Each planet in our solar system has unique properties that provide clues to the formation of the system

The Moon

Key points: Origin of Moon; age of moon; cratering history

The moon has always been a subject of romance: to left, cycle of days in a lunar phase on the ceiling of the Egyptian Temple at Dendara; to the right, the moon over the city hall of Stockholm (both, G. Rieke); below, poets weigh in


"Praised be Diana's fair and harmless light, Praised be the dews wherewith she moists the ground, Praised be her beams, the glory of the night; Praised be her power, by which all powers abound..."

-- Sir Walter Raleigh


"That orbed maiden

With white fire laden

          Whom mortals call the Moon."

-- Shelley

the moon

Reality is harsh:


"the moon is no maiden, but a scarred and wrinkled crone; she is not white, and she bears no fire."

-- Cecelia Payne-Gaposchkin, "Introduction to Astronomy," 1954

moonaldrin.gif (108737 bytes) Still, the moon holds a special place as the only body other than Earth to have been visited by humans.en00500_1.jpg (18578 bytes) (reload page to restart animations)

(Astronaut Aldrin from http://images.ksc.nasa.gov/photos/1969/captions/AS11-40-5903.html;

footprint from http://us.st11.yimg.com/us.st.yimg.com/I/skyimage_1920_9141155

panorama from Apollo 15 crew, NASA, via APOD, http://antwrp.gsfc.nasa.gov/apod/ap040113.html

apollofootprint.jpg (301110 bytes)

apollo15hill.jpg (432563 bytes)

Can you do it? The astronauts landed on the moon without crashing. Try your skill here:


"Lunar lander" is perhaps the first personal computer game. Navigate with the up arrow for more rocket, the down for less, and the left and right ones to pivot your lander module.

earthr2.gif (143404 bytes)

Earth/Moon System

Earth formed by accretion of many small bodies. We believe the moon formed when a large body struck the early Earth. All other theories have significant problems:

If the earth and moon had formed together as a double planet, they should have similar densities, contrary to observation. If the earth split during formation, we have no good explanation of why. If the moon formed elsewhere and was later captured by the earth, we cannot explain where the orbital energy of the moon went.

Large Impact Theory

Artist's concept of formation of moon  

A large body smashes into Earth (illustration from Joe Tucciarone http://www.monkeytime.com/sciencemaster/galleries/jt_space/gallery.php)

Much of the material broken away from the earth goes into orbit. The moon reassembles in this orbit - it takes about a month of violent collisions. 

Earth is speeded up in rotation as a result of the collision.

This theory is consistent with: 1.) composition of lunar rocks, which is similar to the composition of the crust of the earth; 2.) evidence that the moon had a molten surface for 200 million years; and 3.) the lack of magnetic field for the moon, which together with its low density implies it has very little iron in its core. See it in action! (caution, large file: 20 MB - do not try with slow internet connection) en00500_1.jpg (18578 bytes) (reload this page to restart lecture animations)(movie from Eiichiro Kokubo, http://yso.mtk.nao.ac.jp/~kokubo/moon/kit/movie.html)


Age of the moon

We can measure ages from the radioactive decay of uranium 238 to lead (uranium 238 is an isotope of uraniumbuttonbook.jpg (10323 bytes)):

the "half life" for this decay is 4.5 billion yrs ( half-life is the time it takes for half of the decaying material to turn into its final state).

Molten rock tends to carry the lead away from the uranium, but they are locked together in solid rock. Therefore, the time since a rock solidified can be determined from its uranium vs. lead content.

Example (animation by G. Rieke):

raddecay.gif (68319 bytes) We have a sample of rock which we analyze and find that it has 1/4 of its original U238. U238 has a half-life of 4.5 billion years. In one half-life (=4.5billion years), 50% = 1/2 of the U238disappears.buttonex.jpg (1228 bytes)

In the second half-life, half of the remaining half would disappear so 1/2 of 1/2 = 1/4 (25%) would remain. Therefore our rock must have an age of 2 x 4.5 billion years or 9 billion years (note—no rock any where near this old has been found in the Solar System!).

Moon rocks brought back by the astronauts have proven to be better for measuring ages than Earth rocks because the moon is not active geologically ( no plates, no volcanoes, no cycling of the surface though hotter, deeper regions). Thus, the oldest rocks still lie on the surface, rather than having been destroyed or buried deep inside.

The oldest Moon rocks have an age of nearly 4.5 x 109 years. We think that this is essentially the same as the age of the earth and only slightly less than the age of the Solar System.

Cratering History

formation of craters, diagram  

Craters are formed when bodies strike the surface:

Steps in an impact of a body on the surface of a planet, ending with a crater.(From The Essential Cosmic Perspective, by Bennett et al.)

bgboomm.gif (353029 bytes) (from Meteor Crater Enterprises, http://www.meteorcrater.com/index2.htm)

See it happen in this movie en00500_1.jpg (18578 bytes) (from L. Close, http://athene.as.arizona.edu/~lclose/teaching/a202/lect4.html

large scale image of moon, showing Mare and Highlands The cratering history shows a huge number of impacts just after the moon formed -- compare the highlands with the lunar maria (Latin for "seas", singular is mare). The maria are the result of lava flows about 3 billion years ago. The lava resurfaced that part of the moon, obliterating any craters. The maria have fewer craters than the highlands as a result, and also because the rate of cratering has been dropping ever since the moon formed (The prominence of craters near the top and bottom is an artifact of the lighting). (From USGS, SEDS, http://www.seds.org/billa/pics/Luna2.jpg)

The first manned landing was in the "Sea of Tranquility," just to the right of center in this image buttonex.jpg (1228 bytes)

cramoon1.jpg (74338 bytes) The drop in cratering rate corresponds to the sweeping up of the “leftovers” from the formation of the Solar System -- as time passes, fewer and fewer objects that could collide with the moon are left. (illustration adapted by G. Rieke, after J. & C. Lunine, "Earth") Recall that we said that debris disks around other stars decayed away in about 100 million years -- the cratering on the moon indicates that a similar time scale applies to the clearing of small bodies from the solar system and the resulting reduction in the rate with which they collided with the moon. Still, the surface of the moon also shows major "hits" are still occurring at a low rate.
Arizona meteor crater Earth has also been hit with meteors that formed craters but only the most recent impact sites (like Meteor Crater in northern AZ) are still visible -- erosion and geologic processes have removed all traces of most cratering on the earth’s surface. (from David Hathaway, NASA/MSFC, http://science.nasa.gov/ssl/pad/solar/tutorials/lessons/craters.htm)
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Navaho sandpainting: Mother Earth, Father Sky


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Odin, associated with Mercury in Northern Europe, http://www.pitt.edu/~dash/odin.html

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hypertext  copyright.jpg (1684 bytes) G. H. Rieke

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