New Horizons hints at Plutonian clouds and dynamic Eris

One of the big surprises to come from NASA’s New Horizons spacecraft when it flew by Pluto in July 2015 was that Pluto has blue skies and bands of haze extending 200 kilometres into space. 

But now, scientists say, it appears that Pluto may also have clouds – or possibly fog.

The difference is that haze blocks only a tiny fraction of the light, while clouds are more opaque, says Alan Stern, the mission’s principal investigator, at a joint meeting of the American Astronomical Society Division for Planetary Sciences and the European Planetary Science Congress in Pasadena, California.

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For the most part, Pluto is cloud-free. But a close examination of photos taken during the spacecraft’s flyby revealed seven bright patches that appear to be clouds. 

Intriguingly, these bright patches all lie near the terminator – the boundary between Pluto’s day and night sides. 

This means that they are only present at dawn and dusk, when conditions are most likely to be cool enough for atmospheric vapours to condense into clouds. 

These bright spots also occur at altitudes low enough they might even be fog. 

That matches modelling work by planetary scientist Erica Barth of the Southwest Research Institute in Boulder, Colorado, who used New Horizons data on Pluto’s atmosphere to calculate the altitude at which clouds could form. She calculated they’d arise within 10 to 20 kilometres above the surface “instead of much higher or not at all”.

“That,” she says, “is further consistent with these features being clouds.” 

Not that these clouds would be anything like earthly ones. On Pluto, Barth says, they’d most likely be made of hydrogen cyanide, acetylene or ethane. 

Unfortunately, the clouds are too close to the surface for New Horizon’s photos to reveal their exact elevation. In fact, they might actually be oddly coloured patches of terrain that for some reason are bright only under dawn or dusk illumination.

The fact they show up along the terminator and elevations low enough to be consistent with Barth’s model “makes a strong but not airtight case”, Stern says.

In another paper presented at the same meeting, Bonnie Buratti, a planetary scientist from NASA’s Jet Propulsion Laboratory in Pasadena, California, says that New Horizons data suggests that another outer solar system world might be geologically active, just like Pluto.

That world is Eris, a Pluto-sized object that currently lies about three times farther out from the sun – so remote that astronomers didn’t discover it until 2005. 

What is interesting about Eris, Buratti says, is that it reflects nearly all the sunlight falling on it. 

Only two other objects in the solar system are known to have such bright surfaces. One is Saturn’s moon Enceladus, while the other is a region of Pluto informally known as Sputnik Planitia, which forms one lobe of the dwarf planet’s heart-shaped spot.

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In both cases, Buratti says, the brightness is due to the freshness of the material being brought to the surface by active geological processes. Based on that, and Eris’ comparable brightness, she says, “we wouldn’t be surprised if we went to Eris and saw that it’s also active”.

Meanwhile, Stern says, the massive quantities of data collected during the Pluto flyby are within days of being fully downloaded. 

But new findings will undoubtedly continue to be made for years to come. “What rains down is just ones and zeros, and requires people to interpret.”

In the interim, New Horizons is about a third of the way into the next phase of its mission, which will culminate with a flyby of a 30- to 45-kilometre worldlet known as 2014 MU69 around New Year’s Day, 2019. 

2014 MU69 is much bigger than Comet 67P/Churyumov-Gerasimenko, recently explored by the European Space Agency’s Rosetta mission, but vastly smaller than Pluto. 

“We’ve never been to something like MU69,” Stern says. “Something that’s been cold for four billion years, but small enough not to have the processes we see on Pluto and [its moon] Charon. This is a completely new type of world we’re going to.”