no magnetic field (perhaps weak field in earlier epoch)
moon is tidally locked in orbit around earth
center of mass is off-set from center of moon
lunar rocks are from 3 to 4.5 billion years old
In many ways the earth-moon system is more like a double planet than a
planet and moon. The moon is very large (27%) the size of the earth which
is a much larger proportion to the size of a parent planet for any other
moon in the solar system. Further more its size is comparable to the largest
moons of the Jovian planets.
The moon is interesting for a number of reasons. It certainly
is an object of beauty and has been venerated by all peoples. It also represents
a museum of the early history of the solar system.
The Lunar Orbit
The moon travels around the earth with a nearly circular orbit of mean
radius 384, 400 km and with a period of 27.321661 days. The orbit is a synchronous
one which means that the moon's rotation rate is the same as its orbital rate
- this means that we always see the same face of the moon pointing towards us.
This phenomenon is called tidal locking and, as the name suggests is due to
the tidal force that the earth exerts on the moon. This effect is often "seen" or
assumed in the case of close binary stars.
Craters, Maria, Rills and Catastrophes
Galileo angered many when he proclaimed that the moon was not a perfect
sphere but poc-marked, rugged - much like the earth. Later generations
of astronomers have painstakingly mapped the lunar surface both from the
earth as well as from lunar orbiters. The lunar surface reveals a number
of significant features:
craters - one of the most
breathtaking views through a small telescope
is the cratered surface of the moon - especially when craters are seen
along the dividing line between light and dark called the terminator .
maria - dark, smooth regions on the lunar
surface which were once thought to be seas (hence the name maria). These
are easily apparent to the unaided eye.
rilles - under telescopic observation valley-like
lines can be seen wending across the lunar surface.
rays - bright projections in the vicinity of large
craters especially visible when contrasted against the lunar maria.
What produced the craters, maria, rilles
and rays? Are these distinct phenomena
or are they related? Several decades ago the idea was popular that the
craters were the result of volcanism on the lunar surface. Since this time,
however, the incidence of volcanism on the lunar surface has been shown
to be too little to account for the cratering that we observe. Instead
the currently favoured view is that the lunar surface was struck numerous
times in the early phase of the solar system by large "asteroidal" objects.
These numerous collisions throw up blankets of debris that gave rise to
the rays that we see around some craters. In the early epoch of the moon's
history the impact of numerous asteroids and the hotter lunar environment
would have produced massive lava flows and it is believed that the maria
were indeed seas at one time - but seas of lava not water . The ejecta
produced in the early cratering episodes would not be expected to be visible
- these would be obliterated by other impacts and flows. We know realize
that the presence of rays tells us the relative ages of craters. Young
craters are accompanied by the rays produced by their ejecta - old craters
are not. We can put all this together to arrive at a crude lunar chronology:
heavily cratered regions: oldest, formed during early era in the s.s. when
collision incidence was very high
maria: formed later, the
result of massive lava flooding
craters with rays: recently
formed craters
This still leaves un-answered the origin of lunar rills. It is now conceded
that lunar rills are evidence of lunar volcanism. This is further supported
by the observation of lunar domes - small raised regions that are an indicator
of volcanic activity. The two figures below are different views of the
famous Hadley Rille. Consult a lunar atlas and try to view this
the next time you are using a small telescope.
The phenomenon of cratering
has interesting parallels with cratering
observed on earth and during the past decade has led us to realize that "quasi-periodic" catastrophic
meteor impacts are part of the earth-moon
history. This has spawned a number of theories that we will consider
in
a future lecture.
Lunar Geology
The moon has many similarities and some surprising difference with the
earth. The composition of the moon is differentiated and the age of the
moon is essentially the same as that of the earth. The moon's composition
however is decidedly different than is the earth's composition. The moon's
composition favours the refractory elements (solid at high temperature)
but is underabundant (wrt earth) in volatile elements. This suggests that
the moon was formed at a higher temperature than was the earth.
Also, the moon does show seismic activity - "moonquakes" which
suggests that the lunar core may be partially molten. The lunar crust,
on the other hand, is much thicker in relation to the lunar radius than
is earth's. This is the result of the rapid cooling one would expect for
a small body.
The structure of rocks on the lunar surface is also distinctly
different than on earth. Most significant is the large number basalts (indicating
lava flow) and breccias which are basically broken or pulverized rock "glued
together". This would suggest that the lunar surface has been pummeled
vigorously in its past.
The Lunar Atmosphere
Basically - there is none! The moon is a small body gravitationally speaking.
At the temperatures the bright side of the moon would be subject to gases
would heat and reach velocities comparable to the velocity of gases in
the earth's atmosphere. On the moon, however these gases would quickly
escape ( diffuse ) into space. The lack of atmosphere, of course means
that there is no weather and hence no weathering on the lunar surface.
Origin of the Moon
fission hypothesis : spinning semi-molten earth split into two bodies.
Problem: why was the earth spinning so fast (10x) and where did the angular
momentum go?
co-accretion : moon and earth formed together. Problem: why the
very different compositions.
capture hypothesis : moon formed closer to the sun and was capture
by the earth. Problem: requires a very unlikely set of interaction between
the inner planets (3 - body encounter).
large-impact hypothesis : A larger body hit the earth-moon planetesimal.
The moon formed from some of the debris. The ejecta would be primarily crust
. So far this looks like the winner.
Debussy, Claire de Lune
Seeds:
Chp 21; 
