Five surprising discoveries about Pluto

Upon further review, Pluto is still mysterious and exciting.

After three months of releasing photos and data piecemeal, scientists with NASA’s New Horizons mission codified their preliminary observations of Pluto and three of its moons in the Oct. 16 Science. Here are five key (though not necessarily brand-spanking new) findings in the paper that epitomize the surprising complexity of the Pluto system:

1. Pluto has mountains of ice several kilometers high.

Over the years astronomers had gathered evidence that Pluto’s surface is covered with nitrogen, carbon monoxide and methane ice. But those compounds can’t support the towering mountains seen in the pictures New Horizons sent back, leading researchers to conclude that the ices sit atop solid bedrock of frozen water.

2. Pluto likes to resurface.

Scientists spotted evidence of dynamic surface processes familiar to Earthlings such as glaciation and tectonic movement of the crust. A large portion of Pluto’s famous heart-shaped region has no visible craters, which indicates it is no more than a few hundred million years old.

3. The atmosphere is sparse but hazy.

Pluto’s atmospheric pressure at the surface is about 0.001 percent that of Earth’s, a bit lower than expected. But New Horizons also spotted fog on the surface and a global haze that stretches up to 150 kilometers high.

4. Pluto and Charon are similar in density.

Astronomers believe that Pluto and its largest moon Charon formed in a giant impact like the one that produced Earth and the moon. Recent density measurements suggest that the two colliding bodies had similar compositions, offering insight into the planet-building material that was floating around during the solar system’s infancy.

5. Nix and Hydra are bright and wacky.

Objects in space tend to darken over time as they are pelted with radiation and dust, but these two Pluto moons are unexpectedly bright and probably covered in clean water ice. They are also spinning oddly, suggesting that Pluto and Charon are engaging in a gravitational tug-of-war on the smaller moons.