NASA’s New Horizons continues to amaze as it reveals more and more about Pluto and its family of moons. Recently, New Horizons dazzled us as it beamed back the first color images of Pluto, revealing a scaly, snakeskin-like surface. This week, we see Pluto’s hazy atmosphere in color for the first time, and surprise... it’s blue!
"Who would have expected a blue sky in the Kuiper Belt? It’s gorgeous," said Alan Stern, New Horizons principal investigator from Southwest Research Institute (SwRI) in Boulder, Colorado.
While the actual particles that make up the haze are most likely not blue, the color of the haze tells scientists a great deal. They think the particles within the haze are most likely red or gray; however, based on the blue tint, the science team can determine particle size and composition. "A blue sky often results from scattering of sunlight by very small particles," said science team researcher Carly Howett, also of SwRI. "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."
First detected in the upper atmosphere of Saturn’s moon Titan, tholin particles, like the ones on Pluto, are thought to form high in the atmosphere, and are produced by ultraviolet sunlight blasting atmospheric organic compounds. As essentially “complex organic gunk,” tholins can coat a planet or moon’s surface and come in a variety of colors depending on what molecules are present and how much radiation they receive. Lab experiments have shown that nitrogen and methane – both present in Pluto’s atmosphere – can yield red tholins. This can explain the red surface material on both Pluto and its largest moon Charon.
What makes this gunk so fascinating? Well, tholins could have implications for life on other worlds as scientists have produced the basic building blocks of life (amino acids) in tholin experiments. Tholins can also provide valuable insight into a planet's surface age and composition. This means if you have a world where tholins form regularly, any regions lacking the organic gunk would either be very young or could even be subjected to removal processes like rain.
Water ice on Pluto. NASA/JHUAPL/SwRI
As if blue haze wasn’t exciting enough, New Horizons had another surprise for us this week: water ice on Pluto’s surface. While there aren’t large areas of exposed ice, there are many small regions. The science team is currently investigating this discovery further, but thinks the regions of water ice might be more extensive than what we currently see, and it could be masked by other types of ices. Jason Cook, a Pluto science team member from SwRI, said in a statement: "Large expanses of Pluto don’t show exposed water ice, 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."
Spectral analysis shows that the location of the water ice deposits seems to correlate with the bright red areas in the recently released color images. This is surprising and may indicate a relationship between the water ice and tholins that we don’t yet comprehend. "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."
New Horizons, currently 5 billion kilometers (3.1 billion miles) from Earth, is in the midst of a year-long process to beam back all the data collected during its flyby of the Pluto system.