Comets are icy counterparts to asteroids that formed beyond the frost line in the early solar system. They have a frozen nucleus and can grow comas and tails as they enter the inner solar system and warm. Comet showers occur when Earth crosses a comet's orbit and encounters dust ejected along its path. The Kuiper belt contains orderly comet orbits beyond Neptune, while the distant Oort cloud has randomly distributed comet orbits initially placed there by interactions with giant planets. Spacecraft images have revealed diverse geology on comet nuclei including cliffs, mesas, and circular depressions.

1. 1515
CometsComets
Fire and Ice

2. 1515
GoalsGoals
• What are comets?
• How are they different from asteroids?
• What are meteor showers?
• How are they different from typical meteors?

3. 1515
Comets – Hale Bopp
Copyright – Tyler Nordgren
Copyright – Tyler Nordgren
Copyright – John Glerason

4. 1515
Comets
Copyright – Ray Gralak
Copyright – Michael Jager
Copyright – Stefan Seip

5. 1515
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.

6. 1515
Copyright – Tyler Nordgren

7. 1515
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.

8. 1515

9. 1515
Concept Test
• 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

10. 1515
Comet Motion

11. 1515
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.

12. 1515
Comet Disintigration
SW3 - HST

13. 1515
Comets eject small particles that follow the comet around in its
orbit and cause meteor showers when Earth crosses the comet’s
orbit.

14. 1515
Meteors in a shower appear to emanate from the same area of sky
because of Earth’s motion through space

15. 1515
Copyright – Fred Bruenjes

16. 1515
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

17. 1515
Sedna

18. 1515
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.

19. 1515
Concept Test
• Oort cloud comet orbits are ‘random’ compared to
Kuiper Belt comets because…
a. The comets have collided so frequently their orbits became
randomized
b. They formed from the collapsing cloud before it formed an
organized disk
c. They were ejected by the jovian planets onto random orbits
d. Orbital resonances with nearby stars randomized the
orbits
e. None of the above: Kuiper Belt comets have more random
orbits

20. 1515
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:

21. 1515
Deep Impact
• NASA mission to impact comet nucleus.
• Use spectroscopy to determine composition
of nucleus.
Deep Impact movies

22. 1515
What the Impactor saw
A’Hearn et al 2005

23. 1515A’Hearn et al 2005
Geology of theGeology 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.

24. 1515

25. 1515
Homework #20Homework #20
• Due Friday 21-Nov:
• Read Bennett 12.4
• Do 13, 33, 34
• What is the Torino Scale?