Seven years ago, NASA's New Horizons spacecraft made history. After a nearly 10-year journey of more than 3 billion miles, an intrepid, piano-sized robot plunged 7,800 miles closer to Pluto. That way, for the first time, scientists are seeing the surface of this distant world in spectacular color detail. The discovery also includes a detailed view of Pluto's largest moon named Charon, towering mountains, giant ice sheets, holes, steep slopes, valleys and terrain not seen anywhere else in the solar system. And that's only the beginning. New Horizons has completely changed scientists' perception of the former planet of the solar system number 9. The notion that Pluto is a ball of inert ice should be pushed out of the way.
“It is clear to me that the solar system is saving the best for last!” said Alan Stern, principal investigator for New Horizons at the Southwest Research Institute, Boulder, Colorado.
Scientists now know, even though it's really cold, Pluto is an interesting, active, and scientifically valuable world. Amazingly, it even holds some of the keys to better understanding the other small planets in our distant solar system. Pluto is now a small planet or what is often called a dwarf planet orbiting the sun in the Kuiper Belt, the region beyond Neptune that is occupied by icy bodies. Here are 10 of the coolest, weirdest and most unexpected discoveries scientists have made about the Pluto system. This is based on data from New Horizons that scientists have studied since 2015.
1. Pluto has a 'heart' that drives activity on the planet
Sometimes we just need to follow our heart, and Pluto seems to take that advice literally. Pluto's heart, one of the distinctive features that New Horizons observes as it approaches and is imaged in high resolution during its flyby, is a million-square-mile nitrogen glacier. The heart's left ventricle, called Sputnik Planitia, literally forces the dwarf planet to reorient itself so that the basin remains facing almost directly opposite Pluto's moon Charon.
"This is a process called true polar exploration, when a planetary body changes its axis of rotation, usually in response to a large geological process," said James Tuttle, planetary scientist and member of the New Horizons team at NASA's Jet Propulsion Laboratory in Pasadena, California.
Sputnik Planitia's position is no accident. This is a cold trap, where nitrogen ice has accumulated to create an ice sheet about 2.5 miles thick. The constant imbalance of that hefty mass, combined with the tidal pull of Charon as it orbits Pluto. This turns the dwarf planet upside down so that the basin aligns more closely with the tidal axis between Pluto and Charon.
"This event is also likely responsible for the cracking of Pluto's surface and creating the many giant faults in its crust that zigzag across much of Pluto," said Tuttle.
The basin is thought to have formed northwest of its current location, and closer to Pluto's north pole. And if ice continues to accumulate in the basin, Pluto will continue to reorient itself.
2. It is suspected that there is a vast ocean and water flowing beneath Pluto's surface
The accumulated ice may not be the only thing helping to redirect Sputnik Planitia. New Horizons data from the basin suggests there may be a heavier mass beneath it at play, and scientists suspect the heavier mass is an ocean of water.
"It was an amazing find," said Tuttle.
"That would make Pluto an elusive 'sea world', just like Europa, Enceladus and Titan."
The three names are Jupiter's moons. Some other evidence, including the tectonic structures seen in New Horizons images, also suggests an ocean beneath Pluto's crust. Sputnik Planitia was probably created about 4 billion years ago by the impact of a 30 to 60 mile long Kuiper Belt object that carved large chunks of Pluto's ice crust and left only a thin, weak layer at the bottom of the basin. The ocean below the surface appears to have infiltrated the basin from below by pushing on the weakened crust, and then the thick nitrogen ice seen there is now laid on top of it. Recent models based on planetary images suggest that this liquid ocean may have emerged from Pluto's rapid and violent formation.
3. Pluto may still be tectonically active because the ocean is still liquid
The massive fault stretches for hundreds of miles and cuts roughly 2.5 miles into the icy crust that covers Pluto's surface. According to scientists, the only way Pluto gets to the gap is by gradual freezing of the ocean beneath its surface. Water expands as it freezes, and under a layer of ice, that expansion will push against and break the surface, like the ice cubes in our freezer. But if the temperature is low enough and the pressure is high enough, the water crystals can begin to form a more compact crystalline configuration and the ice will once again contract. Modeling using New Horizons data shows Pluto has the conditions for that type of contraction, but has no known geological features that suggest a contraction has occurred. For scientists, that means the subsurface ocean is still in the process of freezing and potentially creating new faults on the surface today.
"If Pluto is an active ocean world, then it suggests that the Kuiper Belt may be filled with other ocean worlds among its dwarf planets, dramatically expanding the number of potentially habitable places in our solar system," said Tuttle.
But while Pluto's molten ocean may still exist today, scientists suspect it is isolated in most places by nearly 200 miles of ice. That means it may not have touched the surface today, but in the past it may have flowed through volcanic activity called cryovolcanism.
4. Pluto used to be volcanically active
Maybe not like a volcanic eruption as we know it. On Earth, molten lava spits, drools, bubbles, and erupts from underwater fissures through volcanoes that lie miles high and protrude from the oceans. But on Pluto, there are many indications that some kind of cold, liquid cryolava has flowed to the surface at various points. Scientists call it cryovolcanism or volcanoes that spew ice crystals. Wright Mons and Piccard Mons, two large mountains south of Sputnik Planitia, each have a deep central hole believed to be a cryovolano mouth, unlike those found in the solar system. To the west of Sputnik is Viking Terra, with its long fissures and handles showing evidence of cryolava once flowing across its surface. And further west of Sputnik Planitia is the Virgil Fossae region, where ammonia-rich cryolava appears to have exploded to the surface and coated an area of several thousand square kilometers in red organic molecules.
