31.03.2022
Existence of volcanoes makes idea that dwarf planet is inert ball of ice look increasingly improbable
Perspective view of Pluto’s icy volcanic region. The surface and atmospheric hazes of the planet are shown in greyscale, with an artistic interpretation of how past volcanic processes may have operated superimposed in blue. Photograph: Nasa/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Isaac Herrera/Kelsi Singer
Strung out in the icy reaches of our solar system, two peaks that tower over the surface of the dwarf planet Pluto have perplexed planetary scientists for years. Some speculated it could be an ice volcano, spewing out not lava but vast quantities of icy slush – yet no cauldron-like caldera could be seen.
Now a full analysis of images and topographical data suggests it is not one ice volcano but a merger of many – some up to 7,000 metres tall and about 10-150km across. Their discovery has reignited another debate: what could be keeping Pluto warm enough to support volcanic activity?
Sitting at the southern edge of a vast heart-shaped ice sheet, these unusual surface features were initially spotted when Nasa’s New Horizons spacecraft flew past in July 2015, providing the first close-up images of the icy former planet and its moons.
“We were instantly intrigued by this area because it was so different and striking-looking,” said Dr Kelsi Singer, a New Horizons co-investigator and deputy project scientist at Southwest Research Institute in Boulder, Colorado.
“There are these giant broad mounds, and then this hummocky-like, undulating texture superimposed on top; and even on top of that there’s a smaller bouldery kind of texture.”
At the time, an ice volcano seemed like the least-weird explanation for these features – there were no impact craters from asteroids or meteors nearby, suggesting these features had been erased by relatively recent geological events; and no evidence of plate tectonics – a key contributor to mountain formation on Earth.
Yet, Singer and her colleagues were cautious about calling them volcanoes: “It’s considered kind of a big claim to have icy volcanism,” she said. “It’s theoretically possible, but there aren’t a tonne of other examples in the solar system, and they are all really different looking, and do not look like the features on Pluto.”
Since those first images were beamed back in 2015, many more have arrived, along with compositional and topographical data. Taking all of this together, the team has concluded that these unusual features really are volcanoes – although their appearance and behaviour is very different to those found on Earth.
“If you look at Mount Fuji from a distance or one of the Hawaiian volcanoes, they look like these big, broad, smooth features, which is just not what we see there,” said Singer, whose findings are published in Nature Communications. “So, we think, probably the material is extruded from below, and the dome grows on top.”
As for the nature of this material, the compositional data suggests it is mainly water ice, but with some additional “antifreeze” components mixed in, such as ammonia, or methanol. “It is still difficult to think that it would be liquid, because it’s just too cold – the average surface temperature on Pluto is about 40 Kelvin (-233 C),” said Singer. “So, it’s probably more, either slushy material, or it could even be mostly solid state – like a glacier is solid, but it can still flow.”
Even this is quite surprising, she added, because, given the extremely low temperature, this material shouldn’t be mobile at all. Possibly, it suggests Pluto’s rocky core is warmer than anticipated, and that heat energy released from the radioactive decay some of its elements is being somehow becoming trapped, for example by an insulating layer of material, and periodically released, triggering volcanic eruptions.
All of this is speculation. “I will freely admit we do not have a lot of information about what’s going on in the subsurface of Pluto,” said Singer. “But this is forcing people to come up with some creative ideas for how [ice volcanism] could happen.”
Whatever the explanation, the old idea of Pluto as just an inert ball of ice is looking increasingly improbable.
Quelle: The Guardian
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Pluto's giant ice volcanos may have formed from multiple eruption events
Scientists on NASA's New Horizons mission team have 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 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."
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 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."
The paper "Large-scale cryovolcanic resurfacing on Pluto" is available today in Nature Communications.
New Horizons mission scientists have determined that cryovolcanic activity most likely created unique structures on Pluto not yet seen anywhere else in the solar system. The amount of material required to create the formations suggest its interior structure retained heat at some point in its history, enabling water-ice-rich materials to build up and resurface the region through cryovolcanic processes. The surface and atmospheric hazes of Pluto are shown here in greyscale, with an artistic interpretation of how past volcanic processes may have operated superimposed in blue.
Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Isaac Herrera/Kelsi Singer
The region studied lies southwest of Pluto’s “heart,” Sputnik Planitia, and contains multiple large domes and rises up to 7 kilometers (about 4 miles) tall and 30 to 100 kilometers (18 to 60 miles) across, with interconnected hills, mounds, and depressions covering the sides and tops of many of the larger structures.
Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Isaac Herrera/Kelsi Singer
As part of their research, Kelsi Singer of the Southwest Research Institute and the New Horizons team proposed the names for two structures in the cryovolcanic region honoring aviation pioneers Bessie Coleman, the first African American and Native American woman to earn a pilot’s license, and Sally Ride, the first American woman in space. The International Astronomical Union approved the names Coleman Mons and Ride Rupes in October 2021. Coleman Mons was key to understanding this region because it may be one of the most recently formed volcanic domes. Ride Rupes is one of the tallest and longest cliffs on Pluto and indicates there may be deep faulting in the area that could allow cryolava to flow up from the subsurface. The elevation values in this region range more than 8 kilometers (nearly 5 miles) from the highest areas in red/orange to the lowest areas in pink/white.
For more information: http://pluto.jhuapl.edu/News-Center/News-Article.php?page=20211025
Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Kelsi Singer
Quelle: NASA