15 Comets Fire and Ice

15 Goals What are comets? How are they different from asteroids? What are meteor showers? How are they different from typical meteors?

15 Comets – Hale Bopp Copyright – Tyler Nordgren Copyright – John Glerason

15 Comets Copyright – Ray Gralak Copyright – Michael Jager Copyright – Stefan Seip

15 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.

15 Copyright – Tyler Nordgren

15 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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15 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

15 Comet Motion

15 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.

15 Comet Disintigration SW3 - HST

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

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

15 Copyright – Fred Bruenjes

15 Kuiper belt: On orderly orbits from 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

15 Sedna

15 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.

15 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

15 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:

15 Deep Impact NASA mission to impact comet nucleus. Use spectroscopy to determine composition of nucleus. Deep Impact movies

15 What the Impactor saw A’Hearn et al 2005

15 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.

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