5. Glaciers cut through Pluto's surface
Pluto joins the ranks of Earth, Mars, and several moons that have active glaciers. To the east of Sputnik Planitia are a number of nitrogen ice glaciers that descend from the plateau pitted into the basin, and pass by carving out the valley. Scientists suspect seasonal and mega-seasonal cycles of nitrogen ice that sublimates into vapor, floats around the dwarf planet and then refreezes on the surface to become a source of glacier ice. But these glaciers are not like our water-ice glaciers on Earth. First, any melt in it will not fall to the bottom of the glacier. It will rise to the top because liquid nitrogen is less dense than solid nitrogen. When liquid nitrogen appears over a glacier, it has the potential to even erupt like a jet or geyser. In addition, there is the fact that some of Pluto's surface is made up of water ice, which is slightly denser than nitrogen ice. As Pluto's glaciers carve the surface, some of that water-ice rock will rise through the glacier and float like icebergs. Such an iceberg is seen in several images of Sputnik Planitia by New Horizons.
6. Pluto has hot convection cells on the giant glacier Sputnik
If you zoom in on the surface image of Sputnik Planitia, you'll see something like no other in the solar system. Among these, a network of oddly polygonal shapes in the ice, each at least 6,214 miles in diameter rotating on the glacier's surface. Although they resemble cells under a microscope, they are not. They are evidence of Pluto's internal heat trying to escape beneath the glacier, and forming bubbles of nitrogen ice that rise and fall. Warm ice rises to the center of the cell while cold ice sinks along the edges. There is nothing like it on any Earth glacier, and or anywhere else in the solar system that NASA has explored!
7. Pluto has a beating 'heart' that controls its atmosphere and climate
It's cold and distant Pluto, but his icy heart still beats every day. It rhythmically drives Pluto's atmosphere and climate just as Greenland and Antarctica help control Earth's climate. The nitrogen ice in the heart-shaped Tombaugh Region of Pluto cycles every day. It sublimates from ice to vapor in the midday sun and condenses again on the surface during cold nights. Each spin acts like a heartbeat, propelling a nitrogen wind that circulates around the planet.
"Pluto's heart actually controls the circulation of its atmosphere," said Tanguy Bertrand, a planetary scientist at NASA's Ames Research Center in Mountain View, California.
Bertrand's sophisticated forecast model using New Horizons data shows ice is sublime in the northern part of Pluto's heart and frozen in the south. This ice then pushes strong winds westward, oddly opposite to Pluto's eastward spin. These westerly winds, hitting the rugged topography on the periphery of Pluto's heart, explain why there are streaks of wind on the western edge of Sputnik Planitia.
"This is remarkable considering that Pluto's atmosphere is only 1/100,000 Earth's atmosphere," Bertrand said.
8. Pluto has sand dunes
This is not the Sahara Desert or the Gobi Desert. This is Pluto. Hundreds of dunes stretch for at least 45 miles from the western edge of Sputnik Planitia. Scientists suspect they formed recently. I mean in astronomical time. Dunes need small, continuous particles, driving winds that can lift and blow the grains. And despite its weak gravity, thin atmosphere, extreme cold, and entire composition of surface ice, Pluto appears to have all the elements needed to form sand dunes. The water-ice mountains on the northwestern rim of the Sputnik glacier can provide the particles, and Pluto's beating nitrogen heart provides the wind. But instead of the quartz, basalt, and gypsum sand that high winds blow on Earth, scientists suspect the dunes on Pluto are sand-sized grains of methane ice.
9. Pluto and Charon have almost no small craters
Finding craters on planetary surfaces is common in outer space. But if there's one thing that's abnormal about the Pluto system, it's that neither Pluto nor the moon Charon has many small craters. Everything is great.
"It surprised us because there were fewer small craters than we expected, which meant there were also fewer small Kuiper Belt objects than we expected," said Kelsi Singer, New Horizons deputy project scientist and investigator with the Southwest Research Institute in Boulder, Colorado.
Analysis of crater images from New Horizons shows several objects less than a mile in diameter bombarding Pluto and Charon. This 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 populations of larger objects, such as Pluto and possibly even Earth," he added.
10. Charon has a volcanic past
New Horizons also captured stunning images of the moon Charon and revealed some surprising geological phenomena there. On the side that New Horizons imaged in high resolution, Charon has two distinct types of terrain, namely a wide plain stretching southward officially called Vulcan Planitia which is at least the size of California and a rugged terrain commonly called Oz Terra that extends to Charon's north pole. Both appear to have formed from the freezing and expansion of an ancient ocean beneath Charon's crust.
"New Horizons transformed Pluto from a telescopic point blur, into a living world of astonishing diversity and surprising complexity," said Hal Weaver, New Horizons project scientist at the Johns Hopkins Applied Physics Laboratory in Laurel, Maryland.
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