Pluto is the solar system’s largest dwarf planet, which despite its cold surface has attracted a lot of interest. A new study shows that icy lava flows may have recently covered large parts of Pluto’s surface, but no one knows how the celestial body generated enough heat to enable volcanic activity.

Pluto is located at the outer edge of our solar system, nearly 40 times further from the Sun than Earth, and is perhaps best known for losing its planet status in 2006. Pluto was originally thought to be about the same size as Earth, but after more detailed study could see that this was not the case. The decision to demote Pluto from planet to dwarf planet was based, among other things, on the celestial body’s size, which reaches only 0.2 percent of Earth’s mass.

NASA’s New Horizons spacecraft became the first to fly close to Pluto when it passed by the dwarf planet in 2015. A team of scientists at the Southwest Research Institute in Colorado has since been studying Pluto using images from the spacecraft, along with older data. A new study presents interesting discoveries about the cold little celestial body.

The new study shows that there may have recently been icy lava flows on Pluto caused by large ice volcanoes, so-called cryovolcanoes. In this context, “recently” certainly means around a billion years. For us, it is a long time ago, and there are no signs that the volcanoes are still active. But at the same time, this is only about a quarter of the total age of the solar system, and no one can explain where the heat came from to generate the eruptions.

Wright Mons

The research team behind the study has primarily focused on a “rock structure” on Pluto called Wright Mons that rises four to five kilometers above the surface. Pluto’s density suggests that there is a core of rock at the center of the dwarf planet, but the outer layers consist of mixtures of various ices of frozen water, methane, nitrogen, and probably also ammonia and carbon monoxide. Calculations say that Pluto may have almost as much water on its surface as we have here on Earth.

Wright Mons measures around 150 kilometers across its base, with a 40–50 kilometer wide hollow in the middle. At the site around Wright Mons, there are no impact craters, indicating that the structure is no older than one to two billion years. Many other areas on Pluto are old enough to have accumulated large numbers of impact craters that have not been covered by new ice since then. All of this supports the research team’s thesis, namely that Wright Mons is a cryovolcano.

By volcano standards, Wright Mons is big. Its volume measures over 20 thousand cubic kilometers, which is about the same as the volcano Mauna Loa in Hawaii. Wright Mon’s size is even more impressive when you factor in the size of Pluto, which is about one-sixth the diameter of Earth.

Slush instead of magma

Instead of spewing molten rock, cryovolcanoes erupt with ice water. Or rather outbreaks of a slushy, crystal-rich “porridge”. Wright Mons and the surrounding area appear to consist of thick layers of water ice. It is likely that it is precisely water, because it freezes more compactly than many other substances. Ices of, for example, nitrogen or methane could not support or preserve a structure the size of Wright Mons. Pluto’s surface temperature is well below -200 degrees Celsius so the water ice here has the ability to form steep mountains that last forever without sinking.

There is currently no known source that could have heated the dwarf planet enough to enable the eruptions. The tidal forces that make Jupiter’s moon Io the most volcanically active celestial body in the Solar System are absent here. Jupiter pulls and tears at Io with its gravity and it is the friction that generates the heat, but Pluto is completely alone at the edge of the solar system without the influence of a nearby celestial body.

Instead, the research team behind the study speculates that Pluto has somehow retained some of the heat from its formation, which for some reason was not able to escape from the celestial body until later in Pluto’s history. This theory is consistent with that of Pluto having a deep subsurface liquid ocean, as previous studies have indicated.

Not the only one of its kind

Although this study focused on Wright Mons, the volcano is not the only one of its kind. South of Wright Mons is another probable cryovolcano, called Piccard Mons. Piccard Mons is much larger than Wright Mons, and like its neighbor, has a large, central cavity.

It will probably be many years before we find out more about Pluto. That a space probe actually found itself at the dwarf planet is nothing less than a technical miracle and today there are no plans for a return visit. We simply have to continue to speculate on what gave rise to these icy volcanoes with the data we already have.