The tension was almost unbearable. At nine o’clock in the morning on July 14, 2015, the leader of the New Horizons mission, Alan Stern, was waiting in the command center at the Johns Hopkins University in Maryland in front of 2000 invited guests.
After a nine-year journey, the spacecraft was now scheduled to make a precision flight right past Pluto, five billion kilometers from Earth. Since the spacecraft was traveling at 52,000 km/h, a collision with even just one speck of dust could destroy two decades of work in a split second.
Suddenly the first invitations from the spacecraft appeared. A few seconds later, powerful computers had begun to decode the invitations, and one by one the watchmen announced at large screens that the scientific instruments on board were working as expected. All seven in number.
Then the cheers broke out.
But if Stern and the other scientists in the room had learned right then and there about all the knowledge that New Horizon would eventually return after its 24-hour trip past Pluto, the cheers could have possibly blown the roof off the house.
Over the next 16 months, 50 gigabytes of information gradually arrived at the control center. The speed was almost unbearably low, only one kilobit per second. The reason was, on the one hand, the distance and, on the other hand, limited electricity on board.
Now we see the back side
The very first close-ups of Pluto’s face facing the spacecraft revolutionized scientists’ knowledge of this distant dwarf planet due to its high resolution, which showed everything down to 75 meters in size.
Before the days of New Horizons, people knew little else than that Pluto had a slightly reddish surface and ice caps in the polar regions. The images from the spacecraft revealed mountain ranges, ice craters and then the huge comet crater Sputnik Planitia where you can find glaciers of frozen nitrogen.
Now the scientists have finally managed to process images from the back side of Pluto , which were taken by the spacecraft from a longer distance in the days before the passage by Pluto. The resolution is lower, but in the images, everything up to two kilometers in size can still be distinguished, but it is 250 times higher resolution than in images from Hubble.
For the first time, it is now possible to view Pluto as a whole, and the results are, to say the least, quite interesting. Pluto is not the spiky lump of ice that scientists thought, but a globe with geologic activity. Underneath a thick sheet of ice lies an inner ocean that could potentially harbor life.
Smaller than the moon
Until the last century, people had no idea that Pluto existed, but at the turn of the century, the American astronomer Percival Lowell came to the conclusion that an unknown planet far out in the solar system disturbed the orbits of Uranus and Neptune around the sun.
Three decades later, a young astronomer, Clyde Tombaugh, noticed a small spot that in a few days moved closer to the constellation Gemini. He had discovered this unknown planet, which was named Pluto after Pluton, the Roman god of death.
At the beginning, people thought that the mass of Pluto was similar to the mass of the Earth, and it was not until 1978, when the American astronomer James Christy discovered the moon Karon, that it became possible to calculate the mass of Pluto, which turned out to be only 0.2% of the mass of the Earth.
The exact diameter is 2,376 km and was not definitively determined until New Horizons measurements.
Pluto is said to be smaller than the Moon, and due to both its small size and its immense distance, this dwarf planet was largely unexplored before New Horizons flew by at a distance of 12,500 km.
Six km high icebergs
The scientists feared that the first close-up images of Pluto would not show another desert rock with countless meteorite craters from the solar system’s infancy. Instead, the images showed a globe where geologic activity flattens the surface and has created irregular mountain ranges of ice, along with miles-high, razor-sharp spikes of frozen methane.
The surface temperature on Pluto is -233 °C and from this extreme cold it follows that the water ice that forms the bedrock, is unlike hard granite and is able to form mountains more than 6 km high. In many places, the surface is covered with softer ice made of nitrogen, which is also the most abundant substance in the atmosphere.
Pluto has fissures deeper than the American Grand Canyon, mountains taller than Mount Everest, and hydrothermal vents that may have erupted liquid water just a few million years ago.
The images also showed clear signs of two violent collisions that have created and shaped the dwarf planet. The first occurred more than four billion years ago when Pluto and its moon Charon were formed by the collision of two ice globes.
A collision created Pluto and the moon
More than 4 billion years ago, two ice globes collided to form Pluto and its large moon Charon. The four small moons were then formed from a large cloud of rock, ice and gas that swirled up during the collision. The heat caused by the collision melted all the ice on Pluto and it became a huge ocean.
1. Comets contributed material to Pluto and Charon
Two ice balls collided to form Pluto and its largest moon, Charon. The two older ice balls were probably formed from a total of over a billion comets. Namely, Pluto’s vaporsphere and the ice plain of Sputnik Planitia are largely made of nitrogen, which matches the content of comets.
2. The asteroids were formed after the collision
When the ice globes collided, a large cloud of rock, ice and gas swirled up, which then gathered into the four small moons: Styx, Nix, Kerberos and Hydra. Kerberos is a very dark moon and may be the remnant of the planet that collided with Pluto. In the foreground is Karon.
