Astronomy midterm question: cosmic ray exposure

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

Astronomy 10 learning goal M2.3
[15 points.] Discuss the evidence that supports the theory that most of the meteorites found on the surface of the Earth are recent fragments of asteroids.

Solution and grading rubric:

• p = 15/15:
  Correct. Exposure to the high energy particles of cosmic rays on the
  exposed surfaces of asteroids induce radioactive decays. The degree of cosmic ray induced decay products corresponds to amount of time spent exposed to outer space. Since meteorites do not have much of this "space tan," they could not have been exposed to outer space for very long, and thus could only have recently fragmented off from deep within larger asteroids.
  
• r = 12/15:
  Nearly correct (explanation weak, unclear or only nearly complete); includes extraneous/tangential information; or has minor errors.
  
• t = 9/15:
  Contains right ideas, but discussion is unclear/incomplete or contains major errors. May confuse the decay products due to cosmic ray exposure with those that determine the solidification age of a material.
  
• v = 6/15:
  Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. Typically argues that a meteorite found on the Earth (which is continuously renewed/resurfaced) must haven been deposited there recently, while not accounting for the possibility that the meteoroid may have broken off of its parent asteroid a long time ago, and spent much of its time until now drifiting through space.
  
• 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 5166
p: 13 students
r: 4 students
t: 6 students
v: 17 students
x: 23 students
y: 0 students
z: 1 student

A sample "p" response (from student 1337):

A sample "p" response (from student 6258) appealing to "The Space Tan Effect":

A sample "x" response (from student 1426), with credit given for effort rather than merit:

Previous post: Astronomy in-class activity: meteorite types and origins