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 fact that liquid water flowed on the surface in the past indicates that Mars must have had a warmer and thicker atmosphere that allowed for such conditions. The current atmosphere is too thin and cold to support liquid water, which suggests a different atmospheric composition in the past.

Answer: C 
 The presence of inactive volcanoes on Mars indicates that the planet must have had a lot of outgassing, which would have released gases into the atmosphere and increased its thickness. This suggests that the past atmosphere was likely warmer and had higher surface pressure.

Answer: None of the above 
 Option D is incorrect because the number of craters on a planet's surface is not necessarily an indicator of its past temperature. Option B may play a role in the discussion, but it is not the only theoretical reason for thinking that Mars had a warmer atmosphere in the past.

What Makes Us Think That Mars Must Once Have Had