What makes us think that Mars must once have had an atmosphere ...

[tfester]

What makes us think that Mars must once have had an atmosphere that was warmer and had higher surface pressure?
 
  A) We think it for purely theoretical reasons, based on calculations showing that the Sun has brightened with time.
  B) The atmosphere is too cold and thin for abundant liquid water today, yet we see evidence of large areas of flowing water in the past.
  C) The presence of inactive volcanoes on Mars tells us that there must once have been a lot of outgassing, and hence a thicker atmosphere.
  D) The fact that parts of Mars have a lot of craters tells us that Mars must once have been much warmer.

Question 2

All the following statements about Mars are true. Which one might have led to a significant loss of atmospheric gas to space?
 
  A) The axis tilt of Mars is thought to change significantly with time.
  B) Mars lost any global magnetic field that it may once have had.
  C) Outgassed water molecules are split apart, and the oxygen then reacts chemically with surface rock on Mars.
  D) Mars probably once had a much higher density of greenhouse gases in its atmosphere than it does today.

[mochi09]

Answer to Question 1

B

Answer to Question 2

B

[tfester]

Excellent

[cdmart10]

Gracias!