Comet strike reveals an ocean under the surface
Less than 10 million years ago, a comet collided with Pluto, forming a huge impact crater. New pictures from the New Horizons probe demonstrate a torn landscape on the far side of Pluto, revealing that the pressure waves passed through an ocean.
Pluto has canyons deeper than the Grand Canyon, mountains higher than Everest, and ice volcanoes that may have erupted and emitted liquid water during the most recent millions of years. The New Horizons images also included clear evidence of two powerful collisions that created and shaped the dwarf planet. The first of these happened more than four billion years ago, when Pluto and its big moon of Charon formed in a collision between two ice worlds.
According to existing theories, the dwarf planet was initially covered in ice, but heat from radioactive decay in its rocky core melted the ice from within, forming an interior ocean under the ice cover. If that was what happened, the ice cover on the surface would contract, causing wrinkles, like on the skin of an old apple. Over time the heat from the core would ease with ever reducing radioactive decay. The ice crust would once again become thicker, and in its expansion would produce large cracks. So scientists expected that Pluto’s surface would be covered in old wrinkles and more recent cracks.
But New Horizons’ cameras spotted only cracks, so a new theory has emerged. When the initial versions of Pluto and Charon collided, so much heat was emitted that the dwarf planet became covered by a deep ocean. The surface then quickly froze into ice, which expanded and cracked.
The theory is supported especially by a huge crack running all the way from pole to pole on both sides of Pluto. The crack is so ancient that it now seems clear that Pluto was formed with a liquid ocean that almost immediately began to freeze. If correct, then the ancient ocean may have included life.
Organic matter makes water red
The images of Pluto’s front side show clear evidence of reddish water that probably gushed from an ocean under the ice and subsequently froze into ice on the surface.
The red colour indicates that the water included considerable quantities of organic matter. And we know from lab experiments that charged particles emitted from the Sun’s atmosphere – the solar wind – and cosmic radiation from the outside universe could have been enough to convert simple matter into complex organic molecules.
Now astronomer Dale Cruikshank from NASA’s Ames Research Center in California has proved the existence of ammonia in the reddish ice, meaning that the genetic building blocks of RNA and DNA could have formed in the ocean’s red soup.
This discovery does not necessarily mean that life originated in Pluto’s ancient ocean, emphasises Cruikshank. But if the miracle did happen, then microorganisms could have taken hold and survived.
The theory is supported by the fact that a red belt of organic matter has also been found on the dwarf planet’s far side. The belt spans the equator, which receives the most sunlight, and where higher temperatures are experienced than on the rest of Pluto.
”Three conditions must be met for life to originate: liquid water, organic matter, and an energy source. We have now ticked off the first two for Pluto,” notes Alan Stern, head of the New Horizons mission.
The initial collision between proto-Pluto and Charon pumped in thermal energy, but that was more than four billion years ago. If life originated in that ancient ocean, it’s not clear whether ongoing heat from radioactive decay in Pluto’s rocky core would generate sufficient energy to maintain it. The chances of finding living micro-organisms are likely higher in the interior oceans of Jupiter’s moon Europa or Saturn’s Enceladus, where tidal powers from the huge gas planets are constantly pumping energy into the moons.
A punch in the heart changed Pluto
The other major collision that shaped Pluto occurred less than 10 million years ago, when a 400km-wide comet collided with the dwarf planet at a speed over 7000km/h. The impact caused the crater of Sputnik Planitia, which is 4km deep and covers an area comparable to all of New South Wales. The crater forms part of the vast heart-shaped ice plain north of the equator, as revealed in New Horizons’ first images.
The new photos of the far side of Pluto show a chaotic landscape torn by seismic waves from the comet strike. Similar phenomena are known from Mars and from Europa, and especially from Mercury, where similarly chaotic terrain exists on the far side of the huge Caloris Basin impact crater.
