Astronomy final exam question: runaway greenhouse or runaway refrigerator

Astronomy 210 Final Exam, Spring Semester 2010
Cuesta College, San Luis Obispo, CA

[Version 1]

[20 points.] If tectonic plate motion continued on Earth (but volcanic activity stopped), discuss how it would 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. Ceasing volcanic activity means no carbon dioxide released back from the mantle into the atmosphere, while tectonic plate motion continues to subduct carbon dioxide in ocean sediments back into the mantle, 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 no longer present in the atmosphere, and thus even colder temperatures will take place; this much detail is not required as long as the lack of source, and continued sink in the carbon cycle are both mentioned.)
  
• r = 16/20:
  Nearly correct (explanation weak, unclear or only nearly complete); includes extraneous/tangential information; or has minor errors. Discusses lack of source and continued sink, but one of these is slightly problematic.
  
• t = 12/20:
  Contains right ideas, but discussion is unclear/incomplete or contains major errors. Typically discusses lack of source, but not continued sink.
  
• v = 8/20:
  Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. Lack of heat from volcanoes that would otherwise maintain warm atmospheric temperatures, or other factors.
  
• 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: 8 students
r: 9 students
t: 6 students
v: 14 students
x: 1 student
y: 2 students
z: 0 students

A sample "p" response (from student 8479):


A sample "r" response (from student 2569), with no discussion on carbon dioxide intake by oceans (and subsequent subduction of carbon-bearing sediments down into the mantle):


A sample "v" response (from student 7109), touching upon end of plant life and nuclear winter scenarios:


A sample "y" response (from student 0000), with a fanciful illustration:


[Version 2]

[20 points.] If tectonic plate motion stopped on Earth (but volcanic activity continued), discuss how it would 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. Tectonic plate motion ceasing to subduct carbon dioxide in ocean sediments back into the mantle, and continuing volcanic activity releasing carbon dioxide from the mantle into the atmosphere would increase the greenhouse
  effect and causing warmer temperatures. (The _runaway_ greenhouse effect is when it becomes warm enough for the oceans to evaporate, such that water vapor (also a greenhouse gas) is now prevalent in the atmosphere, and thus even hotter temperatures will take place; this much detail is not required as long as the lack of sink, and continued source in the carbon cycle are both mentioned.)
  
• r = 16/20:
  Nearly correct (explanation weak, unclear or only nearly complete); includes extraneous/tangential information; or has minor errors. Discusses lack of sink and continued source, but one of these is slightly problematic.
  
• t = 12/20:
  Contains right ideas, but discussion is unclear/incomplete or contains major errors. Typically discusses continued source, but not lack of sink.
  
• v = 8/20:
  Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. Heat from volcanoes would directly cause warm atmospheric temperatures, or other factors.
  
• 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 30676
p: 6 students
r: 6 students
t: 13 students
v: 26 students
x: 12 students
y: 4 students
z: 1 student

A sample "p" response (from student 9998):


A sample "v" response (from student 3455), taking into consideration the heat produced by friction between moving tectonic plates:


A sample "x" response (from student 5611), using uppercase to underscore the importance of convection:


A sample "y" response (from student 4000):