Astronomy final exam question: Mars as runaway refrigerator

Astronomy 210 Final Exam, Spring Semester 2009
Cuesta College, San Luis Obispo, CA

[20 points.] If all volcanic activity ceased on Earth, discuss how it would eventually either become a runaway refrigerator (like Mars), or a runaway greenhouse (like Venus). Explain using properties of the geological/atmospheric cycles of planets.

Solution and grading rubric:

• p = 20/20:
  Correct. Decreasing volcanic activity means less carbon dioxide released from the mantle into the atmosphere, decreasing the greenhouse effect and causing cooler temperatures. The runaway refrigerator effect is when it is cold enough for the oceans to freeze, such that water vapor (also a greenhouse gas) is now missing in the atmosphere, and thus even colder temperatures will take place.
  
• r = 16/20:
  Nearly correct (explanation weak, unclear or only nearly complete); includes extraneous/tangential information; or has minor errors. Explains how Earth's greenhouse effect will get weaker and colder temperatures would result, but not how the freezing of oceans and lack of water vapor would accelerate this effect.
  
• t = 12/20:
  Contains right ideas, but discussion is unclear/incomplete or contains major errors. Understands how lack of carbon dioxide would decrease the greenhouse effect.
  
• v = 8/20:
  Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner.
  
• x = 4/20:
  Implementation/application of ideas, but credit given for effort rather than merit.
  
• y = 2/20:
  Irrelevant discussion/effectively blank.
  
• z = 0/20:
  Blank.
  

Grading distribution:
Section 30674
p: 0 students
r: 8 students
t: 8 students
v: 10 students
x: 1 student
y: 3 students
z: 1 student

A sample "r" response (from student 5398), not quite explaining how lowering temperatures would accelerate ("runaway") once water vapor freezes out of a cooling atmosphere:

A sample "v" response (from student 5238), where the apparently the heat supplied by volcanoes that is now missing would cause temperatures to lower:

Another sample "v" response (from student 9898), where the lack of fertile volcanic soil for plant growth will eventually cause the atmosphere to heat up, much like Venus'(!) atmosphere:

A sample "y" response (from student 1228), going for the cheap joke:

Another sample "y" response (from student 1959), with a visual pun: