Wednesday 18 February 2009

The handouts on the dangers of impacts on Earth are not reproduced here, as all the viewgraphs I showed on that topic were in the handout given out on 13 September.

Meteors are streaks of light in the sky caused when small pieces of rock and debris from space hit the Earths atmosphere. The piece of material, usually no bigger than a grain of sand, is instantly heated and vaporized by friction with the Earths atmosphere. Meteors, commonly called shooting stars, can be seen any clear night. If you find a dark place, well away from city lights, you can expect to see around 5 to 10 meteors per hour. More are usually seen after midnight than before, as the Earth is then turned in the direction that it is moving, and so collides "headon" with many debris particles.

Occasionally, the Earth's path intersects a region of space with an enhanced density of meteors, called a meteor stream. This produces a brief (hours to days) long period of time when there are many more meteors than usual. This period of enhanced meteor activity is called a meteor shower. Most meteor streams are related to comets, and can be thought of as small particles that have come of the comet.

There are about a dozen major regular meteor showers that happen every year about the same time. A calendar of meteor showers can be found here .

Comet orbit and origin of and meteor showers.. Small pieces of material are shed from a comet as it orbits the Sun. This debris trail is concentrated along the orbital path of the comet. If the Earth's orbit happens to intersect this trail, we can get a meteor shower, a time when there is an enhanced number of meteors seen in the Earth's sky. As the Earth orbits the Sun, it intersects a number of debris trails on about the same day each year, leading to well- know meteor showers, such as the Leonids in mid-November, the Geminids in mid-December, and the Perseids in mid-August. Very rarely, the Earth hits a particularly dense clump of debris, and there can be a spectacular meteor storm, with thousands of meteors visible for a relatively brief period. There have been no meteorites known to be associated with meteor showers, indicating that the cometary debris particles are probably small enough to burn up completely in the Earth's atmosphere.


The next few images are of the night sky during meteor showers (or meteor storm). These are wide-angle views, as would be seen with a regular camera, not narrow angle views, as seen through a telescope. All are time exposures, typically of 5 to 15 minutes. Thus, they are misleading as to what you would see if you were watching the sky with your own eyeballs- you might see most of the meteors ahown in the images, BUT NOT ALL AT ONCE- they would be spread out in time.

Leonids with "fisheye" lens.

Meteor shower over Ayers Rock (Uluru) Australia..

Perseid meteor shower.

Perseid meteor shower.

(1) Cosmic Car Conker (2) The car Michelle Knapp, of Peekskill, NY, got her 15 minutes of fame when a football-sized rock from space hit her old junker Chevy Malibu while she watched TV. This rock is now known as the Peekskill Meteorite and the Chevy is known as the Peekskill Meteorite Car and has been exhibited around the world!

Hoba meteorite This is the famous Hoba meteorite, located in Namibia. It is about 2.5 x 2.5 x 1 meter in size, and is thought to have fallen to Earth about 80,000 years ago. This is an "iron" meteorite, with an elemental composition of about 84% iron and 16% nickel.

Chondritic meteorite This is a meteorite, a space rock that made it through the Earths atmosphere to land on the ground, here sliced open to reveal its internal makeup. This type of meteorite has small, BB-sized chunks of rock called chondrites. Radioactive dating shows that these are 4.56 billion years old. These are the oldest rock particles known, and date back to the beginning of the solar system.

Widmanstatten pattern Some meteorites are made almost entirely of iron and nickel. When these are cut open, and the surface polished and etched with acid, this pattern, called a Widmanstatten pattern, is seen. The pattern is formed by cooling of a iron- nickel alloy. It is essentially a crytallization effect. The formation of this pattern requires *VERY* slow cooling (1 to 100 degrees per MILLION YEARS!!) These meteorites are thought to have originated in the differentiated cores of large asteroids, which were then shattered by impacts. No terrestrial irons show this pattern

(1) World impact structures (2) North American meteorite impact structures These two images show known craters on Earth and on North America created by the impact of meteorites. (It appears that the US has gotten more than its "fair share" of hits, but this is due only to fact that US has been studied more thoroughly.) Many of these structures are buried and so are not visible from the surface. These buried craters were usually discovered when drilling brought up pieces of shocked quartz rock, a rare type of quartz that can only be made in the intense pressure of a giant impact. We talked specifically about: #1 - the Ames, Oklahoma buried crater; #3 - the fresh, easily visible (Barringer) Meteor Crater in Arizona; #10 - Chicxulub, the "dinosaur killer", and #30 - Manicouagan, the donut-shaped lake.

