If someone asked you what color Pluto was, you'd probably describe it as a icy blue or gray, which is how it's depicted in the vast majority of illustrations and renderings.
Though this might seem like a surprise, scientists have thought it to be true since at least 2002, when the Hubble Space Telescope imaged Pluto and created some of the first maps of its surface.
Still, in those images, Pluto only appeared as a blurry blob. It was too small to photograph in detail from so far away, and impossible to learn much about its color or surface features. But as the New Horizons probe closes in for its long-awaited flyby on July 14, it's giving us sharper and sharper images — and one of the first things we've learned from them is Pluto's true color.
Why Pluto is brown
Scientists believe that the color is the result of interactions between methane molecules in Pluto's atmosphere and a specific kind of ultraviolet light, emitted by both the sun and distant galaxies. When this light hits the methane, it triggers the formation of solid molecules called tholins, which are reddish brown in color.
New Horizons has measured high levels of this type of light hitting Pluto, providing some evidence for the idea, and scientists have replicated the same chemical process in labs on Earth. The same phenomenon has also been observed on Saturn's moon Titan and Neptune's moon Triton.
This is just the beginning for New Horizons
Over the next week, New Horizons will send back better and better images — before flying past Pluto on Tuesday, coming within 6,200 miles of the dwarf planet's surface at speeds surpassing 31,000 miles per hour.
As it does so, the probe will photograph and map Pluto in unprecedented detail, providing information about the mysterious dark regions recently spotted on its surface. It could reveal polar ice caps, mountains, or perhaps even volcanic activity.
Other instruments will detect particles escaping from the dwarf planet's nitrogen-based atmosphere, while a radio antenna will send signals through it. By analyzing these signals after they pass through the atmosphere and reach radio dishes on Earth, we'll get a better idea of the specific gases present in the atmosphere.
One of the things that makes this mission so amazing is that as large as Pluto looms in the public imagination, we still know very little about it. Two of its moons have actually been discovered since New Horizons was launched in 2006 — and we still have just a rough idea of what Pluto even looks like.
That's about to change in a big way. And in showing us Pluto, New Horizons will mark the end to a momentous, 50-year era in space history.
In the 1960s and '70s, the Mariner missions showed us Mars, Venus, and Mercury, and in the '70s and '80s the Pioneer and Voyager missions showed us Jupiter, Saturn, Neptune, and Uranus in detail for the first time. Though it's technically not a planet, Pluto remains one of the few unexplored worlds in our solar system — but not for long.