MW

Comets and Meteorites

  • Comets and Meteorites Overview

    • Definition of Comet:
    • A body of ice and rock, typically orbiting the Sun in a highly eccentric orbit.
    • Both comets and asteroids are leftover planetesimals from the formation of the Solar System.

  • Asteroids

    • Characteristics:
    • Formed inside the frost line, primarily composed of rock and metal.
    • Majority are located in the asteroid belt.
    • Approximately 1 to 2 million asteroids larger than 1 km exist, with millions of smaller ones.
    • Classification:
      • Carbonaceous (C), Siliceous (S), and Metallic (M).

  • Comets

    • Characteristics:
    • Described as 'dirty ice balls' composed of ice, rock, and dust, formed beyond the frost line.
    • Reside mainly in two regions: the Kuiper Belt and the Oort Cloud, both located beyond Neptune’s orbit.

  • Kuiper Belt and Oort Cloud

    • Kuiper Belt:
    • Disk-shaped region from about 30 to 50 astronomical units (au) containing primarily rock and ice, including dwarf planets and comets.
    • Oort Cloud:
    • Spherical region at the outer limits of the Solar System, 50,000 to 100,000 au in radius, hypothesized to contain trillions of comets with varying inclinations and eccentricities.

  • Types of Comets

    • Kuiper Belt Comets:
    • Short-period comets with orbital periods less than 200 years (e.g., Halley’s Comet with a period of 76 years).
    • Oort Cloud Comets:
    • Long-period comets, orbital periods ranging from 200 years to millions of years.
    • Gravitational influences of nearby stars can alter their orbits, potentially sending them into the inner solar system.

  • Comet Properties

    • Nucleus of a Comet:
    • The solid core of a comet, typically ranges in size from 50 m to 50 km in diameter.
    • Composition includes ice (up to 50%), rock, rock dust, and some metals (H2O, NH3, CH4, CO2, CO).
    • A high albedo indicates a sizeable ice proportion.

  • Observation of Comets

    • ESA spacecraft Giotto was the first to photograph a comet's nucleus (Halley’s Comet) in 1982.
    • Comets typically lose about 6 m of surface material with each Sun passage, leading to meteor showers like Orionids and Eta Aquarids.
    • Comet surfaces often appear dark due to substantial sublimation of ices during their passage close to the Sun.

  • Coma and Tails of Comets

    • Coma:
    • A large nebulous envelope around the nucleus that grows as the comet approaches the Sun, potentially 10,000 times larger than the nucleus.
    • Composed of sublimated gases and ejected dust.
    • Tails of Comets:
    • Ion Tail:
      • Composed of ionized sublimated gas blown away from the Sun by solar wind at high speed.
    • Dust Tail:
      • Formed from dust and small particles pushed away by solar radiation pressure, often appearing curved due to varying speeds of particles.

  • Meteor Showers

    • Occur when Earth's orbit intersects the debris trail left by a comet.
    • Regular events particularly for short-period comets, thus predictable timings.

  • Meteor and Meteorite Definitions

    • Meteoroid:
    • A rock ranging from sand-sized to boulder-sized orbiting the Sun.
    • Meteor:
    • A meteoroid entering a planet's atmosphere, causing a streak of light due to heating.
    • Meteorite:
    • A meteoroid that strikes a planetary or lunar surface.

  • Meteorite Characteristics

    • Dark, pitted crust due to atmospheric entry.
    • Typically high metal content, making them magnetically responsive.
    • Found predominantly in Antarctica, due to easier visibility against snow.

  • Micrometeorites

    • Tiny extraterrestrial particles, sized from 50µm to 2mm, that reach Earth’s surface.
    • Estimates suggest that between 5 and 300 metric tons of cosmic dust and meteorites enter Earth’s atmosphere daily.