4.3 Comparative planetology within our solar system

Planets in our solar system fall into two main categories: terrestrial and Jovian. Terrestrial planets are smaller, rocky bodies closer to the Sun, while Jovian planets are massive gas giants farther out. Their differences stem from their formation and location.

A planet's distance from the Sun greatly affects its properties. Closer planets are hotter and struggle to keep light gases, while farther planets are colder and can hold onto more volatile elements. This influences their atmospheres, surface features, and potential for life.

Planetary Properties and Characteristics

Terrestrial vs Jovian planets

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• Terrestrial planets (Mercury, Venus, Earth, Mars)
  • Smaller in size and mass compared to Jovian planets
  • Higher average densities due to their rocky composition
  • Solid, rocky surfaces that allow for the formation of landforms and geological features
  • Thin or no atmospheres (except Venus) because of their lower gravity and proximity to the Sun
  • Slower rotation rates, which affect the length of their days and nights
  • Few or no moons orbiting them
• Jovian planets (Jupiter, Saturn, Uranus, Neptune)
  • Larger in size and mass compared to terrestrial planets, often referred to as "gas giants"
  • Lower average densities because they are primarily composed of hydrogen and helium
  • No solid surfaces; composed mainly of gas and ice, with possible rocky cores
  • Thick, massive atmospheres that are dominated by hydrogen, helium, and other volatile compounds
  • Faster rotation rates, leading to strong zonal winds and banded appearance
  • Numerous moons and ring systems surrounding them, created by the planets' strong gravitational fields

Planetary characteristics and solar distance

• Distance from the Sun
  • Affects the amount of solar radiation received by a planet
    • Closer planets (Mercury, Venus) receive more energy, leading to higher surface temperatures
    • Farther planets (Mars, Jupiter, Saturn, Uranus, Neptune) receive less energy, resulting in colder temperatures
  • Influences the state of matter and the potential for atmospheric retention
    • Closer to the Sun, lighter elements and compounds (hydrogen, helium) can escape more easily due to higher thermal energy
    • Farther from the Sun, even lighter elements can be retained in the atmosphere because of lower temperatures
• Initial composition
  • Determined by the solar nebula's temperature gradient during planetary formation
    • Closer to the Sun, higher temperatures allowed only refractory elements (metals, silicates) to condense and form terrestrial planets
    • Farther from the Sun, lower temperatures allowed the condensation of volatiles (ices, gases) in addition to refractory elements, leading to the formation of Jovian planets
  • Affects the overall density and structure of a planet
    • Terrestrial planets formed from refractory materials, resulting in higher densities and rocky compositions
    • Jovian planets formed from a mix of refractory and volatile materials, leading to lower densities and gas-rich compositions

Atmospheres and Surface Processes

Planetary atmospheres in solar system

• Atmospheric composition
  • Terrestrial planets
    • Venus and Mars: primarily carbon dioxide (CO2), creating a strong greenhouse effect on Venus
    • Earth: nitrogen (N2), oxygen (O2), and trace gases that support life and moderate temperatures
    • Mercury: virtually no atmosphere due to its proximity to the Sun and low gravity
  • Jovian planets
    • Jupiter and Saturn: primarily hydrogen (H2) and helium (He), similar to the composition of the Sun
    • Uranus and Neptune: H2, He, and substantial amounts of methane (CH4) and other ices, giving them their blue-green appearance
• Atmospheric dynamics
  • Venus: super-rotating atmosphere that completes a full rotation in just 4 Earth days, creating strong winds and a runaway greenhouse effect
  • Earth: moderate greenhouse effect, weather systems, and atmospheric circulation cells that distribute heat and moisture
  • Mars: thin atmosphere that allows for large temperature fluctuations, dust storms, and seasonal carbon dioxide ice caps at the poles
  • Jovian planets: strong zonal winds, storms (Jupiter's Great Red Spot, Neptune's Great Dark Spot), and banded appearance due to rapid rotation and convection

Geological processes on planets and moons

• Volcanism
  • Observed on Earth, Venus, Mars, and Io (Jupiter's moon)
  • Responsible for shaping planetary surfaces and atmospheres through lava flows, ash deposits, and outgassing
  • Examples: Olympus Mons (largest known volcano in the solar system on Mars), Mauna Loa (Earth's largest active volcano), Ionian volcanoes (powered by tidal heating)
• Tectonics
  • Plate tectonics on Earth
    1. Driven by mantle convection, which causes plates to move and interact
    2. Creates features like mountains (Himalayas), rift valleys (East African Rift), and subduction zones (Mariana Trench)
    3. Plays a crucial role in the carbon cycle and the recycling of surface materials
  • Possible tectonic activity on Venus and Mars
    • Venus: coronae (circular volcanic features) and tesserae (deformed highland regions) suggest past or present tectonic activity
    • Mars: Valles Marineris (largest canyon system) and Tharsis bulge (volcanic region) indicate possible tectonic processes in the past
• Impact cratering
  • Common feature on most terrestrial planets and moons, as they lack thick atmospheres to protect against impacts
  • Provides insights into the age and geological history of a surface
    • Older surfaces have more craters because they have been exposed to impacts for a longer time
    • Younger surfaces have fewer craters due to resurfacing events (volcanism, erosion) that erase or bury older craters
  • Examples: Caloris Basin (largest impact basin on Mercury), Chicxulub crater (site of the dinosaur extinction event on Earth), Hellas Basin (one of the largest impact basins on Mars)