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Difficulty: HardThe Solar System and Planets

Although Mercury is the closest planet to the Sun and receives the highest intensity of direct solar irradiance, Venus maintains a substantially higher and more uniform average surface temperature of approximately 465∘C465^\circ\text{C} (870∘F870^\circ\text{F}). Which of the following factors is primarily responsible for this difference in surface temperature?

  1. A
    A low planetary albedo that allows the dark silicate surface to absorb nearly all incident visible sunlight
  2. B
    A rapid axial rotation period that distributes concentrated tidal friction and core heat throughout the lithosphere
  3. A dense atmosphere dominated by carbon dioxide that traps outgoing infrared radiation through a runaway greenhouse effectAnswer
  4. D
    A powerful global magnetosphere that focuses energetic solar wind particles directly into the lower atmosphere

Answer

A dense atmosphere dominated by carbon dioxide that traps outgoing infrared radiation through a runaway greenhouse effect
The correct answer identifies the dense carbon dioxide atmosphere and resulting runaway greenhouse effect. Venus has an atmospheric surface pressure over 90 times that of Earth and an atmosphere composed of over 96% carbon dioxide. This dense blanket allows shortwave solar radiation to penetrate but absorbs and re-radiates longwave infrared energy, raising the surface temperature beyond that of Mercury.

Step-by-Step Solution

1
Analyze the atmospheric conditions of Mercury versus Venus
Mercury possesses an extremely tenuous exosphere incapable of trapping heat, leading to extreme temperature swings between day and night, whereas Venus has a thick atmosphere with a surface pressure roughly 92 times that of Earth.
Atmospheric density determines whether a planet can retain thermal energy and maintain stable temperatures.
2
Evaluate the chemical composition and thermal mechanism of Venus's atmosphere
Venus's atmosphere is over 96% carbon dioxide (CO2CO_2), which is transparent to incoming visible solar radiation but opaque to outgoing infrared (thermal) radiation.
Carbon dioxide is a potent greenhouse gas that drives a runaway greenhouse effect.
3
Synthesize why Venus is hotter despite receiving less solar irradiance than Mercury
Even though Venus reflects a larger fraction of incoming sunlight due to its clouds and is farther from the Sun than Mercury, the trapped infrared energy accumulates until an extreme thermal equilibrium is established.
Thermal trapping efficiency outweighs proximity to the Sun in determining the equilibrium surface temperature of Venus.

Key Concept

Planetary atmospheric composition and the runaway greenhouse effect
Estimated Time:1m 30s
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Topics

The Solar System and Planets

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