Composition of the planets and meteorites | Geology Optional Notes for UPSC PDF Download

Composition and Structure of Planets

Rocky Terrestrial Planets

• These planets, like Mercury, Venus, Earth, and Mars, are primarily composed of rock and metal.
• They have solid surfaces and are relatively small compared to the gas giants.
• Examples include Earth, with its silicate mantle and iron core, and Mars, known for its iron-rich soil.

Jovian Gas Giants

• Gas giants like Jupiter and Saturn are predominantly made up of hydrogen and helium.
• They are much larger in size and primarily consist of gases rather than solid materials.
• Jupiter, for instance, is known for its massive size and its swirling atmosphere of gases.

In summary, rocky terrestrial planets are characterized by their solid surfaces and metal-rich compositions, while jovian gas giants are primarily composed of hydrogen and helium, existing as massive spheres of gas. These distinctions in composition and structure point towards different formation processes for these two types of planets.

The Giant Planets

• Composition of Jupiter and Saturn

  • Jupiter and Saturn are primarily made up of hydrogen and helium, similar to the Sun.
  • About 75% of their mass is hydrogen, and 25% is helium.
  • Despite being called gas planets, the immense size of Jupiter and Saturn causes the gases to compress into a liquid state.
  • Due to the predominance of compressed, liquefied hydrogen, they can be more accurately referred to as liquid planets.

• Core Composition

  • Under gravitational forces, heavier elements within these giant planets sink towards their cores.
  • Jupiter and Saturn have cores consisting of heavier rock, metal, and ice, although these are not directly visible from the surface due to the obscuring atmosphere.
  • Observations of gravity help in inferring the presence of these dense cores.

• Uranus and Neptune

  • Uranus and Neptune, smaller than Jupiter and Saturn, also possess cores made of rock, metal, and ice.
  • These planets have smaller atmospheres relative to their cores as they were less effective at attracting hydrogen and helium.

• Chemical Dominance

  • Hydrogen and its compounds are the dominant chemicals in each giant planet.
  • Most oxygen in these planets is chemically combined with hydrogen to form water (H2O).
  • The prevalent composition of hydrogen signifies a reducing environment throughout the outer solar system.
  • Abundant water, mainly in icy form, along with reducing chemistry, is common in the outer solar system.

The Terrestrial Planets

• Composition of Terrestrial Planets:

  • The terrestrial planets are smaller in size compared to the gas giants.
  • They are primarily made up of rocks and metals.
  • These planets consist of elements like silicates (silicon and oxygen) and metals, with iron being the most common metal.
  • Mercury has the highest proportion of metals, while the Moon has the lowest density.
  • Earth, Venus, and Mars share similar compositions, with around one third of their mass being iron-based and two thirds being silicates.
  • The chemistry of these planets is described as oxidized due to the prevalence of oxygen compounds.

• Internal Structure of Terrestrial Planets:

  • The densest metals are located in a central core, while lighter silicates are closer to the surface.
  • If these planets were liquid, gravity would cause heavier elements to sink towards the core, explaining the current structure.
  • It is inferred that these planets were once in a molten state due to their current layered composition.

• Planetary Differentiation:

  • Differentiation is the process where gravity separates a planet into layers based on composition and density.
  • Heavier metals sink to form a core, while lighter minerals rise to create a crust.
  • This layered structure is preserved as the planet cools down.
  • For a rocky planet to differentiate, it must reach temperatures exceeding 1300 K, the melting point of rocks.

Meteorites

• Meteorites are fragments originating from larger bodies known as parent bodies, which could have been small planets, large asteroids, or comets within our solar system.
• Classification of meteorites:
  • Stony meteorites are the most prevalent type and bear resemblance to rocks found on and within Earth. They primarily consist of minerals like olivine and pyroxene, akin to Earth's mantle. There are two main types:
    • Chondrites: These are the most common stony meteorites, comprising olivine, pyroxene, and magnetic iron-nickel alloys. They contain small spherical structures called chondrules, possibly formed through quick melting and cooling early in the solar system's history, with radiometric age dates around 4.6 billion years.
    • Achondrites: Comprising similar minerals to chondrites, achondrites lack chondrules and resemble Earth's igneous rocks. Their absence of chondrules is due to being subjected to heating, melting, and recrystallization processes.

Meteorite Composition Overview

Iron Meteorites

• Iron meteorites are made up of iron and nickel alloys, possessing a significantly higher density compared to typical rocks found in the Earth's crust.
• These meteorites are magnetic in nature.
• When iron meteorites are cut and polished, they reveal a unique pattern known as the Widmanstätten pattern, which forms due to the slow cooling of hot solid materials.
• Studying iron meteorites provides insights into the composition of Earth's core.

Stony Iron Meteorites

• Stony iron meteorites are a combination of stony silicate materials and iron.
• They contain silicates embedded within an iron-nickel alloy matrix.
• These meteorites can exist as breccias, where fragments of stony and iron materials are fused together through either heat or chemical processes.

Micrometeorites

• Micrometeorites are tiny particles that have not melted despite passing through the Earth's atmosphere.
• They typically range in size from 50 µm to 2 mm.
• These particles are distinct from meteorites due to their smaller size, higher abundance, and differing composition.
• They consist of cosmic dust and smaller interplanetary dust particles.
• At times, when meteoroids and micrometeoroids enter Earth's atmosphere, they can be observed as meteors or shooting stars.