Class Survey Results
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Web access: yes=46 no=1
Core=30 Elective=15 Citation=2
Prior astro: none=27 A100=8 A101=9
other=3
Class 4 : The physics of
collisions & crater formation
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Today, we’ll finish talking about
cratering in the solar system…
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Then, we’ll talk about:
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The factors that determine the nature and
severity of a collision.
The processes that shape impact craters.
III : Volcanism in the solar
system
Io – the most active volcanic region in the Solar System
Examples…
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Io
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Very smooth surface – no craters or rilles.
Thought to be a very young surface – lava
has filled all craters and valleys.
Strong stretching/pulling by Jupiter heats
interior and causes volcanic activity.
Mars
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Possesses some giant dead volcanoes (e.g.
Olympus Mons… 27km high, 600km wide).
Example of a crater not caused by impacts.
IV : The outer moons – ice
geology at work.
Europa
Surface ice sheet occasionally seems to break and float
around on an underlying liquid water ocean.
Now – the details of how
craters are made…
I : Craters are circular…
satellite image of Meteor crater
The Moon – all craters are
circular!
Far side of
The Moon
Mercury – they’re still all
circular!!
II : Energy and Momentum
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A moving object possesses momentum
and (kinetic) energy.
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Suppose the object has mass m and
speed v. Then
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Momentum = mv
Kinetic Energy = ½ mv2
Typically both energy and momentum
are conserved (i.e., cannot be created
or destroyed).
Energy & Momentum in
Impacts
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Consider an impact on the Earth.
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The momentum of the impactor…
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Excavates material from the crater.
Gets soaked up by the Earth (moves it in its
orbit by a very, very small amount).
The energy of the impactor…
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Heats the impactor and surrounding rock.
Excavates material from the crater.
Gets turned into sound and seismic waves.
Hypervelocity Impacts
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Hypervelocity (very fast!) impacts…
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Large momentum but enormous energy.
Impactor completely vaporizes on impact.
Circular crater blown out by the force of
the resulting explosion (not the gouging
action of the impactor!).
Also melt or vaporize some of the preimpact surface rocks.
III : Simple and Complex Craters
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Simple craters
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Simple bowl-like craters.
Small (<2-4 km on Earth, <15-20 km on Moon).
Have diameter-to-depth ratio of 5:1 or so.
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Complex craters…
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Above a certain size, the crater walls
collapse.
Get terraced walls and central peak
Relatively shallower (diameter-to-depth
ratios of 10:1 or so).
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Impact basins – very large craters
IV: Other signatures of impact
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The high temperatures and pressures
caused by an impact leave other signs…
examples are:
Shatter cones
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Shocked quartz
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Geological feature characteristic of impacts.
Sign of very high pressure & temperature.
Impact breccia
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Rock that’s been shattered, mixed, &
melted!
Shatter Cone
Shocked Quartz
Impact Breccia
Iridium
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A rare silvery-white metal (close to
platinum & gold on the Periodic Table).
Much more abundant in asteroids and
comets than on the Earth’s surface.
Later, we’ll learn about the “iridium
anomaly”… a geological signature
coincident with the death of the
dinosaurs!