Scientists on NASA’s New Horizons mission team have revealed a strange world of massive ice volcanoes and a possible hidden ocean. The NASA astronomers have determined that multiple episodes of cryovolcanism may have created some kinds of surface structures on Pluto, the likes of which are not seen anywhere else in the solar system. Material expelled from below the surface of this distant, icy planet could have created a region of large domes and rises flanked by hills, mounds, and depressions. New Horizons was NASA’s mission to make the first exploration of Pluto and its system of five moons with its July, 2015 flyby.
“The particular structures we studied are unique to Pluto, at least so far,” said Kelsi Singer, New Horizons deputy project scientist from the Southwest Research Institute, Boulder, Colorado, and lead author of the paper published today in Nature Communications. “Rather than erosion or other geologic processes, cryovolcanic activity appears to have extruded large amounts of material onto Pluto’s exterior and resurfaced an entire region of the hemisphere New Horizons saw up close.”
The surface and atmospheric hazes of Pluto are shown in the image at the top of the page in greyscale, with an artistic interpretation of how past volcanic processes may have operated superimposed in blue. (NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Isaac Herrera/Kelsi Singer).
Rival the Mauna Loa Volcano in Hawaii
Singer’s team analyzed the geomorphology and composition of an area located southwest of Pluto’s bright, icy “heart,” Sputnik Planitia – “One of Pluto’s crown jewels, and understanding its origin is a puzzle,” said New Horizons Principal Investigator Alan Stern, of the Southwest Research Institute, Boulder, Colorado. “Whatever caused Sputnik to form, nothing like it exists anywhere else in the solar system. Work to understand it will continue, but whatever that origin is, one thing is for certain—the exploration of Pluto has created new puzzles for 21st century planetary science.”
The geological map below covers a portion of Pluto’s surface that measures 1,290 miles (2,070 kilometers) from top to bottom, and includes the vast nitrogen-ice plain informally named Sputnik Planum and surrounding terrain. Image credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute
“It is not clear if there is a connection between the cryovolcanic region south of the large impact basin filled with a giant nitrogen ice sheet known as Sputnik Planitia as the cryovolcanic region does not have deep nitrogen ice deposits like in Sputnik Planitia,” Kelsi Singer, Deputy Project Scientist and Co-Investigator on NASA’s New Horizons spacecraft mission, told The Daily Galaxy. “Also,” she explains, “the cryovolcanic region extends for a very large distance away from Sputnik Planitia. However, the large impact crater underlying Sputnik Planitia could be a source of fracturing or stress in the crust of Pluto, and that could enable the movement of icy materials in the subsurface, which is critical for cryovolcanism.”
Wright and Piccard Volcanoes Formed Billions of Years after Sputnik Planitia
New Horizons is the first and only spacecraft (to date) to whiz past the dwarf planet, sending back images and data that transformed Pluto from a round smudge seen through a telescope to a beguiling rock and ice world, dominated by a vast heart-shaped ice sheet resembling a polar ice cap, the Sputnik Planitia. By the time the New Horizons spacecraft reached Pluto, the spacecraft had traveled farther away and for a longer time period (more than nine years) than any previous deep space spacecraft ever launched.
Just south of the Sputnik Planitia ice sheet, there looked to be two gigantic volcanoes. One, called Wright Mons after the Wright brothers, was about 150 kilometers wide and 4km high. Its volume is around the same as Hawaii’s Mauna Kea, one of Earth’s largest volcanoes. To the south of Wright Mons was another, larger volcano-looking feature, this one around 7km tall at its highest point, dubbed Piccard Mons, after the 20th-century physicist and balloonist Auguste Piccard.
The cryovolcanic region contains multiple large domes, ranging from 1 to 7 kilometers (about one-half to 4 miles) tall and 30 to 100 or more kilometers (about 18 to 60 miles) across, that sometimes merge to form more complex structures. Irregular interconnected hills, mounds and depressions, called hummocky terrain, cover the sides and tops of many of the larger structures. Few if any craters exist in this area, indicating it is geologically young.