(1) Meteor Crater (2) Meteor Crater (3) Meteor Crater Three views of the almost mile-wide Meteor Crater in northern Arizona. This is the most famous impact crater on Earth, produced when a meteor about the size of the physics building hit the Earth about 50,000 years ago. If you ever get to northern AZ, its definitely worth a look! (Just go up to I40, hang a left, and drive about 840 miles).

(1) Roter Kamm (2) Roter Kamm This is an impact crater in Namibia. It is somewhat larger than the Arizona Crater, at 2.5 km diameter. The crater is thought be be about 4 million years old. Because of wind blown sand and erosion, the crater looks much "softer edged" than the Arizona Crater and is well along on the process of slowly disappearing by being filled in and the rim eroded away.

Geologic Time Geologists and paleontologists divide the Earths 4.5 billion year history into periods and subperiods. This is a simplified chart of geologic time. The time 65 million years ago when the dinosaurs met their end marks an important division, sometimes called the K/T boundary.

(1) Chicxulub (2) Chicxulub (3) Dinosaurs last supper? The enormous (over 100 miles across) buried crater called Chicxulub off the Yucatan Peninsula, Mexico. The second image shows a "gravity map" that reveals details of underground rocks. The "horseshoe shaped" structure is the remains of the crater. Thought by many people to be the scar left by the impact of a 10 mile wide rock or iceball that slammed into the Earth 65 million years ago, leading to catastrophic environmental change on our planet, which probably led to the demise of the dinosaurs.

(1) Manicouagan (winter) (2) Manicouagan (summer) This unusual donut-shaped lake about 60 miles across is the remains of a giant impact crater formed 200 million years ago. This structure is in eastern Canada. Both images are taken from the space shuttle- the first one in winter, the second in summer.

Tunguska aftereffects In 1908, in a remote part of Siberia, something hit the Earth, perhaps a small comet. The object appears to have broken up in the atmosphere, so it did not produce a crater, but the impact felled trees over an area about the size of a large city.

How often do big impacts happen on Earth? This graph shows the typical time between impacts of different energy. The downward sloping line shows that bigger impacts are rarer than smaller impacts (which is a Good Thing!). For example an impact of the energy of the 1908 Tunguska impact in Siberia (equivalent to the energy of an asteroid about 50 meters (150 feet) in size is expected to happen every few hundred years. (If you find the point labeled Tunguska and go over horizontally to the "Typical Impact Interval", you see that such events happen every few centuries.) Such impacts might cause great local damage and loss of life, IF the impact took place in a densely populated area. The impact of a 1 to 3 kilometer sized asteroid (roughly 0.5 to 1.5 miles across)(this is about where the line changes from solid to dotted) will happen every few hundred thousand years. Such an event would cause immediate and total devastation over a area the size of a large state. It could also cause large loss of human life through indirect effects- such an impact might fill our atmosphere with dust and acid rain which could cause widespread crop failure, and subsequent mass starvation. If we find such an object on a collison course with Earth, our response will depend on the amount of time we have until impact. If we get only days or weeks of warning, there is little that we can do. With months or a few years of warning, we might at least save some select group of humans by hiding them in caves with ample food supply and air filtering systems (but think about being left on the outside!). With a number of years or decades of warning, we can hope to take direct action to avoid the impact- this would probably involve some sort of rocket mission to the object, perhaps to nudge it into a different orbit so that it misses the Earth.