Exploring Pluto

For my money, the New Horizons mission to Pluto and beyond is one of the most audacious and exciting missions sent out through the solar system.  I say that as someone who cut her teeth doing science reporting on the Voyager missions! This time last year, we all waited for the spacecraft to arrive at the planet. When it did, on July 14, 2016, it whizzed past at a speed of 49,600 kilometers per hour, and then continued on its way. Hundreds of us — team members, friends, family, and the press were absolutely exhilarated at the dazzling visions of the distant world we were seeing. The data have been streaming back ever since then and each download provides an amazing look at a world that nobody expected. Here’s a “highlights” tour of what we know about Pluto so far.

Revealing Pluto

When the Pluto research papers came out  in late 2015 and spring 2016, they revealed a curious and interesting place. Pluto is a real world, with diverse surface features and active geology. It has really fascinating surface chemistry. There’s a complex layered atmosphere, a somewhat puzzling interaction with the Sun, and a collection of small moons that are fascinating places in their own right.

Mind you, those facts (and the science behind them) are what the Pluto science team know after only a few months of data returned by the spacecraft. The full data load won’t finish relaying back to Earth until late 2016. So, there’s still lots to see — and learn. In the meantime, the mission team racks up awards and recognition by the scientific community for their contributions to planetary science. All of it is well-deserved.

Where’s New Horizons Now?

Right now, New Horizons is outbound from Pluto. It already has another target in sight — a Kuiper Belt object called 2014 MU69. NASA gave the official go-ahead for the extended mission to the next world “out there”. That flyby happens on January 1, 2019. I hope that we’ll all gather again to cheer the spacecraft on as it makes the first-ever close encounter with a KBO beyond Pluto. It’s been an amazing ride, and it’s all thanks to the amazing spacecraft and its Earthbound support team.

Answering the Big Questions

The New Horizons mission was the result of a big idea that asked a big question: what is Pluto like? Ask that question, and you have to ask other questions: why does it look and behave the way it does? What’s happening with its atmosphere? What about those moons?

The mission has already answered the first one quite well: Pluto is like nothing we’ve seen elsewhere in the solar system. The spacecraft’s data and images are a treasure chest and each day the team lifts out a new jewel. So, let’s take a look at some of the riches they’ve found and answers to some other questions about this distant world.

Why Does Pluto Look the Way It Does?

Pluto was a major surprise in many ways. Yes, it is known to be a cold, icy place in the Kuiper Belt. However, nobody expected the strange surface appearance of this distant ice ball. We kept hearing Alan Stern (principal investigator for the mission) ask the same question each time a new picture came up showing an amazing piece of Pluto terrain: “Who ordered that?!”

Some regions contain nitrogen ice, others have methane-rich or water-rich ices. It’s quite clear that Pluto’s surface appearance is the result of interactions between its upper layers of ice and warmer material deep inside.

Pluto does not have a smooth, shimmering surface. Rather, the close-up, high-resolution images show a complex arrangement of mountains and highlands surrounding pitted plains and hexagonal-shaped or cells. It’s not as heavily cratered as you might expect from an object that has undoubtedly been bombarded throughout its history. The heart-shaped Sputnik Planum doesn’t seem to have any craters. It’s so smooth that it has to be a fairly young surface; scientists estimate it to be no more than about 10 million years old.

Pluto, the Active World

Taken together, the hexagonal cells, the highlands, and the smoothness of Sputnik Planum, all point to a geologically active place. It uses nitrogen in various states to rebuild itself. Something’s happening inside Pluto that results in surface repaving, the formation of “plates”, and the building of craggy mountains the height of the Colorado Rockies, with peaks topped by methane snows.

The Cosmic Lava Lamp

That “something”  inside Pluto is a process called “convection”. Think of Pluto like a giant lava lamp, slowly circulating heated “blobs” of its soft and pliable nitrogen ice. That action replaces older ice on the surface with fresher ice from the interior. The result is the Pluto we see in the New Horizons images.

There’s likely a reservoir beneath the surface several miles deep in some places. In that lake solid nitrogen is warmed by Pluto’s modest internal heat. It rises in great blobs, and when they reach the surface, they cool off and sink again. The borders of the warmed surface freeze in place, which is what causes the Y- or X-shaped features. These are where three or four convection cells rubbed together. Pluto’s surface, therefore, shows the history of convection, frozen in place.

Searching out the Inner Heat

What’s causing the interior heating? That’s a question the team hopes to answer soon. However, it’s clear there’s enough energy in the interior to warm the nitrogen ice. Just to be clear — it’s not “hot” inside Pluto in the sense that we think of heat inside a planet. The melting point of nitrogen is around 63 K (-210 C or -346 F), so it doesn’t take much heat to get it going. It’s around 35 K in the Pluto region, a deep-freeze for water, nitrogen, and methane.

What’s Happening with Pluto’s Atmosphere?

One of the most evocative images of Pluto was a “farewell” view taken by the spacecraft as it left the system. It shows the layered atmosphere of the planet. Careful study of the blanket of air (with cameras and a special instrument that measured the light from a star as it filtered through the atmosphere) reveals a mix of nitrogen, methane, and other gases. Nitrogen flows away from Pluto at a rate of hundreds of tons per hour and this seasonal loss happens as solar ultraviolet light heats the atmosphere.

That brings up another question: how does the planet resupply the nitrogen in the atmosphere? At first, scientists thought that perhaps comets could deliver enough nitrogen to do the trick. Upon further review, that theory didn’t work out. It’s far more likely that geologic activity brings up fresh nitrogen supplies from deep inside Pluto through the action of cryovolcanism (with ice as the “lava” rather than basaltic rock we see flowing from Earth volcanoes).

The layers in Pluto’s atmosphere? It seems that they form as haze particles are concentrated by the action of “gravity waves” in the atmosphere, and spurred by winds blowing over Pluto’s mountains.

What about Those Plutonian Moons?

Pluto wasn’t the only thing that New Horizons checked out on its swoop through the system. The companion moon Charon has an ancient surface, not much changed in the last few billion years. This is quite a change active Pluto. A great equatorial expanse of smooth plains on Charon, which has been nicknamed Vulcan Planum, is likely a vast cryovolcanic flow or flows that erupted onto Charon’s surface about 4 billion years ago. These flows are probably related in some way to the freezing of an internal ocean. That action globally ruptured Charon’s grayish crust.

The other moons: Hydra, Styx, Nyx, and Kerberos, are more reflective than expected of small bodies in the Kuiper Belt. Most other objects “out there” are darker. That brightness may mean that the small moons of Pluto probably formed as a result of a giant collision between two bodies early in the history of the solar system. The result was Pluto and its moons.

The Gift of Pluto

There’s so much more to tell and learn about Pluto and its moons — more than I have space to retell here. This system is an amazing gift from the New Horizons mission to the world. It’s a third “region” of the solar system for planetary scientists to explore. Far from being an outpost populated with “leftovers”, Pluto shows that the Kuiper Belt is a vibrant and much more “alive” place than we expected. The results of this flyby show us a new world to explore.

The hard work of the science team is what allows me to tell you this story. I recommend you visit the New Horizons web pages to read through all the press releases and look at the amazing imagery stored there. You’ll come away asking the same question we heard at the flyby: Who ordered THAT!?!

I, for one, can’t wait to see what’s still to come from New Horizons as it speeds on to its next destination in the Kuiper Belt. Far from being the boring and forgotten outskirts of the solar system, this region is now known as a treasury of information about our complete solar system and its evolution.