Why the Dark Spots on Pluto?

What’s at Work on this Distant Planet?

As Pluto comes into sharper focus through the eyes of New Horizons, we’re seeing more definition of the dark and light areas on this distant world. The lighter areas are very likely ice patches, but the dark ones are more intriguing. The most interesting image THIS week, shows four distinct markings that look like craters, or mountains, or… something on one side of the planet. What could be causing these dark markings?

Think about this: Pluto is covered with ices — water, nitrogen, and methane. That means it’s a cold place, indeed. At a surface temperature range of -287 to -369, that’s cold enough for nitrogen and methane to freeze out and coat the landscape. Of course, water is frozen as well.

Nitrogen and methane are susceptible to what’s called “irradiation” by sunlight, and in particular, the ultraviolet radiation emitted by the Sun. That process darkens the two ices and can create molecules called “tholins”. If you could be there and pick up some of this stuff, it might be as Carl Sagan once described: “A complex, organic goo.” Astronomers have found a great deal of evidence of these tholins on Pluto, which could account for the reddish color and dark markings we’re seeing.

Patterns of Tholins?

These tholins aren’t new discoveries. We’ve seen them on other our solar system moons such as Iapetus at Saturn and Umbriel at Uranus. They’re also present in the atmosphere of Titan. So, it makes sense that we’d see them at Pluto, too. And, they’re very likely what is forming the dark areas we’re seeing and turning the overall surface a nice red color. Now, why they’re in those exact configurations? Maybe there are vents or some other sources in those areas that periodically spew water ice over the other types of ice. Maybe those dark areas are where the water ice has gone away, leaving leaving behind the sun-darkened ice.

That’s my educated guess, and I can’t wait to see if it’s right or wrong. If it’s wrong, then, wow — maybe there’s something even MORE interesting going on there than everybody expects. In any case, I’m just part of the “Public Imaging Team”, taking my best educated guess at what’s going on there.

The image I’ve posted here was released on July 1 and was part of a pair taken on June 25 and June 27th. I’ve overprocessed it a wee bit just to bring out the light and dark areas, so don’t read too much into the image based on what I’ve done. In point of fact, the day they were released,  Principal Investigator Alan Stern made it clear that the team was really puzzled by all the dark and light markings on the planet. I suspect they won’t be for long!

It might be tempting to say, “Oh, those look like craters!” Or “Mountains on Pluto!”, the truth is, nobody quite knows yet what they are. The good news is, there are more images on the way, and in 11 days (or less) we’ll know exactly what those things are. Go New Horizons!

To learn more about Pluto’s coloration and markings, check out NASA’s Pluto: The Other Red Planet page. It goes into more scientific detail about the darkening of Pluto’s surface.