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Gravity Assist: Pluto with Alan Stern (1)
Jan. 24, 2018
NASA's New Horizons spacecraft captured this image of Pluto on July 14, 2015.
Credits: NASA/JHUAPL/SWRIPluto -- which is smaller than Earth’s Moon -- has a heart-shaped glacier that’s the size of Texas and Oklahoma. Fascinating Pluto also has blue skies, spinning moons,
mountains as high as the Rockies, and it snows—but the snow is red! In this episode of Gravity Assist, Jim Green talks with New Horizons Principal Investigator Alan Stern of the Southwest Research Institute about what the July 2015 flyby of Pluto revealed about this mysterious and diverse world. TRANSCRIPT:
Jim Green: Our solar system is a wondrous place with a single star, our Sun, and everything that orbits around it, planets, moons, asteroids and comets. What do we know about this beautiful solar system we call home? It’s part of an even larger cosmos with billions of other solar systems. Hi, I’m Jim Green, Director of Planetary Science at NASA, and this is Gravity Assist.
I’m here with Alan Stern, the principal investigator of the New Horizons mission, and it’s all about Pluto today. What an amazing body that has turned out to be. You know, just a matter of a few years ago we knew virtually nothing about Pluto. But in July 2015 New Horizons changed all that and changed everything in our lives in planetary science. What really surprised me is what we found. What were your biggest surprises, Alan?
Alan Stern: You know, 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 people really wanted to participate in it in the public and just be a part of this exploration. We expected it would be a big response, but it was much bigger than we thought. And even for months -- I would say at least a year afterwards -- there was this completely unparalleled public reaction that our team members would go places; we were getting requests for literally hundreds of public presentations.
Jim Green: Yeah.
Alan Stern: We just couldn’t fulfill it all.
Jim Green: I think some of that is still going on. I mean, you know when I was in Japan just this last week I went to a girls’ school, and they wanted to know about Pluto.
Alan Stern: Yeah?
Jim Green: Yeah.
Alan Stern: Wow. That’s cool.
Jim Green: It really is. I mean you’re right; it’s just absolutely gone international. What really shocked me, actually, was the heart feature, but also the context around it. In a few craters, you know, a body smaller than the Moon, and yet it looked nothing like our Moon, so different.
Alan Stern: Oh, Pluto has its own personality, and the heart is probably one of the biggest parts of that. You know we named that heart Tombaugh Regio after the discoverer of Pluto, Clyde Tombaugh. When we were far away and first training our cameras on the planet in the distance, we were 100 million miles away -- as far as the Earth to the Sun -- and every time that part of Pluto would come into view we could see this bright, massive feature on the surface. And as it 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.
Jim Green: It really does look like a heart.
Alan Stern: But what it is, is a massive glacier made of nitrogen ice that’s a million square kilometers in scale. 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. And there’s not a single crater we can find there, which means this massive piece of real estate was born yesterday geologically. It’s amazing.
Jim Green: Yeah, it really is amazing. The other thing that I really liked about the heart that you guys have found out is 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: Yeah, you know, the whole planitia is surrounded by soaring mountains that are four or five kilometers tall, as tall as the Rockies. And it looks like they were uplifted in a gigantic impact onto the surface of Pluto that formed a big basin, as I said, about 1 million kilometers in scale -- 1,000 kilometers in every direction -- that 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. And it’s filled up over time just like you would be filling up a bathtub. But 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 this, if you will, the sea level of the nitrogen -- of course it’s frozen -- but the sea level can rise and fall by thousands of feet.
Jim Green: Well, is that due to its interaction with the atmosphere?
Alan Stern: Yes. As the sunlight, the amount of sunlight on its surface changes with either where Pluto is in its elliptical orbit or how it’s pole is tilted over time, as that varies, you get more or less heating into the basin. And that can either drive condensation flow in to fill it up, or what’s called the sublimation, kind of an evaporation process, that can draw it down.
Jim Green: Well, you know the haze on Pluto was really fantastic, and it’s really quite structured. So that also just blew me away that, you know, such a beautifully small body has such a beautiful atmosphere associated with it. And what I mean by small, I mean you know it’s just smaller than the Moon, but yet it’s so dynamic. It’s got almost everything a planet ever wanted to have.Close-up images of a region near Pluto’s equator reveal a giant surprise: a range of youthful mountains rising as high as 11,000 feet (3,500 meters) above the surface of the icy body. Credits: NASA/JHUAPL/SwRIAlan Stern: Yeah, I think it shows that small planets can be as interesting as big planets.
Jim Green: Yeah.
Alan Stern: You know, 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 literally half a million feet into Pluto’s sky. And when we took color pictures of it it’s blue. So there aren’t many places with blue skies. Earth is one of them; Pluto is another. And that, along with the structure and all the fine layering in the haze really caught us by surprise.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. Credits: NASA/JHUAPL/SwRIJim Green: I think the blue light is being backlit, but what’s happening in the atmosphere is quite a chemical reaction going on interacting with the solar wind, but also with the UV light, 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.
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. The reason the atmosphere -- you know, the Earth’s atmosphere is blue, but the air isn’t blue. It’s that the scattering properties of the molecules create a blueness through a process called Rayleigh scattering that Lord Rayleigh figured out almost 200 years ago as an early atmospheric scientist and physicist. And Pluto --
Jim Green: Yeah, it lets the red through, but the blue gets -- the blue wavelengths get scattered.
Alan Stern: They get scattered much more efficiently.
Jim Green: Right.
Alan Stern: And 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 is colorless, the actual molecules. And the same is true in a body of water. The same kind of process makes the ocean blue or a swimming pool blue. In Pluto’s case it’s different. It’s the fine suspended particles which themselves, even though they are red, the way they interact with sunlight through a different process discovered by a different physicist called Mie scattering, generates the blue color. And as you say, it’s primarily 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 the analogy of standing in some places on Pluto and it’s snowing red.
Alan Stern: It’s red snow.
Jim Green: Red snow, wow.
Alan Stern: Yeah, that’s a sci-fi planet for you.
Jim Green: Yeah, it really is. Yeah, that’s fantastic. So you had been thinking about going to Pluto for quite a while, and you were, I have to admit, the driving force that really made that happen.
Alan Stern: Well, a lot of people worked on it.
Jim Green: Well, I know, but I don’t know we’d be there without you. I’ve got to tell you that right now. I’m sure you’d say it’s all worth it. But it takes quite a toll because it takes an enormous amount of energy and an enormous amount of concentration to pull off.
Alan Stern: It does, and it took a toll on our families. I think for many of us, you know, we signed up for nights and weekends, and long hours, and lots of travel. And for kids and partners and spouses and relatives, you know, you’re absentee a lot. But I think it was well worth it, and I wouldn’t trade it. I would do it again in a minute. I think we really did something good for science, and we did something good for exploration, and we inspired a lot of people. And you can’t help but be proud of that.
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