Astronomy midterm question: comparing atmospheric CO_2 abundances

Astronomy 10 Midterm 2, Spring Semester 2008
Cuesta College, San Luis Obispo, CA

Astronomy 10 learning goal Q5.5

[15 points.] Currently, carbon dioxide (CO_2) makes up about 95% of Venus' atmosphere, and also makes up about 95% of Mars' atmosphere. Explain why carbon dioxide makes up a much smaller percentage of the Earth's atmosphere

Solution and grading rubric:

• p = 15/15:
  Correct. Earth has oceans and tectonic plate motion, while Venus and Mars do not. CO_2 in the Earth's atmosphere is absorbed by the oceans, then is incorporated into sedimentary rock on the ocean floor, which is then subducted back into the mantle.
  
• r = 12/15:
  Nearly correct (explanation weak, unclear or only nearly complete); includes extraneous/tangential information; or has minor errors. Recognizes importance of oceans or tectonic plate motion, but does not explicitly describe both. Or as (p), but also cites less CO_2 released by volcanos relative to Venus and/or Mars as contributing factor.
  
• t = 9/15:
  Contains right ideas, but discussion is unclear/incomplete or contains major errors. At least recognizes that CO_2 is somehow being cycled out of the atmosphere back into the mantle.
  
• v = 6/15:
  Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. Appeals to plant life, greenhouse effect, or attaining escape velocity as the primary removers of CO_2 from the atmosphere.
  
• 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 distributions:
Section 4160
p: 13 students
r: 15 students
t: 0 students
v: 12 students
x: 0 students
y: 0 students
z: 0 students

Section 5166
p: 15 students
r: 20 students
t: 5 students
v: 22 students
x: 1 student
y: 0 students
z: 1 student

A sample "p" response (from student 1223, who memorized the Earth carbon cycle chart from In-class activity 12):

Another "p" response (from student 6221):

And one more "p" response (from student 8994), who, despite discussing plants as an important factor in cycling excess carbon dioxide out of the Earth's atmosphere, emphasizes the contribution of oceans, sedimentary rocks, and tectonic plate motion with an illustration:

Previous posts:
Astronomy in-class activity: geo-atmospheric cycles and histories
Astronomy quiz question: carbon dioxide source
Astronomy quiz question: carbon dioxide sink
Astronomy quiz question: Venus' atmosphere
Astronomy quiz question: Mars' atmosphere