It was the dominant event in spaceflight last year: New Horizons’ rapid-paced, intense scientific investigation of the icy, dwarf planet Pluto – the mysterious little world that once bounded the planets of our solar system. This year, New Horizons completed its transmission of all the data it gathered during the Pluto encounter, information that has revealed Pluto to be an even stranger world than our imaginations conjured for so long.
Pluto – an icy little world with an intriguing present and past:
As soon as it zipped past Pluto on 14 July 2015, New Horizons immediately reoriented itself and began transmitting the massive amount of information it gathered about the dwarf planet back to Earth.
However, because of the downlink rate, amount of data collected, and the immense distance from Earth, it would take New Horizons a full 15 months to transmit all of its stored data.
On 25 October 2016, at 05:48 EDT, the moment came: the final piece of Pluto data – a segment of the Pluto-Charon observation sequence from the Ralph/Linear Etalon Imaging Spectral Array (LEISA) imager – arrived at mission operations at the Johns Hopkins Applied Physics Laboratory (APL).
The final piece of data traveled over 5.5 billion km (3.4 billion mi), or 5 hours, 8 minutes at light speed, and arrived through NASA’s Deep Space Network station in Canberra, Australia.
“The Pluto system data New Horizons collected has amazed us over and over again with the beauty and complexity of Pluto and its system of moons,” said Alan Stern, New Horizons principal investigator.
“There’s a great deal of work ahead for us to understand the 400-plus scientific observations that have all been sent to Earth.”
Even though the complete set of data didn’t arrive until 25 October, scientists have been hard at work analyzing all of the data that’s come streaming in from New Horizons this year.
The more data that came in from New Horizons, the more scientists were able to combine observations from the host of instruments carried aboard the craft.
Specifically, data from the Long Range Reconnaissance Imager (LORRI) and Ralph instruments revealed a collection of dark red tholins (small soot-like particles generated from reactions involving methane and nitrogen in the atmosphere) in low areas of Pluto’s surface – like the bottoms of craters.
Surprisingly, the images revealed that the thickest deposits of tholin seem to flow into channels and craters, which is surprising as thick tholin isn’t usually mobile on large scales.
This suggests that the deposits might be riding on top of ice or are being blown by Pluto’s winds – both intriguing possibilities.
January also brought new, hi-resolution images of Sputnik Planum (the heart-shaped region), revealing darker colors in portions of the surface feature, possibly implying a change in composition or surface texture.
The same images also revealed occasional raised, dark blocks that scientists believe are probably dirty water “icebergs” floating in denser solid nitrogen.
Moreover, the images also revealed new information about Sputnik Planum’s cell block structure.
Sputnik Planum itself lies at a lower elevation than the surrounding terrain, and its surface is separated into cells or polygons 16 to 40 km (10 to 25 mi) in width.
When New Horizons viewed these structures at low sun angles, the cells were seen to have slightly raised centers and ridged margins, with about 100 meters (100 yards) of overall height variation.
Scientists believe this pattern stems from the slow thermal convection of the nitrogen-dominated ices that fill Sputnik Planum – which are heated at depth by Pluto’s modest internal heat.
The heated nitrogen then becomes buoyant and rises in great blobs – the cell block structures – to the surface, much the same way a lava lamp works.
Moreover, while this area of Pluto might act like a lava lamp, the dwarf planet may also possess a recently active – in geologic terms – cryovolcano, Wright Mons, in its southern hemisphere.
Returned hi-resolution photos of Wright Mons show it to have a relatively young surface – with only one identifiable impact crater.
A lack of impact craters indicates a recently renewed surface in this region – possibly by icy, cryo material ejected from Wright Mons.
While more study is needed to confirm if Wright Mons is indeed a cryovolcano, if it is, it would be the largest volcano discovered to date in the outer solar system; however, its existence wouldn’t exactly be a surprise given Pluto’s abundance of water.
In fact, data returned in January revealed far more water ice on Pluto’s surface than previously thought.
Infrared observations by the Ralph/LEISA instrument revealed that spectral features of water ice are abundant on Pluto’s surface and are indeed part of the dwarf planet’s crustal bedrock, the layer on which other volatile ices’ leave their seasonal footprint.
What’s more, the new information shows that the amount of exposed water ice is considerably more widespread across Pluto’s surface than previously known.
