No. 13 - Fall 1989

© 1989, Astronomical Society of the Pacific, 390 Ashton Avenue, San Francisco, CA 94112

The Surface of the Moon

Editor's Note

Earth and Moon Compared

The Earth, too, must have experienced a wealth of such impacts, but our dynamic planet has several characteristics that erase craters relatively quickly (on the geologic time scale) — or even prevent them from forming in the first place:

• Earth's atmosphere burns up the smaller pieces of asteroidal metal and rock or cometary ice by friction. A typical speed of such a meteoroid is about 25,000 kilometers per hour — and intense heat from friction with the air vaporizes them; as they burn up, we see their quick fiery trails as meteors (sometimes called "shooting stars"). The Moon has no atmosphere, so even the tiniest bit of high-speed dust can hit the surface and cause a crater to form.

• On Earth, erosion due to water and wind eventually erases the craters caused by impacts of objects large enough to hit the ground. Depending on the climate conditions at an impact site, a crater about a kilometer across (such as the famous Meteor Crater in Arizona) can be eroded beyond recognition in a hundred thousand years or so. (Note that a hundred thousand years is only 1/10,000 of a billion years — and we believe the age of the solar system to be about four and a half billion years.) The Moon has no air or water to erode a crater away — it still bears scars billions of years old.

• Surface activity on the Earth — that is, volcanoes, earthquakes, and the slow motions of its crust called continental drift — eventually masks even the largest impact craters over periods of millions to hundreds of millions of years. The Moon is a geologically dead world now, so it lacks this "healing" process, too.

Crater Formation

However, the impacts that formed most craters on the Moon (and elsewhere) did so by literally exploding, not just by gouging up the ground. Craters formed by such detonations — similar to those made by artillery shells — are circular regardless of what direction the projectiles came from.

The energy source for an impact's explosion is the raw speed of the impacting object. An interplanetary chunk headed toward the Moon is accellerated by the Moon's gravity; astronomers' calculations indicate that an object falling from deep space will have a minimum speed of about 5,000 mph on impact. At that speed, the sudden collision of a meteoroid (as these cosmic pieces of debris are called) with the Moon's surface generates enough heat to vaporize much or all of itself and some of the ground it penetrates. The vaporized material violently expands — explodes — forming a circular crater.

Craters' Features

Craters larger than about 10 kilometers across often have central peaks, which are hills or mountains pushed up by pressure within the Moon when the weight of the rocks that were blasted away was removed.

The very largest impact features on the Moon are the enormous impact basins: great circular plains from 300 to more than a thousand kilometers across. There are only about two or three dozen of these — and all of the largest ones are on the Moon's "nearside" (the hemisphere that faces Earth). Astronomers believe that many of these great basins formed about four billion years ago in a relatively short period time — only two-tenths of a billion years or so. Why the largest impacts took place only then is not certain, but it is clear that after that only smaller impacts took place.

Maria (the Lowlands) and the Highlands

The highlands make up about 80% of the Moon's surface (including virtually all of the farside) and are saturated with ancient impact craters.

History of the Lunar Surface

Much more recently, there were a few, brief visits by "objects" that left footprints instead of craters — twelve American astronauts.

Exploring the Moon

From 1959 (when the Luna 1 probe made the first flyby of the Moon) until 1976 (when Luna 24 returned the last lunar sample to Earth), 32 unmanned probes and 9 manned missions to the Moon or near-Moon space were successfully carried out. It was an intensely exciting time to be alive, as we human beings first exercised our ability to navigate the void of space. The accompanying tables list those 41 early missions to the Moon.

Notice that all of the Moon missions listed in this section's tables were launched by the Soviet Union or the United States. Much — if not all — of the impetus behind the 1959-76 rush of spacecraft to the Moon was political: space (and the Moon in particular) was looked at as a great proving-ground of the technological might of the "superpowers." In a sense, the United States "won" the race to the Moon; the Apollo program produced the only manned lunar landings so far. However, it can be argued that the Soviets were more consistent and persistent in their Moon program (their series of automated Luna spacecraft continued for four years after the Apollo program).

Among the genuine scientific benefits that came about as a result of this largely political contest was a much more detailed knowledge of our neighbor world's composition and structure, a clearer view of its geological history and future — and hundreds of thousands of breathtaking photographs. The photographs returned by Apollo crews, Surveyor robots, Luna orbiters and landers, and by all the others in these data tables constitute a landmark in the history of our species. They are our collective memory of our first visits to another world.

Automated Lunar Exploration: Notable Unmanned Spacecraft

[Spacecraft named Luna or Zond were Soviet missions; all others were U.S.]

The Apollo Project: Manned Lunar Landing Missions

Activities & Resources

The Apollo Program: Men on the Moon

It's especially instructive to get students to go back and survey their hometown newspaper or read the articles from major newsmagazines from 1968 to 1972. Many libraries today can also provide sound and video recordings from that era. NASA makes films available to schools on loan (for more information, contact the NASA center nearest you).

Books for Youngsters

• Apfel, N. The Moon and Its Exploration. 1982, Watts.
• Asimov, I. The Earth's Moon. 1988, Gareth Stevens.
• Bean, A.: My Life As An Astronaut. 1988, Simon & Schuster.
• Becklake, J. Man and the Moon. 1980, Silver Burdett.
• Branley, F. The Moon Seems to Change. 1987, Crowell.
• Branley, F. What the Moon is Like. 1986, Harper & Row
• Darling, D. The Moon: A Spaceflight Away. 1984, Dillon Press.
• Simon, S. The Moon. 1984, Four Winds/Macmillan.

Books About the Moon as a World and Its Exploration

• Cadogan, P. The Moon: Our Sister Planet. 1981, Cambridge U. Press. An introductory textbook, slightly technical in places.
• Cooper, H. Apollo on the Moon and Moon Rocks. 1970, Dial. Accounts of the Apollo 11 mission and the material they brought back from the lunar surface; written by a science journalist.
• French, B. The Moon Book. 1977, Penquin. A basic primer.
• Hockey, T. The Book of the Moon. 1986, Prentice Hall. Good introduction to Moon science and Moon lore.
• Lewis, R. The Voyages of Apollo: The Exploration of the Moon. 1975, Quadrangle. Another science journalist's account.
• Long, K. The Moon Book. 1988, Johnson Books, 1880 S. 57th Court, Boulder, CO 80301. Moon observing and Moon lore.
• Mazursky, H., et all. Apollo Over the Moon: A View from Orbit. 1978, NASA Special Publication SP-362.
• Moore, P. New Guide to the Moon. 1976, Norton. A basic book for beginners.
• Morrison, D. & Owen, T. The Planetary System. 1988, Addison-Wesley. A superb textbook on our solar system.
• Preiss, B., ed. The Planets. 1985, Bantam. Includes a chapter by G. Taylor: "Earth's Moon: Doorway to the Solar System."

   

  

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