Five years since New Horizons saw Pluto

It’s exactly five years since NASA’s New Horizons spacecraft gave us a close-up view of Pluto, after a 10-year, 4.8-billion-kilometre journey.

Moving to within 12,500 kilometres of the dwarf planet’s surface, it revealed an icy world of towering mountains, giant ice sheets, pits, scarps, valleys and terrains seen nowhere else in the Solar System. And just for good measure, it took a detailed look at the largest of Pluto’s five moons, Charon.

To celebrate the anniversary, NASA has listed “10 of the coolest, weirdest and most unexpected” things we now know about the Pluto system thanks to New Horizons data.

1. Pluto has a “heart,” which drives activity

This is one of the signature features New Horizons observed on approach and imaged in high resolution during the flyby.

Pluto’s heart is a vast, 1.6-million-square-kilometre nitrogen glacier. The “left ventricle”, called Sputnik Planitia, forced the dwarf planet to reorient itself so the basin now faces almost squarely opposite Charon.

It’s a process called true polar wander: it’s when a planetary body changes its spin axis, usually in response to large geologic processes. Nitrogen ices have accumulated in Sputnik Planitia to make an ice sheet that’s at least four kilometres thick.

2. There’s probably a vast, liquid, water ocean sloshing beneath Pluto’s surface

New Horizons data indicated there may be a heavier mass beneath the surface that helped reorient Sputnik Planitia, and scientists suspect it is a water ocean. Several other lines of evidence, including tectonic structures seen in New Horizons imagery, support this.

Sputnik Planitia was likely created some four billion years ago by the impact of a Kuiper Belt object 50 to 100 kilometres across that carved out a massive chunk of Pluto’s icy crust and left only a thin, weak layer at the basin’s floor.

A subsurface ocean likely intruded into the basin from below by pushing up against the weakened crust, and later the thick layer of nitrogen ice seen there now was laid on top.

3. Pluto may still be tectonically active because that ocean is still liquid

Enormous faults stretch hundreds of kilometres and cut deeply into the icy crust covering Pluto’s surface. One of the only ways they could have got there, scientists reason, is through the gradual freezing of an ocean beneath its surface.

Water expands as it freezes, and under an icy crust that expansion will push and crack the surface. But if the temperature is low enough and the pressure high enough, water crystals can start to form a more compact crystal configuration and the ice will once again contract.

Models using New Horizons data show Pluto has the conditions for that type of contraction, but it doesn’t have any known geologic features that indicate that contraction has occurred. That suggests the subsurface ocean is still in the process of freezing and potentially creating new faults.

4. Pluto was – and still may be – volcanically active

On Earth, molten lava emerges from underwater fissures through volcanoes. On Pluto, there are indications that a kind of cold, slushy cryolava has poured over the surface at various points. Scientists call it cryovolcanism.

Wright Mons and Piccard Mons, two large mountains to the south of Sputnik Planitia, bear deep central pits that scientists believe are likely to be the mouths of cryovolcanoes unlike any others found in the Solar System.

To the west of Sputnik sits Viking Terra, with its long fractures and grabens that show evidence of once-flowing cryolavas all over the surface, and even further west is the Virgil Fossae region, where ammonia-rich cryolavas seem to have burst to the surface and coated an area of several thousand square kilometers in red-coloured organic molecules.

5. Glaciers cut across Pluto’s surface

Pluto joins the ranks of Earth, Mars and a handful of moons that have actively flowing glaciers.

East of Sputnik Planitia are dozens of (mostly) nitrogen-ice glaciers that course down from pitted highlands into the basin, carving out valleys as they go. Scientists suspect seasonal and “mega-seasonal” cycles of nitrogen ices that sublimate from ice to vapor, waft around the dwarf planet and then freeze back on the surface are the source of the glacier ice.

