During its water-propelled journey here on Earth, our rock is pummeled by particles and thrown into obstructions only to be pushed farther along by the force of water. Each impact it endures, however minute, leaves its mark. Many years later, the rock has been transformed into the smooth-surfaced creation that begs to be flung at a low angle across the water. In the geology world this process is known as abrasion.

So when we see rounded rocks on Mars we know that at that location a river ran through it, at some point in its history. From the size and the shapes of the mixture of stones found together come clues to the speed of the water flow and the distance that the stones have traveled to reach their present location. Working the data we have received so far, we believe the water source that propelled our skipping stone to its present location was anywhere from ankle to hip-deep and flowed at about 3 feet per second. The distance traveled and the source of the water is unknown, but in my mind’s eye I can picture the different scenarios that could bring that little stone into existence. And to a planetary geologist, each and every one of them has its own inner and outer beauty. Perhaps one day, our insatiable Curiosity will help us learn which of these scenarios is correct.

In the meantime, we’ve put our skipping stones in the rover’s rearview mirror (several of our 17 cameras watched them disappear over the horizon) and moved forward in both time and space.

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Just the other day, we crossed another significant threshold of the mission by processing our first sample of Mars in one of Curiosity’s onboard chemistry labs. Essentially, we’re eating dirt and loving it. With this step, we’ve begun doing the kind of exacting scientific research you would usually only be able to accomplish in a fully stocked laboratory at a university or research center. Now, we’re doing it on Mars.

What these findings will tell us we’re not sure. But we do know that Gale Crater, the skipping stones and the baby aspirin-sized sample of Martian dirt the rover just ingested all have a story to tell.

Norman Maclean wrote in his seminal work, “A River Runs Through It and Other Stories”: “Eventually, all things merge into one, and a river runs through it. The river was cut by the world’s great flood and runs over rocks from the basement of time. On some of the rocks are timeless raindrops. Under the rocks are the words, and some of the words are theirs. I am haunted by waters.”

Our waters may be over a hundred million miles away and over a thousand million years back in time from the Big Blackfoot River Maclean was referring to, but that makes little difference. Those of us working on NASA’s Curiosity mission are haunted by waters as well. Where those ancient waters lead us remains to be seen, but they provide us with our roadmap. Now it’s just up to our instruments and our patience with a few detours if we feel the need to explore tributaries.

I imagine that the pursuit of this clear liquid and the stories and worlds it helps recreate will go on well beyond our rover’s mission. That someday, an astronaut may very well walk in the treaded footsteps of Curiosity and find these stones or ones like them. When that day happens, 180 million miles away from home, in Gale Crater or elsewhere on the planet of Mars, for at least a moment, they might think back to their own simpler time when, on the shoreline of some body of water, they first learned the value of the perfect stone.

Then, I imagine, they’ll get back to work because on Mars there is a lot to see, a lot to do, and a lot to learn.

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John Grotzinger is the project scientist for the Mars Science Laboratory mission.