Right: Impact crater on Mercury (Caloris Basin). Credit: NASA/Messenger

Transcription

1 Impact Craters Comets, asteroids and meteoroids are the debris left over from the formation of our Solar System. Once in a while, a piece of this debris will strike a planet, a moon or an asteroid at a speed of thirty thousand miles an hour, blasting out soil and producing a pit that we call a crater. Earth bears many scars from impacts like this although most of them have eroded away and are no longer visible. Because the Moon and Mercury have no atmosphere (and no weather) craters there last much longer and their surfaces are covered by thousands of craters -- some on top of others. Right: Impact crater on Mercury (Caloris Basin). Credit: NASA/Messenger Some of Earth s craters are young enough to still be fairly well preserved and recognizable. Fifty thousand years ago, a 30 ton asteroid struck the desert of northern Arizona at a speed over 30,000 miles per hour. The subsequent explosion excavated a hole threequarters of a mile across and nearly 600 feet deep now known as the Barringer Meteor Crater. Left: Barringer Crater in northern Arizona near Winslow. Credit: Shane Torgerson

2 Even more impressive is the 40 mile wide Manicouagen Reservoir in Quebec, Canada. The artificial lake fills the remnants of a 214 million-year-old impact crater nearly 60 miles wide and provides hydo-electric power to the residents of Quebec. The impactor is estimated to have been a 3-mile-wide asteroid. The crater predates the Triassic/Jurassic extinction event by approx. 12 Million years. A crater as large as the Manicouagen Reservoir is best seen from space. Left: Manicouagen Reservoir. Credit: NASA However, one of the most famous and largest impact features on Earth lies buried beneath the town of Chicxulub on the northern tip of the Yucatan Peninsula. Because it is so large (over 100 miles in diameter) and lies entirely below ground, it is difficult to see and so it remained undetected until fairly recently. It was spotted in 1981 by a group of geologists searching for oil. Chicxulub Crater was formed when an asteroid or a comet the size of a city plunged to Earth 65 million years ago. Right: Chicxulub Crater located just off the northern coast of the Yucatan Peninsula, Credit: NASA, JPL-Caltech

3 This catastrophic event wrought global devastation and many scientists believe it may have led to the extinction of the dinosaurs. Extinction, Credit: Joe Tucciarone The force of an impact throws soil away from a crater in all directions. This soil may be lighter in color than the ground on which it falls because surface soils are slowly darkened by exposure to sunlight. That is why crater rays often look like bright splashes around their craters. Copernicus and Tycho (below) are two wellknown lunar craters surrounded by bright crater rays.

4 Make Your Own Flour Craters You can demonstrate how craters and crater rays are produced. You will need a few marbles, baking flour, some dark cocoa powder and a deep pie pan. Fill the bottom of the pie pan with 2 or 3 inches of flour and cover it with cocoa powder. A sifter will spread the cocoa powder evenly. The cocoa powder represents the sun-darkened surface of a moon or planet. Drop a marble onto the flour from a height of 2 feet. The falling marble will produce a crater surrounded by rays of flour sprayed outward by the impact. The white flour rays represent underground soil that has not yet been darkened by sunlight. ove Right: Tycho. Each is nearly 100 miles across. Credit: NASA Marbles dropped from different heights show what happens when space debris strikes a moon or a planet at different speeds. The amount of flour thrown out of a crater is directly proportional to the energy of the impacting marble. For example, a marble dropped from a height of 4 feet has twice the impact energy, and will make a crater with twice the volume, as the same marble dropped from a height of 2 feet. Verify this by dropping the marble from the two heights, then carefully measure and pour just enough flour to fill the small crater. Twice this amount of flour should be enough to fill the larger crater. In addition, the energy of an impacting marble is directly proportional to its weight (mass). You can verify this by choosing two marbles so that one is twice as heavy as the other. Then, drop both marbles into the flour from the same height. Once again, carefully measure and pour just enough flour to fill the small crater. Twice this amount of flour should be just enough to fill the larger crater. The rims of your flour craters might be raised above the level of the rest of the floury surface. Many lunar craters have raised rims which show they were created by high-energy impacts. You can reuse the flour for several more crater demonstrations. Go to the following website to watch a NASA educator perform this experiment: Sunshine State Standards addressed in this activity: Kindergarten: K.E.5.1 Explore the Law of Gravity by showing how objects are pulled to the ground unless something holds them up. K.P.12.1 Investigate that things move in different ways, such as fast and slow. 1 st Grade: 1.E.5.2 Show how Earth's gravity pulls objects to it without anything touching the object.

5 1.E E nd Grade: 2.E P rd Grade: 3.E P P th Grade: 4.P th Grade: 5.E P P th Grade: 6.P P.13.3 Identify harmful & beneficial properties of the Sun. Recognize that water, rocks, soil & living organisms are found on Earth s surface. Recognize that Earth is made up of rocks; rocks come in many sizes and shapes. Recognize that objects are pulled toward the ground unless something holds them up. Explain that our Sun is a star that emits light energy. Identify some basic forms of energy such as light, heat, sound electrical and mechanical. Recognize that energy has the ability to cause motion or create change. Observe & describe some basic forms of energy including light, heat sound, electrical and energy of motion. Identify solar system objects: Sun, planets, moons, asteroids & comets. Identify Earth's place in the Solar System. Investigate & describe some basic forms of energy including light, heat sound, electrical, chemical and mechanical. Investigate & explain that energy has the ability to cause motion or create change. Explore the Law of Gravity by noting that every object exerts a gravitational force on every other object and that the force depends on the masses of the objects and the distance between them. Investigate & describe that an unbalanced force acting on an object changes its speed, direction of motion or both.