Asteroids and Comets Debris of the Solar System Chapter 9

Asteroids Small, rocky objects –like terrestrial planets –size much smaller shape not round no atmosphere –little volatile material Also called “Minor Planets” Early solar system remnants Four largest: –Ceres, Pallas, Vesta, Hygeia

Discovery of Asteroids Ceres discovered in 1801 –by Piazzi –thought it was “missing planet” –more discovered later 300 known by 1890 today >10,000 known Most in asteroid belt –between Mars and Jupiter ( AU)

Composition & Classification Three main types: C-type (carbon-rich): –silicates mixed with dark carbon compounds –“primitive” (unchanged since formation of solar system) S-type (stony): –mostly silicates, no dark carbon compounds –“primitive” M-type (metallic): –rare –fragments from core of differentiated asteroid –potential mining resource

Comets Observed from earliest recorded times –frequently interpreted as “harbingers of doom”, or bad omens Primordial remnants of early solar system Three main components: –nucleus –coma –tail (often two parts) gas (ion) tail dust tail

Comet Nucleus Small, solid body (few km) –“dirty snowball” model nucleus made of ices mixed with rocks and dust When heated (near sun) –ices vaporize –release gas & dust Gas released in “jets” –not steady –like geysers

Comet Atmosphere (Coma) Escaping gases –forms “cloud” around nucleus Coma mostly water vapor

Comet Tails Sunlight pushes gas & dust Gas & dust flows away from nucleus Forms tail Tail points away from Sun –gas (ion) tail straighter, bluer –dust tail curves, yellower

Comet Orbits Orbits highly elongated –most have aphelion well beyond Pluto 50,000 AU! –perihelion near sun

Origin of Comets Comets come from large distances Jan Oort (1950) –proposed “cloud” (reservoir) of ancient icy bodies Oort Cloud –extends half way to nearest star –contains 10 trillion (10 13 ) comets total mass: 1000 Earth masses more mass than all the planets –Occasionally comets fall into inner solar system

The Kuiper Belt Second source of comets –in plane of solar system –beyond the orbit of Pluto –proposed by Kuiper Pluto probably largest Kuiper belt object

Meteors Rocks from space falling to Earth –vaporized by air friction –glow from heat –produces “shooting stars” Typical size < 1 gram (pea-sized) –100 tons/day fall to Earth Occasional larger objects –produce fireballs (bolides)

Meteor Showers Comets leave debris in their orbits Meteor showers –occur when Earth passes through comet debris –produces large number of meteors –meteors appear to come from a point in the sky (called the radiant) shower named for location of radiant (Leonids come from Leo)

Meteorites Meteors that land on Earth called meteorites Origin –early solar system debris (asteroids / comets) –planets (e.g. Mars) –the Moon

Meteorites Three types: –iron (almost pure nickel-iron) clearly extraterrestrial easy to identify –stony (silicate or rocky) difficult to find, look like rocks –stony-iron (mixture) Age ~ 4.5 billion years