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Crater Counting Chronologies
● Planetary radius determines the duration over which radioactive decay can maintain geological activity.
Crater density on a surface is a proxy for “age” presuming you know the local impact rate as a function of time.
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Highlands vs. Maria / “Old” vs. “Young”
“age” = time since last resurfacing
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Lunar Elevations
Blue = low Red = high
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Cratering Isochrons
● The degree to which a surface is cratered depends on...
– you tell me...
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A Natural Evolution of Cratering Rate
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D = 0.7 ( g earthg local )1 /6
(K.E. ρimpactorρtarget )1
3.4
Crater Size vs. Energy
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D = 0.7 ( g earthg local )1 /6
(K.E. ρimpactorρtarget )1
3.4
Crater Size vs. Energy
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Cratering Mechanics
http://www.youtube.com/watch?v=QfDoQwIAaXg
http://www.popsci.com/science/article/2011-02/giant-granite-balls-help-scientists-study-asteroid-collisions-and-planet-formationhttp://video.mit.edu/watch/milk-drop-to-splash-3105/
http://www.astro.virginia.edu/class/skrutskie/images/SLOW_M_1.MOV
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Cratering Mechanics
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Cratering Mechanics
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Ejecta
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Secondary Craters
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Complex Craters
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Multi-ring Basins
Mare Orientale – Earth's Moon
Valhalla Basin – Jupiter's Moon Callisto
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Saturation
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Crater Counts vs. Size vs. Surface “Age”
● Craters accumulate over time on a “fresh” surface.
● Eventually the crater density saturates
● Large impacts can also produce large numbers of secondary craters.
Lunar Highlands
Lunar Maria
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Scaling to other Worlds (isn't easy)
● We have the advantage of being able to radiometrically date lunar samples – thus providing an absolute age scale for the lunar crater isochrons.
● We don't know how the cratering rate varies throughout the solar system, so other cratering chronologies are relative.
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Scaling to other Worlds (isn't easy)
● We have the advantage of being able to radiometrically date lunar samples – thus providing an absolute age scale for the lunar crater isochrons.
● We don't know how the cratering rate varies throughout the solar system, so other cratering chronologies are relative.
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Scaling to other Worlds (isn't easy)
● We have the advantage of being able to radiometrically date lunar samples – thus providing an absolute age scale for the lunar crater isochrons.
● We don't know how the cratering rate varies throughout the solar system, so other cratering chronologies are relative.
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Vesta Crater Counts
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http://www.psrd.hawaii.edu/May11/Mars_volc_timeline.html
Timeline Timeline ofof Martian Martian VolcanismVolcanism
• A crater-count study on Mars dates the calderas on 20 major volcanoes.
• High-resolution images from the Context Camera (CTX), onboard NASA’s Mars Reconnaissance Orbiter, allow a larger range of crater sizes than were ever used before.
• The timing of the last major summit eruptions correlates well with the crystallization ages (~165-170 million years) of the youngest basaltic Martian meteorites.
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http://www.psrd.hawaii.edu/May11/Mars_volc_timeline.html
Timeline Timeline ofof Martian Volcanism – Olympus Martian Volcanism – Olympus MonsMons
Six calderas on Olympus Mons show ages ranging from about 140 - 420 million years.
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http://www.psrd.hawaii.edu/May11/Mars_volc_timeline.html
Timeline Timeline ofof Martian Volcanism – Elysium Martian Volcanism – Elysium MonsMons
Three calderas on Elysium Mons show ages around 3 billion years.
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http://www.psrd.hawaii.edu/May11/Mars_volc_timeline.html
Timeline Timeline ofof Martian Volcanism and Martian Volcanism and Meteorites Meteorites
• After Apollinaris Mons shut off, major volcanism ended throughout the highlands and Syrtis Major, then in the smaller Elysium volcanoes, and finally in the largest of the Tharsis volcanoes.
• Martian meteorites a few 100 million years old likely came from the younger flows of the Tharsis Montes.
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Late Heavy Bombardment and the “Lunar Cataclysm”
Lunar Impact Basin Chronology
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Results of the “Nice” Model
Uranus
Neptune