CRATER ORIGIN

The craters on the Moon are due to impacts rather than to say, volcanism despite the fact that the craters are, for the most part, circular in shape. A compelling argument against volcansim is that the craters are all below sea level, the average level of the lunar surface. They are not built-up above the sea-level as are volcanoes.

A puzzle had been, Why are craters circular in shape if they are produced by impacts?

Crater formation is more like a bomb exploding rather than something simply digging out a trench or a hole. A meteor carries a huge amount of energy. A smallish asteroid, diameter of 2 kilometers, traveling around 6.9 kilometers per second (the escape speed of the Earth), has kinetic energy E = 5x1020 Joules or, in units of Megatons the energy of one Million tons of TNT (=4.2x1015), the energy yield of this smallish asteroid is roughly 100,000 Megatons. A largish nuclear weapon carries energy ~ 50 to 100 Megatons. The conversion of this kinetic energy to heat after the asteroid buries itself leads to a huge explosion which digs the crater, thrwoing material into the atmosphere. Comments Note that rays can show the direction from which the objects approached. The centers of some craters show raised mounds in their centers.

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Impacts and Dinosaurs This scenario is proposed for the dinosaur killer (which produced the Chicxulub crater near the Yucatan, diameter ~ 180 km, depth ~ 900 meters). It is proposed that the dinsoaur killer asteroid was about 10 to 12 kilometers in diameter, not large by any means, but this object is thought to had kinetic energy of 1027 Joules or around 250 billion Megatons!! In 1980, a team of researchers consisting of Nobel prize-winning physicist Luis Alvarez, his son geologist Walter Alvarez, and chemists Frank Asaro and Helen Michel discovered that sedimentary layers found all over the world at the Cretaceous-Tertiary boundary contained a concentration of iridium many times greater than normal (30 times and 130 times background in the two sections originally studied). Iridium is extremely rare in the Earth's crust because it is a siderophile, and therefore most of it sank with the iron into the Earth's core during chemical differentiation. As iridium is abundant in most asteroids and comets, the Alvarez team suggested that an asteroid struck the earth at the time of the K-T boundary. Exactly how the Chicxulub impact led to the dinosaur's demise is not known. Scientists imagine at least three scenarios: (i) the impact threw massive quantities of dust into the atmosphere which blocked the Sun and arrested plant growth (Nuclear Winter). (ii) Sulfur released by the impact led to global sulfuric acid clouds that blocked the Sun and fell as acid rain. (iii) Red-hot debris from the falling asteroid or comet triggered global wildfires. The above lead to atmospheric changes which last up to 10 years, and then dissipate. Impacts can only explain rapid extinctions.