Comets –Messengers

from the Cold.Dark Past

• Primitive objects containing substantial quantities ofwater ice & other volatiles

• Volatiles evaporate as the comets approach the sun –forms “atmosphere”

• Volatile composition – comets formed at lowtemperatures, & spend most of their lives in outer solarsystem

Outline

• Components• Orbits• Death• Meteor Showers• Classification• Origin

Appearance• Nucleus – small solid body from which the atmosphere

is released

• Coma – the atmosphere that surrounds the nucleus(OH, CN, NH)

• Tail – long streamers of gas & dust sweeping away fromthe sun

→ not directly seen from Earth (radar, spacecraft)→ Size = 5 – 10 km→ Dirty snowball – the nucleus is believed toconsist of equal quantities of silicates & ice

→ can have lengths up to 1 AU→ typically very prominent by the time the cometpasses the orbit of Mars

Tails

• Plasma Tail

• Dust Tail

→ Straight: points away from Sun→ Ionized gas caught up in solar wind→ Blue in color (CO+)

→ Curved tail→ Dust grain motion influenced by gravity& pressure from solar wind→ Yellow-white in color (reflected sunlight)

Non-Gravitational Forces

• Mass loss from the side of the nucleus facing the Suncan alter the comet’s orbit

Hale Bopp Nucleus (Video)

Deep Impact - July 3, 2005

• Comet consists ofunprocessed materialfrom the protoplanetarydisk

• Dig deep into cometnucleus to view materialunheated by the Sun

• And to determinephysical properties (e.g.material strength) ofnucleus

Comet Tempel 1

• Orbital Period ~ 5.5 yr• Perihelion ~ 1.6 AU• Size ~ 11x4x4 km• Albedo ~ 4%• Spin period ~ 2 days

Impact Ejecta Fan

-1 day impact +1 day

What have we learned thus far?• Craters observed on surface• Surface density is low ~ 0.6 g cm-3

• Temperature, color, albedo, &spectroscopic characteristics areindicative of no ice on the surface

• However, the presence of ice inthe ejecta are indicative of icenear the surface

What have we learned thus far?

• Dust-to-gas ratio larger thanbefore impact - volatile contentdepleted even several m belowsurface

• Small outbursts seen on sun-facing side of comet

• Organic molecules, water ice, andsilicate dust composition &abundance indicate that some ofthe comets originated close to orwithin giant planet region

Death of Comets: 3 Possibilities• Solid material is carried away with evaporating ices,

leaving nothing• Ices evaporate & leave behind rocky material which

ultimately become a Near Earth Asteroid• Impact with a planet or the Sun

Impact – Comet Shoemaker-Levy 9

• SL9 Discovered by the Shoemakers & Davy Levy• 20 separate nuclei

• Crashed into Jupiter in 1994

→ Disrupted by gravity of Jupiter (1992)→ trapped in orbit of Jupiter

→ impact speed ~ 60 km / s

SL9 impactwith Jupiter

Infrared

Jupiter – SL9 Impact (Video)

SL9 impactgeometry

Meteor Shower: Dust from comet that trailsthe comet in its orbit (pea-sized)

Dust from Comets also contribute to Zodiacal Dust

Classification

• Long-Period Comets

• Short-Period Comets

→ extremely eccentric orbits→ high inclinations (relative to ecliptic plane)→ Periods as long as 1 million years→ Derived aphelion of 50,000 AU

→ Small inclinations→ Periods of 20-100 years or so

Origin of Comets

• Oort Cloud

• Kuiper Belt

→ Source of Long-Period Comets→ Spherical distribution with a 50,000 AU radius→ Cloud is perturbed by passing stars

→ disk-like distribution

• Oort Cloud →source of long-period comets

• Kuiper Belt →source of short-period comets

Detection of 1st Kuiper Belt Object(Jewitt & Luu 1992)

• How were they found?- CCD images of the same

region of sky at 15-60minute intervals

- Look for slow-movingobjects

Video

Orbits ofKuiper Belt

Objects

• Beyond the orbit of Neptune• Many lie in the orbit of Pluto. Is Pluto just a Kuiper Belt

Object?• Classification: Classical & Scattering KBOs• Origin: leftover from circumstellar disk