Gravity Assist Podcast: Pluto, with Alan Stern, Part 1

The Gravity Assist Podcast is hosted by NASA’s Director of Planetary Science, Jim Green, who each week talks to some of the greatest planetary scientists on the planet, giving a guided tour through the Solar System and beyond in the process. This week, he’s joined by New Horizons Principal Investigator Alan Stern of the Southwest Research Institute, to chat about what the July 2015 fly-by of Pluto revealed about this mysterious and diverse world, which has a heart-shaped glacier that’s the size of Texas and Oklahoma, blue skies, spinning moons, mountains as high as the Rockies, and red snow!

You can listen to the full podcast here, or read part one of the transcript below.

Pluto’s heart is a vast planitia 1,600 kilometers across. Image credit: NASA/JHUAPL/SwRI.

Jim Green: Just a few years ago we knew virtually nothing about Pluto, but in July 2015 New Horizons changed all that when it flew by the dwarf planet. What were the biggest surprises, Alan?

Alan Stern: I think my two biggest surprises were first, just how utterly amazing Pluto turned out to be – how many different kinds of features were on the surface and even in the atmosphere. There was something for everyone. And the second amazing finding was how many members of the public really wanted to participate in it and just be a part of this exploration. We expected it would be a big response, but it was much bigger than we thought. I would say for at least a year afterwards there was this completely unparalleled public reaction that our team members would [receive when] going places; we were getting requests for literally hundreds of public presentations. We just couldn’t fulfill it all.

Jim Green: I think some of that is still going on. When I was in Japan just this last week I went to a girls’ school, and they wanted to know about Pluto. It’s just absolutely gone international.

What really shocked me, actually, was the heart feature, but also the context around it. Pluto is a body smaller than the Moon, and yet it looked nothing like our Moon.

Alan Stern (left) joined Jim Green in the recording booth for the latest episode of the Gravity Assist podcast. Image credit: NASA.

Alan Stern: Pluto has its own personality, and the heart is probably one of the biggest parts of that. We named that heart Tombaugh Regio after the discoverer of Pluto, Clyde Tombaugh. When we were far away, 100 million miles away – as far as the Earth from the Sun – and first training our cameras on Pluto in the distance, every time that part came into view we could see this bright, massive feature on the surface. As we got closer and closer it started to take this heart shape, and we decided to run with that and call it Pluto’s heart. And it really does look like a heart, but it’s really a massive glacier made of nitrogen-ice that’s a million square kilometers in area. It’s the size of Texas and Oklahoma combined. And the glacier is flowing. We see places where there are avalanches onto it and where it runs up against the mountains and subducts under them, and we see where it’s overturning. There’s not a single crater we can find there, which means this massive piece of real estate was born yesterday, geologically speaking. It’s amazing.

Jim Green: The other thing that I really liked about the heart is that it’s a planitia. That means it’s a lower area. So that has led you to some really neat ideas as to how that came about.

Alan Stern: The whole planitia is surrounded by soaring mountains that are four or five kilometers tall, as tall as the Rockies. It looks like they were uplifted in a gigantic impact onto the surface of Pluto that formed a big basin that is, as I said, about one million square kilometers, [which is] 1,000 kilometers in every direction, and which dug this big hole out. And then that hole became a cold trap for snows. Primarily the atmosphere is made of nitrogen, so that’s what snows the most. It’s filled up over time just like you would be filling up a bathtub. Our mathematical models show that as Pluto goes around its orbit, and then as it has these longer seasonal cycles, the amount of nitrogen in the basin can actually ebb and flow back and forth thousands of times over billions of years where the sea level, if you will, of the frozen nitrogen can rise and fall by thousands of feet.

Jim Green: Is that due to its interaction with the atmosphere?

Alan Stern: Yes. As the amount of sunlight on Pluto’s surface changes with either where it is in its elliptical orbit or how its pole is tilted over time, as that varies, you get more or less heating into the basin. That can either drive condensation to flow in and fill it up, or what’s called sublimation, which is a kind of evaporation process that can [empty it].

Pluto’s blue haze, produced by back-scattering from small particles in the atmosphere in a process known as Mie scattering. Image credit: NASA/JHUAPL/SwRI.

Jim Green: The atmosphere haze on Pluto was really fantastic, and it’s really quite structured. It that also just blew me away that such a small body has such a beautiful atmosphere associated with it.

Alan Stern: We expected that haze for decades. There were hints of it in ground-based data, and you look at artist conceptions of Pluto and they’ll often show a low-lying kind of haze layer towards the surface, but what we found was a soaring structure with dozens of layers that stretch up half a million feet (150 kilometers) into Pluto’s sky. When we took color pictures of it [we saw that] it’s blue. There aren’t many places with blue skies. Earth is one of them; Pluto is another. that, along with the structure and all the fine layering in the haze, really caught us by surprise.

Jim Green: I think the blue light is being backlit, but what’s happening in the atmosphere is quite a chemical reaction going on as a result of interacting with the solar wind, but also with the ultraviolet light [from the Sun], and then complex carbon molecules are coming together and creating what are called the tholins. So that’s my understanding of what’s happening, and those are red.

A close-up of a section of Pluto’s heart, showing the slowly flowing nitrogen-ice butting up against a ‘coastline’ of hills and mountains. Image credit: NASA/JHUAPL/SwRI.

Alan Stern: Yeah, that’s spot on. And that’s what is probably silting out onto the surface of Pluto and making the surface red. Earth’s atmosphere [sky] is blue, but the air isn’t blue. It’s that the scattering properties of the [atmospheric] molecules scatter blue light more efficiently than red light through a process called Rayleigh scattering that Lord Rayleigh figured out almost 200 years ago as an early atmospheric scientist and physicist. So the red passes through, and the blue is what gets scattered around. It’s what paints the color of the atmosphere, if you will, even though the air itself, the actual molecules, is colorless. The same is true in a body of water. The same kind of process makes the ocean blue or a swimming pool blue. But in Pluto’s case it’s different. It’s the fine suspended particles [in the atmosphere] which, even though they are red, the way they interact with sunlight through a different process called Mie scattering, which generates the blue color. [Editor’s note: Mie scattering takes place when the particles doing the scattering are large than the wavelength of the light. On Earth Mie scattering can be caused by water droplets, or particles of smoke or dust.] It’s primarily a forward-scattering effect, so you see it from the far side of Pluto looking back when sunlight is filtering through the atmosphere. But if you were an astronaut and you were there, it would literally appear blue just like the Earth’s atmosphere.

Jim Green: What I really like, too, is in some places on Pluto it’s snowing red [as a result of those fine suspended particles].

Alan Stern: It’s red snow. Yeah, that’s a sci-fi planet for you.

You can read the second part of the transcript here.