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 the transcript below.

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? [Soar Over Pluto and Charon in Dazzling New Horizons Videos]

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.

New Horizons' best close up of Pluto's surface. This is looking at the ice that forms the Heart. Small islands can bee seen where ridges in the ice intersect. (Image credit: NASA/JHUAPL/SwRI)

Jim Green: 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'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, we signed up for nights and weekends, and long hours, and lots of travel. For your kids and partners and spouses and relatives, you're absent 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.

Jim Green: We have our decadal surveys conducted by the National Academy (of Sciences) in terms of what the next steps in the various sciences should be, and some consideration has got to be made about going back to Pluto and Charon. What are your thoughts on that? How can we possibly pull that off?

Alan Stern: I think we know how to do it from a technology standpoint, and there are studies going on at the Southwest Research Institute where I work that are funded by internal research funds to get a head start on that, as well as at the Goddard Space Flight Center, at NIAC and some other places. It's clear that we have the technology to put an orbiter around Pluto and even to fly it there about as fast or maybe even faster with the SLS (Space Launch System) than [we did] with New Horizons [which was launched in January 2006 and reached Pluto in July 2015]. The interesting thing is that we've learned from these studies that the big satellite, Charon, can play the same role at Pluto that Titan did for Cassini. It's your motorboat that lets you tour the system by gravity assists so you can visit all the small satellites and go out in the plasma tail, and dip down into the atmosphere with a mass spectrometer and do all these things virtually for free in terms of fuel because Charon is giving you gravity assist after gravity assist after gravity assist.

A simulated image of Pluto's moon Charon, based on real observations and data as New Horizons flew between the two. (Image credit: NASA/JHUAPL/SwRI/Lunar and Planetary Institute)

Jim Green: I really like that concept already. But Charon also is pretty spectacular. It's very different to Pluto in terms of its color and a variety of features. What did you think about that when you saw it?

Alan Stern: Well, we knew the two would be different. From all the ground-based data, we could see that Pluto had more personality, if you will, that it had brighter colors and a more reflective surface and much more varied bright and dark areas [that were visible] as it rotated. But Charon's got its own personality, and it's got some really unique features that we don't see anywhere else in the Solar System. This vast canyon across the equator that stretches for more than 1,000 miles is probably the result of the freezing of an interior ocean early after its formation, as it went from a hot interior liquid to a solid, cooled-off interior. And it's got this tremendous big red polar cap, like something somebody would dream up, not something that would actually happen. It turns out the polar cap is made of stuff from Pluto that's flowing across space in between them and sticking to Charon's surface at the poles where it's coldest, and turning red for the same reason those tholins on the surface of Pluto are red. So Charon's got a lot to teach us, as well.

Jim Green: In addition to Charon, Pluto's got some other moons, and they're spinning, and they're not all tidally-locked, which was also quite a surprise. What's that all about?

Alan Stern: Well, if I knew I would go out and write a paper. We sort of understand the problem, and once you admit that there's a problem, you can then start to cope with it. We expected the small satellites would tidally spin down and behave like normal satellites of a giant planet, but it looks like the Pluto—Charon binary system, by being a binary and being so unusual in that it's got a lumpy gravitational field because you've got two masses there, creates these kicks and bumps as the small satellites orbit that keep working against the tidal forces that would damp down their rotational periods. Instead it excites them. It gives them kicks. Take [Pluto's moon] Hydra. It's the farthest out and it's tumbling almost 100 times every time it rolls around Pluto once in its orbit. It's like a football. It's just tumbling and tumbling and tumbling every 10 hours.

A high-resolution image of Pluto's moon Charon, courtesy of New Horizons. (Image credit: NASA/JHUAPL/SwRI)

Jim Green: When I first saw that I thought well, this must have been the result of some sort of recent impact that has done that and it hasn't quite slowed down. But then when many of the other moons also didn't show that tidally-locked position [Editor's note: tidal locking is when an objects rotational period and orbital period are locked in unison so that they last the same length of time, which results in them showing the same face to their parent body, like our Moon does to Earth], then you're right: something else has to explain that.

Alan Stern: A lot of people thought what you just said, that it must be evidence for a recent impact, and yet, as you say, because all four [small moons of Pluto] are doing it, it means that's too improbable. But also some people thought that maybe the system [of small moons] had just formed. But when our crater counters did the work to determine the surface ages — because the more craters on the surface, the longer it's been out in the rain of impactors — they got an age of billions of years old for the small satellites, just as old as Charon, which meant that they were all formed together and very, very long ago. And something else is going on to make those unusual spin rates.

Jim Green: The other part to that too is it doesn't look like there's any debris laying around. So if there was a recent impact, where did all the debris go?

Alan Stern: Exactly. The system is clean as a whistle.

A simulated oblique view of the landscape on the edge of the Heart, built from data collected by New Horizons. Notice the mountains jutting up into the sky on Pluto's limb. (Image credit: NASA/JHUAPL/SwRI/Lunar and Planetary Institute)

Jim Green: One of the things that I always like to do with my guests is talk about how they got into their field, how they really got excited about what they're doing and the drive that they obtained. So Alan, what was your gravity assist that made that happen for you?

Alan Stern: Gosh, there are probably a lot, but one that comes to mind right away is my dad, who was not in the space business, not in the science business, he was in sales and management, and one day he came home from a business trip with his little handheld voice recorder, and he said, "guess who I sat next to on this airplane — the Apollo astronaut, Wally Schirra. And he left a message for you." And then my dad played this [recording], and there was Wally Schirra, pretty much saying, "Hey, Alan, I understand you live in Dallas and you're interested in science, and you study hard and you know you can do anything you want. You can come be an astronaut like I am, or be a scientist, or an engineer, whatever you want to work on. Go get 'em. I hope I meet you someday." And wow, for a 9- or 10-year-old kid, boy was that a gravity assist.

Jim Green: Did you ever get back and meet Wally?

Alan Stern: I never did meet him. I've met quite a number of the Apollo astronauts, and I don't know if the tape still works, but I know we still have that little handheld recorder. I wouldn't let my dad throw it away. I'm sure it's in my parents' attic somewhere.

Jim Green: New Horizons is now past Pluto, and is heading out into the Kuiper Belt. What's next for it?

Alan Stern: Well, the best-kept secret about New Horizons is that we're using the telescopes on board as an observatory actually in the Kuiper Belt. We just woke the spacecraft up in September, and all the way through the end of the year it's observing small planets and even smaller Kuiper Belt objects across our trajectory (Editor's note: the spacecraft has now returned into hibernation until 4 June 2018). And then at the end of 2018 we're going to be bearing down on our next fly-by, a billion miles beyond Pluto, at one of the building blocks of these planets like Pluto. It's an object that doesn't have a name yet, just a license plate, 2014 MU69. It's probably the most pristine relic of the formation of the Solar System ever to be explored. It's been out in that — 400 degree deep-freeze for four billion years, and we've never been to anything like that before. We don't quite know what to expect. It's a little bit like Pluto in that regard. I'm sure we'll see some surprises. It all happens during the holidays in 2018, so you can spend your Christmas in the Kuiper Belt and your New Year's Eve with NASA. The flyby is actually on New Year's Eve and New Year's Day turning 2019. And New Horizons is in spectacularly healthy shape and ready to go back in the ring for the next round.

This story was provided by Astrobiology Magazine, a web-based publication sponsored by the NASA astrobiology program. This version of the story published on Space.com. Follow us @Spacedotcom, Facebook oe Google+.