3. The entire surface of Pluto was covered by a large ocean
The collision heated Pluto so much that the ice melted and covered the globe with a vast ocean. When the sea level froze, cracks formed in the ice (arrows), as the ice expanded. The dark crater, Sputnik Planitia, was formed when a comet crashed into it much later.
According to the generally accepted theory, the dwarf planet was covered in ice from the beginning, but heat from the decay of radioactive materials in the rock core melted the ice from the inside so that an ocean formed under the ice.
If this had happened like this, it could be assumed that the ice cover on the surface would have shrunk and formed wrinkles like on an old, crusted apple.
Over time, the disintegration of radioactive materials would have decreased and then the ice sheet would have thickened again and now formed large cracks.
The scientists therefore assumed that the surface of Pluto was covered with old wrinkles and more recent cracks, but the cameras of New Horizons only detected cracks, and that is why a new theory has now gained wind in its sails.
For 24 hours, New Horizon was passing by Pluto. The closest was 12,500 km.
When the progenitors of Pluto and Charon collided, the collision created so much heat that the dwarf planet was initially covered by a deep ocean, but its surface soon froze into ice that expanded and formed cracks.
What seems to particularly support this idea is a huge crack that runs all the way around and extends between the poles on both the front and back sides. The crack is so old that it seems obvious that Pluto started life with a liquid ocean that immediately began to freeze. If this theory turns out to be correct, it is possible that life could be hidden in this ancient ocean.
Organic matter turns the water red
The images of Pluto’s front surface clearly show traces of reddish water that has probably erupted from an ocean beneath the ice sheet and then frozen on the surface.
The red color indicates that there was a significant amount of organic matter in the water. Laboratory experiments suggest that the solar wind—charged particles emitted from the Sun’s vaporosphere—and cosmic rays, which are radiation from the outer universe, may have transformed simple substances into more complex, organic molecules.
Multiple signs of activity on Pluto
Despite its location on the icy outskirts of the solar system, images from New Horizons show considerable Earth activity on this planet. In addition, there are traces of organic materials that could be the building blocks of living things, signs of glacial eruptions and other relatively recent geologic activity. Finally, there is a vast vaporsphere.
Click on an image to see it larger with description
Now, astronomer Dale Cruikshank at NASA’s Ames Research Center in California has demonstrated the presence of ammonia in this reddish ice. It opens up the possibility that the building blocks of the nucleic acids RNA and DNA could have formed in the red soup of this ocean.
Cruikshank doesn’t think his discovery necessarily means life formed in Pluto’s ancient ocean, but if it did, the best microbes could have lived there.
It strengthens the theory that a red belt of organic matter has now also been found on the back side of Pluto. The belt is at the equator, where it receives the most sunlight and temperatures are higher than anywhere else on Pluto.
“Three conditions must be met for life to arise. There must be liquid water, organic matter and an energy source. On Pluto, we can now check the two previous conditions and that is a big step,” says Alan Stern, who leads the New Horizons mission.
In the beginning, the collision that created Pluto and Charon generated heat energy inside the dwarf planet, but more than four billion years have passed since then. If life did start in the early ocean, it is unclear whether heat from radioactive decay in Pluto’s rocky core releases enough energy to support life in the ocean today.
Therefore, the chances of finding life far out in the solar system are probably higher in the oceans of Jupiter’s moon Europa and Saturn’s moon Enceladus. There, the tidal effects from the large gas giants continuously pump energy into the moons.
A blow that changed Pluto
The other major collision that changed Pluto occurred less than two billion years ago, when a comet about 400 meters across collided with the dwarf planet at a speed of more than 7,000 km.
The impact left behind a four km deep crater, Sputnik Planitia covering 797,000 square kilometers, an area larger than France.
This crater is part of a huge, heart-shaped ice plain north of the equator that was clearly visible in the first images from Pluto.
Now, new images from New Horizons show that on the other side of Pluto is a chaotic landscape that appears to have been broken up by seismic waves from the comet’s impact.
“Three conditions must be met for life to arise. On Pluto, we can now check both conditions.”
Alan Stern, commander of the New Horizons mission
Similar phenomena are known on Mars and Europa, Jupiter’s moon, but especially on Mercury, where a chaotic landscape that has no parallel on the planet can be found directly opposite the huge Caloris crater.
Experts believe the extensive destruction on the back side of Pluto is only possible because there is a deep ocean of liquid water under the ice. The reason is that earthquake waves are extremely weak depending on the material through which they travel.
The pressure waves travel more slowly through water than rock cores, but the destructive power is greatest if they manage to travel at the same speed through the entire globe.
