The Question

I like postcards

Yes, as I said, I like postcards (also fridge magnets) as this photo of my fridge will attest.

Sending them, receiving them, it's all joy to me.

As a geologist, I also have a penchant for rocks. And as planetary geologist, I get to explore something that I love: Space.

We planetary geologists have several tools at our disposal to learn about the other worlds in our own system and further afield. For instance, we study little pieces of rock from these other worlds for clues to tell us what those faraway bodies are like.

So here, is my question, and the reason for starting this blog:

If we were to send little—or not so little—pieces (postcards, if you will) of Earth into space for a would-be alien planetary geologist in some distant place to discover, what would they be? 

What rocks (or things) on Earth reveal the most, or the best, things about our home—this magical world that can sustain life?

Answers on a postcard!