Mars is approximately the same size as Mercury, but Mars has more of an atmosphere because

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Question restatement: Mars is approximately the same size as Mercury, but Mars has more of an atmosphere because

Option 1: 'Mercury is made of carbon' — This is incorrect because Mercury's composition being carbon-rich is not the reason Mars retains a thicker atmosphere. In fact, Mercury’s composition does not explain why Mars would have a larger atmosphere; atmospheric retention depends more on gravity and temperature than on surface composition.

Option 2: 'Mars is cooler' — This is the best-supported factor. Cooler temperatures mean lower average particle speeds, which reduces the rate at which atmospheric particles achieve escape velocity. Since Mars is farther from the Sun than Mercury, the solar heating is less intense and the average kinetic energy of gas molecules in Mars’ upper atmosphere is lower, helping it retain a thicker atmosphere compared to Mercury.

Option 3: 'Mercury is much less massive' — While Mercury does have far less mass and weaker gravity, this alone would suggest Mercury loses atmosphere more easily, not that Mars would have the thicker atmosphere. Saying Mercury is much less massive does not explain why Mars ends up with more atmosphere; in fact, the weaker gravity of Mercury is a reason Mercury cannot retain a substantial atmosphere.

Option 4: 'Mars is continually creating its atmosphere' — This overstates the situation. Planets do not typically continually generate atmospheres in a way that would create a thicker atmospheric layer relative to their past. The current presence of Mars’ atmosphere is due to retention over time, not ongoing, rapid, planet-wide atmospheric creation.

Synthesis: The comparison hinges on atmospheric retention, which is strongly influenced by surface temperature (and solar heating), gravity, and atmospheric loss processes. Among the options, identifying cooler global temperatures on Mars as a contributing factor aligns with the general understanding that lower thermal speeds reduce escape probability, thereby allowing Mars to maintain more of its atmosphere relative to Mercury. The other options either misattribute the cause (composition), misstate the physical consequence (mass alone), or assert an unrealistic ongoing creation process.

ASTRON 2140 AU2025 (23428) Quiz 3 - 2025 Nov 3

Mars is approximately the same size as Mercury, but Mars has more of an atmosphere because

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