Enormous Impact Basin
“We recognized quite early on that Sputnik Planitia is a vast deposit of nitrogen and carbon monoxide ice in solid solution (with some methane ice as well) that is filling an enormous impact basin”, SETI research scientist Oliver White, wrote in an email to The Daily Galaxy. “The basin formed very early in Pluto’s history (around 4 billion years ago),” he continued, “and acted as a vast “cold trap” for nitrogen/carbon monoxide ice, which migrated to fill the basin across tens of millions of years. This has been demonstrated by the climate modeling of Tanguy Bertrand (The nitrogen cycles on Pluto over seasonal and astronomical timescales, published in Icarus in 2018). To this day, the vast majority of Pluto’s nitrogen/carbon monoxide ice is contained within Sputnik Planitia, where it has continuously undergone glacial flow and convection. Based on the near total absence of craters from them, Wright and Piccard Mons must have formed billions of years after Sputnik Planitia appeared, so their formation cannot be said to be related to the formation of Sputnik Planitia.”
Even with the addition of ammonia and other antifreeze-like components to lower the melting temperature of water ices — a process similar to the way road salt inhibits ice from forming on streets and highways — the extremely low temperatures and atmospheric pressures on Pluto rapidly freeze liquid water on its surface.
Similar to Ice Glacier Flows on Earth
Because these are young geologic terrains and large amounts of material were required to create them, it is possible that Pluto’s interior structure retained heat into the relatively recent past, enabling water-ice-rich materials to be deposited onto the surface. Cryovolcanic flows capable of creating the large structures could have occurred if the material had a toothpaste-like consistency, behaved somewhat like solid ice glaciers flow on Earth or had a frozen shell or cap with material that was still able to flow underneath.
Other geologic processes considered to create the features are unlikely, according to the team. For example, the area has significant variations in the highs and lows of the terrain that could not have been created through erosion. Singer’s team also saw no evidence of extensive glacial or sublimation erosion in the hummocky terrain surrounding the largest structures.
“One of the benefits of exploring new places in the solar system is that we find things we weren’t expecting,” said Singer. “These giant, strange-looking cryovolcanoes observed by New Horizons are a great example of how we are expanding our knowledge of volcanic processes and geologic activity on icy worlds.”
Images obtained in 2015 by the New Horizons spacecraft revealed diverse geological features populating across Pluto, including mountains, valleys, plains and glaciers. They were particularly intriguing because the frigid temperatures at Pluto’s distance were expected to produce a frozen, geologically inactive world.
“This newly published work is truly landmark, showing once again how much geologic personality Pluto for such a small planet has, and how it has been incredibly active over long periods,” said New Horizons Principal Investigator Alan Stern, of the Southwest Research Institute. “Even years after the flyby, these new results by Singer and coworkers show that there’s much more to learn about the marvels of Pluto than we imagined before it was explored up close.”
Exotic, Pre-Cellular Life in Pluto’s Ocean?
“Life can tolerate a lot of stuff: It can tolerate a lot of salt, extreme cold, extreme heat”, says William McKinnon, professor of earth and planetary sciences at Washington University, in a 2016 paper, Could There Be Life in Pluto’s Ocean, “but I don’t think it can tolerate the amount of ammonia Pluto needs to prevent its ocean from freezing — ammonia is a superb antifreeze. Not that ammonia is all bad.”
“On Earth, microorganisms in the soil fix nitrogen to ammonia, which is important for making DNA and proteins and such,” writes McKinnon. “The idea that bodies of Pluto’s scale, of which there are more than one out there in the Kuiper Belt, could all have these kinds of oceans. But they’d be very exotic compared to what we think of as an ocean,” McKinnon adds.
“If you’re going to talk about life in an ocean that’s completely covered with an ice shell, it seems most likely that the best you could hope for is some extremely primitive kind of organism. It might even be pre-cellular, like we think the earliest life on Earth was,.” McKinnonn notes.
The Last Word
“We think Pluto does have an ocean, though the evidence is much more inferential and less direct than it is, say, for Europa or Enceladus (where we are pretty sure),” William McKinnon wrote in an email to The Daily Galaxy. “Preservation of the ocean on a smallish world, and one not tidally heated, may require special conditions,” he explained. “One idea is that Pluto’s ocean is very ammonia-rich and very cold (ammonia is a fantastic antifreeze) – but concentrated ammonia is pretty nasty stuff with respect to even microbial life. But a new idea hypothesizes that Pluto’s ocean is insulated by a layer of ice clathrate, an unusual ice cage structure filled with guest molecules like N2 or CO. Clathrates have very low thermal conductivity, so there is no need for dissolved ammonia or exceeding cold, and the ocean waters would be much more pleasant (meaning, habitable, at least in principle).”