Small Debris in the Solar System

• Meteoroids are small solid particles in the interplanetary medium.
  • Origin: break up of comets (the smallest particles) or asteroids (all others)
• They can enter the Earth’s atmosphere at speeds up to 72 km/sec (the local speed limit - why?).

Meteors

• Meteors are meteoroids as they enter the atmosphere and vaporize.
  • Typical size: grain of sand
  • Typical altitude: 100 km

Why Study Meteoroids?

• The only extra-terrestrial objects that can be found on the earth
• Called meteorites when they are found on the Earth
• They are very old - learn about formation and early history of the solar system
• Large ones hitting the Earth (called meteorites) can cause global catastrophes.

Meteorites and Impacts

• Meteorites are more massive meteoroids (more than a few grams) that reach the ground intact.
• Large impacts can have disastrous results:
  • Possible relationship to large-scale extinctions?
  • Tunguska (1908) in Northern Siberia

Identified Impact Sites on Earth

Barringer Meteor Crater Northern Arizona. 50,000 yrs old

The Tunguska Impact Area and its Size

Meteorites

• A large meteorite (about 1 km in radius) at maximum impact velocity:
  • Crater 10 km in diameter.
  • Explosive effect of ~30,000 1–Megaton bombs.

Meteors and Meteorites

• There are over 25 million observable meteors per day (but only about 100 from any one spot).
• Only one or two per day reach the ground intact.
• Earth accumulates 10 – 100 tons daily.

Composition of Meteorites

Meteorites come in two major varieties:

• Chondrites: Most (95%) of these are Stony: asteroid-like material density ~ 3.6 gm/cm³
• Iron: Primarily iron and some nickel; density 7.6 gm/cm³.
  • Crystal patterns in iron-type give clues to formation

• Stony Chondrites:
  • Could not have been heated to T > 1,300K
• Carbonaceous Chondrites:
  • Only ~5% of all Chondrites - easily destroyed
  • Very dark, nearly black - carbon based materials, some organic molecules
  • Could never have been warm.
  • Low density - ~2.4 gm/cm³ - means never pressurized.
  • Most "primitive" and least altered objects

Origin of Meteorites

• Some (irons) formed in object that had differentiated before break up.
• Heat from radioactivity in small object?
  • Could not have been large because iron (from centers?) shows no evidence for high pressures
• Chondrites from mantle or outer solar system
• Carbonaceous chon. Only from outer S.S.
• How can we draw these conclusions?

Break-Up of a Differentiated Body

Meteors and Meteorites

• Most meteorites originate in asteroids.
• Asteroids were broken up (by collisions?)
• Most meteors (the ones you see flashing through the night sky) originate in comets.

Meteor Showers

• When Earth crosses the path of a comet, a meteor shower occurs.
  • Annual: meteoroids distributed along the whole orbit.
  • Remnants from tail and evaporation from head.
  • Periodic: "clumped" in certain locations in orbit; showers do not occur every year.

Asteroids

• Bigger, but still primitive
• Asteroids, or minor planets, were discovered as a result of a search for a "missing planet", between Mars and Jupiter.
• Planets can be searched for by trying to detect motion relative to the background stars - eg. Pluto
• Led to discovery of Ceres, at a = 2.8 AU
• Most are found in the Asteroid Belt.
• Largest asteroids are small compared with smallest planets - commonly called "minor planets"
  • Low surface gravity
  • Irregular shapes
• Orbits of about 2000 asteroids have been determined.
  • A careful search would turn up more than 100,000, mostly between Mars and Jupiter

  

The Asteroid Ida

• Shows discovery of Dactyl, its little satellite
• Colors of surface show that Ida has some iron bearing minerals on surface
• Dactyl is similar but not identical
• Was parent body a heated, differentiated object, or unaltered primitive chondrite material?
• Age from crater density (how?)

Dactyl

• Image taken from distance of 3,900 kms
• 1.2 x 1.4 x 1.6 kilometers
• Resolution of 39 meters, ~90 kms. from Ida
• More than a dozen craters > 80 meters diameter
• Large crater is about 300 meters across

Sizes of Asteroids

• Measure:
  • measure optical light - tells you how much solar radiation is reflected and absorbed
  • absorbed radiation has to = amount reradiated in the infrared (energy equilibrium, conservation)
  • measure infrared radiation - tells you the size (determine distance from orbit)
  • Light variations -> shapes are irregular

Size Distribution of Asteroids

• Only one (Ceres) is about 1000 km in diameter.

• About two hundred are 100-1000 km

• About 1000 are between 30 and 100 km.

• The rest are smaller.

Composition of Asteroids

• Asteroids are made of rocky and metallic material.
  • No ices since they are too close to Sun.
  • They would be much brighter if they had icy surfaces.
• Optical/infrared spectra show that they are similar to meteorites found on the Earth.

Types of Asteroids

• Parallels to types of Meteoroids:
  • S: stony, chondrites
  • M: irons, stony-irons
  • C: carbonaceous chondrites

Distribution of Asteroids

Orbits of Asteroids

• Perturbations by Jupiter remove asteroids from orbits that are in resonance with Jupiter.
• These are Kirkwood Gaps.
• Ratio of orbit period of Jupiter to orbital periods in Kirkwood gaps are integers or ratio of two integers.

• Some asteroids are in orbits that are identical to Jupiter’s, but lead Jupiter or follow Jupiter by 60°.
  • These are called the Trojan asteroids.
  • The points 60° from Jupiter are two of the five quasi-stable Lagrangian points.
• Earth-crossing asteroids are known as Apollo asteroids.

Origin of Asteroids

• Not much known!
• Jupiter probably responsible for absence of planet between it and Mars
• • Perturbations by Jupiter could have thrown many objects into the asteroid belt causing collisions, break up of any large proto-planet
  • Perturbations by Jupiter may not have allowed large proto-planet to form in the first place
• In either case, large number of collisions would cause very irregular shapes.

Next Lecture

Comparative Planetology Home Page