WHAT IS COMET?A comet is an icy small Solar System body that,when close enough to the Sun, displays a visiblecoma (a thin, fuzzy, temporary atmosphere) andsometimes also a tail. These phenomena are bothdue to the effects of solar radiation and the solarwind upon the nucleus of the comet. Comet nucleirange from a few hundred meters to tens ofkilometers across and are composed of loosecollections of ice, dust, and small rocky particles.Comets have been observed since ancient timesand have traditionally been considered bad omens.
Who discovered comet? In 1704 Sir Edmond Halley hypothesizedthat the comets of 1456, 1532, 1607, 1682were the same object. He calculated a 75.7 year orbit andpredicted that it would return in 1758. Halley died; but comet returned.
HALLEY’S COMETHalleys Comet or Comet Halley is the best-known of the short-period comets, and isvisible from Earth every 75 to 76 years.Halley is the only short-period comet that isclearly visible to the naked eye from Earth,and thus the only naked-eye comet thatmight appear twice in a human life time.Other naked-eye comets may be brighterand more spectacular, but will appear onlyonce in thousands of years.
Impacts on Earth Meteors burn up in the atmosphere;meteorites don’t; impacts can and dohappen Meteor Crater, Arizona– Diameter is 1.2 km; 50,000 years old– Projectile was 50 meter diameter, metallicasteroid– Crater diameter is 1.2 km, 200 m deep
In 1908, an 80 meter stony asteroid exploded overTunguska Siberia. The air blast knocked downtrees over 30 km. Windows were broken 600 kmaway. People heard the noise 1000 km away
What was impact of comet 65million yrs. ago? Disappearance of 70% species from thefossil 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 clayShocked quartz—indicates heat & pressureSoot—global wildfires
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 fallsback down as a fiery rain igniting forests Lighter dust stays suspended=>dark & cold Photosynthesis & food chain disrupted Soot, shocked quartz, iridium dust settlescreating 65 million year old clay layerexposed at over 100 places on Earth