Venus Overview

True Color Image of Venus

• Obtained by Mariner 10.

• Appears yellowish-orange due to sulfur dioxide (SO2).

• SO2 can combine with water to form sulfuric acid (H2SO4).

Orbital Properties

• Average Orbit Radius: 0.72 astronomical units (au)

  • Receives approximately 1.9 times the solar radiation compared to Earth.

• Orbital Period: 0.615 years (225 Earth days)

  • Lies between Mercury and Earth's orbital periods, consistent with Kepler’s third law.

• Rotational Period: 243 Earth days

  • Venus rotates slowly; its day is longer than its year.

• Axis Tilt: 177.4°

  • Spins retrograde (clockwise).

  • Hypothesis: A massive impactor may have flipped Venus and slowed its rotation.

Physical Properties

• Radius: 0.95 times that of Earth (RE);

  • Sometimes called sister planets with Earth due to size similarity but with notable differences.

• Planet Mass: 0.82 times that of Earth (ME);

  • Compressed density is about 95% that of Earth.

• Surface Temperature: Approximately 740 K (870 °F)

  • Uniform temperature day and night; similar across equator and poles.

• Atmospheric Pressure: 92 times that of Earth.

  • Simplified as pVenus ≈ 100 pEarth.

  • Dense CO2 atmosphere prevents visible light observation; creates a runaway greenhouse effect.

• Magnetosphere: Absent

  • Indicates Venus lacks a global magnetic field.

Spacecraft Visits

• Over 40 spacecraft have explored Venus, notably:

  • Mariner 2 (1962): First spacecraft to provide surface temperature data.

  • Magellan (1989): Mapped the surface using radar (couldn't be seen in visible light due to clouds).

  • First to image entire surface, developed topological maps by measuring travel time of microwaves.

  • Mapped 95% of gravitational field and was destroyed to quantify atmospheric density at mission's end.

Venera Program

• Conducted by the former Soviet Union (1961-1984)

• 16 spacecraft: 13 transmitted atmospheric data, 10 transmitted surface data.

• Notable achievements:

  1. First to enter another planet's atmosphere (Venera 4, 1967).

  2. First to achieve a soft landing (Venera 7, 1970).

  3. First to send images from the surface (Venera 9, 1975).

  4. High-resolution radar mapping (Venera 16, 1983).

• Surface conditions harsh; earlier probes survived from 30 minutes to 2 hours.

Challenges of the Venera Missions

• Many failed attempts were not recognized as Venera missions.

• Venera 7: Overbuilt to withstand surface conditions, survived impact but encountered signal loss and internal failures.

Interior and Surface

• Structure: Iron core, rocky mantle, and crust similar to Earth.

  • No consensus on the core's state: solid, molten, or partially molten.

• Lack of a magnetosphere suggests a solid metal core.

• Thin crust, likely fragmented, no plate tectonics.

• Radar data reveals mountains, valleys, and craters from historical volcanism; surface 0.5-1 billion years old.

• Only about 1,000 craters detected, mostly large due to:

  1. Smaller craters covered by lava flows.

  2. Dense atmosphere incinerates smaller meteoroids before reaching the surface.

Volcanoes on Venus

• Most abundant in the Solar System with over 1,600 large features.

  • All are shield volcanoes, flatter than Earth's with wider calderas and coronas.

• Formation of coronas involves mantle material pushing crust upward, while calderas form from collapses above magma chambers.

Ishtar Terra

• Smallest continent-sized highland region on Venus, about the size of Australia.

• Contains Lakshmi Planum, a high, flat lava-covered plateau, about 3.5 km above average radius.

• Surrounded by mountains, including Maxwell Montes (tallest range on Venus at 11 km).

Atmosphere of Venus

• Composition: 96.5% CO2, 3.5% N2, trace amounts of H2SO4 and H2O.

• Thickness: ~250 km, much thicker than Earth's atmosphere.

• Atmospheric pressure approximately 100 times that of Earth's.

• Opaque sulfuric acid clouds impede surface visibility; with a runaway greenhouse effect maintaining high surface temperatures.

• Wind speeds vary drastically with altitude; upper atmosphere: 360 km/h, middle layer: >700 km/h, surface: slight breezes.

• Lightning detected involving sulfuric acid vapor; no rainfall.

Comparison with Earth

• Both planets formed similarly, but differing solar distances led to stark atmospheric contrasts.

• Increased solar energy caused more reactive atmospheric gases on Venus, inhibiting water vapor formation and contributing to greenhouse gases.

• The prevailing hypothesis addresses water vapor loss; UV light breaks down H2O, allowing hydrogen to escape, possibly assisted by electric fields.

  • Replacement of escaping water vapor with inner atmosphere's water resulted in a dense CO2 environment, explaining current atmospheric conditions.