NASA's Pluto mission is already surprising scientists with what it has found

NASA's New Horizons spacecraft made its closest approach to Pluto Tuesday morning, and even though we've only seen a small trickle of data so far, the findings are already surprising scientists.

A group of New Horizons scientists took to Reddit on July 14 to answer questions about the Pluto flyby, and it quickly became clear that the preliminary data holds some curiosities.

For starters, we didn't know Pluto had craters until today.

"We didn't really start seeing craters until the image released today - the pictures simply weren't sharp enough till now," Curt Niebur, a NASA program scientist, wrote on Reddit. "The images that come down later this week will be better for crater counting, and then we can compare to other planets."

We've also learned more about Pluto's very thin atmosphere - it's even thinner than we thought, about 700 times thinner than Earth's. The New Horizons atmospheric data ruled out some scientists' theories it could be thicker:

"Also, we have gotten great pictures back, and we have also gotten tons of info about the plasma and particle and dust environment around Pluto - AND great info about the atmosphere - already constraining a lot of our theories," Kelsi Singer, a post-doc student on the New Horizons team, wrote.

Scientists are also surprised by what they have not found.

"I think most people thought we would find at least one small moon - so far no new moons...!!" Singer wrote on Reddit.

But the team discovered that a Pluto moon that we already knew about - Charon - has a very dark pole. Amanda Zangari, a post-doc researcher on the New Horizons team, said scientists expected Charon's surface to be pretty uniform and featureless.

And Charon isn't the only moon that's not living up to expectations. Nix and Hydra are much smaller than expected, Pluto geologist John Spencer said during a scientific panel discussing the results that we have so far.

New Horizon's Principle Investigator Alan Stern is famous for not making predictions. But Zangari says he has an envelope from 2006 full of Pluto predictions that he's going to open soon. It'll be exciting to see how well (or not well) those predictions line up with the data.

"We've found that even experts can't predict everything perfectly," Zangari said.

We're going to be learning more and more about Pluto and its moon as New Horizons continues to send back more data over the next 16 months.