Comets
Fire and Ice
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Goals
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What are comets?
How are they different from asteroids?
What are meteor showers?
How are they different from typical meteors?
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Copyright – John Glerason
Comets – Hale Bopp
Copyright – Tyler Nordgren
Copyright – Tyler Nordgren
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Comets
Copyright – Stefan Seip
Copyright – Michael Jager
Copyright – Ray Gralak
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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
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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
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Why are comets icier than asteroids?
a. Asteroids were once as icy but the solar wind blasted
it away
b. Planetesimals that formed closer to the Sun contained
fewer ices
c. The sun’s gravity attracts dense objects more
d. Comets and asteroids formed in the same region but
asteroids were flung outward
e. Comets are not icier: comets are actually less icy on
average
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Comet Motion
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Concept Test
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Suppose there were no solar wind. How would
the appearance of a comet in our inner solar
system be different?
a.
b.
c.
d.
e.
It would not have an ion tail.
It would not have a nucleus.
It would not have a coma.
It would be much brighter in appearance.
It would not have a dust tail.
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Comet Disintigration
SW3 - 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
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Copyright – Fred Bruenjes
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Only a tiny number
of comets enter the
inner solar system most stay far from
the Sun
Oort cloud:
On random orbits
extending to about
50,000 AU
Kuiper belt:
On orderly orbits
from 30-100 AU in
disk of solar
system
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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
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Oort cloud comet orbits are ‘random’ compared to Kuiper
Belt comets because…
a.
b.
c.
d.
e.
The comets have collided so frequently their orbits became
randomized
They formed from the collapsing cloud before it formed an
organized disk
They were ejected by the jovian planets onto random orbits
Orbital resonances with nearby stars randomized the orbits
None of the above: Kuiper Belt comets have more random orbits
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Halley
Borrelly
Wild 2
Each nucleus had some (but not all!) of these:
• Mesas
• “Craters” (circular depressions)
• “Smooth” terrain as sources of jets
• Pinnacles
• Dark Spots; Bright Spots (small albedo features)
• Sharp Edges
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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
surface
• 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.
A’Hearn et al 2005
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Homework #20
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Due Friday 21-Nov:
Read Bennett 12.4
Do 13, 33, 34
What is the Torino Scale?
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