Chapter 9 Notes: Olivia Ward

Asteroids, Comets, and Dwarf Planets: Their Nature, Orbits, and Impacts

9.1 Asteroids and Meteorites
Why is there an asteroid belt?

• Discovering Asteroids
  
  • Asteroids leave trails in long-exposure images because of their orbital motion around the Sun.
• Asteroid Facts
  
  • Rocky leftovers of planet formation
  • The largest is Ceres (diameter of about 1000 km).
  • There are 150,000 listed in catalogs and probably over a million with diameters > 1 km.
  • Small asteroids are more common than large asteroids.
  • All the asteroids in the solar system wouldn't add up to the size of a small terrestrial planet.
  • Asteroids are cratered and not round.
• Asteroids with Moons
  • Some large asteroids have their own moons.
  • Asteroid Ida has a moon named Dactyl.
• Asteroid Orbits
  
  • Most asteroids orbit in a belt between Mars and Jupiter.
  • They are not the debris of previous planets.
  • Trojan asteroids follow Jupiter's orbit.
  • Orbits of near-Earth asteroids cross Earth's orbit.
  • The belt is where all the asteroids happened to survive (resonance).
• Orbit Resonance
  
  • 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 planetesimals between Mars and Jupiter did not form into a planet.
  • Jupiter's gravity through influence of orbital resonances, stirred up asteroid orbits and prevented their accretion into a planet.

How are meteorites related to asteroids?

•  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 tail left by a meteorite.
• Meteor Types
  
  • Primitive: Unchanged in composition since they first formed 4.6 billion years ago. 
  • Strong Primitive Meteorite: Made of rocky material embedded with shiny metal flakes.
  • Carbon-rich Primitive Meteorite: Also rock with dark carbon compounds and small whitish spheres.
  • Processed: Younger, have experienced processes such as volcanism or differentiation.
  • Metal-rich Processed Meteorite: Made of iron and other metals that came from a shattered asteroid's core.
  • Rocky Processed Meteorite: Resembles volcanic rocks found on Earth.
• 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.

9.2 Comets
Was there every geological activity on the Moon or Mercury?

• Comet Facts
  • Formed beyond the frost line, comets are icy counter parts 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 (cold to hot causes tails).
• Nucleus of a Comet
  
  • A "dirty snowball"
  • Source of material for comet's trail (dust tail, plasma tail)
• Anatomy of a Comet
  
  • Coma is atmosphere that comes from heated nucleus.
  • Plasma tails: gas escaping from coma, pushed by solar winds.
  • Dust tails are pushed by protons.
• Deep Impact
  
  • Mission to study nucleus of Comet Tempel 1
  • Projectile hit surface on 4 July 2005
  • Many telescopes studied the aftermath of impact
  • Comets eject small particles that follow the comet around in its orbit and cause meteor showers when Earth crosses the comet's orbit.

Where do comets come from?

• 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 50,000 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 planets.
  • Oort Cloud comets were once closer to the Sun, but they were kicked father out by gravitational interactions with Jovian planets.
    
    • Spherical distribution
    • Orbiting in any direction

9.3 Pluto: Lone Dog No More
How big can a comet be?

• Pluto's Orbit
  
  • Pluto's orbit is tilted and significantly elliptical.
  • Neptune orbits 3 times during the time Pluto orbits twice - resonance prevents a collision.
• Is Pluto a Planet?
  • Much smaller than the 8 major planets
  • Not a gas giant like the outer planets
  • Has very elliptical inclined orbit
  • Pluto has more in common with comets than the 8 major planets.
  • 2006: Pluto was named a dwarf planet.
• Discovering Large Ice Balls
  • Summer 2006: Astronomers discovered Eris, an ice ball larger than Pluto.
    
    • Eris has a moon: Dysnomia
• Other Icy Bodies
  
  • There are many icy objects like Pluto on elliptical, included orbits beyond Neptune.
  • The largest ones are comparable in size to Earth's moon.
• Kupier Belt Objects
  
  • These large, icy objects have orbits similar to the smaller objects in the Kuiper Belt that become short period comets.

What are Pluto and other large objects of the Kuiper Belt like?

• What is Pluto like?
  
  • Its largest moon, Charon, is nearly as large as Pluto itself.
  • Pluto is very cold (40 degrees 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 and little is known about them.
  • NASA's New Horizons mission will study Pluto and a few other Kuiper Belt objects.

9.4 Cosmic Collision: Small Bodies Vs. The Planets
Have we ever witnessed major impact?

• Comet SL9 caused a strong 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.

Did an impact kill the dinosaurs?

• Mass Extinctions
  
  • Fossil records show occasional large dips in the diversity of species: mass extinction
  • The most recent was 65 million years ago, ending the reign of the dinosaurs.
• Iridium: Evidence of an Impact
  
  • Iridium is very rare in Earth's 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 are all below this layer.
• Iridium Layer
  
  • No dinosaur fossils in upper rock layers
  • Thin layer containing the rare element Iridium
  • Dinosaur fossils in lower rock layer
• Consequences of an Impact
  
  • A meteorite 10 km in size would send large amounts of debris into the atmosphere.
  • Debris would reduce the amount of sunlight reaching Earth's surface.
• Likely Impact Site
  
  • Geologists found a large subsurface crater about 65 million years old in Mexico.
  • Size of crater suggests impacting object was about 10 km in diameter.
  • Impact of such a large object would have ejected debris high into Earth's atmosphere.

Is the impact threat a real danger or just media hype?

• Facts About Impacts
  
  • Asteroids and comets have hit Earth.
  • A major impact is only a matter of time. It's not 'If', but 'When'
  • Major impacts are very rare.
  • Extinction level events can be expected in millions of years.
  • Major damage can be expected in tens of hundreds of years.
• Frequency of Impacts
  
  • Small impacts happen almost daily.
  • Impacts large enough to cause mass extinction are many million years apart.
• The Asteroid With Our Name on It
  
  • We haven't seen it yet.
  • Deflection is more probable with years of advanced warning.
  • Control is critical: Breaking a big asteroid into a bunch of little asteroids is likely to help.
  • We get less advanced warning of a killer comet.

How do other planets affect impact rates?

• Influence of Jovian Planets
  
  • The gravity of a jovian planet (especially Jupiter) can redirect a comet.
  • Jupiter has directed some comets.