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CometsComets

Fire and Ice

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GoalsGoals

• What are comets?• How are they different from asteroids?• What are meteor showers?• How are they different from typical

meteors?

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Comets – Hale Bopp

Copyright – Tyler Nordgren

Copyright – Tyler Nordgren

Copyright – John Glerason

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Comets

Copyright – Ray Gralak

Copyright – Michael Jager

Copyright – Stefan Seip

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Comet Facts

• Formed beyond the frostline, comets are icy counterparts to asteroids.

• “Dirty snowballs” = the nucleus• Most comets do not have tails.• Most comets remain perpetually frozen

in the outer solar system. Only a few enter the inner solar system, where they can grow tails.

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Copyright – Tyler Nordgren

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Concept Test

• Suppose we discover a new comet on an orbit that brings it closer to the Sun than Mercury every 125 years. What can we conclude? a. It has been on its current orbit for only a

very short time compared to the age of our solar system.

b. It has a coma and tail during most of each orbit.

c. It came from the Oort cloud.d. It came from the Kuiper belt.e. None of the above.

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Concept Test

• Why are comets icier than asteroids?a. Asteroids were once as icy but the solar

wind blasted it awayb. Planetesimals that formed closer to the

Sun contained fewer icesc. The sun’s gravity attracts dense objects

mored. Comets and asteroids formed in the same

region but asteroids were flung outwarde. Comets are not icier: comets are actually

less icy on average

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Comet Motion

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Concept Test

• Suppose there were no solar wind. How would the appearance of a comet in our inner solar system be different? a. It would not have an ion tail.b. It would not have a nucleus.c. It would not have a coma.d. It would be much brighter in appearance. e. It would not have a dust tail.

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Comet DisintigrationSW3 - HST

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Comets eject small particles that follow the comet around in its orbit and cause meteor showers when Earth crosses the comet’s orbit.

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Meteors in a shower appear to emanate from the same area of sky because of Earth’s motion through space

1515Copyright – Fred Bruenjes

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Kuiper belt:On orderly orbits from 30-100 AU in disk of solar system

Oort cloud:On random orbits extending to about 50,000 AU

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

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Sedna

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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 out there by gravitational interactions with jovian planets: spherical distribution, orbits in any direction.

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Concept Test

• Oort cloud comet orbits are ‘random’ compared to Kuiper Belt comets because…a. The comets have collided so frequently their orbits

became randomizedb. They formed from the collapsing cloud before it

formed an organized diskc. They were ejected by the jovian planets onto

random orbitsd. Orbital resonances with nearby stars randomized

the orbitse. None of the above: Kuiper Belt comets have more

random orbits

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Halley Borrelly Wild 2

• Mesas• “Craters” (circular depressions)• “Smooth” terrain as sources of jets• Pinnacles• Dark Spots; Bright Spots (small albedo features)• Sharp Edges

Each nucleus had some (but not all!) of these:

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Deep Impact

• NASA mission to impact comet nucleus.• Use spectroscopy to determine

composition of nucleus.

Deep Impact movies

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What the Impactor saw

A’Hearn et al 2005

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Geology of the Geology of the surfacesurface

• Circular “craters”: due to impacts? Like Wild 2? Unlike Borrelly.

• Cliffs, mesas: like Borrelly, maybe like Wild 2.

• Smooth and rough terrains, like Borrelly.

• Fairly uniform reflectivity, like Wild 2, unlike Borrelly.

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Homework #20Homework #20

• Due Friday 21-Nov:• Read Bennett 12.4• Do 13, 33, 34• What is the Torino Scale?