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"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 orbéd maiden With white fire laden Whom mortals call the Moon." -- Shelley
"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 |
Holds a special
place as the only body other than Earth to have been visited by humans.
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animations)
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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.
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A glancing blow by a large body shatters 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 (they show moon had a molten surface for 200 million years) 2.)
lack of magnetic field, low density (implies little iron core). See it in action! (caution, large file: 20 MB - do not try over internet)
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Measure age from radioactive decay of uranium 238 to lead
(uranium 238 is an isotope of uranium
):
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 removes lead from uranium, so the time since a rock solidified can be determined from uranium vs. lead content.
Example (animation by G. Rieke):
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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. 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.
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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.)
See it happen in this movie |
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The cratering history shows a huge number of impacts just after the moon formed -- compare the highlands with the lunar maria (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) |
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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. |
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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. |
Pioneering space art by Chesley Bonestell, http://www.bonestell.org/, http://www.dreamstone.com.au |
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Odin, associated with Mercury in Northern Europe, http://www.pitt.edu/~dash/odin.html |
Click to return to syllabus |
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| Click to return to Exploring Planets | hypertext © G. H. Rieke |
Click to go to Mercury |
(from Meteor Crater Enterprises, 
