20071023

Astronomy midterm question: runaway greenhouse or refrigerator

Astronomy 10 Midterm 2, Fall Semester 2007
Cuesta College, San Luis Obispo, CA

Astronomy 10 learning goal Q5.5

[Version 1]
[15 points.] If volcanic activity drastically increased on the Earth, 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 = 15/15: Correct. Increasing volcanic activity means more carbon dioxide released back from the mantle into the atmosphere, increasing the greenhouse effect and causing cooler temperatures. The runaway greenhouse effect is when it is hot 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.
  • r = 12/15:
    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 stronger and hotter temperatures would result, but not how the evaporation of oceans and prevalence of water vapor will accelerate this effect.
  • t = 9/15:
    Contains right ideas, but discussion is unclear/incomplete or contains major errors. Understands how carbon dioxide produces a greenhouse effect, and how an increase in volcanic activity would generate higher temperatures.
  • v = 6/15:
    Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner.
  • x = 3/15:
    Implementation/application of ideas, but credit given for effort rather than merit.
  • y = 1.5/15:
    Irrelevant discussion/effectively blank.
  • z = 0/15:
    Blank.
Grading distribution:
Section 0135
p: 7 students
r: 8 students
t: 3 students
v: 13 students
x: 6 students
y: 1 student
z: 0 students

A sample of a "p" response (from student 2001) is shown below:

[Version 2]
[15 points.] If volcanic activity drastically increased on the Earth, 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 = 15/15: Correct. Tectonic plate motion recycles sedimentary rock of ocean floors (which contains carbon dioxide from the atmosphere) down into the mantle. Decreasing volcanic activity means less carbon dioxide released back 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 no longer present in the atmosphere, and thus even colder temperatures will take place.
  • r = 12/15:
    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 cooler temperatures would result, but not how the freezing oceans and lack of water vapor will accelerate this effect.
  • t = 9/15:
    Contains right ideas, but discussion is unclear/incomplete or contains major errors. Discusses how no new carbon dioxide put into the atmosphere by volcanos would result in lower temperatures, without mentioning that the process that removes carbon dioxide from the atmosphere is still continuing.
  • v = 6/15:
    Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. May discuss how lack of volcanos and continuing tectonic plate motion will result in a runaway greenhouse effect.
  • x = 3/15:
    Implementation/application of ideas, but credit given for effort rather than merit.
  • y = 1.5/15:
    Irrelevant discussion/effectively blank.
  • z = 0/15:
    Blank.
Grading distribution:
Section 1073
p: 1 student
r: 13 students
t: 13 students
v: 15 students
x: 5 students
y: 0 students
z: 0 students

A sample of a "p" response (from student 7823) is shown below:

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