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.