These are not like glaciers on Earth, however. For one, any melt within them won’t fall toward the bottom of the glacier; it will rise to the top, because liquid nitrogen is less dense than solid nitrogen. As that liquid nitrogen emerges on top of the glacier, it potentially even erupts as jets or geysers.

Also, some of Pluto’s surface is composed of water ice, which is slightly less dense than nitrogen ice. As Pluto’s glaciers carve the surface, some of those water-ice “rocks” will rise up through the glacier and float like icebergs.

6. Pluto has heat convection cells on its giant glacier

There is a network of strange polygonal shapes, each at least 10 kilometres across, churning the icy surface of the giant glacier Sputnik Planitia. They’re evidence of Pluto’s internal heat trying to escape from underneath the glacier, and forming bubbles of upwelling and downwelling nitrogen ice, something like a hot lava lamp.

Warm ice rises up into the centre of the cells while cold ice sinks along their margins. There’s nothing like it in any of Earth’s glaciers, or anywhere else in the Solar System that we’ve explored.

7. Pluto’s “heart” controls its atmosphere and climate

Nitrogen ices in the heart-shaped Tombaugh Regio go through a cycle every day, subliming from ice to vapour in the daytime sunlight and condensing back on the surface during the frigid night. Each round acts like a heartbeat, driving nitrogen winds that circulate around the planet at up to 30 kph.

Sophisticated weather forecast models created using New Horizons data show that as these ices sublime in the northern reaches of Pluto’s icy heart and freeze out in the southern part, they drive brisk winds in a westward direction – curiously opposite to Pluto’s eastward spin.

Those westward winds, bumping up against the rugged topography at the fringes of Pluto’s heart, explain why there are wind streaks on the western edge of Sputnik Planitia, a remarkable finding considering Pluto’s atmosphere is only 1/100,000th that of Earth’s.

8. Pluto has dunes

Hundreds of dunes stretch over at least 75 kilometres of the western edge of Sputnik Planitia, and scientists suspect they formed recently.

Dunes require small particles and sustained, driving winds that can lift and blow the specks of sand or whatever else along. Despite its weak gravity, thin atmosphere, extreme cold and entire surface composition of ices, Pluto apparently had (or still may have) everything needed to make dunes.

Water-ice mountains on the northwest fringes of the Sputnik glacier may provide the particles, and Pluto’s beating nitrogen “heart” provides winds. Instead of quartz, basalt and gypsum sands blown by sometimes gale-force winds on Earth, though, scientists suspect the dunes on Pluto are sand-sized grains of methane ice carried by winds that blow at no more than 30 kph, although given the size of the dunes, the winds may have been stronger and atmosphere much thicker in the past.

9. Pluto and Charon have almost no little craters

Analyses of crater images from New Horizons indicate that few objects less than about 1.5 kilometres in diameter bombarded either Pluto or Charon. Scientists have no reason to believe tectonic activity would have preferentially wiped the surface clean of small craters, so it could mean the Kuiper Belt is mostly devoid of very small objects.

These results give us clues about how the Solar System formed because they tell us about the population of building blocks of larger objects.

10. Charon had a volcanic past

There are two distinct terrain types on the side of Charon that New Horizons imaged in high resolution: an immense, southward-stretching plain officially called Vulcan Planitia and a rugged terrain colloquially called Oz Terra that stretches northward to Charon’s north pole. Both seem to have formed from the freezing and expansion of an ancient ocean beneath Charon’s crust.

Moderate expansion in the north created the rugged, mountains terrain of Oz Terra seen today, whereas the expansion in the south forced its way through vents, cracks and other openings as cryolava, spilling across the surface. In fact, Vulcan Planitia is thought to be a giant cryoflow that covered the entire region early in Charon’s history.

Similar features exist on some icy satellites all around the solar system, including Neptune’s giant moon Triton, Saturn’s moons Tethys, Dione and Enceladus, and Uranus’ moons Miranda and Ariel.

This is an edited version of an article prepared by NASA. You can read the original here.