Lecture 9

Comets:  As Primitive as you Can Get

Comets

• Beyond Jupiter, icy bodies can survive in the vacuum of interplanetary space.
• However, if icy bodies approach closer to the Sun than ~4 – 6 AU, they begin to sublimate.
  • Sublimation: solid becomes a gas (snow on ski slope)
  • This is why asteroids have no ice
  • Such icy bodies are comets.

Historical Remarks

• Originally thought to be one time visitors to the solar system
• Edmund Halley used Newton’s laws (and Kepler) to show:
  • orbits of comets seen in 1531, 1607, 1682 were the same
  • predicted return in 1758-1759
• Space craft now valuable tool

Comet P/Halley

• Comet Halley: most famous periodic comet.
  • Named after Edmund Halley (Astronomer Royal of Britain, friend of Newton, champion of Universal Gravitation).
  • P = 76 years
  • a = 17.9 AU, so aphelion distance is about 36 AU
  • Every return recorded since 240 BC (except 164 BC).
  • Most recent return in 1986.

Orbit of Comet P/Halley

A Typical Comet

• Mass: 10 – 100 billion tons
• Size: 1– 10 km

Comet Structure

• Close up view of Halley’s comet - first time nucleus imaged
• The original icy body forms the "nucleus".
• Darkest object in solar system
• Sublimation of the nucleus forms the coma,, of the comet.
• Gas escapes from regions on heated, sunlit side.
• Gas drags dust with it forming dust tail - shines by reflected and scattered sunlight.

Comet Tails

• 
• "Gas" or "ion" tail:
  • Straight and narrow
  • Consists of ions swept up by solar wind
• "Dust" tail:
  • Wide and diffuse
  • Solid particles pushed by radiation pressure
• Comet tails always point away from the Sun
  • gas tail in direction of Solar wind
  • when comet leaves the inner Solar System, it goes tail first
• Near the Sun, coma can expand to 100,000 km (but very low density).
• Tail can be millions of kilometers long (up to 1 – 2 AU in extreme cases).
• 

Comet Hale-Bopp:

Why Study Comets?

• They are nice to look at

• More importantly:  Clues to formation and early evolution of solar system

  • Vital role in allowing life on earth?

    • Atmosphere?

    • Oceans?

Origin of Comets

• Need to explain: two classes of orbits
• Long period comets
• • Periods > 200 yrs, can be millions of years
  • semi-major axes up to 10⁵ AU, e close to 1
  • orbits randomly inclined to ecliptic
• Shorter period comets
• • Periods < 200yrs, usually 5-20 yrs
  • Have been "captured" by planets, usually Jupiter
  • small semi-major axes
  • low inclination to ecliptic

Shorter period Comets

• 
  
• Orbital periods P < 200 years
• Implies aphelion distances < 70 AU.
• They have been captured into a smaller orbit by Jupiter (or another planet).

Oort Cloud - the Long Periods

• 
  
• The Oort Cloud is a "comet reservoir"
  • quasi-spherical distribution of icy bodies
  • centered on Sun, radius 50,000 AU
  • distance to nearest star is ~270,000 AU
  • Could be 1 trillion comets with total mass ~25 earth masses
  • Perturbations by passing stars kick comets into inner solar system.
  • Formed by ejection of comets during formation of solar system.

Formation of Comets

• Cometary bodies formed in pre-Solar nebula, about 20 – 30 AU from center
  • distance of Uranus and Neptune
  • temperature T ~ 100 K
    • At higher temperature, ices do not survive.
    • A lower temperatures, H₂O locked into solid hydrates of ammonia and methane.

Formation of Oort Cloud

• Perturbations by Uranus and Neptune scatter the orbits of the icy bodies:
  • smaller orbits:  ice will sublimate quickly, don’t last.
  • large orbits will form Oort Cloud
  • very large orbits will escape from Solar System

Kuiper Belt - Short Period Comets

• Ring of comets beyond orbit of Neptune
  • Inner region: 34-45 AU, ~6x10⁹ objects
  • Outer region: 45 -> 10³(?) AU with >10¹³ objects, total mass ~ 100 Earth masses
  • Population of icy planetesimals that formed via accretion at their present location -
  • different process from Oort cloud

Evidence for Kuiper Belt

• IRAS discovery of "disks" around other stars that can be seen in the IR
• Disks around 25-50% of proto stars
• HST shows images of disks around stars that are just forming
• Discovery of 21 trans-Neptunian objects in our Solar System
• Orbital elements of Jupiter’s comets
• Such disks are common and part of normal process of formation of planetary systems

How Do Comets Get Back to the Inner Solar System?

• Perturbations of cometary orbits probably alter their orbits so they can re-enter the inner Solar System.

Impacts by Comets on Planets

• Recent Dramatic example -
  Comet Shoemaker-Levy 9 (SL/9)

Comet SL/9

• Captured by Jupiter in 1992
• Collided with Jupiter after first orbit
• First perijove fragmented the comet into several smaller pieces, all on the same orbit.
• Fragmented comet struck Jupiter in the summer of 1994
• Comet fragments hitting the upper cloud deck of Jupiter resulted in fireballs easily seen in the infrared.

What Would a Similar Collision on Earth Look Like?

Comets and the Earth:

• Did they supply much of early gases on earth?
• Are they responsible for Earth’s oceans?
• Are they the source of complex organic molecules that are necessary for life?

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