It's a strange world out there on the outer edge of our solar system, in the middle of the Kuiper Belt, rich in ancient and icy rocks.

Orbiting at least thirty times farther from the Sun than Earth is the dwarf planet Pluto, along with its moon Charon, which is a good half its size, and four other moons.

With a diameter of just 2370 kilometers, Pluto is almost a fifth of Earth's size.

Icy temperatures of between minus 240 and minus 210 degrees Celsius prevail on its surface.

Sibylle Anderl

Editor in the feuilleton, responsible for the "Nature and Science" department.

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Not much was known about this surface of the distant celestial body since its discovery in 1930.

Even seen with the Hubble Space Telescope, Pluto remained a distant, fuzzy speck.

That changed dramatically in 2015, when the NASA probe "New Horizons" flew past Pluto at a distance of 12,500 kilometers and delivered surface images of astonishing quality: It resolved details on length scales of less than 100 meters and showed a surprisingly varied range Geology.

In comparison: the Hubble Space Telescope had previously only delivered a resolution of around 500 kilometers.

At the time, Pluto was best known to the public for a bright region shaped like a heart, the Tombaugh region, named after the discoverer of Pluto.

The data recorded at that time revolutionized our understanding of the ex-planet degraded in 2006.

Their scientific analysis continues today and keeps raising new questions.

In "Nature Communications", a US-French group of scientists has now investigated an area southwest of the ice-filled Sputnik plain, where a ridge several kilometers high with hilly flanks extends.

There are few impact craters in this region, suggesting that the surface is relatively young.

Their age is estimated at a maximum of one to two billion years.

By analyzing images from various instruments on board New Horizons, the researchers attempted to identify what process might have shaped this landscape in this way.

Accordingly, the elevations must consist of water ice with its relatively high melting temperature, since only this is stable enough under the conditions prevailing on Pluto not to melt over time - unlike the nitrogen ice that also frequently occurs on Pluto, whose melting point is minus 210 degrees, or the equally widespread frozen methane with a melting point of minus 183 degrees.

According to the observations, the latter seems to form at most a thin layer on the hills, which has probably settled there from the atmosphere.

Patches of nitrogen ice are also visible.

The scientists interpret all of this as evidence of cryovolcanism, i.e. eruptions from several ice volcanoes that, despite the freezing temperatures, have ejected mobile material from below the surface.

The flows of freezing lavas from different ice volcanoes merged in some areas, forming the complex structure of the region.

The mounds on the flanks of the domes would then be due to muddy or solid material.

Where does the tiny creature get its heat from?

Even before the launch of New Horizons, it was suspected that there could be such ice volcanoes on Pluto.

Evidence of active cryovolcanism has also been found on other cold bodies in the outer reaches of the solar system, such as on Saturn's moon Enceladus or Neptune's moon Triton.

However, the cryovolcanism on Pluto is very different from that on the satellites of these gas planets, the scientists write.

This is shown by the fact that it is unclear which heat source was able to drive the ice volcanic activity at Pluto.

Inside those moons of Saturn or Neptune, heat is generated by gravitational interaction with their home planets.

In the case of Pluto, on the other hand, this heat source is out of the question, since it moves in equilibrium with its main moon Charon in such a way that tidal friction does not occur.

Even the heat generated by radioactive elements in its interior should hardly be enough for volcanism.

The researchers therefore speculate that there may have been heat storage inside Pluto, the energy of which was only released later.

The problem of the mysterious heat source in Pluto's interior is not new.

This had already been discovered when attempting to understand the structure of the Sputnik plane.

In this region too, the small number of impact craters suggests that the icy surface here cannot be very old.

It is also divided into irregular polygons, which are interpreted as convection cells: like in a pot of boiling water, clumps of material rise to the top, cool down and then sink again - only at much lower temperatures and much more slowly in viscous nitrogen ice.

Theoretical models can explain the appearance of the Sputnik plane well in this way.

But here, too, the question arose as to how the ice is warmed from below.