# Mars Craters Common Problem

The most common problem encountered when working with students on this assignment is that they initially derive the depth-diameter equation by inputing data (depth, diameter) in

In some instances when I teach the class, if they submit a short write-up explaining the two equations they derived and why they're different, then re-perform their depth analysis for the Xanthe Terra craters, I amend their grade on the assignment.

*kilometers*. Because they are then measuring small craters in the Xanthe Terra images, however, many students then use the depth-diameter equation incorrectly by inputting diameter in*meters*. When students encounter this difficulty, after I evaluate their homework I have them go back to the part of the assignment where they derived the depth-diameter equation, have them alter all values in Excel to meters instead of kilometers, and re-derive the equation. Voila! Not the same equation.In some instances when I teach the class, if they submit a short write-up explaining the two equations they derived and why they're different, then re-perform their depth analysis for the Xanthe Terra craters, I amend their grade on the assignment.

**This is often the first time that students realize that an equation's derivation is sensitive to the units employed when the equation is constructed, and it's a highly useful lesson to have them learn for themselves.**