Does Pluto Have Clouds? Well …

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Mar 9, 2016
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The surprises from Pluto keep on coming. This time: Clouds.

Maybe.

An article in New Scientist gives some of the details. Briefly, images of Pluto taken during the July 2015 close encounter by the New Horizons spacecraft show tantalizing hints of clouds over the surface. I’ll be honest: The evidence is interesting, but I’m not convinced.

Let me start off right away by saying I sent an inquiry about this to scientists with New Horizons, but they’re not ready to comment yet; obviously and understandably they want more time (and probably more images too) before issuing an official statement. I also could not find the images in the New Horizons raw images database, but there are thousands of images there, and I may have simply missed it (for the sake of discussion I'll assume it's real). Given all this as a caveat, in my opinion the New Scientist headline (“Exclusive Photos: Clouds Seen on Pluto for First Time”) is misleading in this regard (and they've been guilty of this sort of thing before). It’s not at all clear that’s what we’re seeing here. Possible, but not conclusive.

Pluto has an atmosphere, but it’s thin. I mean, really thin; atmospheric pressure at the surface is about 0.00001 (one one-hundred thousandth!) that of Earth’s. The air there is mostly nitrogen, which is oddly similar to the air here on Earth. However, temperatures on Pluto peak at roughly -220°C (-360° F), so don’t get too cozy comparing our climate with Pluto’s.

Still, there’s enough atmosphere to have effects we can see. For example, there are haze layers, a dozen of them or more, above the surface, seen best when New Horizons passed Pluto and saw its atmosphere backlit by the Sun (and yes, the sky there really is blue, for the same reason ours is).

And therein lies the issue. How do you distinguish between haze and clouds? The haze over Pluto is probably mostly made of carbon-based molecules, created by ultraviolet light from the distant Sun zapping simpler molecules in the air and on the surface. Are the “clouds” seen—if they are seen—similar to that, or a different phenomenon?

Haze tends to be widespread, and clouds more localized. But the haze isn’t distributed evenly, and could be thicker in some places than others. That makes it hard to distinguish from a cloud. The images of features on Pluto’s limb are hard to clarify.

However, there is a bright feature in one photo that seems to cut across both a brighter smooth and a darker bumpy region on the surface, which is interesting. If that were surface ice or something like that you wouldn’t expect it to overlap both regions. And it does look cloudlike … but looks can, as you might expect, be deceiving.

I’d love to see two shots taken a few minutes apart of the same area. That could settle it. The cloud may not move much in that time, but the motion of New Horizons, barreling past Pluto at 14 kilometers/second, might literally give us some perspective on it. As the spacecraft took images from different angles, a cloud well above the surface could be IDed due to its parallax.

Perhaps we’ll know more soon. Images are still being sent back to Earth from New Horizons (data rates from 5 billion kilometers are a tad on the slow side), so hopefully some will reveal more about these features.

So again, to be clear, we don’t know if these images show clouds on Pluto or not. However, I do hope this pans out. Clouds over Pluto! What a thought.