Astronomy final exam question: closer Mars?

Astronomy 210 Final Exam, Fall Semester 2008
Cuesta College, San Luis Obispo, CA

[20 points.] If Mars had originally formed closer to the Sun, discuss whether its atmosphere would now be thinner than, thicker than, or the same as it is today. Explain your reasoning.

Solution and grading rubric:

• p = 20/20:
  Correct. Mars lost its atmosphere due to two effects--its weak gravity (due to its small mass) is unable to hold on to molecules in its atmosphere, and lack of an ozone layer allowed ultraviolet light to break down molecules into constituent atoms, allowing them to more easily escape. Having Mars form closer to the Sun would exacerbate both effects, as a higher temperature would cause molecules/atoms in the atmosphere to move faster. (May refer to escaping molecules as due to the Sun "burning off" the atmosphere, or calls this process "outgassing").
  
• r = 16/20:
  Nearly correct (explanation weak, unclear or only nearly complete); includes extraneous/tangential information; or has minor errors. Discusses how Mars would then be warm enough to have oceans and water vapor would contribute to a temporarily more thicker atmosphere.
  
• t = 12/20:
  Contains right ideas, but discussion is unclear/incomplete or contains major errors. Some discussion of difference in lifetimes, but compounded/confounded with other factors. Garbled discussion, but recognition of related factors such as mass, ozone layer, distance from Sun, temperature, and greenhouse effect.
  
• v = 8/20:
  Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. Involving unrelated factors, such as an inverse correlation between distance from Sun and thickness of atmosphere, Mars now being in the habitable zone and thus sustaining life, or heat causing Mars' surface to begin outgassing.
  
• 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 70158
p: 18 students
r: 11 students
t: 15 students
v: 24 students
x: 0 students
y: 0 students
z: 0 students

A sample "p" response for Mars having a thinner atmosphere (from student 3259):

A sample "p" response for Mars having a thicker (albeit, temporarily so) atmosphere (from student 7070):

A sample "r" response (from student 0013):

Examples of sample "t" responses, discussing how the position would determine the thinness/thickness of an atmosphere, but with no discussion as to why this would be so (here, from student 0725):

(...and from student 1616):

(...and from student 9933):