What makes us think that Mars must once have had an atmosphere that was warmer and had higher surface pressure?

Question

Quizplus · Accepted Answer

The evidence of large areas of flowing water in the past on Mars suggests that the planet must have had a warmer and thicker atmosphere, which enabled the existence of liquid water. However, the atmosphere is too cold and thin for abundant liquid water today, indicating a significant change in the Martian climate.

Answer: C
 The presence of inactive volcanoes on Mars suggests that the planet must once have had a significant amount of outgassing, which contributed to the development of a thicker atmosphere. The outgassing would have released volatiles and greenhouse gases, which could have kept Mars warm and helped to maintain liquid water on the surface.

Answer: A and B
 Theoretical calculations suggest that the Sun has brightened with time, which would have warmed the atmospheres of planets in the inner solar system, including Mars. Additionally, the evidence for flowing water in the past on Mars supports the idea that the planet must once have had a warmer and wetter climate, which is consistent with the idea that there may have been a thicker atmosphere.

What Makes Us Think That Mars Must Once Have Had