Impacts
RULE 1: Impacts Happen
RULE 2: There are Consequences to Rule 1
Due in large part to the adoption of the “gradualism” model of evolution by natural election in biology, any hint of “catastrophism” in the science was met with derision. The big pock marks on the Moon were generally thought to be volcanic in origin, and similar pits on the Earth were simply mysteries. As recently as the 1960s there were some scientists who thought that the craters on the moon were largely volcanic. We know now that this is wrong.
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Barringer Crater, near Winslow, Arizona (just east of Flagstaff) |
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More than any other individual, the geologist Eugene Shoemaker changed all that. He carefully examined the famous Barringer Crater in Arizona, also known as Canyon Diablo, and similar features around the world, and concluded that they (and by inference those on the Moon) were due to impacts by large meteorites. Barringer Crater is now more generally known as Meteor Crater, and pieces of the impacting body (an iron meteorite) can be purchased by anyone for $1/g (smaller specimens) to $500/lb (larger fragments). |
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A young Gene Shoemaker |
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RULE 3: The History of the Solar System is Written on the Face of the Moon |
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Q: Why are all craters circular, if impacts come from all directions?
Actually, they don’t…..
Secondly, because the impactor penetrates deep before releasing most of its energy, it produces a buried explosion with little “memory” of its trajectory. Nevertheless, if it comes in within 15° of the horizontal, the “splash” can be asymmetric.
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And at grazing angles less than 5°, elongated craters can occur. |
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Physics of Impacts
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A typical asteroidal impactor (hitting at 20 km/s) will deposit its kinetic energy, which will be shared among various mechanisms. It can crush up to 104 times its own mass of target rock, melt roughly 100 times its mass , or vaporize about 10 times its mass.. It has the ability to accelerate 100 times its mass to a speed of 0.1 times the impact speed 2 km/s), a significant fraction of the escape speed from the surface of the Earth.
Usually equipartition of energy occurs in physical situations. “Thus an impactor may crush 1000 times its own mass of rock, melt 10 times its own mass, vaporize a few times its own mass, and eject 100 times its mass at speeds of tehs to hundreds of meters per second and still give off a substantial amount of energy as seismic waves and radiation from the fireball.” (J.S. Lewis, Physics and Chemistry of the Solar System)
For a rocky impactor hitting a rocky surface, the momentum will be will be largely dissipated by the time it has penetrated to about its own diameter, so for a 100-m impactor with v=20 km/s the deceleration is
or 4x106 cm s-2 (4000 earth gravities). The pressure will be
which is about 4x1012 dyn cm-2 or 4 Mbar. |
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The Tunguska Event
On the morning of June 30, 1908, a brilliant fireball appeared in the morning skies of Siberia, and exploded at an altitude of about 6 km. At a distance of 60 km from “ground zero” people were knocked to the ground. Those within 30 km were thrown into the air (and one person was killed as a result).
No impactor body was ever found.
The impact flattened trees for many kilometers around, although a few at “ground zero” where the airburst would have run along the length of the tree remained standing, but stripped of their branches. It was 19 years before the first scientific expedition to the area (this was the time of the Russo-Japanese war, the Communist Revolution, etc.) Leonid Kulik led an expedition in 1927 and took these photos:
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An analysis of the tree-fall pattern has provided valuable information about the trajectory of the impactor.
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This is one image from a nice little Russian site where the investigators have looked at the tree fall pattern.
http://www.orc.ru/~azorcord/page_sob.htm
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Being the equivalent of a 10-20 Megaton (TNT) explosion, it is the largest impactor event in recent history. While some people have proposed such exotic causes as the impact of a black hole or a crashed alien spacecraft, it seems to be most consistent with the impact of a comet or perhaps a large stony meteorite/asteroid.
Visit the Tunguska web page of the Planetary Science Institute. The University of Bologna had a recent expedition to the site.
http://www.psi.edu/projects/siberia/siberia.html http://www-th.bo.infn.it/tunguska/tu99foto.htm
Although Tunguska had been around a while, and a few small programs were underway to find and track Near Earth Asteroids (NEAs) as potential hazards to Earth, progress in this area was relatively quite and small-scale until 2 event occurred: SL 9 and the KT boundary discovery.
For a “personal” perspective see: http://www.lpl.arizona.edu/impacteffects/
Comet Shoemaker-Levy 9 (SL9)
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Discovered by Eugene & Carolyn Shoemaker and David Levy on March 24, 1993 using the 18-inch Schmidt telescope at Mt. Palomar, California, this was the comet to shake the foundations of the non-astronomical world. It’s peculiar fan-shaped appearance prompted them to contact Jim Scotti at the Spacewatch Telescope at Kitt Peak, Arizona (the telescope is operated by the University of Arizona).
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In fact, the comet was a string of fragments produced when the comet came to close to Jupiter on a previous encounter.
Hubble WFPC2 image of the SL9 fragments, with labels assigned them.
The comet continued to fragment in its way toward a collision with Jupiter.
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Eventually, the fragments would impact Jupiter in a sequential manner, as shown in this animation. http://www.physics.uc.edu/~sitko/AdvancedAstro/12-Impacts&Atmospheres/sl9.mpg
The impacts were particularly impressive in the infrared, as shown in this image (probably the most popular one ever published) from the Calar Alto Observatory. See also their movie. http://www.physics.uc.edu/~sitko/AdvancedAstro/12-Impacts&Atmospheres/mpia_q.mpg |
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The Hubble Space Telescope recorded this plume from one impacting fragment.
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At visible wavelengths, very visible “scars” were produced in the atmosphere of Jupiter.
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Impacts & Extinctions
The K-T boundary, which divides the geologic strata for the Cretaceous and Tertiary eras was discovered by Walter Alvarez in 1980 to have a very high abundance of iridium, an element relatively rare on the surface of the earth, but much more common in meteorites. His father, Lius suggested that the layer, which is found world-wide, could have been caused by the impact of a large asteroid.
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Also found in abundance in the KT layer was “shocked quartz”, rock that had been high-pressure shock passing through rock.
The importance of this layer is that it marks the boundary beyond which no dinosaur remains are found. Was the impact of an asteroid the cause of the extinction of the dinosaurs?
The matter was hotly debated in the 1980’s when many people simply refused to believe that mass extinctions (in this case over 10% of all species on Earth) could occur in the course of what must have been a few years.
At that same layer was a complete change in the ratio of pollen to fern spores (although this may not have been global: see this nice site),suggesting the flora of the earth was severely impacted. (http://taggart.glg.msu.edu/isb200/kt.htm )
In addition to the dinosaurs, a large fraction of marine animals and plankton perished. Mammals and amphibians seemed less effected.
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Effects of huge impact: tidal waves, dust clouds, increased volcanism, acid rain.
The theory that the KT boundary layer was produced by an impact gain additional support when the Chicxulub Crater in the Yucatan. Datting to 65 Myr BP (before present), it coincides roughly with the KT event.
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Gravity anomaly around the ‘buried” Chicxulub Crater. The Yucatan coastline is the white line. White dots indicate the locations of cenotes, essentially sinkholes where the bedrock has collapsed, leaving an area where water will pool |
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North American ejecta map |
Shocked quartz (above) and tektites (below) |
The Chicxulub impact would certainly be a good candidate for the event that produced the worldwide KT boundary layer. While there is some evidence that many species of dinosaur were becoming extinct prior to the KT event, there is diminishing doubt that it may have been the final blow to many species on Earth.
