What are comets?

1. What are comets? • In 1704 Sir Edmond Halley hypothesized that the comets of 1456, 1532, 1607, 1682 were the same object. • He calculated a 75.7 year orbit and predicted that it would return in 1758. • Halley died; but comet returned. • perihelion=0.59 AU; aphelion=35.3 AU (closest to Sun) (farthest from Sun)

2. Halley’s comet was at perihelion in 1986 It will return in 2061.

3. Comets are dirty iceballs! • Nucleus (very small, about 10 km across) rock & ices (mostly H2O and CO2 [dry ice], some methane CH4 & ammonia NH3) • Coma (103 to 105 km across) is gaseous. As comet approaches Sun ices sublime, change from solid to gas, dust grains loosen and move away • Tails (105 km to 1 AU long)—solar wind (steady stream of solar particles) pushes gas away; dust continues to orbit Sun

4. 1997—Comet Hale-Bopp Can you spot the coma, gas tail, and curved dust tail?

5. Asteroids show up as streaks on photos. Since they orbit the Sun, they are observed to move with respect to the stars.

6. Most asteroids lie in the asteroid belt. • 1801 Ceres was discovered first. Approx. 100,000 lie between Mars and Jupiter.

7. Asteroids are typically cratered and irregularly shaped. • Composition • Carbonaceous—containing carbon • Rocky—mostly silicates • Metallic—iron and nickel rich • NEAR probe orbited Eros (2000) and crash landed on it (2001) Eros is a near-Earth asteroid Orbit brings it close to Earth!

8. View of 3 asteroids from spacecraft 50 km

9. Impacts on Earth • Meteors burn up in the atmosphere; meteorites don’t; impacts can and do happen • Meteor Crater, Arizona • Diameter is 1.2 km; 50,000 years old • Projectile was 50 meter diameter, metallic asteroid • Crater diameter is 1.2 km, 200 m deep

10. How does a small body produce a very large impact crater? • A huge amount of energy is released as the body explodes on impact. • The energy released depends on a body’s mass (m) and velocity (v). • Energy of motion, kinetic energy (KE) is given by the formula KE = ½ m v2

11. The projectile that exploded on impact and produced Meteor Crater was equivalent to the energy of 1000 Hiroshima bombs. 20 megatons!

12. In 1908, an 80 meter stony asteroid exploded over Tunguska Siberia. The air blast knocked down trees over 30 km. Windows were broken 600 km away. People heard the noise 1000 km away

13. Italian clay—65 million years old Iridium, shocked quartz, & soot in layer support impact hypothesis (Italian coin is the size of a quarter)

14. Did an impact cause mass extinctions 65 million yrs. ago? • Disappearance of 70% species from the fossil record, including dinosaurs • Evidence for 10 km asteroid impact in • Worldwide 65 million yr old clay layer with: • Iridium (element #77) rare in crust, but in clay • Shocked quartz—indicates heat & pressure • Soot—global wildfires

15. What happens when a large (10 km) asteroid or comet strikes? • Object explodes (energy = billion megatons) on impact and vaporizes a portion of crust • Molten rock is thrown into atmosphere; it falls back down as a fiery rain igniting forests • Lighter dust stays suspended=>dark & cold • Photosynthesis & food chain disrupted • Soot, shocked quartz, iridium dust settles creating 65 million year old clay layer exposed at over 100 places on Earth

16. Chicxulub Crater is identified off Yucatan Peninsula • 300 km diameter ringed basin • About 65 million year old • Could this happen again? • Yes! It is estimated that more than 10,000 objects approach Earth • Tunguska/Meteor Crater events happen about every 1000 years • A “Big One” predicted once every 100 million years

17. Crater is buried beneath Yucatan Penisula and Gulf of Mexico. This map was made by studying gravity measurements made in the region. Compressed layers show crater.