Unique phenomenon: Analysis of data from NASA’s New Horizons spacecraft suggests a never-before-seen form of ice volcanism may be occurring on the dwarf planet Pluto. Slowly escaping, viscous water ice piled up mountains up to seven kilometers high over time, creating ice volcanoes that have more volume than the largest fiery mountains on earth. The young age of some of these formations also suggests that Pluto may still be warm on the inside today.
The dwarf planet Pluto is a surprisingly dynamic world despite its distance from the sun and freezing cold, as images from the NASA New Horizons spacecraft have revealed. On it there are flowing glaciers, shaped by overturning currents ice floes and even winch, powered by outgassing of frozen nitrogen from the great ice sheet Sputnik Planitia. Surface stretch marks and planetary models also suggest that Pluto’s interior is still warm enough for one subglacial ocean or at least could be softened layers of ice.
Riddles about icy mountain giants
But one question has remained unanswered: are there ice volcanoes on Pluto? This question is prompted by two mountains with large craters in their peaks, located southwest of the Sputnik Planitia ice plain. With the Wright Mons, which is almost five kilometers high and around 150 kilometers wide at the base, this summit depression even has a diameter of 56 kilometers. As early as 2015, the shape of these mountains awoke the suspicionthat these could be ice volcanoes – vents that bring half-thawed ice to the surface.
Kelsi Singer of the Southwest Research Institute in Boulder and her colleagues have now looked again for evidence of such cryovolcanism on Pluto in the data and images from the New Horizons probe. To do this, they primarily analyzed images in which the potential ice volcanoes were illuminated obliquely by weak sunlight, so that surface structures clearly stand out due to the shadows cast.
Unlike known volcanic forms
The result: At least the volcanoes and ice volcanoes in the classic sense do not resemble the Pluto mountains, as the team reports. The peak depressions of Wright Mons and Piccard Mons, which is even higher at seven kilometers, differ significantly from calderas or volcanic vents on Earth and Mars. Among other things, the openings are too large and irregularly shaped and do not match the craters that subsequently collapsed, according to Singer and her colleagues.
“In addition, there is no obvious evidence of a directed flow or the location of exit centers,” the researchers report. Traces of escaping material or ejected sputum are completely absent. Also strange: The entire terrain of this region, including the mountain flanks and the inside of the crater depressions, is covered by lumpy elevations, a few hundred meters high and up to 20 kilometers wide – this is also rather unusual for volcanoes and ice volcanoes.
Terrain is still relatively young
“The geological features of the Wright-Mons region thus differ from all other regions on Pluto and also from the terrains on most other celestial bodies in the solar system,” say Singer and her team. The almost complete absence of impact craters in this area also suggests that the landscape here is relatively young – it must have been reshaped long after the formation of the dwarf planet.
This late formation is supported by spectral data. Thus, the base material of these mountains consists of rock-hard frozen water ice covered by a thin covering of organic deposits and more volatile types of ice such as nitrogen ice and frozen methane. According to the researchers, differences in the thickness and composition of these layers suggest that different areas of this terrain did not form together, but are of different ages.
Novel form of cryovolcanism
But how does all of this fit together? According to the research team, Wright and Piccard Mons and the terrain between them may have been formed by a novel form of cryovolcanism. In the process, tough but still mobile water ice swelled upwards from several crevasses in the subsoil and formed dome-like elevations there. Over time, these individual domes merged into the hilly landscape seen today.
“This scenario provides a consistent explanatory mechanism for all major peaks and troughs – both dome-shaped or ring-shaped and more complex,” write Singer and her colleagues. The two high mountains were formed because there were several ice outcrops close together and constantly pushing new masses of water ice upwards from below. The result was huge elevations like the Wright Mons and the even larger Piccard Mons.
“The volume of Wright Mons alone is 24,000 cubic kilometers – that’s similar to the volume of Mauna Loa in Hawaii,” the researchers explain.
Indication of internal heat?
If this scenario is confirmed, then there could be a previously unique form of ice volcanism on the dwarf planet Pluto. At the same time, this also means that Pluto is actually warmer inside than long thought. Because this heat is the prerequisite for water ice to become mobile enough to be able to at least slowly swell to the surface.
“The outpouring of icy material onto the surface of an extremely low-temperature, low-atmospheric-pressure, low-gravity celestial body, combined with the abundance of volatile ice on Pluto’s surface, make it unique among any previously explored locations in the Solar System,” the scientists state. (Nature Communications, 2022; doi: 10.1038/s41467-022-29056-3)
Source: Nature Communications