Jessica Horn: Chapter 9 Notes

Chapter 9: Asteroids, Comets, and Dwarf Planets: Their Nature, Orbits, and Impacts
Discovering Asteroids

• Asteroids leave trails in long exposure images because of their orbital motion around the Sun.

Asteroid Facts

• Asteroids are rocky leftovers of planet formation
• The largest is Ceres, diameter is approximately 1000 km
• There are 150,000 listed in catalogs, and probably over a million with diameter > 1 km
• Small asteroids are more common than large asteroids
• All the asteroids in the solar system wouldn't add up to even a small terrestrial planet
• Asteroids are cratered and not round

Asteroids with Moons

• Some large asteroids have their own moons
• Asteroid Ida has a tiny moon named Dactyl

Asteroid Orbits

• Most asteroids orbit in a belt between Mars and Jupiter
• Trojan asteroids follow Jupiter's orbit
• Orbits of near-Earth asteroids cross Earth's orbit

Orbital Resonances

• Asteroids in orbital resonance with Jupiter experience periodic nudges
• Eventually those nudges move asteroids out of resonant orbits, leaving gaps in the belt

Origin of Asteroid Belt

• Rocky planetesiamals between Mars and Jupiter did not accrete into a planet
• Jupiter's gravity, through influence of orbital resonances, stirred up asteroid orbits and prevented their accretion into a planet

Origin of Meteorites

• Most meteorites are pieces of asteroids

Meteor Terminology

• Meteorite: A rock from space that falls through Earth's atmosphere
• Meteor: The bright trail left by a meteorite

Meteor Types

1. Primitive: Unchanged in composition since they first formed 4.6 billion years ago
2. Processed: Younger, have experienced processes such as volcanism or differentiation 

Meteorites from the Moon and Mars

• A few meteorites arrive on Earth from the Moon and Mars
• Composition differs from the asteroid fragments
• This is a cheap (but slow) way to acquire moon rocks and Mars rocks

Comet Facts

• Formed beyond the frost line, comets are icy counterparts to asteroids
• The nucleus of a comet is like a "dirty snowball" Most comets do not have tails
• Most comets remain perpetually frozen in the outer solar system
• Only comets that enter the inner solar system grow tails

Nucleus of Comet

• A "dirty snowball"
• Source of material for comet's tail

Anatomy of a Comet

• Coma is atmosphere that comes from heated nucleus
• Plasma tail is gas escaping from coma, pushed by solar wind
• Dust tail is pushed by photons

Deep Impact

• Mission to study nucleus of Comet Tempel 1
• Projectile hit surface on July 4, 2005
• Many telescopes studied aftermath of impact

Only a tiny number of comets enter the inner solar system; most stay far from the Sun

• Oort Cloud: Comets on random orbits extending to about 50,00 AU
• Kuiper Belt: Comets on orderly orbits at 30-100 AU in disk of solar system

How did they get there?

• Kuiper Belt comets formed in the Kuiper Belt. Flat plane aligned with the plane of planetary orbits. Orbiting in the same direction as the planets
• Oort Cloud comets were once closer to the Sun, but they were kicked farther out by gravitational interactions with Jovian planets. Spherical distribution, orbiting in any direction

Pluto's Orbit

• Pluto's orbit is tilted and significantly elliptical
• Neptune orbits three times during the time Pluto orbits twice-resonance prevents a collision

Is Pluto a Planet?

• Much smaller than the eight major planets
• Not a gas giant like the outer planets
• Has an icy composition like a comet
• Has a very elliptical, inclined orbit
• Pluto has more in common with comets than with the eight major planets 

Discovering Large Iceballs

• In summer 2005, astronomers discovered Eris, an iceball even larger than Pluto
• Eris even has a moon: Dysnomia

Other Icy Bodies

• There are many icy objects like Pluto on elliptical, inclined orbits beyond Neptune
• The largest ones are comparable in size to Earth's Moon

Kuiper Belt Objects

• These large, icy objects have orbits similar to the smaller objects in the Kuiper Belt that become short period comets

What is Pluto like?

• Its larges moon, Charon, is nearly as large as Pluto itself (probably made by a major impact)
• Pluto is very cold (40 K)
• Pluto has a thin nitrogen atmosphere that refreezes onto the surface as Pluto's orbit takes it farther from the Sun

Other Kuiper Belt Objects

• Most have been discovered very recently so little is known about them
• NASA's New Horizons mission will study Pluto and a few other Kuiper Belt Objects in a planned flyby

Comet SL9 caused a string of violent impacts on Jupiter in 1994, reminding us that catastrophic collisions still happen

Tidal forces tore it apart during a previous encounter with Jupiter

This crater chain on Callisto probably came from another comet that tidal forces tore to pieces

Impact plume from a fragment of comet Sl9 rises high above Jupiter's surface

Did an impact kill the dinosaurs?

       Mass Extinctions

• Fossil records show occasional large dips in the diversity of species: mass extinctions
• The most recent was 65 million ears ago, ending the reign of the dinosaurs

Iridium: Evidence of an Impact

• Iridium is very rare in Earth surface rocks but is often found in meteorites
• Luis and Walter Alvarez found a worldwide layer containing iridium, laid down 65 million years ago, probably by a meteorite impact
• Dinosaur fossils all lie below this layer 

Consequences of an Impact

• A meteorite 1 km in size would send large amounts of debris into the atmosphere
• Debris would reduce the amount of sunlight reaching Earth's surface
• The resulting climate change may have caused mass extinction 

Likely Impact Site

• Geologists fund a large subsurface crater about 65 million years old in Mexico
• Comet or asteroid about 10 km in diameter approaches Earth

Likely Impact Site

• Size of crater suggest impacting object was about 10 km in diameter
• Impact of such a large object would have ejected debris high into Earth's atmosphere 

Facts About Impacts

• Asteroids and comets have hit Earth
• A major impact is only a matter of time: not IF but WHEN
• Major impacts are very rare
• Extinction level events about millions of years
• Major damage about tens to hundreds of years

Frequency of Impacts

• Small impacts happen almost daily
• Impacts large enough to cause mass extinctions are many millions of years apart

The Asteroid with Our Name on It

• We haven't seen it yet
• Deflection is more probable with years of advance warning
• Control is critical: Breaking a big asteroid into a bunch of little asteroids is likely to help
• We get less advance warning of a killer comet