The Secrets of Pluto’s Thin Blue Line

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“Like a summer’s evening here on Earth just after sunset. A faint blue glow follows the Sun below the horizon — the only bit of color within an otherwise black sky,” Tanguy Bertrand imagines a view from the surface of Pluto, a picture more fully realized following recent man-made visitors to the icy dwarf planet, and further enhanced through new modeling from his team 4.7 billion km away in Paris.

Pluto’s haze layer shows its blue color in this picture taken by the New Horizons Ralph/Multispectral Visible Imaging Camera (MVIC). The high-altitude haze is thought to be similar in nature to that seen at Saturn’s moon Titan. The source of both hazes likely involves sunlight-initiated chemical reactions of nitrogen and methane, leading to relatively small, soot-like particles -- called tholins -- that grow as they settle toward the surface. This image was generated by software that combines information from blue, red and near-infrared images to replicate the color a human eye would perceive as closely as possible. Image credit: NASA / Johns Hopkins University Applied Physics Laboratory / Southwest Research Institute.

Pluto’s haze layer shows its blue color in this picture taken by the New Horizons Ralph/Multispectral Visible Imaging Camera (MVIC). The high-altitude haze is thought to be similar in nature to that seen at Saturn’s moon Titan. The source of both hazes likely involves sunlight-initiated chemical reactions of nitrogen and methane, leading to relatively small, soot-like particles — called tholins — that grow as they settle toward the surface. This image was generated by software that combines information from blue, red and near-infrared images to replicate the color a human eye would perceive as closely as possible. Image credit: NASA / Johns Hopkins University Applied Physics Laboratory / Southwest Research Institute.

Pluto’s atmosphere was characterized scientifically during 2015’s New Horizons’ flyby that analyzed both its emissions into space (its airglow), and the dimming of background stars viewed through it.

However, it was the view from Pluto’s far side, the atmosphere backlit by our own Sun, which provided a more illustrative, even familiar picture of a thin blue line encircling the dwarf planet.

That illuminated sky blue ring results from the scattering of sunlight by layers of organic haze within the atmosphere, making Pluto the latest in a growing list of hazy solar system bodies.

A world where haze is a far more pronounced atmospheric feature, tinting the sunlight illuminating its frozen host, can be found orbiting Saturn.

Titan’s thick orange-brown haze was captured by the Huygens probe during its descent and time on the surface.

Described like ‘L.A. smog on steroids,’ by Scott Edgington, Cassini deputy project scientist, the solid organic molecules are suspended in Titan’s atmosphere for far longer than on Pluto due to intense Earth-like vertical winds, creating a thicker haze.

On Pluto, haze particles quickly fall to the surface after their production.

Clues to the origin of atmospheric hazes have mostly come from the analysis of Titan, where evidence suggests methane and nitrogen molecules are dissociated and ionized by the sun’s UV radiation, a process known as photolysis. The molecules then react with each other to form larger precursor hydrocarbon and nitrile molecules which eventually, through aggregation, produce solid organic aerosols heavy enough to form a haze.

Similar processes are also thought to occur on Neptune’s moon Triton, but like on Pluto, yield less haze, whilst organic chemistry models suggest the early Earth’s nitrogen and methane heavy atmosphere might have been hazy too.

In fact, it has been suggested our own ancient haze may have played a role in the formation of life, protecting the surface from deadly UV and countering the build-up of greenhouse gases to ensure a habitable temperature.

Despite their misty nature, organic hazes are thought to be extremely revealing of the surface and atmospheric state of the bodies they envelope. Their ability to produce and deposit complex hydrocarbons, known as tholins, is thought to give Pluto’s surface its reddish appearance.

To aid the investigation of Pluto’s haze and what it might reveal about its host dwarf planet, Bertrand and his team at the Laboratoire de Météorologie Dynamique in Paris set out to reproduce New Horizons flyby observations.

The model they used was developed 30 years ago for the Earth’s own atmosphere, before versions were created for Mars, Venus, the gas giant planets, their satellites and recently discovered exoplanets.

“We worked hard to have models ready in anticipation of the New Horizons flyby in the hope we could provide explanations of any observations made.”

In a new paper, published in the journal Icarus, Bertrand used aerosol properties similar to those observed in the upper layers of Titan, which most closely resemble Pluto’s haze to get a close enough fit to constrain certain haze parameters on Pluto. These include haze particle size, to around 10-50 nm, and the amount of time the haze precursor molecules took to become solid, to around 3 months.

“This paper is a nice example of the power of comparative planetology where we apply what we learn from one planet to discover new things about another,” says Giada Arney from NASA Astrobiology Institute’s Virtual Planetary Laboratory, who has herself looked at models of Earth’s ancient haze to study similar atmospheres around exoplanets.

“This paper an important step forward in understanding the processes that occur in Pluto’s atmosphere.”

Bertrand’s model also showed that the methane photolysis reactions peak at an altitude of 250 km, and occur mostly in the sunlit summer hemisphere, which the team believes explains the higher density hazes observed in the current sun facing north.

Despite this uneven production, the team suggest that the low level of atmospheric circulation should still be sufficient, when combined with indirect UV flux from the interplanetary medium to ensure haze material is falling down to the surface everywhere at all times, covering any icy surface material with a thin layer of darker organics.

“This really confirms our inclination that shiny parts of Pluto must be evidence of recent resurfacing,”

However, the wide scale reddening of Pluto’s equatorial region remains a mystery.

Previous suggestions that direct photolysis of the surface could be a cause are contradicted by Bertrand’s model, which shows Pluto’s entire UV flux would be blocked through absorption by its atmospheric methane.

Earlier this year an alternative mechanism was proposed that linked the dark equator to the impact that formed Pluto’s moon Charon.

As well as answering these questions, Bertrand hopes that refined versions of their model could be applied to another wispy atmosphere at the edges of our solar system.

“We have very little data on Triton – if we understand what is going on with Pluto we may better understand what is going on there as well.”

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Tanguy Bertrand François Forget. 2017. 3D modeling of organic haze in Pluto’s atmosphere. Icarus 287: 72-86; doi: 10.1016/j.icarus.2017.01.016