Comets – Messengers from the Cold. Dark Past€¦ · Comets – Messengers from the Cold. Dark...
Transcript of Comets – Messengers from the Cold. Dark Past€¦ · Comets – Messengers from the Cold. Dark...
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