From possible ice volcanoes to geologically diverse surfaces to oddly behaving moons that could have formed through mergers of smaller moons, Pluto system discoveries continue to surprise scientists on NASA’s New Horizons mission team.
“The New Horizons mission has taken what we thought we knew about Pluto and turned it upside down,” said Jim Green, director of planetary science at NASA Headquarters in Washington. “It’s why we explore – to satisfy our innate curiosity and answer deeper questions about how we got here and what lies beyond the next horizon.”
The New Horizons team is discussing numerous findings at the 47th Annual Meeting of the Division for Planetary Sciences (DPS) of the American Astronomical Society (AAS) this week in National Harbor, Maryland. Just four months after the spacecraft encountered Pluto, science team members are presenting more than 50 reports on exciting discoveries.
“It’s hard to imagine how rapidly our view of Pluto and its moons are evolving as new data stream in each week. As the discoveries pour in from those data, Pluto is becoming a star of the solar system,” said mission Principal Investigator Alan Stern of the Southwest Research Institute, Boulder, Colorado. “Moreover, I’d wager that for most planetary scientists, any one or two of our latest major findings on one world would be considered astounding. To have them all is simply incredible.”
In one such discovery, New Horizons geologists have combined images of Pluto’s surface to make 3-D maps that indicate that two of Pluto’s most distinctive mountains could be cryovolcanoes—ice volcanoes that may have been active in the recent geological past.
The two cryovolcano candidates are large features measuring tens of miles (tens of kilometers) across and several miles or kilometers high. “These are big mountains with a large hole in their summit, and on Earth that generally means one thing—a volcano,” said Oliver White, New Horizons postdoctoral researcher with NASA’s Ames Research Center, Moffett Field, California. While their appearance is similar to volcanoes on Earth that spew molten rock, ice volcanoes on Pluto are expected to emit a somewhat melted slurry of substances such as water ice, nitrogen, ammonia, or methane on Pluto.
White stresses that the team’s interpretation of these features as volcanoes is tentative. However, “If they are volcanic, then the summit depression would likely have formed via collapse as material is erupted from underneath. The strange hummocky texture of the mountain flanks may represent volcanic flows of some sort that have travelled down from the summit region and onto the plains beyond, but why they are hummocky, and what they are made of, we don't yet know.”
If Pluto is proven to have volcanoes, it will provide an important new clue to its geologic and atmospheric evolution. “After all, nothing like this has been seen in the deep outer solar system,” said Jeffrey Moore, New Horizons Geology, Geophysics and Imaging team leader, also from NASA Ames.
Pluto’s Long History of Geologic Activity
Another of the more surprising findings from New Horizons is the wide range of surface ages found on Pluto, from ancient to intermediate to relatively young in geological terms. Crater counts used to determine surface unit ages indicate that Pluto has ancient surface areas dating to just after the formation of the planets, about 4 billion years ago. In addition, there’s a vast area that was geologically born “yesterday,” meaning it may have formed within the past 10 million years. This area – informally named Sputnik Planum – appears on the left side of Pluto’s “heart” and is completely impact-free in all images returned to date.
Scientists wondered if Sputnik Planum’s smooth, icy plains were an oddity; did a recent geological episode form the plains long after all other geologic activity ceased?
Apparently not. New data from crater counts reveal the presence of intermediate or “middle-aged” terrains on Pluto as well. This suggests that Sputnik Planum is not an anomaly—that Pluto has been geologically active throughout much of its more than 4-billion-year history. “We’ve mapped more than a thousand craters, which vary greatly in size and appearance,” said postdoctoral researcher Kelsi Singer, of the Southwest Research Institute (SwRI) in Boulder, Colorado. “Among other things, I expect cratering studies like these to give us important new insights into how this part of the solar system formed.”
Building Blocks of the Solar System
Crater counts are giving the New Horizons team insight into the structure of the Kuiper Belt itself. The dearth of smaller craters across Pluto and its large moon Charon indicate that the Kuiper Belt likely had fewer smaller objects than some models had predicted. This leads New Horizons scientists to doubt a longstanding model that all Kuiper Belt objects formed by accumulating much smaller objects of less than a mile wide. The absence of small craters on Pluto and Charon support other models theorizing that Kuiper Belt objects tens of miles across may have formed directly, at their current—or close to current—size.
In fact, the evidence that many Kuiper Belt objects could have been “born large” has scientists excited that New Horizons’ next potential target – the 30-mile-wide (40-50 kilometer wide) KBO named 2014 MU69 – which may offer the first detailed look at just such a pristine, ancient building block of the solar system.
Spinning, Merged Moons
The New Horizons mission is also shedding new light on Pluto’s fascinating system of moons and their unusual properties. For example, nearly every other moon in the solar system, including Earth’s moon, is in synchronous rotation, but not so of Pluto’s small moons. These small satellites are spinning much faster, with Hydra – the most distant moon - rotating an unprecedented 89 times during a single lap around Pluto. Scientists believe these spin rates could be chaotic (i.e., variable) because Charon exerts a strong torque that prevents each small moon from settling down into synchronous rotation, which means keeping one face toward the planet.
Another oddity of Pluto’s moons: scientists expected the satellites to wobble, but not to this degree. “Pluto’s moons are behaving like spinning tops,” said co-investigator Mark Showalter of the SETI Institute in Mountain View, California.
Images of Pluto’s four smallest satellites also indicate that several of them could have been born from mergers of two or more former moons, suggesting the presence of more moons at some point. “We suspect from this that Pluto had more moons in the past, in the aftermath of the big impact that also created Charon,” said Showalter.
Pluto’s Frigid, Extended Atmosphere
The New Horizons team is presenting new data at DPS that reveal Pluto’s upper atmosphere is significantly colder and therefore more compact than Earth-based models had indicated. As a result, scientists have discovered that Pluto’s atmospheric escape rate is thousands of times lower than had been thought. It now appears that Pluto’s atmosphere escapes by the same mechanism as do gases from the atmospheres of Earth and Mars – rather than the previously believed escape process that more resembled escape from cometary atmospheres.
New Horizons is part of NASA's New Frontiers Program, managed by the agency's Marshall Space Flight Center in Huntsville, Alabama. The Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland, designed, built, and operates the New Horizons spacecraft and manages the mission for NASA's Science Mission Directorate. The Southwest Research Institute leads the science mission, payload operations, and encounter science planning.
Johns Hopkins University Applied Physics Laboratory, Laurel, Md.
Southwest Research Institute, San Antonio