The last image of Pluto sent back by NASA’s New Horizons spacecraft, which made a historic flyby of the dwarf planet in July, reveal unexpected detail: Pluto has brilliant blue skies similar to those of Earth.

Who would have expected a blue sky in the Kuiper Belt? It’s gorgeous.

said Alan Stern ~ New Horizons principal investigator.

So, how is that possible? Why Pluto’s sky is blue? Is due to what’s called scattering. The particles found in Pluto’s atmosphere — called tholins — are probably gray or red, but the way that scatter light in blue wavelengths has drawn the attention of New Horizons’s scientific team.

Tholins forms high up in Pluto’s atmosphere as a result of ultraviolet radiation from the sun that breaks apart nitrogen and methane. Tholins and other complicated molecules react with each other to form more complex negatively and positively charged ions.

When sunlight passes by them, the red light gets through, but the blue light hits those particles and caroms off in a different direction. The tholins eventually drift down to Pluto’s surface, which explains why the dwarf planet sports a reddish-brown hue.

That striking blue tint tells us about the size and composition of the haze particles,

Carly Howett said in a statement.

A blue sky often results from scattering of sunlight by very small particles. On Earth, those particles are very tiny nitrogen molecules. On Pluto they appear to be larger — but still relatively small — soot-like particles we call tholins.

I think the biggest surprise to us has been that even as far as Pluto is—it’s the most distant object we’ve explored—the ultraviolet light from the sun is still strong enough to be powering these kind of chemical reactions,

Curt Niebur said.

The second significant finding, as NASA said, is that the probe has detected numerous small, exposed regions of water ice on Pluto’s surface. The exposed water ice appears to be red!! The discovery was made from data collected using a tool called a spectral composition mapper on New Horizons.





Large expanses of Pluto don’t show exposed water ice,

said science team member Jason Cook of SwRI.

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.

The areas that seem to contain the most water ice also appear bright red in recent color images of Pluto.

I’m surprised that this water ice is so red,

said Silvia Protopapa.

We don’t yet understand the relationship between water ice and the reddish tholin colorants on Pluto’s surface.

New Horizons sent back a small portion of its flyby data shortly after the epic encounter but stored most of this treasure trove on board for later transmission. New Horizons is currently 3.1 billion miles (5 billion kilometers) from Earth and in good health.

As confident as we get in our knowledge of the solar system, the solar system continues to surprise us,

Niebur says.

There’s such wonderful diversity and such wonderful complexity out there, even all the way at the edge of our solar system.

And as for new data from New Horizons,

there’s more to come. This is going to keep happening every week for the next year.