Study Notes on the Formation and Characteristics of the Moon and Mercury

Moon's Origin

• Giant Impact Hypothesis

  • Explains the Moon's origin through a massive collision with Earth.

  • A ring of material formed around Earth subsequently condensed into the Moon.

  • While not definitively proven, it provides solutions to several problems regarding lunar chemistry.

• Key Problems Addressed by the Hypothesis:

  • Absence of Metals:

  • Raw materials for the Moon derived from Earth's mantle and the impacting projectile.

  • Lack of metals on the Moon can be understood through this context.

  • Volatile Elements:

  • Many volatile elements were lost during the high-temperature phase post-impact, accounting for their scarcity on the Moon.

  • Oxygen Isotopes:

  • The Moon being primarily composed of terrestrial mantle material leads to similarities in isotopic abundances of oxygen between Earth and the Moon.

Overview of Mercury

Learning Objectives

• By the end of this section, students should be able to:

  • Characterize Mercury’s orbit around the Sun.

  • Describe Mercury’s structure and composition.

  • Explain the relationship between Mercury’s orbit and rotation.

  • Describe the topography and features of Mercury’s surface.

  • Summarize ideas about the origin and evolution of Mercury.

Mercury's Orbit

• General Characteristics:

  • Similarities to the Moon: no atmosphere, heavily cratered surface, violent history.

  • Closest planet to the Sun, shares traits of a violent birth.

• Orbital Properties:

  • Shortest period of revolution around the Sun: 88 Earth days.

  • Highest average orbital speed: 48 kilometers per second.

• Eccentricity:

  • Mercury’s orbit has a high eccentricity of 0.206, its distance from the Sun varies from:

  • Perihelion: 46 million kilometers.

  • Aphelion: 70 million kilometers.

  • Semimajor Axis: 58 million kilometers (0.39 AU).

Composition and Structure

• Physical Characteristics:

  • Mercury’s mass: one-eighteenth that of Earth (smallest terrestrial planet).

  • Diameter: 4878 kilometers (less than half of Earth).

  • Density: 5.4 g/cm³, significantly higher than the Moon, indicating different compositions.

• Interior Structure:

  • High density suggests composition includes heavy materials, mainly metals.

  • Core composition model: metallic iron-nickel core constitutes 60% of total mass.

  • Core diameter: 3500 kilometers, extending 700 kilometers from the surface.

  • Mercury can be conceptualized as a metallic sphere surrounded by a rocky crust.

  • Magnetic Field:

  • Presence of a weak magnetic field indicates a partially liquid core, consistent with the metallic core presence.

Rotation of Mercury

• Historically thought to keep one face toward the Sun, similar to the Moon.

• Current Understanding:

  • Radar observations from mid-1960s showed it does not keep one side fixed:

  • Rotation Period: 59 days (2/3 of revolution period).

• Doppler Effect:

  • Method used to measure rotation rate through radar wave frequencies.

• Day/Night Cycle:

  • Daytime temperature can reach 700 K (430 °C); nighttime can drop to 100 K (−170 °C), exhibiting a temperature range of 600 K (600 °C).

• Unique relationship between rotation and orbit:

  • Rotates three times for every two orbits (2:3 ratio).

Observations on Mercury's Surface

• First Images:

  • Captured by Mariner 10 in 1974, mapping with 150-meter resolution.

• Surface Features:

  • Resembles the Moon, covered with thousands of craters and basins up to 1300 kilometers.

  • Notable features include:

  • Ray craters, central peaks, scarps (cliffs), ridges, and plains.

  • Significant Discoveries by MESSENGER (2011-2015):

  • High-resolution mapping, verification of water ice in shadowed polar craters.

  • Detection of organic carbon compounds in conjunction with water ice.

Geological Features of Mercury

• Scarps and Craters:

  • Long scarps indicate compression of Mercury’s crust post-cratering, no evidence of plate tectonics.

  • Craters named after artists, contrasting with scientific honors on the Moon.

• Compression History:

  • Indicates Mercury must have shrunk after formation of craters approximately 4 billion years ago.

Origin Theories of Mercury

• Mercury’s composition primarily metallic differs from what would be expected based on its proximity to Earth and Venus.

• Possible explanations for its high metal content:

  • Early giant impacts stripping away some of its less dense silicate material.

  • Differential evaporation of lighter silicates due to the young Sun's heat.

• NASA's ongoing research continues to investigate this planetary formation process, highlighting the chaotic nature of the early solar system.

• Final Thought: Both Mercury and the Moon showcase the violent impacts and resultant compositional differences that characterized the solar system's formation.