March 29, 2022 — Southwest Research Institute scientists led a New Horizons mission team that determined 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.
“The particular structures we studied are unique to Pluto, at least so far,” said Dr. Kelsi Singer, New Horizons Deputy Project Scientist from the Southwest Research Institute, 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.”
Singer’s team analyzed the geomorphology and composition of an area located southwest of Pluto’s bright, icy “heart,” Sputnik Planitia. 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. The largest structures in the region rival the Mauna Loa volcano in Hawaii.
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.
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 has for such a small planet, and how it has been incredibly active over long periods,” said New Horizons Principal Investigator Dr. 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.”
The paper “Large-scale cryovolcanic resurfacing on Pluto” by Singer and the New Horizons team is available in Nature Communications.