Pluto: A World of Blue Skies and Red Ice

A tiny frozen world three billion miles away from the sun isn't the place where you might expect to find a brilliant blue sky, but that's exactly what NASA's New Horizons team has discovered on far-off Pluto. Continue reading →

A tiny frozen world three billion miles away from the sun isn't the place where you might expect to find a brilliant blue sky, but that's exactly what NASA's New Horizons team has discovered on far-off Pluto.

Of course, if you were to somehow stand on Pluto and look up, the sky above your head would still look black. Pluto's atmosphere is much too thin to actually fill in with scattered light. But at sunset and sunrise -- which, on Pluto, are about 3.2 Earth-days apart -- you might see the horizon illuminated by a lovely blue glow.

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The image above is a color-composite made from data acquired by New Horizons' Ralph/Multispectral Visible Imaging Camera (MVIC) as the spacecraft was traveling away from Pluto after its historic close encounter on July 14. It shows the silhouette of Pluto backlit by the sun, surrounded by a sky-blue multilayered haze.

"Who would have expected a blue sky in the Kuiper Belt? It's gorgeous," said Alan Stern, principal investigator for the New Horizons mission from the Southwest Research Institute in Boulder, Colorado.

Here on Earth, the daytime sky appears blue because of the way molecules of nitrogen and oxygen in the atmosphere disperse visible light from the sun, an effect called Rayleigh scattering. Shorter blue wavelengths are scattered more effectively, giving the sky its blue color.

On Pluto, 33 times farther from the sun than Earth is, light from our star is scattered through layers of atmospheric haze in much the same way - except there it's not by molecules of oxygen or nitrogen but by very tiny soot-like particles called tholins.

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The tholins are created from the breakdown of nitrogen and methane in Pluto's atmosphere by ultraviolet radiation from the sun, a complex chemical process also found at Saturn's moon Titan. Individual tholin particles themselves aren't blue but rather grey or red in color; eventually they grow large and heavy enough to "rain" down onto Pluto's surface - the likely source of its large red-stained areas.

In addition to a cerulean sky the New Horizons team also identified areas of exposed water ice on Pluto. While you might expect ice to be covering a world located in the far frozen reaches of the Kuiper Belt, on Pluto much of the surface hasn't been found to have ice made of water but rather other more volatile substances, like nitrogen.

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Curiously, the areas that show signatures of water ice are also regions that have shown up in high-resolution color image as particularly bright red - have these also been stained by tholins from Pluto's atmosphere?

"I'm surprised that this water ice is so red," says Silvia Protopapa, a science team member from the University of Maryland, College Park. "We don't yet understand the relationship between water ice and the reddish tholin colorants on Pluto's surface."

CLARIFICATION ADDED: Paragraph added to clarify that if you were on Pluto's surface, looking up, the sky would still look black. At dawn and dusk (and as New Horizons looked back at the Pluto-sun eclipse) Pluto's atmospheric haze would appear blue.

Source: NASA/New Horizons

A backlit Pluto shows off its beautiful blue sky in a new color image from New Horizons’ Ralph/Multispectral Visible Imaging Camera (MVIC).

After several false starts, NASA in 2001 agreed to fund an independent effort to fly a spacecraft to Pluto, the only member of the solar system’s original nine planets that hadn’t been explored. Five years later, New Horizons blasted off to begin a nearly 3 billion mile journey to Pluto, farther than any probe has traveled since the 1970s-era Pioneer and Voyager spacecraft.

Here’s a look at the New Horizons mission by the numbers.

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Launching a small spacecraft on a big rocket is one way to get going fast. Slingshotting off giant Jupiter’s gravity is another. New Horizons did both, and still the journey to distant Pluto took nearly 10 years. It is zipping along at about 31,000 mph -- fast enough to fly from New York City to Los Angeles in less than 5 minutes.

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Image: Viewed from the top of the Vehicle Assembly Building at Kennedy Space Center, NASA’s New Horizons spacecraft roars off the launch pad aboard an Atlas V rocket on Jan. 19, 2006.

At its closest approach, New Horizons will pass about 7,750 miles from Pluto and about 17,900 miles from its orbital mate Charon. The view will be about 500 times better than this image, taken on July 7 when New Horizons was just less than 5 million miles from Pluto. New Horizons will pass through the Pluto system in about 30 minutes. The probe carries seven science instruments, including LORRI, the Long Range Reconnaissance Imager, telescope.

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During the encounter, New Horizons will take hundreds of pictures in both visible and near-infrared wavelengths. The best images should depict surface features as small as 200 feet across. With nearly 3 billion miles between New Horizons and Earth, a radio signal, which travels at the speed of light, will take about 4.5 hours to reach Earth.

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Image: An artist's impression of Pluto's surface reveals an icy surface -- we're about to find out what Pluto is really made of.

With just one shot to get a close-up view of Pluto, New Horizons is designed to gather as much data as possible, as quickly as possible. In all, scientists expect the spacecraft to collect 100 times more data during closest approach than it can transmit back to Earth just after the encounter. A few high-priority images and data will be sent back just before and after closest approach, but the rest will trickle in over the next 16 months.

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Image: Diagram showing the sequence of events during New Horizons' encounter with the Pluto system.

New Horizons draws electricity from a single radioisotope thermoelectric generator, or RTG, which converts heat given off by the natural decay of about 24 pounds of radioactive plutonium. It runs on less power than a pair of 110-watt light bulbs.

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Image: Artist's impression of New Horizons flying past Jupiter, with its RTG visible in the lower right of the image.

After its Pluto flyby, New Horizons will continue out into the Kuiper Belt region of the solar system. Scientists hope to extend its mission so it can pass by at least one of the thousands of icy bodies that orbit in this vast domain. Eventually, New Horizons will end up leaving the solar system. It is expected to remain viable until the late 2030s.

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Image: Artist's impression of New Horizons encountering a Kuiper Belt object beyond Pluto.