ASTR 1210 (O'Connell) Study Guide 13
Apollo 17 landing site in
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.
- Size: Diameter 3500 km = 1/4 Earth; largest satellite relative to
its primary; 5th largest overall.
"Moons" and "planets" are not intrinsically different kinds of
bodies; a "moon" simply orbits a planet, rather than the Sun.
"Planet" Mercury, at 4900 km in diameter, is smaller than two
- Telescopic exploration: begins 1609 (Galileo). Telescopes
permitted identification of a complex topography,
including mountains, craters, maria ("seas"), and numerous
- 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
Apollo Program. There were no further spacecraft studies
until the 1990's.
A new NASA program intended to send humans back into deep space revived
US scientific investigations of the Moon, and other nations (including
Japan, India, and China) have recently begun lunar exploration.
- Atmosphere: ~none. Lunar gravity is too small to retain
atmospheric gases. These escape to space
- Surface: Despite appearances, the Moon's surface is
actually very dark. It has a low albedo
(reflectivity) of only ~7% because it is is covered by
a regolith, a layer of powdery fragments from
impacts, a few meters deep
- For a large selection of maps and other data on the Moon's
surface, see the USGS Moon Information Page.
B. Main Terrain Types
The two main lunar terrain types are highlighted in the image of the
full Moon at right. Click for
more images and information.
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.
- Highlands: rough, light colored
- Maria: dark, round, smoother (variations < 150 meters).
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, and 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 the mountains and maria.
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
cratering it will have accumulated.
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)
Click for the full image.
D. Topographic Features
Click for illustrations
- Preserved: no weathering by rain, wind
- 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 range from
millimeters to 150 miles diameter and larger. 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, and 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
- Rilles: canyons produced by lava flows, not water
E. Geology of the Moon
- During the Apollo Program, human crews landed &
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, which
is lower than the mean value for the Earth (implying fewer
heavy metals) but comparable to its outer layers.
- 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
details). Deposited by comets? Possibly useful for human colonies.
But results on recoverable reservoirs of water are inconclusive so
- 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.
- Maria: higher density rocks "basalts" (more Fe, Mg rich);
igneous; younger ~ 3.8 Byr
- Differentiated, but with a thick lithosphere
(~ 10 x Earth's)
- Not tectonically active: has cooled too much.
- Fission from Earth? No: mean chemical content differs.
- Capture? Unlikely. Captured satellites exist (e.g. of
Mars, Jupiter) but are small, asteroidal bodies.
- 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
Click here for a Quicktime
animation of the birth of the Moon and
here for a brief documentary on recent supercomputer simulations of
the Moon's formation.
- The Earth and Moon mutually interact gravitationally.
- "Tides," are the bulges in the liquid or solid surface of the
Earth caused by the differential effect of the gravitational forces of
the Moon and Sun across the Earth's diameter. Tides were first
explained by Newton in 1687.
- The tides are most obvious in the rise and fall
of the ocean surface as the
Earth rotates under the bulges.
- 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 and Bennett Chapter 6 are optional reading]
Illustrations of Lunar Topography for ASTR 1210 (O'Connell)
Phases of the Moon (Lecture 5)
Google Moon (interactive Moon viewing)
Info at Views of the Solar System
Geology (Federation of American Scientists)
(and comparison to Grand Canyon, C. Cowley)
Resources on Impact
Cratering in the Solar System
Apollo Program and
Lunar Science (U. Wash. Astronomy 105, Toby Smith)
the Moon (Robot and human missions to the Moon; LPI)
Lunar Exploration (History, photos, other data; NASA, NSSDC)
Apollo Mission Gallery
(3D images, panoramas, maps; by USGS)
Chariots for Apollo (description of Apollo Program spacecraft; NASA)
Apollo Over the Moon (photographs from the Apollo mission orbiters)
The Project Apollo
Archive (images, videos, info, links)
Lunar Reconnaissance Orbiter Mission
Moon, Mars, and Beyond Project
Moon Hoax---response from "Bad Astronomy" to the people who have
nothing better to do than imagine up improbable government
conspiracies about NASA faking the Moon landings.
May 2014 by rwo
Cratering rate drawing by Barbara Cohen (Univ. of Tennessee) .
Lunar formation drawing by Toby Smith (Univ. of Washington). Text
copyright © 1998-2014 Robert W. O'Connell. All rights reserved.
These notes are intended for the private, noncommercial use of
students enrolled in Astronomy 1210 at the University of