Homework 3 Solutions

Homework 3 Solutions - A stronomy 100 H omework 3 Sol...

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Astronomy 100 Homework 3: Solutions Dr. Warner 1. A large meteorite impact (that is, the impact of a meteorite a few kilometers across) will, of course, cause a massive explosion and shock wave that will kill everything within many miles of the blast, but this, in itself, will not cause a mass extinction. The blast of the impact with throw huge amounts of dust into the upper atmosphere and this will block a significant amount of sunlight for months or even years. This causes cooling of the Earth on a global scale and would lead to a phenomenon called “Nuclear Winter” in which plants and animals die back dramatically because of the lack of sunlight and the consequent harsher, colder weather and flooding. One of the signatures of such an impact and global climate change 65 million years ago is the presence of a layer of iridium rich clay at the Cretaceous-Tertiary boundary at locations all over the world. Clay is a signature of flooding events, and iridium is more abundant in asteroids and meteorites than in the Earth’s crust (because chemical differentiation on Earth has depleted it). Enriched iridium is thus a signature of material from an meteorite or asteroid. The fact that it can be found all over the world means that the dust from the impact spread everywhere and the flooding events and climate change were felt globally. The final piece of evidence was the discovery and analysis of the Chicxulub crater. The crater is 180 km across, which is consistent with an impact by a meteorite 10km across. Impact glasses (rapidly melted and violently shocked rock) were found around the site and radioactive dating showed that the impact happened 64.98 million years ago. While there still some scientists who debate the importance of this meteorite impact in causing the extinction of the dinosaurs, there is a consensus that the meteorite-impact explanation is the best theory for the mass extinction at the Cretaceous-Tertiary boundary and that it fits all the available data extremely well.
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2. A planet, P, exerts a gravitational force on an object, O, that depends upon the distance of the object from the center of the planet. If the object is actually a large extended object, like a moon, then there will be a significant difference in the gravitational force on opposite sides of the object (see diagram): This difference in forces causes a “tidal stress” inside the object and tends to pull the object apart. Indeed, if the tidal forces (the difference between the attraction of opposite sides of the object) are large enough then the object will indeed be pulled apart. The Roche limit of a planet is the distance from the center of the planet inside of which any “gravitationally cohering” object will be pulled apart by tidal forces. To say that something is “gravitationally cohering” means that it is being assembled or held together by its own gravitational forces. Examples of such objects include moons, large asteroid and comets. Ring systems are generated inside the Roche limit of a planet in two
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This note was uploaded on 11/27/2009 for the course ASTR 50800 at USC.

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Homework 3 Solutions - A stronomy 100 H omework 3 Sol...

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