Astronomy final exam question: comparing relative potential habitability of Kepler-62e, Kepler-62f

Astronomy 210 Final Exam, spring semester 2013
Cuesta College, San Luis Obispo, CA

According to a NASA press release[*], the star Kepler-62 is a medium-mass main-sequence star somewhat similar to the sun, and:

Kepler-62 is home to two habitable zone worlds, Kepler-62f and Kepler-62e. Kepler-62f [orbiting farther out] is only 40 percent larger than Earth... Kepler-62e [orbiting closer in] is roughly 60 percent larger than Earth.

Determine which planet (62f or 62e) would be more likely to have an atmosphere with a moderate greenhouse effect and a moderate temperature for liquid oceans to be present. Explain using the properties of greenhouse gases and geological activity.

[*] http://www.nasa.gov/mission_pages/kepler/multimedia/images/kepler-62-diagram.html.

Solution and grading rubric:

• p = 20/20:
  Correct. Recognizes how (1) mass determines atmosphere thickness (more massive planet Kepler 62e would outgas more atmosphere, and better gravitationally retain this atmosphere); and how (2) distance from sun determines the temperature of an atmosphere (Kepler 62e being closer to its star would absorb and retain more heat). Thus Kepler 62e would be more susceptible to higher temperatures and more likely evaporate its oceans, while less mass and further out Kepler 62f would be more likely to have moderate temperatures for liquid oceans.
• r = 16/20:
  Nearly correct (explanation weak, unclear or only nearly complete); includes extraneous/tangential information; or has minor errors. One of two points (1)-(2) correct, other is problematic/incomplete. May argue that Kepler 62f would have an atmosphere that is too thin, and too little heat absorbed for moderate temperatures for liquid oceans, making Kepler 62e the more ideal candidate.
• t = 12/20:
  Contains right ideas, but discussion is unclear/incomplete or contains major errors. Both points (1)-(2) problematic/incomplete, or one point correct while 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 30674
Exam code: finalnNz2
p: 4 students
r: 7 students
t: 8 students
v: 6 students
x: 1 student
y: 1 student
z: 0 students

Section 30676
Exam code: finals56G
p: 3 students
r: 7 students
t: 17 students
v: 9 students
x: 0 students
y: 1 student
z: 2 students

A sample "p" response (from student 0211):
Another sample "p" response (from student 6608):
A sample "y" response (from student 0426):
Another sample "y" response (from student 9251):