I am not a geologist by any stretch of the imagination, yet I do help operate a mobile geology lab on Mars and occasionally get asked about why we should explore Mars in the first place. I usually mumble something about geology and why rocks are important and how water was probably there once and then move on to the engineering topics I’m comfortable with. In the future though, I’ll be using an argument similar to the following passage by John McPhee in Assembling California. Although he’s talking about the moon here, I think the passage applies equally well to other bodies in the solar system.

“Some people have claimed that geology is actually a historical science, and not a physical science. And they’re halfway right because you are really looking back through time. You look at a landscape and try to see through time, all the things that happened in that place… On Earth the distant past can be hard to read. The forces of erosion, glaciation, volcanism, plate tectonics-they all conspire to obscure the nature of a given spot over time. The moon, however, has been dead since infancy. Its rocks are snapshots taken billions of years ago, and, because they’re part of what may well be the least changed major body in the solar system, they testify not only to its own history but, to ours.”