Importantly, too, the high sensitivity of these observations revealed little or no water ice in Sputnik Planum and Lowell Regio – indicating that Pluto’s icy bedrock is well hidden beneath a thick blanket of other ices, such as methane, nitrogen and carbon monoxide, in these regions.
But January wasn’t just about Pluto. During the July 2015 flyby, the dwarf planet’s reflected sunlight shone brightly on Charon’s nightside, allowing New Horizons to capture images of the moon’s dark side – some of which was illuminated in Plutoshine.
A few of the Plutoshine illuminated areas of Charon were otherwise hidden from New Horizons during close approach – including the moon’s south pole, which entered polar night in 1989 and won’t witness sunlight again until 2107.
Plutoshine thus enabled scientists to construct better maps of the moon due to the more visible surface features on Charon’s nightside.
The month began with the release of more information regarding Sputnik Planum’s floating water-ice hills, which were discovered to likely be miniature versions of the larger, jumbled mountains on Sputnik Planum’s western border.
New information on the water-ice hills showed them to have a similar composition to the rugged uplands on the Planum’s western border, suggesting that they broke away from the uplands and are being carried away by the nitrogen glaciers.
This overall better understanding of Sputnik Planum spread to a global understanding of Pluto’s overall geology as a global map of the planet began to take shape – with a level of detail that allowed scientists to start identifying specific terrains, textures, and morphology.
By studying how the terrain boundaries between various surface textures crosscut one another, scientists began to determine which terrains overlie others – which in turn created better estimates to location-specific ages of Pluto’s surface.
This also helped create a better understanding of the processes at work on Pluto and when certain terrains formed in relation to others.
This is especially evident in a series of long canyons in Lowell Regio.
The canyons run vertically across the northern polar area and were found to have degraded walls that appear to be much older than the more sharply defined canyon systems elsewhere on Pluto – potentially indicating that the polar canyons are made of weaker material as well.
Moreover, the polar canyons appear to show evidence of an ancient period of tectonics on the dwarf planet.
Additionally, large, irregularly-shaped pits were found to scar the polar region, potentially indicating locations where subsurface ice has melted or sublimated from below, causing the ground to collapse.
The color and composition of this north polar region are also unusual as high elevations are distinctively yellow – something not seen elsewhere on Pluto.
The yellowish terrain fades to a uniform bluish gray at lower elevations and latitudes.
“One possibility is that the yellow terrains may correspond to older methane deposits that have been more processed by solar radiation than the bluer terrain,” said Will Grundy, New Horizons composition team lead.
Leaving Pluto, Charon once again yielded surprises in February: not the least being the discovery that it once harboured a subsurface ocean that has long since frozen and expanded, causing the moon’s surface to stretch and fracture on a massive scale.
The side of Charon viewed by New Horizons is characterized by a system of “pull apart” tectonic faults, which are expressed as ridges, scarps, and valleys – the latter sometimes reaching more than 6.5 km (4 mi) deep.
Charon’s tectonic landscape shows that the moon expanded in its past, and that its surface fractured as it stretched.
It was already known from previous New Horizons data that Charon’s outer layer is primarily water ice. Now, scientists think that when Charon was younger, this layer was kept warm via heat provided by the decay of radioactive elements and Charon’s own internal heat.
Therefore, Charon could have been warm enough to cause the water ice to melt deep down, creating a subsurface ocean.
Then, as Charon cooled over time, the ocean froze and expanded, lifting the outermost layers of the moon and producing the massive chasms present today.
With March came the identification of methane snow or frost on a prominent mountain range in the southeast section of Cthulhu.
Cthulhu’s overall appearance is characterized by a dark surface, though the region is geologically complex from towering mountains to smooth plains.
One particular mountain range in Cthulhu contains a series of high peaks that are coated with a bright material composed predominantly of methane that has condensed as ice. (Note: This was confirmed by other New Horizons data released in August 2016.)
“That this material coats only the upper slopes of the peaks suggests methane ice may act like water in Earth’s atmosphere, condensing as frost at high altitude,” said John Stansberry, a New Horizons science team member.
Moving from Cthulhu to Pluto’s far western hemisphere, a strange series of “bite marks” could mark an area where active sublimation – the process by which a solid transitions to a gas while skipping the liquid stage altogether – is occurring on a large scale.
