06/08/03 COMETS AND IMPACT CRATORS Mullard Space Science Laboratory University College London

06/08/03 What is a Comet ? Comets are small, fragile, irregularly shaped bodies composed mostly of a mixture of water ice, dust and carbon- and silicon- based compounds. Comets have highly elliptical orbits that repeatedly bring them close to the Sun and then swing them into Space.

06/08/03 Parts of a Comet Hydrogen Cloud Ion Tail Nucleus Coma Dust Tail

06/08/03 Ingredients for the Nucleus of a Comet 1.Water 2.Ammonia 3.Sand or Fine-grained Dirt 4.Salt 5.Methane 6.Crushed Charcoal briquettes 7.Organic compounds 8.Alcohol 9.Dry Ice

06/08/03 Shoemaker-Levy 9 Collision with Jupiter Date: July 16 th – July 22 nd 1994 Diameter: 20 fragments up to 2 km The large dark “scars” seen on the above image were the result of the impact of fragment G

06/08/03 Tunguska Comet Impact Area : Earth, Siberia, Tunguska Date : 7.17AM, June 30 th, 1908 A mysterious explosion occurred in the skies over Siberia. This explosion was caused by the impact and breakup of a large meteorite, at an Altitude of roughly 6km in the atmosphere.

06/08/03 Chicxulub Impact Crater Area : Earth, Mexico, Yucatan Peninsula Age : 65 million years Diameter : 170 km Thought to be the cause of the extinction of the dinosaurs !

06/08/03 How is an Impact Crater Formed ?

06/08/03 Two Types of Impact Crater 1)Simple Crater – Bowl-shaped, smooth-walled craters of a small size. 2) Complex Crater – Larger craters that can collapse, form terraces, central peaks, central pits, multiple rings and peak ring basins.

06/08/03 Making an Impact Crater 1.Fill a pan with surface material (flour) to a depth of about 2.5cm. 2.Smooth the surface, then tap the pan to make the material settle evenly. 3.Sprinkle a fine layer of coloured material (dry paint, or powdered drink mix), evenly and completely over the surface. (Use a sieve or sifter for more uniform layering). 4.Then sprinkle a fine layer of sand or flour over the top. 5.Use different side balls and drop from different heights, and observe the impact craters produced.

06/08/03 Barringer Meter Crater Area : Earth, Arizona Age : 49,000 years Diameter : km Simple meteorite impact crater

06/08/03 Wolfe Creek Crater Area : Earth, Australia Age : 300,000 years Diameter : km Simple impact crater

06/08/03 Aorounga Crater Area : Earth, Chad Age: 200 million years Diameter : 17 km A buried complex crater with dark streams

06/08/03 Bessel Crater Area : Moon Diameter : 16 km In between a simple and complex crater

06/08/03 Copernicus Crater Area : Moon Age: > 1 billion years Diameter : 93 km Complex crater with central peak and rays

06/08/03 Yuty Crater Area : Mars, North Hemisphere Diameter : 10 km Complex crater with central peak and fluid flow

06/08/03 Meitner Crater Area : Venus Diameter : 150 km 3rd Largest impact feature on Venus with 2 circular rings

06/08/03 What is a Meteor, a Meteoroid, a meteorite, and an Asteroid. Meteor - a flash of light in the night sky due to the friction of a meteoroid passing through the atmosphere. Meteoroid - an interplanetary chunk of matter smaller than 1km and frequently millimeters in size. Meteorite – an part of a Meteoroid that survives the fall through the atmosphere and lands on the Earth. Asteroid – a larger chunk of rock that comes from the asteroid belt located between the orbits of Mars and Jupiter.