ASTR 121 (O'Connell) Study Guide 13


MoonTitle

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.


A. GENERAL

  • Size: Diameter 3500 km = 1/4 Earth; largest satellite relative to its primary; 5th largest overall.

      "Moons" and "planets" are not necessarily intrinsically different; "moons" orbit a planet, rather than the Sun. Planet Mercury is 4900 km in diameter---smaller than two "moons."

  • Telescopic exploration: begins 1609 (Galileo). Identify mountains, craters, maria ("seas"), etc.

  • 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 to the Moon was begun in 2004.

  • Atmosphere: ~none. Gravity is too small to retain. Escapes to space

  • Surface: Low albedo (reflectivity) = ~10%; covered by regolith = powdery fragments from impacts, few meters deep


B. MAIN TERRAIN TYPES

The two main lunar terrain types are highlighted in the image of the full Moon at right. Click for enlargements and information.

  • 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 by asteroids, planetesimals, or comets. Although they should have realized this earlier, astronomers have only 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.

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 plot at right---click for enlargement)

  • 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

  • Large differences in crater densities on a given object, e.g. Enceladus, is evidence of re-surfacing activity.

  • Earth has little surface impact cratering because plate tectonics recycles its surface material.


D. TOPOGRAPHIC FEATURES

Click for illustrations

  • Preserved: no weathering by rain, wind

  • Craters: on Earth, find craters mostly on volcanic mountaintops. On Moon, craters are everywhere. Lunar craters were produced by impacts, not volcanic activity. Scales from millimeters to 150 miles diameter. Circular shape, raised rims, ejecta blankets typical of impact events.

  • Maria: large, round basins; produced by major impacts after lunar surface had solidified, filled by subsequent dense, dark lava flow from interior.

  • Mountains: to 25,000 feet. Extended ranges tend to lie at borders of maria basins; formed during mare impacts.

  • Rilles: canyons produced by lava flows, not water


E. GEOLOGY OF THE MOON

  • The Apollo Program: Launch of Apollo 8 at right

    • Humans land & return rock samples.

    • This was of great scientific value, but it 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.

  • Overall density 3.3 gr/cc, lower than mean value for Earth (implies fewer heavy metals), but comparable to outer layers of Earth

  • More refractory (hard to melt) materials than Earth, fewer volatiles

  • NO sedimentary rocks; NO water in rocks at Apollo sites

  • Highlands: Lower density rocks, anorthosites (Ca, Al rich); igneous (deposited molten).

      OLD! ~ 4.5 Byr.

      Oldest Earth rocks are younger, but Earth surface tectonically recycled and such old rocks are rare.

  • Maria: higher density rocks "basalts" (more Fe, Mg rich); igneous; younger ~ 3.5 Byr


F. INTERIOR

  • Differentiated, but with thick lithosphere (~ 10 x Earth's)

  • NOT tectonically active


G. ORIGIN

  • Fission from Earth? No: mean chemical content differs.

  • Capture? Unlikely. Captured satellites exist (e.g. Jupiter) but are small.

  • Collisional ejection. Favored. Large planetoid hits Earth; heats and expells material from outer layers. Consistent with chemistry: non-refractory material evaporates. See simulation images here.

      Click here for a Quicktime animation of the birth of the Moon.


H. HISTORY

  • Molten after accretion. Highlands produced from low density magma "foam"; solidifies to solid crust.

  • Intense bombardment of fragments/asteroids to 4 Byr ago. Declines with time.

  • Large impacts penetrate surface, allow upwelling of dense interior magma to form the maria. 3-3.8 Byr ago.

  • Continuing, declining bombardment of smaller objects erodes surface and creates regolith

    • Note: expect number of impacts per unit area on Earth to be somewhat higher than on Moon. Maria-scale impacts must have happened on Earth.


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 Command Module after first human landing on lunar surface


Reading for this lecture:

    Seeds textbook, Sec. 21-1

    Study Guide 13

Reading for next lecture:

    Seeds textbook, Chapter 6 (Telescopes)

    Study Guide 14


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Last modified March 2008 by rwo

Cratering rate drawing from Fraknoi text (copyright © Harcourt, Inc). Text copyright © 1998-2008 Robert W. O'Connell. All rights reserved. These notes are intended for the private, noncommercial use of students enrolled in Astronomy 121 at the University of Virginia.