The region in question contains a series of cratered plateau uplands (Vega Terra), jagged scarps and wall cliffs (Piri Rupes), and a young, nearly crater-free plain (Piri Planitia).
Compositional data from the Ralph/LEISA instrument indicates that the Vega Terra uplands are rich in methane ice and that the sublimation of methane from the uplands may be causing the uplands to erode and retreat – leaving Piri Planitia in their wake.
The same data also confirms that Piri Planitia is more rich in water ice than the surrounding terrain – indicating that it’s made of water ice bedrock.
Furthermore, March brought new photographic evidence that liquids might have once flowed across Pluto’s surface millions or even billions of years ago when the dwarf planet was much younger, had a higher surface temperature, and had a thicker atmosphere.
Sprawling across Pluto’s icy landscape is an unusual geologic fracture pattern.
“The pattern these fractures form is like nothing we’ve seen in the outer solar system, and shows once again that anywhere we look on Pluto, we see something different,” said Oliver White, a member of the New Horizons geology team.
The feature in question consists of at least six extensional fractures converging to a point near the center. The longest fractures are aligned roughly north-south, with the shortest one aligned east-west.
To the north and west, the fractures extend across the mottled, rolling plains of the high northern latitudes, and to the south, they intercept and cut through the bladed terrain informally named Tartarus Dorsa.
New Horizons scientists think fractures seen elsewhere on Pluto – which tend to run parallel to one another in long belts – are caused by a global-scale extension of Pluto’s water-ice crust.
Instead, the unusual fractures in the high northern latitudes may be caused by a focused source of stress in the crust under the point where the fractures converge.
While not seen anywhere else in the outer solar system, the fractures do somewhat resemble radially fractured novas on Venus as well as the Pantheon Fossae formation on Mercury.
From the surface to the atmosphere, April also returned more information on the complex nitrogen atmosphere surrounding Pluto, specifically the layers of haze first discovered in July 2015.
Scientists have now observed that the layers of haze vary in brightness depending on illumination and viewpoint while they consistently maintain their overall vertical structure.
Observations of Pluto’s atmosphere occurred after closest approach when New Horizons swung behind the dwarf planet so that its atmosphere was backlit by the sun.
Data from New Horizons shows that the brightness variations – up to 30% during a three hour period – may be due to buoyancy waves, also known to atmospheric scientists as gravity waves, which are typically lofted by the flow of air over mountain ranges.
For the New Horizons/Pluto team, May began with a fascinating understanding of how Pluto interacts with the solar wind in comparison to the major planets of the solar system.
As Pluto is wont to do, the dwarf doesn’t quite fit the exact profile of either a planet or a comet when it comes to the solar wind – though it does behave more like a planet than it does a comet.
During the flyby, New Horizons’ Solar Wind Around Pluto (SWAP) instrument observed the material coming off of Pluto’s atmosphere and studied how it interacted with the solar wind.
The main reason scientists thought Pluto would behave more comet-like is because of its distance from the sun and its small size. With these two elements at play, scientists thought Pluto’s gravity wouldn’t be strong enough to hold heavy ions in its extended atmosphere – the ions that help abruptly diverge the solar wind around the major planets.
When the solar wind meets a major planet, the heavy ions in their atmospheres help deflect the brunt of the solar wind; at the same time, some of those heavy ions are stripped away from the atmosphere by the solar wind.
And this is what was seen at Pluto in the form of heavy ions of methane, though scientists were right in that Pluto’s small size and distance from the sun does make it behave somewhat comet-like with the solar wind.
Thus, Pluto is a hybrid between planet and comet. “This is a type of interaction we’ve never seen before anywhere in our solar system,” said David J. McComas, vice president for the Princeton Plasma Physics Laboratory. “The results are astonishing.”
Moreover, Alan Stern said, “These results speak to the power of exploration. Once again we’ve gone to a new place and found ourselves discovering entirely new kinds of expressions in nature.”
The solar wind observation also allowed scientists to learn that: Pluto (like Earth) has a long ion tail that extends downwind that is loaded with heavy ions from the atmosphere and has a “considerable structure”; Pluto’s obstruction of the solar wind upwind of the dwarf planet is smaller than thought, with the solar wind not being completely blocked until ~3,000 km (1,844 mi); Pluto has a very thin boundary between its tail of heavy ions and the sheath of the shocked solar wind, which actually serves as an obstacle to the tail’s flow.