According to researchers, this damage on Pluto’s far side would be possible only if there is a deep ocean of liquid water under the ice crust. Seismic waves have different properties depending on the material through which they propagate. Pressure waves move more slowly through water than through a rocky core, but their destructive force is larger if they are able to travel at a constant speed across the whole world.
Simulations carried out by astronomer Adene Denton from Purdue University in the US indicate that Pluto’s core consists primarily of the rock serpentinite, through which seismic waves travel more slowly than through other types of rock.The combination of this special rock and water in Pluto’s interior would allow the pressure waves to send enough energy through the dwarf planet to rip up the surface on the far side.
Based on New Horizon’s measurements of the dwarf planet’s diameter and mass, scientists have simulated Pluto’s interior
ALAN STERN, HEAD OF THE NEW HORIZONS MISSION “Three conditions must be met for life to originate. We have now ticked off the first two for Pluto.”
structure and calculated its mass. The rocky core makes up 70% of the mass, while the rest largely consists of water. The core is surrounded by a 150km-deep ocean, which is covered by a crust of bedrock water ice more than another 200km thick.
After the comet strike, the impact crater filled with soft, heavy nitrogen ice from the atmosphere, and from glaciers that flow into the hole from the surrounding mountain ranges.
This heavy ice lump would have changed Pluto’s mass distribution. It is thought that the weight from the nitrogen ice in the crater gradually made the dwarf planet tilt, so the tidal axis, where Pluto and its big moon of Charon are pulling most strongly at each other, now passes through the heavy ice in the Sputnik Planitia crater.
Enigmatic ice spikes
New Horizons’ photos have revolutionised our knowledge of Pluto, but they have also raised new questions. When the first images of the dwarf planet were analysed, scientists discovered ’reefs’ on the eastern part of the planet’s front, with sharply pointed spikes a kilometre high. From those early images the spikes were just an oddity on the edge of the map. But now the scientists can see Pluto’s far side, they realise that the huge spikes form a belt through the elevated areas of the equator all the way back round to the western part of the front of Pluto.
This discovery has led to mission leader Alan Stern characterising the sharp spikes, which are three times taller than the Empire State Building, as Pluto’s biggest mystery. And if rovers or humans were ever to land on the remote dwarf planet, it would be a nightmare trying to cross such a landscape of huge pointed reef spikes.
New Horizons’ data shows that the spikes consist of methane ice, which is also connected with Pluto’s atmosphere. While the lower part of the atmosphere consists primarily of the nitrogen which filled the deep impact crater of Sputnik Planitia with nitrogen ice, there are major quantities of methane higher up, which probably contributed to the huge spikes in the highlands. But how they formed remains a mystery.
Perhaps the spikes are the remains of a thick layer of methane ice that once covered the high plateaus but which has slowly been eroded by the sunlight. Or perhaps the methane was frozen out of the atmosphere in the same way that water vapour in the air can freeze to ice on Earth’s surface. Only one thing is for sure: the spikes took millions of years to form, and climate change might have played a role.
More Plutonic revelations to come
In spite of the new revelations, the exploration of this remote dwarf planet has only just begun. Scientists will undoubtedly continue to make new discoveries based on the data that New Horizons has already sent back to Earth. And later this year, NASA’s new large James Webb space telescope will be launched, and will closely study Pluto. Although
the James Webb telescope will have a lower resolution than the close fly-by of New Horizons, it will allow direct comparisons with observations of other dwarf planets in the Kuiper Belt such as Ceres, Eris, Haumea and Makemake.
And planet researchers would like to know still more. Some scientists are hoping for a NASA-supported satellite that could orbit Pluto and produce even higher-resolution images of the entire planet, in combination with more accurate measurements of the substances on the surface and in the atmosphere. And by making observations over several years, such a satellite would be able to identify changes, if any, in Pluto’s atmosphere and weather over time.
If the satellite is approved and built, it would launched in the 2030s at the earliest. Subsequently, there would be a 15-year space mission to this surprisingly active dwarf planet on the freezing outskirts of the Solar System, where life might just exist – against all previous expectations.