Astronomy final exam question: why Mars inhospitable?

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

An astronomy question on an online discussion board(*) was asked and answered:

CARTART: Why is Mars [too] cold to support life?
yeah: [B]ecause [M]ars is farther from the sun.
swbarnes2: Another factor for Mars is...its small mass...

Discuss why the answers from "yeah" and "swbarnes2" both correctly answer the question from "CARTART," and how you know this. Explain using the properties of greenhouse gases and geological activity.

*Source: http://answers.yahoo.com/question/index?qid=20080913120029AAeOGlQ.

Solution and grading rubric:

• p = 20/20:
  Correct. Discusses answers from both "yeah" and "swbarnes2." Being farther from the sun, Mars would receive less radiation for a greenhouse effect. Mars' small mass would mean less outgassing, and also be unable to retain an atmosphere sufficient for greenhouse effect.
  
• r = 16/20:
  Nearly correct (explanation weak, unclear or only nearly complete); includes extraneous/tangential information; or has minor errors. One of two explanations is problematic/incomplete, the other is correct.
  
• t = 12/20:
  Contains right ideas, but discussion is unclear/incomplete or contains major errors. Both answers are problematic/incomplete, or one answer correct, while the other is missing.
  
• v = 8/20:
  Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. At least understands factors that contribute to atmosphere density and greenhouse effect.
  
• x = 4/20:
  Implementation/application of ideas, but credit given for effort rather than merit. Discusses factors other than relevant to the greenhouse effect.
  
• y = 2/20:
  Irrelevant discussion/effectively blank.
  
• z = 0/20:
  Blank.
  

Grading distribution:Section 70158
Exam code: finalsf4R
p: 13 students
r: 4 students
t: 5 students
v: 7 students
x: 3 students
y: 0 students
z: 1 student

Grading distribution:Section 70160
Exam code: finalN3aR
p: 9 students
r: 6 students
t: 5 students
v: 5 students
x: 1 student
y: 0 students
z: 0 students

A sample "p" response (from student 2286):
A sample "p" response (from student 2003):