Unit 7 Terrestrial Planets PT 1 (copy)

Astronomy Unit 7: The Terrestrial Planets (Part 1)

The Terrestrial Planets

• Four inner Solar System planets are categorized as rocky planets: Mercury, Venus, Earth, Mars.

• These planets are visually smaller and denser compared to Jovian planets (Gas giants).

• Terrestrial planets have a higher percentage of metallic elements than Jovian planets.

Internal Structure

• Common internal structure includes:

  • Central Metallic Core: Mostly iron, partial melting.

  • Rock Mantle: Surrounding the core.

  • Crust: Composed mainly of silicates, leading to lower density metal compounds.

Geological Features

• Distinct geological features observed on terrestrial planets include:

  • Craters

  • Volcanoes

  • Mountains

  • Canyons

  • Valleys

  • Flat plains

• Mars, Venus, and Earth have significant atmospheres, whereas Mercury has only a very tenuous atmosphere.

• Only Earth has liquid water on its surface.

Comparison of Terrestrial Planets

Mercury: Overview

• Diameter: 4879 km (0.38 of Earth's diameter)

• Mass: 3.285 × 10^23 kg (5.5% of Earth's mass)

• Moons: 0

• Orbital Distance: 58 million kilometers (0.39 AU)

• Orbital Velocity: 48 km/s, fastest in the Solar System.

• Orbital Period: 88 Earth days.

• Length of Day: Not tidally locked; 58d 15h 30m.

Atmospheric Conditions

• Mercury has virtually no atmosphere (extremely thin and temporary).

• Daytime temperature can reach up to 430ºC (806°F), while nighttime temperatures can plunge to -170ºC (-274°F).

Unusual Orbit of Mercury

• Mercury has a significantly elliptical orbit, exhibiting a higher eccentricity than other planets (e = 0.206).

• Unique visibility in the night sky aligned with the Sun, observable during greatest elongation.

Orbital Mechanics

• Mercury's orbit precesses due to warping of space caused by the Sun's gravity.

• Cycle of precession occurs approximately every 502 years.

Rotation and Day Length

• Mercury demonstrates a unique spin, rotating 3 times for every 2 orbits.

• A solar day on Mercury lasts 176 Earth days due to its spin-orbit resonance, making an observer see one solar day every 2 Mercurian years.

• Long periods of nighttime lead to drastic temperature drops due to lack of insulating atmosphere.

Polar Regions

• Mercury’s axial tilt is very minimal (approximately 0.01º).

• Some polar craters contain water ice, confirmed by radar studies and the Messenger spacecraft.

Magnetic Field and Exosphere

• Mercury possesses a weak magnetic field (approximately 1% of Earth's), indicating a partially liquid core that contributes to magnetic activity.

• The planet's exosphere is tenuous and varies due to solar wind interactions, predominantly consisting of hydrogen, helium, sodium, calcium, and magnesium.

Processes Affecting Exosphere

• Charged solar particles can accelerate inward along magnetic field lines, impacting the surface near magnetic poles and releasing surface elements into space.

• Variability in the atmosphere results from solar activity, with certain elements intermittently released.

• Sodium atoms form a tail that changes size and shape influenced by solar wind intensity.

Mercury's Composition

• Mercury is the smallest terrestrial planet with a density greater than the Moon, primarily composed of heavier materials like metals.

• The core consists of approximately 70% iron/nickel, with varying compositions in the crust/mantle.

Geological Surface Features

• The surface shows signs of intense bombardment with visible regolith from meteorite impacts.

• The Caloris Basin is a significant impact feature formed from a large comet or asteroid collision, characterized by extensive lava flows.

Unique Cratering

• Mercury's craters are heavily cratered, displaying features unique to the planet, such as ray craters and regions of intense volcanic activity from its early history.

Venus: Characteristics and Conditions

• Venus is known as the "Mysterious Planet," presenting a hellish environment, hot, pressurized, and corrosive.

• It is similar in size to Earth (12,104 km diameter) but has radically different atmospheric conditions.

• The planet has no moons and a retrograde rotation, possibly resulting from a significant impact or atmospheric tidal forces.

Atmospheric Details

• Primary Composition: 96.5% CO2, 3.5% Nitrogen, with clouds primarily made of sulfuric acid.

• The atmospheric pressure is about 92 times that of Earth.

Surface Features

• Rugged mountainous terrain with extensive volcanic regions, dry lava flows, and various geological formations.

• Surface temperatures average about 462ºC (864ºF) making it the hottest planet in the Solar System.

Atmospheric Circulation

• Venusian atmosphere experiences intense circulation characterized by massive convective currents.

• Winds can exceed speeds of 400km/h at high altitudes, with negligible movement at the surface due to the dense atmosphere.