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ASTR 1210, O'CONNELL. Study Guide 13 [Spring 2013]
ASTR 1210 (O'Connell) Study Guide 13
Apollo 17 landing site in
Taurus-Littrow Valley
After the Sun, the Moon is the most important extraterrestrial object.
It has important practical consequences for humans, since it controls
the tides and provides illumination at night. Its surface is
remarkable seen in a small telescope because it has fantastic topography,
with towering mountain peaks, thousands of craters, and deep valleys
which have never been subject to weathering. It is of critical
astrophysical importance because its surface contains a fossilized
history of the early solar system. It is also unique as the only
extraterrestrial body to have been visited by humans.
"Moons" and "planets" are not necessarily intrinsically different
kinds of bodies; a "moon" simply orbits a planet, rather than the
Sun. "Planet" Mercury is 4900 km in diameter---smaller than two
"moons."
Telescopic exploration: begins 1609 (Galileo). Telescopes permit
identification of mountains, craters, maria ("seas"), and other structures.
Spacecraft exploration: Extensively studied 1960-1976 by US and
USSR using both robot and human spacecraft. The US landed humans on
the Moon 6 times 1969-1972
during the
Apollo Program. No further spacecraft studies until the
1990's.
A new program intended to send humans back into deep space, has revived
scientific investigations of the Moon.
Atmosphere: ~none. Gravity is too small to retain. Escapes to space
Surface: Despite appearances, the Moon's surface is very dark.
It has a low albedo (reflectivity) of only ~7% because it is
is covered by a regolith = powdery fragments
from impacts, a few meters deep
The maria, which make up the conspicuous dark pattern we see as the
"Man in the Moon," are mainly confined to the near-side of the
Moon. The far-side
consists almost entirely of highlands regions.
C. Impact Topography
The Moon's surface testifies to
the fact that the surface topography of most rocky planets is shaped largely by
brutal impacts of asteroids, planetesimals, or comets.
Although they should have realized this earlier, astronomers have only
widely accepted the importance of impacts for the last 50 years.
On the Moon and most other solar system bodies with hard surfaces the
impact history is preserved in the form of extensive
cratering. But impacts are responsible for most of the other
surface features as well, including mountains.
Click here for a
comparison of the appearance of six surfaces.
The surface density of craters (i.e. number per square km) can be used
to crudely age-date different regions on planetary surfaces:
The impact rate was higher at early times (4 Byr ago), when the
solar system was filled with many planetesimals not yet accreted by
planets (see discussion below).
But even if the impact rate were constant in time, the longer
a surface has been exposed to impacts, the more it will have
accumulated.
Therefore:
Older
surfaces have higher crater densities (number per square mile).
Younger surfaces have lower crater densities
Crater counts vs. size can
be calibrated to provide age estimates.
A large regional difference in crater densities on a given object
is evidence of re-surfacing activity. We see such differences
between the highlands and maria on the Moon and in even more dramatic
form in cases such as Enceladus, the sixth
largest satellite of Saturn, which has an icy, rather than rocky,
crust. Enceladus is shown in the image above right. Click for an
enlargement and note the large differences in cratering density
caused by ice flows covering over ancient cratered terrain.
Earth has little surface impact cratering because plate tectonics
recycles its surface material and atmospheric weathering
erodes all structures.
Topographic Map of East Limb of Moon (Lunar Orbiter Laser Altimiter)
Craters: On Earth, we find craters mostly on volcanic
mountaintops. On the Moon, craters are everywhere. Lunar craters
were produced by impacts, not volcanic activity. Scales from
millimeters to 150 miles diameter. The circular shapes, raised rims,
and ejecta blankets are typical of impact events.
Maria: Large, roughly round basins; produced by major
impacts after the lunar surface had solidified, subsequently filled by
dense, dark lava flows from the interior.
Mountains: Altitudes to 25,000 feet. All are related to
impacts, not to plate tectonics, as on Earth. Extended ranges tend to
lie at borders of maria basins and were formed during huge mare
impacts.
Rilles: canyons produced by lava flows, not water
E. Geology of the Moon
During the Apollo Program, human crews landed &
returned rock
samples from six different sites on the Moon's surface.
The launch of Apollo 8, with the first crew to circumnavigate the
Moon, is shown at the right.
The lunar landings were of great scientific value, but this was
the only significant contribution of human space flight to
planetary science. Most of the important scientific discoveries about
the planets have come from
robotic spacecraft and telescopes.
The overall density of the Moon is 3.3 gr/cc, lower than the mean
value for the Earth (implying fewer heavy metals), but comparable
to the outer layers of Earth
The Moon's surface contains more refractory (hard to melt)
materials than Earth and fewer volatiles
No sedimentary rocks; only trace amounts of water in rocks
at Apollo sites
In the last 20 years, observations from a number of spacecraft have
suggested the presence of water on the Moon, e.g. ice lying in constantly shadowed
craters (T ~ 40K) near the poles (click
here for
details). Deposited by comets? Possibly useful for human colonies.
But results on recoverable reservoirs of water are inconclusive so
far.
Highlands: Lower density rocks, anorthosites (Ca, Al rich);
igneous (deposited molten).
Very old ~ 4.5 Byr.
Oldest Earth rocks are younger, but Earth's
surface is tectonically recycled and such old rocks are rare.
Differentiated, but with thick lithosphere
(~ 10 x Earth's)
Not tectonically active: has cooled too much.
G. Origin
Fission from Earth? No: mean chemical content differs.
Capture? Unlikely. Captured satellites exist (e.g. Jupiter) but
are small.
Collisional ejection favored. A large (perhaps Mars-size)
planetoid hits the young Earth about 4.5 Gyr ago, heating and
expelling material from the outer layers, which goes into orbit and
accumulates to form the Moon (see drawing). This is consistent with
lunar chemistry, since it implies the Moon will contain lighter,
refractory materials; the non-refractory materials evaporate. For
more details,
click here.
Click here for a Quicktime
animation of the birth of the Moon.
H. History
Molten after accretion. The highlands are produced from low
density magma "foam," which solidifies to a solid crust.
Based on Apollo return samples, the large maria formed this way
3.8-4.1 Byr ago. It is now thought that this narrow range in the dates
of big lunar impacts indicates a special event called the
Late Heavy Bombardment, during which an unusual number of large
impactors penetrated the inner Solar System. The LHB may have been
caused by a large
scale gravitational
dynamical instability produced by changes in the orbits of Jupiter
and Saturn.
The LHB would have had catastrophic consequences for the surface of
Earth as well. Overall, we expect the number of impacts per unit area on Earth to
have been somewhat higher than on the Moon.
For more discussion of impacts on the Earth and their consequences
for our biosphere, see Study Guide 22.
Continuing, declining bombardment of smaller objects completely
erodes the lunar surface and creates the global regolith.
I. Tides
Earth and Moon interact gravitationally
"Tides" = differential effect of gravity on an extended
object
Raise bulges in water, rocks. Most important in rise/fall
of ocean surface.
Over time, tides act to slow the Moon's spin, so now is
locked in "synchronous" rotation (spin period = orbital period)
Apollo 11 Lunar Module returns to the Command Module
after the first human landing on the Moon (July 1969)
Reading for this lecture:
Bennett textbook, Secs. 9.2, 9.3, 10.3
Study Guide 13
Reading for next lecture:
[Study Guide 14 is optional reading]
Study Guide 15
Bennett textbook: Secs. 9.3, 9.5.
Click for illustrations
Click here for a Quicktime
animation of the birth of the Moon.
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