Pluto, the largest dwarf planet in the solar system, has been getting more and more interesting lately. Recently, glacial lava flows covered large areas of its surface. In this context, “modern” probably means no more than a billion years ago. This is old, of course – and there’s no indication that the volcanoes are still active – but it’s only a quarter of the age of the solar system, and no one knows how Pluto fermented the heat needed to power these eruptions.
This news, nearly seven years after NASA’s New Horizons probe spectacularly flew past Pluto on July 14, 2015, thanks to an analysis of images and other data by a team led by Kelsey Singer of the Southwest Research Institute in Boulder, Colorado. Team Singer discovered something special In a mountainous region on Pluto called Wright Mons, it rises 5 kilometers above its circumference. Wright Mons covers about 150 km and has a crater 40 to 50 km wide in the middle, with ground at least as low as the surrounding terrain.
Wright Mons is a volcano, according to Singer, and the lack of archaeological craters is evidence that it is probably no more than 1-2 billion years old. Wright Mons is no small volcano. Its volume exceeds 20,000 cubic kilometers. Although it is much less than the size of the largest volcanoes on Mars, this is comparable to the total size of, for example, the largest active volcano on Earth, Mauna Loa in Hawaii. This is particularly impressive given Pluto’s small size, being about a third the diameter of Mars and one-sixth the diameter of Earth.
The slopes of Wright Mons and much of the surrounding area are spread out in elevations of up to 1,000 m in height and usually 6 to 12 km in width. The team concluded that these ridges consist primarily of water ice, rather than the nitrogen or methane ice that covers some of the other young regions on Pluto, but that there are small patches of much weaker nitrogen ice, especially in the central depression.
The cores are likely caused by a type of ice volcano known by the technical term “ice volcanoes”. Instead of molten rock, a volcanic eruption spews out ice water. Pluto’s bulk density shows that there must be rocks inside, but its outer regions are a mixture of ice (water, methane, nitrogen and possibly also ammonia and carbon monoxide, all less dense than a third of rock). Compare it to the way the crust of the Earth and other rocky planets is made up of a mixture of different silicate minerals.
At the surface temperature of Pluto below -200 ° C, ice made of frozen water is very strong. It can form (and it does on Pluto) steep mountains that last forever. Ice naturally melts at temperatures much lower than rock. And when there is a mixture of two glaciers, melting can begin at a lower temperature than with either pure ice (the same principle applies to silicate rocks made of different minerals). Despite this, it comes as a surprise to find evidence of a relatively small, water-rich icy volcano erupting on Pluto, because there is no known heat source that could trigger those eruptions.
There is only very limited space for heating the interior of Pluto by tidal forces – the gravitational effect between orbiting objects, such as the Moon and the planet – which heat the interiors of some of the moons of Jupiter and Saturn. And the amount of rock on Pluto isn’t enough to produce a lot of heat from radioactivity.
Singer and her colleagues speculate that Pluto has somehow retained heat from its birth, heat that could not have escaped until much later in the dwarf planet’s history. This would be consistent with the suggestion that Pluto has a deep inner ocean of liquid water.
If the hills around and around Wright Mons do indeed represent water-ice volcanic eruptions, it is clear that these objects do not flow as freely as liquid water. It must have been some sort of sticky crystal “mush”, probably within a completely frozen, but pliable outer shell, containing any effusion of fluid (mud forming).
Incidentally, there is a potentially larger volcano, Picard Mons, south of Wright Mons which also has a large central depression. We know less about Piccard Mons than about Wright Mons because by the time New Horizons approached its closest orbit, Pluto’s rotation had thrown Mons into the dark. Only the side of Pluto that was facing the Sun in time can be seen in detail. However, New Horizons was able to image Piccard Mons thanks to the weak reflection of sunlight on Earth from the nebula in Pluto’s atmosphere.
It makes us curious about more. What additional details are lurking in the bad half of Pluto? It will likely be decades before we discover or learn much about how these ice volcanoes form. No follow-up missions to Pluto are officially planned, but at least two mission concepts have been explored. At best, we won’t return to the dwarf planet until 2030, and the journey alone will take 8 years.