The distant dwarf planet Pluto has surprised many times with exotic landforms and processes. Now new evaluations of images from NASA’s New Horizons spacecraft provide evidence of another unusual phenomenon. Accordingly, the dwarf planet could have a new form of ice volcanism that towered mountains of rock-hard water ice up to seven kilometers high. The ice must have swelled out of the ground like viscous lava and over time formed a hilly landscape with some larger mountains such as the Wright Mons and Piccard Mons. If this is confirmed, this form of ice volcanism would be unique in the solar system.
For a long time, the distant dwarf planet Pluto was considered a cold, dead world. But the flyby of NASA’s New Horizons spacecraft in 2015 and the data and images collected by the spacecraft paint a completely different picture. Accordingly, Pluto, which is a good 2370 kilometers across, is a surprisingly dynamic celestial body despite temperatures of less than minus 215 degrees. There are glaciers of frozen nitrogen flowing in slow motion and young areas that are constantly renewed by convection currents in the ice, such as in the large plain Sputnik Planitia. The nitrogen outgassing from this ice sheet could also drive local and global wind currents, as scientists have determined. According to planetary models, it even seems possible that the dwarf planet still carries enough heat from its early days to have semi-frozen or even liquid water inside.
Riddles about icebergs with peak depression
Another phenomenon that has long puzzled planetary scientists is a mountainous region southwest of the Sputnik Planitia ice sheet. There are several high elevations resembling flat-topped mountains or large domes. The two highest mountains in this area are the Wright Mons, 3 miles high and 90 miles wide at the base, and the Piccard Mons, 4.5 miles high and 145 miles wide. “The volume of Wright Mons alone is 24,000 cubic kilometers – that’s similar to the volume of Mauna Loa in Hawaii,” explain Kelsi Singer of the Southwest Research Institute in Boulder, Colorado and her colleagues. Both mountains have large, deep-reaching holes at their peaks; on the Wright Mons, this peak depression is 56 kilometers in diameter. Due to these crater-like structures, planetary researchers already suspected earlier that these mountains could be volcanic formations. However, given Pluto’s coldness and icy nature, these volcanoes should be pouring out ice rather than lava.
In order to investigate whether these mountains on Pluto could really be ice volcanoes, Singer and her team have now again subjected the images of the region around Wright and Piccard Mons available from New Horizons to a detailed analysis. To do this, they primarily used the images on which the mountains of this region were illuminated at an angle by weak sunlight, so that surface structures are clearly visible due to the shadows cast. The analyzes initially confirmed that both the mountain flanks and the neighboring depressions are covered by numerous smaller, hill-like elevations and that the central depressions of Wright Mons and Piccard Mons also have a rather irregular clumped-appearing surface. Spectral data also suggest that most of these mountains are made up of water ice, while more volatile types of ice, such as nitrogen ice and frozen methane, form a thin covering in only a few places.
Leaks of viscous ice
According to Singer and her team, the characteristics of the mountains speak against them being ice volcanoes in the classic sense. Both the unusually large depressions in the summit area and the absence of traces of ejected material or ejected ejecta indicate that no liquid material erupted or explosive eruptions occurred in this region. Instead, according to them, the hilly, lumpy terrain indicates the slow exit of semi-frozen or frozen material. Accordingly, there must have been several exit points for such a tough but still mobile mass of ice under the mountains and hills of this region. The scientists compare their consistency and shape with the pillow lava of some submarine lava outlets on earth. From the partly different thick coatings of the mountain flanks with organic deposits and more volatile types of ice as well as the irregular shape of the summit depressions, they also conclude that Wright Mons and Piccard Mons were formed by several ice outflows at different times. “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.
If confirmed, the dwarf planet Pluto could be experiencing a form of ice volcanism not previously observed on any other celestial body in the solar system. At the same time, the young age of some of these elevations could mean that there is even more warmth inside Pluto than had long been suspected. Because this heat is the prerequisite for water ice to become mobile enough to be able to at least slowly swell to the surface.
Source: Kelsi Singer (Southwest Research Institute, Boulder) et al., Nature Communications, doi: 10.1038/s41467-022-29056-3