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. The Moon 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 began in 1609 (by Galileo). Telescopes
revealed a complex topography, including mountains, craters, maria
("seas"), and numerous smaller-scale structures --- all easily
visible because the Moon has no perceptible atmosphere.
- Spacecraft exploration: The Moon was 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
- Atmosphere: ~none. Lunar gravity is too small to retain
atmospheric gases, so these escape to space
- Surface reflectivity: Despite its appearance in our night sky,
the Moon's surface is actually very dark. A good comparison of
the relative brightness of the Earth and Moon is shown in the picture
at the upper right, taken from a spacecraft on its way to Jupiter.
The Moon 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. Click
here for a
photomosaic of the lunar far-side.
- 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
lunar surface features as well, including the mountains and
Click here for a
comparison of the appearance of six planetary surfaces.
(by Emily Lakdawalla) for more experienced planetary surface
aficionados that asks you to identify high-resolution (1 km) images of
18 different solar system bodies. The tremendous variation in appearance
is mainly due to the differences in surface composition.
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 younger 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)
The "near-side" is at the left; the "far-side" on the right. 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 over 180 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, 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
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, 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.
The 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 mechanism 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 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 painting at the right is a visualization by famous space
artist Chesley Bonestell of the Moon shortly after it had begun to
solidify as seen from the still-molten surface of the Earth.
- The Moon is initally molten after accretion.
The highlands are produced from low density magma "foam," which
solidifies to a solid crust.
- Intense bombardment by fragments/asteroids continues to
about 3 Byr ago, gradually declining with time. See the drawing at
- The largest impacts penetrate the surface and
allow upwelling of dense interior
magma to form the maria.
- 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
sudden changes in
the orbits of Neptune and Uranus, which induced a large-scale
gravitational instability, scattering planetesimal bodies throughout
the solar system.
- 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.
- The extensive bombardment of smaller objects completely erodes
the lunar surface and creates the global regolith.
- The gravitational interaction between the Earth and Moon
does more than simply determine their mutual orbits.
- "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 -- e.g. the fact that the Moon's gravitational force on the
side of the Earth facing the Moon is larger than on the opposite side
of the Earth. 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. They are largest when
the Earth, Moon, and Sun are co-aligned, as shown in the
- Over billions of years, the distortions in shape caused by tidal
interactions between the Earth and Moon have important
- The tides tend to transfer energy from the Earth's spin to the
orbital motion of the Moon. This causes the Moon's orbit to
increase in size, with the Moon slowly moving farther from Earth.
In the Bonestell painting of the young Moon above, the Moon is
portrayed as considerably nearer Earth than it is today.
- Tides raised in the body of the Moon by the Earth's gravity act
to reduce the Moon's spin. Over time, this has slowed the Moon's
rotation to the point that it is now locked in "synchronous"
rotation, with a spin period equal to its orbital period, meaning
that the same face is always pointed toward the Earth. Synchronous
rotation is common among the satellites of the outer planets and also
many of the exoplanets we have discovered
(Study Guide 11), which always present one
face to their host star.
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]
Slides shown in lecture
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
Ex Luna Scientia -- Apollo Program and Lunar Sample
Science (textbook for U. Wash. Astro 105, Toby Smith)
(and comparison to Grand Canyon, C. Cowley)
Resources on Impact
Cratering in the Solar System
the Moon (robot and human missions to the Moon; LPI)
The Saturn Moon rockets (the most
The (failed) USSR N1 Moon rocket
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 2017 by rwo
Cratering rate drawing by Barbara Cohen (Univ. of Tennessee) .
Lunar formation drawing by Toby Smith (Univ. of Washington). Tide
diagram © 2006 by Encyclopedia Britannica. Text copyright ©
1998-2017 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 Virginia.