Even more with Pluto’s atmosphere, scientists also announced in May that data from Pluto’s occultation of two ultraviolet stars returned confirmation of several major findings about the dwarf planet’s atmosphere.
The occultation occurred 4 hours after New Horizons made its closest approach to Pluto when the Alice ultraviolet spectrometer watched as two bright ultraviolet stars passed behind Pluto and its atmosphere.
The light from each star dimmed as it moved through deeper layers of Pluto’s atmosphere, absorbed by various gases and hazes.
Much like the solar occultation Alice observed a few hours before, these stellar occultations provided information about the composition and structure of Pluto’s atmosphere, revealing the ultraviolet spectral fingerprints of nitrogen, hydrocarbons (methane and acetylene), and haze.
The results from the solar and stellar occultations were consistent in terms of vertical pressure and temperature structure readings of Pluto’s upper atmosphere, meaning the upper atmosphere’s vertical profiles of nitrogen, methane, and the observed hydrocarbons are similar over many locations on Pluto.
Also confirmed by the stellar occultations is that the escape rate of nitrogen from Pluto’s atmosphere is about 1,000 times lower than expected before the flyby.
Back to the surface, Pluto’s unique composition continued to surprise, revealing more never before seen terrain in the outer solar system.
This time, the Venera Terra region was found to sport an expanse of bright plains divided into polygon-shaped blocks by a network of dark, connected valleys that are spotted with numerous impact craters – indicating that this surface area formed early in Pluto’s history.
Moreover, scientists haven’t seen this type of terrain anywhere else on Pluto, and it’s actually quite rare terrain across the entire solar system – with the only other well-known example being the Noctis Labyrinthus on Mars.
The distinct interconnected valley network was likely formed by extensional fracturing of Pluto’s surface, and the valleys separating the blocks may then have been widened by movement of nitrogen ice glaciers or flowing liquids or possibly by ice sublimation at the block margins.
The blocks themselves were found to be rich in methane ice, which is especially susceptible to sublimation at Pluto surface conditions.
Lastly, May finally saw the return of the first data points regarding one of Pluto’s four small satellites – in this case, Hydra: the outermost moon of the Plutonian system.
Specifically, the data confirmed what was first suspected in 2015 – that Hydra’s surface is dominated by nearly pristine water ice.
Infrared spectra analysis of Hydra showed the unmistakable signature of crystalline water ice – a spectrum return very similar to Pluto’s largest moon, Charon… which is also dominated by crystalline water ice.
Hydra’s water-ice absorption bands, however, are deeper than Charon’s, suggesting that ice grains on Hydra’s surface are larger or reflect more light at certain angles than the grains on Charon.
Hydra is thought to have formed in an icy debris disk produced when water-rich mantles were stripped from the two bodies that collided to form the Pluto-Charon binary ~4 billion years ago.
Hydra’s deep water bands and high reflectance imply relatively little contamination by the darker materials that have accumulated on Charon’s surface.
But exactly why Hydra is so much cleaner than Charon is a mystery.
“Perhaps micrometeorite impacts continually refresh the surface of Hydra by blasting off contaminants,” said Simon Porter, a New Horizons science team member. “This process would have been ineffective on the much larger Charon, whose stronger gravity retains any debris created by these impacts.”
With June came an even better understanding of the Sputnik Planum region’s nitrogen ice – specifically the icy, churning, convective cells that pepper its surface.
“We found evidence that even on a distant cold planet billions of miles from Earth, there is sufficient energy for vigorous geological activity, as long as you have ‘the right stuff,’ meaning something as soft and pliable as solid nitrogen,” said William McKinnon, deputy lead of the New Horizons Geology, Geophysics and Imaging team.
New information studied from New Horizons revealed that these cells are about 500,000 years old and that their pattern is due to the slow thermal convection of the nitrogen-dominated ices on Sputnik Planum.
What was first understood to be a lava lamp-like convection earlier in the year yielded new information that this convection can occur in areas where the nitrogen ice is only a few kilometers (miles) deep.
“Sputnik Planum is one of the most amazing geological discoveries in 50-plus years of planetary exploration, and the finding by McKinnon and others on our science team that this vast area is created by current day ice convection is among the most spectacular of the New Horizons mission,” said Alan Stern.
From here, information in June shifted mostly to Pluto’s small moon, Nix.
