Blue skies and patches of water ice on Pluto

Pluto has blue daytime skies, just like the Earth, Nasa revealed today, and there are patches of water ice on the distant world too.

Pluto's blue atmosphere
Pluto’s Blue Sky: Pluto’s haze layer shows its blue colour in this picture taken by the New Horizons Ralph/Multispectral Visible Imaging Camera (MVIC). Image credit: NASA/JHUAPL/SwRI

The first colour pictures sent back by the New Horizons probe, which flew past the dwarf planet in July, show the atmosphere as a dazzling ring of blue light.

The hazy air was pictured by the spacecraft’s Ralph/Multispectral Visible Imaging Camera (MVIC).

Some people had been expecting a more profound announcement from Nasa, such as a detection of life, which was really never likely. But mission chief Alan Stern tweeted that was due to a misunderstanding.

Pluto’s blue skies do not have the same cause as our own. On Earth they are due to sunlight scattered by tiny molecules of nitrogen in the air.

On Pluto, which lies five billion miles away in a band of icy bodies called the Kuiper Belt, the sunlight is scattered by larger particle of a sooty substance called tholins.

Commenting on the discovery, Principal Investigator Stern said: “Who would have expected a blue sky in the Kuiper Belt? It’s gorgeous,”

Water ice on Pluto
Blue false colour is used to indicate the regions where exposed water ice has been found in this composite image from New Horizons’ Ralph instrument, combining visible imagery from the Multispectral Visible Imaging Camera (MVIC) with infrared spectroscopy from the Linear Etalon Imaging Spectral Array (LEISA). Image credit: NASA/JHUAPL/SwRI

Areas of water ice on Pluto’s surface were found in a cratered region alongside a huge heart-shaped feature. Previous images have shown a terrain resembling an icy Earth.

“Large expanses of Pluto don’t show exposed water ice,” said science team member Jason Cook, of the Southwest Research Institute (SwRI) in Boulder, Colorado, “because it’s apparently masked by other, more volatile ices across most of the planet. Understanding why water appears exactly where it does, and not in other places, is a challenge that we are digging into.”

Nasa’s robot probe has also found that Pluto’s atmosphere is escaping and falling from space onto its largest moon, Charon, like snow.

But experts believe the prominent patch, named Mordor after the dark zone in The Lord of the Rings, is material stripped from Pluto’s own atmosphere.

Their theory is that this “air”, made up of nitrogen, methane and carbon monoxide, then fell like snow onto Charon’s cold north polar region where it never gets warmer than minus 213 C.

Pluto, and Charon, the largest of its five satellites, have both intrigued scientists with their peculiar alien landscapes following the flyby. They were expected to be dead worlds, because they lie so deep in the outer reaches of the Solar System. Instead their complex surfaces appear to have been resurfaced by geological upheavals in relatively recent times.

New Horizons scientist Dr Carly Howett, of the SwRI, blogged after seeing the first spectacular photos: “We know that Charon’s surface is too cold for anything other than solids to exist. We think that the colour variation is due to a change in surface composition, which leads to the conclusion that the surface of Charon’s northern polar region is made up of different material than the rest of Charon.

“Unlike at Charon’s warmer equator, any gases that arrive on the winter pole would freeze solid instead of escaping.”

Dr Howett said that the dumped icy material might have been expected to sublimate – a process similar to evaporation – back into space when the pole began to get more sunlight. But instead the sunlight modified the ice to produce a new substance called a tholin which was unable to escape.

She added: “Charon likely has gradually built up a polar deposit over millions of years as Pluto’s atmosphere slowly escapes, during which time the surface is being irradiated by the Sun.”

Some features on Pluto have been unofficially named after top scifi shows including Star Trek and Doctor Who.

Paul Sutherland

Paul Sutherland

I have been a professional journalist for nearly 40 years. I write regularly for science magazines including BBC Sky at Night magazine, BBC Focus, Astronomy Now and Popular Astronomy. I have also authored three books on astronomy and contributed to others.
Paul Sutherland

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Paul Sutherland

I have been a professional journalist for nearly 40 years. I write regularly for science magazines including BBC Sky at Night magazine, BBC Focus, Astronomy Now and Popular Astronomy. I have also authored three books on astronomy and contributed to others.

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