Question at position 20 Which planet has the highest average surface temperature, and why?Venus, because of its dense carbon dioxide atmosphereMercury, because of its dense carbon dioxide atmosphereJupiter, because it is so bigMercury, because it is closest to the SunMars, because of its red colorClear my selection

Question

BlackTom AI · Accepted Answer

Venus has the highest average surface temperature because its very thick, CO2-rich atmosphere produces an extreme runaway greenhouse effect.

1) Relevant concepts
- Greenhouse effect: atmospheric gases (especially CO2) absorb outgoing infrared radiation, trapping heat and raising surface temperature.
- Radiative equilibrium (blackbody) temperature: Teq = [(1 − A) S / (4σ)]^(1/4), where A is albedo, S is solar flux, σ is the Stefan–Boltzmann constant.
- Atmosphere mass and composition matter much more than proximity to the Sun alone.

2) Step-by-step reasoning with calculations
Compare Mercury and Venus using equilibrium temperature (ignoring greenhouse) to show the role of atmosphere:
- Solar constant at 1 AU: S0 ≈ 1361 W/m^2.
- Mercury at 0.387 AU: S_Mercury = 1361 / (0.387^2) ≈ 9090 W/m^2. Mercury albedo ≈ 0.07, so (1 − A)=0.93. 
  Teq^4 = (0.93 × 9090) / (4σ) ≈ 3.726×10^10 ⇒ Teq ≈ 439 K.
- Venus at 0.723 AU: S_Venus = 1361 / (0.723^2) ≈ 2604 W/m^2. Venus albedo ≈ 0.76, so (1 − A)=0.24.
  Teq^4 = (0.24 × 2604) / (4σ) ≈ 2.756×10^9 ⇒ Teq ≈ 229 K.

3) Intermediate steps shown above (flux calculations, (1−A) factors, division by 4σ, fourth-root extraction).

4) Why Venus is correct
Although Venus reflects more sunlight (high albedo), its dense atmosphere (~96–97% CO2) produces a massive greenhouse warming that raises its actual surface temperature to ~735 K (≈462 °C), far above the blackbody Teq. Mercury, despite being closer to the Sun, has almost no atmosphere and large day–night temperature swings; its average is much lower (~440 K). Thus the atmospheric greenhouse on Venus is the decisive factor.

5) Why the other options are incorrect
- Mercury, because of its dense CO2 atmosphere: false — Mercury lacks a dense CO2 atmosphere.
- Jupiter, because it is so big: size does not produce high surface temperature; Jupiter’s cloud-top temperatures are much colder.
- Mercury, because it is closest to the Sun: proximity alone doesn’t produce the highest average temperature without an atmosphere.
- Mars, because of its red color: color is irrelevant; Mars is cold due to a thin atmosphere.

6) Conclusion
Therefore the correct answer is: Venus, because of its dense carbon dioxide atmosphere — which matches the provided answer.

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