Runs in the computer model of the astronomer Adene Denton at Purdue University in the USA indicate that the core of Pluto is mainly made of serpentine, but earthquake waves travel more slowly through that type of rock than others.
The interaction between the motion of the seismic waves in the water and serpentine in Pluto’s bowels managed to carry so much force through the globe that the surface of the back side was torn apart.
Comet tracks reveal an ocean beneath the ice
Less than ten million years ago, a comet hit Pluto and left behind a huge crater. New images from New Horizons show a fragmented landscape on the other side of Pluto, and it shows that the seismic waves were transmitted through water.
1. Collision sent shock waves across Pluto
A comet, about 400 km in diameter, hit Pluto and formed a 4 km deep crater, Sputnik Planitia. The crater is 797,000 square kilometers and on the bottom is hard frozen water ice, as opposed to hard granite. Shock waves from the collision traveled right through Pluto.
2. The impact shattered the ice on the back
New images of Pluto’s backside show a chaotic and fragmented landscape in front of the crater on the front side. There, the surface has disintegrated due to the impact of pressure waves from the collision. It is therefore only possible that the wave has traveled through an ocean between the rock core and the ice crust.
3. Pressure waves provide insight under the ice
Based on New Horizons observations, astronomers have calculated the structure of Pluto. On the outside of the dwarf planet, there is an ice crust at least 200 km thick, and under it a 150 km deep sea of water, where organisms could be hiding. The core is made of silicates, mainly serpentine.
Based on New Horizons measurements of the diameter and mass of the dwarf planet, scientists have calculated the internal structure of Pluto and concluded that the rocky core is about 70% of the mass, while the rest is mostly water.
Surrounding the core is a 150 km deep ocean, which in turn is covered by an ice sheet more than 200 km thick, which forms a kind of crust or bedrock.
After the comet formed this crater, it has been filled with soft and heavy nitrogen ice from the vaporosphere and from glaciers that flow down from the surrounding mountain slopes.
After the comet collision, Pluto’s mass ratios were disrupted. The weight of the nitrogen ice in the crater turned the globe sideways so that the tidal axis – where the weights of Pluto and Charon are most influential – now runs straight through this heavy ice in the Sputnik Planitia crater.
Mysterious icicles rise high
The images from New Horizons have completely changed our knowledge of Pluto, but also raised a number of questions that scientists are now trying to answer.
When the first images were analyzed, people discovered ice fields with kilometer-high, jagged spikes on the eastern front.
However, these spikes were just small, odd blips on the map until scientists were able to look at images from Pluto’s back side, where these giant spikes form a belt on the equatorial plateau all the way to the western front.
This discovery led project director Alan Stern to call the spikes the greatest mystery on Pluto, stretching more than a kilometer into the air.
If SUVs or other man-made vehicles are ever sent to Pluto, trying to get around such high and sometimes steep mountain ridges will be an absolute nightmare.
Video: Fly over Pluto with the New Horizons probe
New Horizon’s measurements show that the spikes are made of methane ice that comes home and merges with Pluto’s vapor atmosphere.
The lower part of the vaporsphere is mainly made of nitrogen, which has, among other things, formed the nitrogen ice in Sputnik Planitia. Above, there is a lot of methane that has probably formed these giant spikes in the highlands. However, it is a very difficult mystery how it has happened.
The spikes may be the remains of a thick layer of methane ice on the highlands, which the sunlight would have absorbed so that the methane would have gradually evaporated. But then the spikes may have formed similar to stalactites here, when the methane in the vaporosphere has frozen. It is certain that the spikes have taken millions of years to form, and possible climate changes in Pluto’s long history may have played a role.
A satellite in orbit
Despite this tidal wave of information about Pluto, research on the dwarf planet is just beginning.
In the coming years, scientists will continue to squeeze more information from the data received from New Horizons.
The new James Webb space telescope, which was launched in December 2021, will be able to be used for long-term observation of Pluto, although the resolution will not be as good as in images from New Horizons. That information can be compared with observations of other dwarf planets, such as Ceres in the meteorite belt between Mars and Jupiter and Eris, Haumea and Makemake in the Kuiper belt outside Pluto.
But the scientists have found out and want to know more about the king of the ice dwarves.
Scientists are now dreaming of support from NASA to fund a satellite that would orbit Pluto and deliver high-definition images of the entire globe and even more precise measurements of materials on the surface and in the atmosphere. Such a satellite could be in operation for many years and, among other things, show changes in the surface and in the weather for a long time.
But if such a satellite becomes a reality, it will not be launched until the 1940s, and it can be assumed that it will be 15 years on the way to this remarkable active dwarf planet in the frost at the far end of the solar system, where, despite everything, life could possibly be hidden.
