HK

Geodesy of the Terrestrial Planets

Solar System Overview

  • The Solar System consists of the Sun and the celestial bodies that orbit it, including eight planets, their moons, dwarf planets, asteroids, comets, and meteoroids.

  • Each planet has unique geodetic features, influenced by its size, gravity, rotation, and surface composition. Among the terrestrial planets, Mercury, Venus, Earth, and Mars showcase a diverse range of topographic elements such as mountains, valleys, and impact craters.

  • The Solar System consists of 8 planets, planetary satellites, asteroids, comets, and meteoroids.

  • A planet is defined by orbiting the Sun, having sufficient mass for hydrostatic equilibrium, and clearing its orbit.

  • Dwarf planets meet the first two criteria but have not cleared their orbits.

  • Pluto is now considered a dwarf planet and a member of the Kuiper Belt.

Solar System Dimensions and Structure

  • Planets orbit the Sun in the same rotational sense, with near-circular orbits and inclinations near zero.

  • Exceptions exist, like long-period comets.

  • Earth's mean solar distance is 1.0 AU (Astronomical Unit), Mars is ~1.5 AU.

  • Jupiter is ~5.2 AU, and Pluto is ~39.5 AU.

  • Astronomical Unit (AU): Roughly the Earth-Sun distance, ~150 million km.

  • Ecliptic: The plane of Earth's orbit around the Sun, serving as a reference plane.

Planetary Motion and Rotation

  • Most planets rotate in the same direction as their orbit around the Sun.

  • Exceptions include Venus, Uranus, and Pluto.

  • Obliquity: The angle between a planet's rotational axis and its orbit normal vector.

  • Precession and nutation are caused by the Sun and Moon's effects on a planet's equatorial bulge.

Planet Statistics

  • Inclinations are relative to the Earth's orbit plane.

  • Axis tilt, also known as obliquity, refers to the planet's orbit plane.

Orbital Relationships and Resonances

  • Planets' orbital and rotational periods usually lack specific relationships.

  • Notable exceptions include:

    • 3:2 Orbital Resonance between Neptune and Pluto.

    • 3:2 Spin/Orbit Resonance of Mercury.
      *What are “Resonances“

Titius-Bode Law

  • Formula predicting spacing between planets: a = 4 + x, where a is 10 times the semimajor axis.

Types of Planets and Objects

  • Terrestrial Planets: Mercury, Venus, Earth, Mars, composed of rock and iron.

  • Asteroid Belt: Fragments of a destroyed planet or remaining building blocks.

  • Jupiter System: Gas giant with 67 known satellites, including Galilean satellites.

  • Saturn: Ring planet with 62 known satellites.

  • Uranus: Gas giant with rotation axis almost within the orbit plane.

  • Neptune: Farthest known planet, a near-twin of Uranus.

  • Trans-Neptunian Objects (TNOs): Beyond Neptune, including the Kuiper Belt and Oort Cloud.

Solar System Neighborhood

  • The Solar System resides in the Milky Way galaxy.

  • The closest neighbor is the Alpha Centauri triple star system.

History of Solar System Astronomy

  • Heliocentric Model: Earth and planets orbit the Sun.

  • Geocentric Model: Earth is the center of the universe (Claudius Ptolemy).

  • Kepler's Laws: Physical models of heliocentric distances and orbital periods.

  • Galileo Galilei: Observed craters on the Moon, phases of Venus, and Saturn's rings.

  • Isaac Newton: Discovered universal gravity and the inverse square law.

Planet Discoveries

  • Terrestrial planets, Jupiter, and Saturn were known since ancient times.

  • Uranus: Discovered by William Herschel in 1781.

  • Neptune: Discovered in 1846.

  • Pluto: Discovered by Clyde Tombaugh in 1930.

Formation of the Solar System

  • Formed from gravitational collapse of a molecular cloud 4.568 billion years ago.

  • A protoplanetary disk formed, with a protostar at the center.

  • Protoplanets formed by accretion.

  • Frost line: Temperatures cold enough to allow volatile icy material to remain solid inside the soot line

  • Soot line: Carbon compounds are present .