to explain a different phenom-
enon. Why doesn’t the moon have a magnetic fi eld?Mercury
Earth’s moon and mercury are good subjects for comparative
planetology. Th ey are similar in a number of ways. Most impor-
tant, they are small worlds (Celestial Profi le 4); the moon
21-2
Mercury is a bit over one-third the diameter of Earth, but its high density must
mean it has a large iron core. The amount of heat it retains is unknown. (NASA)Celestial Profi le 4: Mercury
Motion:
Average distance from the sun 0.387 AU (5.79 107 km)
Eccentricity of orbit 0.206
Inclination of orbit to ecliptic 7.0°
Average orbital velocity 47.9 km/s
Orbital period 0.241 y (88.0 days)
Period of rotation 58.6 d (direct)
Inclination of equator to orbit 0°Characteristics:
Equatorial diameter 4.89 103 km (0.382 D⊕)
Mass 3.31 1023 kg (0.0558 M⊕)
Average density 5.44 g/cm^3 (5.4 g/cm^3 uncompressed)
Surface gravity 0.38 Earth gravity
Escape velocity 4.3 km/s (0.38 V⊕)
Surface temperature 170° to 430°C (275° to 805°F)
Average albedo 0.1
Oblateness 0Personality Point:
Mercury lies very close to the sun and completes an orbit in only 88 days.
For this reason, the ancients named the planet after Mercury, the fl eet-
footed messenger of the gods. The name is also applied to the element
mercury, which is also known as quicksilver because it is a heavy, quickly
fl owing silvery liquid at room temperatures.