Volcanic mountains

Pluto: how can such a cold body fuel volcanic eruptions?

Pluto, the largest dwarf planet in the solar system, has become even more interesting with a report that icy lava flows have recently covered much of its surface. In this context, “recently” probably means no more than a billion years ago. It’s old, of course – and there’s no indication that volcanoes are still active – but it’s only a quarter of the age of the solar system and no one knows how Pluto produced the heat needed to fuel these eruptions.

The news, which comes nearly seven years after NASA’s New Horizons probe made its spectacular flyby of Pluto on July 14, 2015is through the analysis of images and other data by a team led by Kelsi Singer of the Southwest Research Institute in Boulder, Colorado.

Singer’s team draws particular attention to a mountainous feature named Wright Mons, which rises 4-5 km above its surroundings. It measures about 150 km at its base and has a central depression (a hole) 40-50 km wide, with a floor at least as low as the surrounding terrain.

The team claims that Wright Mons is a volcano and cites the lack of impact craters as evidence that it is probably no more than 1-2 billion years old. Many other areas of Pluto have existed long enough to accumulate large numbers of impact craters – no recent icy lava flows have covered them.

As far as volcanoes go, Wright Mons is a big one. Its volume exceeds 20,000 cubic kilometers. Although considerably smaller than the volume of the largest volcanoes on Mars, it is similar to the total volume of Hawaii Mauna Loaand much larger than the volume of its part above sea level. This is particularly impressive considering Pluto’s small size, with a diameter about one-third that of Mars and one-sixth that of that of the Earth.

Height profile of Wright Mons (blue line), relative to the above sea level portion of Hawaii’s Mauna Loa (blue line) and the largest volcanoes on Mars (red lines). Singer et al. (2022)

Wright’s stuff

In detail, the slopes of Wright Mons and much of its surroundings are cluttered with buttes up to 1 km high and mostly 6-12 km wide. The team concludes that these hummocks consist mostly of water ice, rather than the nitrogen or methane ice that covers other young regions of Pluto. They argue that this is consistent with the material strength needed to form and preserve these domes, but they recognize small patches of much weaker nitrogen-ice, mostly in the central depression.

The hummocks were likely created by some kind of glacial volcanism, known technically as “cryovolcanism” – erupting from icy water rather than molten rock. Pluto’s bulk density shows that there must be rock inside, but its outer regions are a mixture of ices (water, methane, nitrogen, and probably also ammonia and carbon monoxide, which are all less than one-third as dense as rock) similarly to the crust of Earth and other rocky planets is a mixture of several silicate minerals.

250 km wide image centered on Wright Mons. NASA/JHUAPL/SwRI

At Pluto’s surface temperature well below -200°C, ice made of frozen water is extremely strong. It can (and on Pluto does) form steep mountains that will last for eternity without collapsing like a glacier on the much less frigid Earth, where the water ice is weaker.

What makes ice melt?

Ice, of course, melts at much lower temperatures than rock. And when there is a mixture of two ices, melting can start at a lower temperature than for either pure ice (the same principle applies to silicate rocks composed of different minerals). This makes melting even easier. Despite this, it is surprising to find evidence of relatively young, water-rich cryovolcanic eruptions on Pluto, as there is no known heat source to fuel them.

There is only a very limited possibility that the interior of Pluto will be heated by tidal forces – a gravitational effect between orbiting bodies, such as a moon and a planet – which heat the interior of some of the moons of Jupiter and Saturn. And the amount of rock inside Pluto is not enough to produce much heat from radioactivity.

Singer and his colleagues believe that Pluto somehow retained heat from birth, which was only able to escape late in the body’s history. This would be consistent with Pluto having depth internal liquid water oceansuggested based on other evidence.

If the hummocks that Wright Mons is built from depict eruptions of water ice, this stuff clearly wasn’t flowing freely like liquid water, but must have been some kind of “mud” rich in gooey crystals, maybe in a place that was completely frozen, but still flexible outer skin that confined each fluid outpouring in a dome-shaped hummock.

A hole in the argument?

The team cites the depth and volume of the central Wright Mons depression to dismiss earlier suggestions that it is a volcanic crater (a caldera) or that it was carved out by explosive eruptions. Instead, they view it as a gap that somehow avoided being covered by erupting hummocks.

I have my doubts about this because there is an even larger probable volcano, Piccard Mons, south of Wright Mons, which also has a large central depression. It seems too much of a coincidence to me that there are two adjacent volcanoes both with fortuitous holes in the middle. I think it’s more likely that these central depressions are somehow integral to the growth or eruption of these volcanoes.

Height map showing the ring-shaped Wright Mons in the northern half and the even larger Piccard Mons in the southern half. NASA/Johns Hopkins University Applied Physics Laboratory/Southwestern Research Institute

Piccard Mons is less well characterized than Wright Mons because, by the time New Horizons made its closest approach, Pluto’s rotation had dragged Piccard Mons into darkness. The flyby was so fast that only the side of Pluto facing the Sun at the right time could be seen in detail. However, New Horizons was able to image Piccard Mons thanks to sunlight reflected weakly on the ground by haze in Pluto’s atmosphere.

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