Explain why a planet with a thin atmosphere has a much greater diurnal (day–night) temperature variation than a planet with a thick atmosphere containing greenhouse gases.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Astronomers compare the surface temperatures of two similarly sized rocky planets orbiting the same star at the same distance. Planet A has a thick atmosphere rich in greenhouse gases; Planet B has a very thin atmosphere with negligible greenhouse gas concentration.
Model answer (4 marks)
Planet B has a very thin atmosphere, so the surface absorbs the star’s short‑wave radiation during the day and its temperature rises. Because there are almost no greenhouse gases to trap the surface’s long‑wave radiation, the heat is radiated straight into space at night and the surface cools rapidly.
In contrast, Planet A’s thick atmosphere contains many greenhouse gases that absorb the surface’s outgoing infrared radiation. The atmosphere re‑emits this energy in all directions, including back toward the surface, so the surface loses heat more slowly. The atmosphere therefore acts as a thermal buffer, lowering the daytime maximum and raising the nighttime minimum. Consequently, the diurnal temperature range on Planet A is much smaller than on Planet B.
In contrast, Planet A’s thick atmosphere contains many greenhouse gases that absorb the surface’s outgoing infrared radiation. The atmosphere re‑emits this energy in all directions, including back toward the surface, so the surface loses heat more slowly. The atmosphere therefore acts as a thermal buffer, lowering the daytime maximum and raising the nighttime minimum. Consequently, the diurnal temperature range on Planet A is much smaller than on Planet B.
Examiner tips
- Use the term ‘greenhouse gases’ and ‘infrared radiation’ to show understanding of the mechanism.
- Explain both the daytime heating and nighttime cooling processes.
- Show how the atmosphere’s back‑radiation reduces the temperature swing.
Common mistakes
- Failing to mention the back‑radiation from the atmosphere.
- Confusing short‑wave with long‑wave radiation.
- Giving a qualitative answer without linking it to the greenhouse effect.
Mark scheme (4 marks)
- The surface of Planet B absorbs shortwave radiation from the star during the day, causing its temperature to rise, but with no significant atmosphere it cannot retain this energy at night.
- Greenhouse gases in Planet A's atmosphere absorb outgoing longwave (infrared) radiation emitted by the surface.
- The atmosphere of Planet A re-emits this infrared radiation in all directions, including back towards the surface, keeping the surface warmer at night.
- Planet A's atmosphere therefore acts as a thermal buffer, reducing both the daytime maximum and raising the night-time minimum, so the overall diurnal range is much smaller than for Planet B.
Key terms in this question
Related
- All IB DP Physics Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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