Maps of Pluto charted using data from NASA’s New Horizons mission appear to show two huge mountains scientists said Monday could be ice volcanoes, a discovery that would set the distant dwarf planet apart from its neighbors in the outer solar system.
Two sprawling mountain peaks, each about 100 miles across and several miles high, have deep depressions carved from their centers, a telltale marker of a volcano, at least on more familiar geologically active worlds closer to the sun.
“Whatever they are, they’re definitely weird, and volcanoes is maybe the least weird hypothesis at the moment,” said Oliver White, a scientist on the New horizons team from NASA’s Ames Research Center.
But Pluto itself is an enigma.
Geologists did not expect to see apparent glacial ice flows, and scientists are making unexpected findings about Pluto’s tenuous atmosphere. With the discovery that Pluto is geologically alive, more signs are pointing toward the presence of volcanic activity.
“I’m having difficulty unseeing volcanoes,” White said Monday. “As somebody who did his Ph.D in volcanic morphology, when you see a big mountain with a hole on the top, it generally points to one thing.”
In their faraway perch 3 billion miles from the sun, Pluto and Charon, the largest of its five moons, are locked in an orbital dance, each presenting the same face to its companion during every 6.4-day cycle.
“It’s just astounding that in all of the exploration that we’ve done, that the nearest neighbor analogy to these constructs occurs on Mars,” said Alan Stern, New Horizons’ principal investigator from the Southwest Research Institute. “You have to look to the other red planet to find something similar. Across all the worlds of the middle solar system, we’ve seen nothing like this. It’s truly amazing. It’s like something on a terrestrial planet.”
If the two mountains highlighted by White, informally dubbed Wright Mons and Piccard Mons, are actually volcanoes — whether active or extinct — their output is not molten rock but a slurry of water ice, nitrogen, ammonia, or methane, scientists said.
Using three-dimensional maps created with stereo imagery captured by New Horizons on its July 14 flyby of Pluto, geologists estimate Wright Mons is 13,000 feet high — about 4 kilometers — while nearby Piccard Mons looms even taller.
The geometry of New Horizons’ encounter with Pluto put both mountains in twilight, with the insides of their central pits in darkness.
The announcement Monday bolsters evidence that Pluto harbors a liquid ocean deep underneath its outer shell of ice. Measurements show Pluto’s icy crust is mainly made of frozen nitrogen, methane and carbon monoxide, volatile species with super-cold melting points.
Scientists believe the ices are energized from a meager heat source buried inside Pluto’s core emitting warmth from the natural radioactive decay of elements cocooned inside the dwarf planet at its formation during the dawn of the solar system 4.5 billion years ago.
The immense tug of gravity from Jupiter and Saturn pulls on their moons, a force known as tidal heating that deforms the interiors of moons like Io and Enceladus, triggering their famous eruptions of lava and icy plumes.
Pluto has no such nearby giant to mush its insides and give rise to volcanoes.
“An internal radioactive heat source is, at the moment, the only heat source that we can really think of given that tidal heating has probably not had much effect on Pluto,” White said. “Pluto is very small. It would have a silicate core, and the heat source may have died off quite a bit over the 4.5 billion years over Pluto’s existence.”
While there’s maybe less heat to go around (than on planets in the inner solar system), perhaps you get more bang for your buck with the heat that is available given the nature of these ices that are on the surface,” White said.
White said that ice volcanoes “would be one of the most phenomenal discoveries of New Horizons, and would make Pluto an even more fascinating and unique place than it’s already proven itself to be.”
“After all, nothing like this has been seen in the deep outer solar system,” said Jeff Moore, leader of New Horizons’ geology, geophysics and imaging team from NASA Ames.
Scientists shared their latest interpretations of Pluto at the 47th annual meeting of the American Astronomical Society’s Division for Planetary Sciences near Washington, D.C.
The suspected volcanoes Wright Mons and Piccard Mons sit to the south of Sputnik Planum, the nickname for an expansive frozen plain composed of current-like flow features and shaped ice blocks stretching across a swath of Pluto the size of Texas.
The ice field makes up the western half of a bright heart-shaped feature spotted as New Horizons approached Pluto in July, and it lies next a dark belt pelted with craters believed to be billions of years old.
