Even now, eight years on, there are countless images captured by New Horizon, moving at 52,000 kilometres per hour, that scientists have yet to fully examine. These include photographs of Pluto's dark hemisphere, taken before the probe's closest approach, which offer great potential for ongoing study.
While the resolution of these images of Pluto's "shadowed side" is not as high as those taken of the sunlit hemisphere during the closest pass, they still offer a detailed view of the terrain, with clarity ranging from 2 to 30 km. This represents a 250-fold enhancement compared to the Hubble Space Telescope's images.
For the foreseeable future, perhaps the next 30 to 40 years, these are the most detailed resources available to astrophysicists. That's unless a new mission is launched towards Pluto, which at its nearest point is 29 astronomical units from Earth.
Pluto, discovered by Clyde Tombaugh in 1930 and viewed as the ninth planet in our solar system for 76 years, had its planetary status challenged from 1992. The final blow came with the discovery of Eris, another dwarf planet found in the scattered disk, a region abundant with icy planetoids.
The arrival of New Horizon marked a turning point in our understanding of Pluto. Observations of the heart-like formation, specifically Sputnik Planitia, a frozen basin filled with massive, swirling glaciers, revealed the profound influence of these movements on Pluto.
As the Sun warms this icy expanse, vapor plumes rise into the atmosphere before falling back to the ground as the day ends. This process is believed to contribute to Pluto's axial tilt.
Initial images of Sputnik Planitia showed the ice plain's near-perfect alignment with Charon, one of Pluto's moons. The chances of such alignment occurring randomly are slim, at around 5 percent. Current theories suggest an underground ocean may have once existed beneath this depression.
As this frigid basin froze, it's likely that gaseous nitrogen from Pluto's atmosphere followed suit. The resulting weight of the water and ice may have resulted in Pluto's current tilt.
The idea of the presence of an underground ocean was further strengthened after analyses of images of Pluto's "dark side," the chaotic ground morphology where cracks, ridges and flat areas alternate is typical of other planets in the Solar System, such as Mars, Mercury and Europa, one of Jupiter's moons.
The low resolution of the images of the planet's far side, however, does not allow all doubts about this interpretation to be dispelled, and the enigma can only be clarified if we decide to visit this icy world again, perhaps with an ad hoc mission.
A giant rift running from the North Pole back toward the South Pole on the far side of Pluto suggests that it is a scar due to the freezing and expansion of a liquid ocean, which would have begun freezing almost immediately after its formation. The possible presence of an underground ocean has reopened the debate about the possible presence of alien life even on a planet as extreme as Pluto.
The presence of ammonia, also found by images on the hidden side of the dwarf planet, detected as a long red spot running around the equator, the area with the highest insolation and the most "temperate" climate on Pluto, reinforces the possible presence of organic molecules.
In short, current analyses show that two of the conditions for the development of life may be present on Pluto: water and organic molecules. The third is currently missing: energy.
Analysis of photos taken by New Horizon on the dark side of Pluto, however, have uncovered other mysteries. One of them is the terrain dotted with sharp ridges of ice, separated from each other by a few kilometers, about a kilometer high and up to thirty kilometers long. Well on the far side the area occupied by this steep terrain of sharp ridges is about 3.5 times larger than the near side.
Spectral data revealed that these ice blades are made of methane ice and form a kind of belt around the equator. The genesis of these formations is still shrouded in mystery. Whatever theories there are about their formation (there are several) all have to deal with the meteorology of the planet.
According to a climate model published some time ago, methane accumulates at high altitudes, while nitrogen accumulates in the lower atmosphere, and this would explain why the basin of Sputnik Planitia is rich in frozen nitrogen while the land occupied by the blades is pervaded by frozen methane.
Pluto's atmosphere is warmer than the surface and therefore downwinds dominate, meaning that nitrogen cannot move from the lower areas and high-altitude methane condenses on the higher mountains before reaching the surface.
According to a study, a collaboration of scientists from several countries, published in 2019, the evolution of the atmosphere over the time span from 1988 to 2016 suggests that Pluto's atmosphere is expected to collapse to the surface and freeze completely by 2030.
Ultimately, the data collected by the New Horizon probe have opened up a rich harvest of studies and analyses, but these cannot be satisfactorily completed without a mission specifically dedicated to this extraordinary celestial object.
Nasa has commissioned a team of scientists to conduct a feasibility study for an orbital probe that could map the entire planet. Assuming this preliminary study translates into an actual mission, the launch will not occur until 2030-2040, then an additional 15 years of travel will be required before this probe enters orbit around Pluto.
Looking forward to 2055, however, the material currently already available will allow scientists a large harvest of work that will occupy much of this long time frame.
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