Astronomy final exam question: Earth avoiding Venus' runaway greenhouse?

Astronomy 210 Final Exam, spring semester 2011
Cuesta College, San Luis Obispo, CA

[20 points.] An astronomy question on an online discussion board (http://answers.yahoo.com/question/index?qid=20101122180043AAVRumU) was asked and answered:

Samantha: How did Earth avoid the runaway greenhouse effect that made Venus so hot?
Paul: The reason that Earth managed to avoid a catastrophic runaway greenhouse effect was its level of geologic activity... Venus, on the other hand, is a geologically inactive planet, with no way of recycling [carbon dioxide].

Discuss whether or not if this answer is correct, and how you know this. Explain using the properties of greenhouse gases and geological activity.

Solution and grading rubric:

• p = 20/20:
  Correct. Understands how oceans absorb carbon dioxide (which eventually gets subducted into the mantle), and specifically discusses how Earth avoided runaway greenhouse effect by being farther away from the sun than Venus.
• r = 16/20:
  Nearly correct (explanation weak, unclear or only nearly complete); includes extraneous/tangential information; or has minor errors. As (p), but does not discuss how location was key for Earth to avoid runaway greenhouse effect as on Venus.
  
• t = 12/20:
  Contains right ideas, but discussion is unclear/incomplete or contains major errors. At least recognizes factors that contribute to and/or control greenhouse effect.
  
• v = 8/20:
  Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. Discusses factors that do not contribute to and/or control greenhouse effect.
  
• 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
Exam code: finaln4St
p: 12 students
r: 7 students
t: 8 students
v: 2 students
x: 0 students
y: 0 students
z: 1 student

Section 30676
Exam code: finalS45s
p: 10 students
r: 12 students
t: 8 students
v: 12 students
x: 0 students
y: 0 students
z: 1 student

A sample "p" response (from student 1440):

A sample "r" response (from student 0701):