Does it feel that your life's become a catastrophe? Oh, it has to be for you to grow, boy. Supertramp, Take the Long Way Home

Assignment: Explorations, pp. 252-266 Problem Set #7 is due on Thursday at 1pm. Sign up for a time slot for Observing Lab #3.

In Class: Question to Ponder Why are there no long mountain ranges on the surface of Venus? a) With no water, the surface cannot be transformed via erosion. b) There are no large volcanoes to build Venusian mountains. c) The lithosphere of Venus is not broken into plates. d) It's so hot on Venus that the mountains melt. The Mystery of the Craters Like all of the terrestrial planets, Venus has craters on its surface. However, the crater density (i.e., number of craters per are on the surface) is much lower than on Mercury or the Moon. Some smaller impactors might have burned up in Venus' thick atmosphere, but large impactors should have gotten through, and even after accounting for the atmosphere's effects, there are too few craters on Venus. Crater Counts as an Age-Determining Mechanism Crater density can tell you how old a surface is. More craters means that the surface has been around longer, and has gotten smacked more. Fewer craters -- younger surface. The fact that Venus' surface has few craters suggests that it is, on average, only about 500 million years old. That's pretty young, given that the terrestrial planets have been around for 4-4.5 billion years. Volcanoes, which are seen dotting the surface, could "repave" the surface with lava flows. Crater Uniformity and Catastrophe Not only is the crater density low, but it's also very uniform. This suggests that every part of the surface is equally old. So the entire surface of Venus was repaved all at the same time. Catastrophic meltdown of Venus' crust? One possibility: since Venus has no plate tectonics, the interior has no good way to cool. Thus it heats up over time (due to radioactive decay), eventually becoming so hot that the overlying crust (a.k.a. the "lithosphere") melts. The melted lithosphere cools and hardens, leaving a cooler core surrounded by a thick crust. The process starts up all over again. One interesting feature of this model: a melting lithosphere would release all trapped CO2 into the atmosphere, and might explain why Venus has so much CO2 in its atmosphere. This explanation is consistent with the basic evidence observations of Venus we've described here, but it's by no means demanded by these observations. That is, there are other ways to explain the crater count mystery and the atmospheric composition. Still, this way is consistent with the observations, and it's pretty wild. Catastrophism vs. Uniformitarianism Catastrophism: massive change occurs quickly. Uniformitarianism: change takes place very slowy, but occurs consistently for a long time and ultimately produces substantial change. The Grand Canyon is a uniformitarian process: slow erosion of sandstone by the Colorado River over time produces the Big Gash. Traditionally, scientists prefer uniformitarian processes. Easier to understand. Catastrophes seem too biblical. However, many new results in many branches of science show that rapid, catastrophic change may be more prevalent that previously thought. Some scientists would even go as far as to say that it's the dominant mode of change in nearly all physical systems.