But the lack of impacts in Sputnik Planum tells geologists the region is relatively young, perhaps forming within the last 10 million years, scientists said Monday.
Tectonics, resurfacing and slow glacial movement can erase the record of craters seen elsewhere on Pluto, according to Kelsi Singer, a scientist based at the Southwest Research Institute.
“We’ve mapped more than a thousand craters, which vary greatly in size and appearance,” Singer said in a statement. “Among other things, I expect cratering studies like these to give us important new insights into how this part of the solar system formed.”
Alex Parker, a planetary astronomer at SWRI, studied the crater record on Pluto and Charon to glean insights into the the wider population in the Kuiper Belt, a ring of icy bodies outside the orbit of Neptune, of which Pluto is the largest.
Nearly all of the craters on Pluto and Charon came from impacts with Kuiper Belt Objects, Parker said, but there are surprisingly few small impact markings. The finding could mean many such worlds in the Kuiper Belt may have formed not as an accumulation of many smaller bodies, but in one piece, challenging a longstanding model on the history of outer frontier of the solar system.
If true, the theory means medium-sized residents of the Kuiper Belt, such as the next target of the New Horizons mission, are likely primordial relics left over from the beginning of the solar system, the very building blocks that came together to form larger worlds.
A sequence of rocket firings in the last three weeks nudged the New Horizons spacecraft on course toward a Kuiper Belt Object the size of Rhode Island named 2014 MU69. NASA still must grant final approval for the mission to proceed toward the next flyby target, which New Horizons will reach on New Year’s Day 2019, after weighing its costs and benefits.
“If an extended mission is approved, and we can actually fly to this object and explore it, it will be the first of this class of objects we’ve ever seen,” Parker said.
Data crunching has also revealed Pluto’s rarefied nitrogen atmosphere hangs closer to the surface than expected.
“We thought the exobase — the top of the atmosphere — was seven to eight times larger than Pluto,” said Leslie Young, deputy project scientist on the New Horizons mission at SWRI. “Now we know it’s only about two-and-a-half times larger than Pluto. It’s still an extended atmosphere, but much more compact.”
Molecules are escaping from Pluto’s atmosphere at a much lower rate than previously thought, scientists said, and the atmosphere is being stripped by the same mechanism that blasts gases away from Earth and Mars.
New Horizons also glimpsed Pluto’s four tiniest moons during its brief flyby, turning the dim points of light only discovered within the last decade into worlds worthy of study.
Images show two of the satellites, Kerberos and Hydra, likely coalesced when two smaller objects merged some time long ago. Pluto’s other two small moons, Styx and Nix, may have formed the same way, according to Mark Showalter, a New Horizons co-investigator from the SETI Institute.
The formation theory suggests Pluto once had more moons in the wake of an ancient cataclysmic collision that siphoned off part of Pluto to form Charon, Showalter said.
The quartet of diminutive moons also behave chaotically, with wildly rotating motions that might be influenced by the pull of Charon, which could keep the satellites from “de-spinning” into slower rotation rates, as scientists predicted.
Hydra, the outermost of Pluto’s moons, spins around 89 times in a single orbit.
“The way I would describe this system is not just chaos but pandemonium,” Showalter said.
The data pipeline streaming down from New Horizons will continue through 2016, with the communications rate limited by the specifications of the probe’s antenna and its vast distance from Earth.
About 20 percent of the Pluto flyby data has reached the ground so far, Stern said Monday.
“I want to try to frame this by talking about grades,” Stern said. “We’re four months past the flyby. I think it’s fair to say that we can tell New Horizons gets an ‘A’ for exploration. All our flyby plans seem to have succeeded.”
Pluto had surprises, and Stern expects scientists to wrestle with the onslaught of data for years.
“But I also think we get a couple of ‘Fs’. One ‘F’ is we get an ‘F’ for predictive ability,” Stern said.
“This is when the debates begin,” said Curt Niebur, New Horizons program scientist at NASA Headquarters. “This is when the heated discussions begin. This is when the entire science community starts staying up throughout the night.”
Regarding the self-assigned “F” grade, Stern said: “Pluto and its system of satellites have really outsmarted us. It’s the best bad grade I’ve ever taken credit for in my life.”
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