Newly arrived spectral investigations revealed Nix’s surface to be startlingly composed of nearly pure water ice – far more so than spectral returns from Charon and Hydra.
This observation allowed for a more detailed understanding of Pluto’s four small moons.
“Pluto’s small satellites probably all formed out of the cloud of debris created by the impact of a small planet onto a young Pluto,” said New Horizons Project Scientist Hal Weaver. “So we would expect them all to be made of similar material. The strong signature of water-ice absorption on the surfaces of [Charon, Nix, and Hydra] adds weight to this scenario.”
However, the striking difference between water ice spectral readings of Nix and Hydra raises new questions, specifically why Nix and Hydra apparently have different ice textures on their surfaces and why Hydra’s surface reflectivity at visible wavelengths is higher than Nix’s even though Nix’s surface appears to be icier.
After a quiet science release period from NASA in July and August, more information on Pluto was released in September – the first of which came from NASA’s Chandra X-Ray Observatory.
The telescope revealed the first detections of X-rays emanating from Pluto, offering new insight into the space environment surrounding the largest and best-known object of the Kuiper Belt.
Overall, low-energy x-rays were detected each of the four times Chandra was pointed at Pluto from February 2014 to August 2015.
“We’ve just detected, for the first time, X-rays coming from an object in our Kuiper Belt and learned that Pluto is interacting with the solar wind in an unexpected and energetic fashion,” said Carey Lisse of APL.
The Chandra detection is especially surprising not just because scientists didn’t expect to find x-ray emissions but because, while Pluto is releasing enough gas from its atmosphere to make the observed X-rays possible, the solar wind at the distance of Pluto isn’t prevalent enough to make them.
Scientists have several hypotheses for the enhanced X-ray emissions from Pluto, including a much wider and longer tail of gases trailing Pluto than New Horizons detected using its SWAP instrument; that interplanetary magnetic fields are focusing more particles than expected from the solar wind into the region around Pluto; and/or that the low density of the solar wind in the outer solar system could allow for the formation of a doughnut, or torus, of neutral gas centered around Pluto’s orbit.
September also brought new information regarding the large, reddish polar region on Charon – which, when first discovered, was odd as it matched no other color feature on the moon.
Now, scientists believe that Charon’s polar coloring comes from Pluto in the form of methane gas that escapes from Pluto’s atmosphere, becomes trapped by Charon’s gravity, and then freezes to the cold, icy surface at the moon’s pole.
Once on Charon’s surface, ultraviolet light from the sun transforms the methane into heavier hydrocarbons and eventually into reddish organic materials called tholins.
“The methane molecules bounce around on Charon’s surface until they either escape back into space or land on the cold pole, where they freeze solid, forming a thin coating of methane ice that lasts until sunlight comes back in the spring,” Grundy said.
When sunlight returns to the polar region, the methane ice quickly sublimates, leaving the heavier hydrocarbons – which eventually irradiate into the reddish material seen today.
Like the middle of the year, October and November brought no new science releases from NASA; but December did yield more information about Sputnik Planum – renamed in the middle of the year to Sputnik Planitia due to its better understood low elevation.
The formation of Sputnik Planitia – a deep basin containing nitrogen, methane, and carbon monoxide ices – is widely believed to have occurred following an impact event from a large Kuiper Belt Object.
However, new research suggests that no impact would have been needed to form Sputnik Planitia – which is tidally locked on the opposite side of the planet from Charon. Instead, Sputnik Planitia could have formed early in Pluto’s life when the icy world had a greater rotation speed.
“Once the ice cap forms, it provides a slight asymmetry that either locks toward or away from Charon when Pluto’s spin slows to match the orbital motion of the moon,” said Douglas Hamilton, a New Horizons team member.
Moreover, the complex interaction of this ice cap and its relationship to Pluto’s atmosphere and the dwarf planet’s 120° axial tilt could explain why Sputnik Planitia formed in the first place as well as why it’s so much lower than the surrounding terrain.
Hamilton and his team modeled Pluto’s temperatures over the world’s 248-year orbit and found that latitudes near 30° north and south are the coldest places on the planet – far colder than either pole.
Therefore, ice would naturally form around these latitudes, including at the center of Sputnik Planitia, which is located at 25° north latitude.
Then, as a result of ice forming at these regions, temperatures there would remain lower throughout the year, which would attract more ice – a process known as the runaway albedo effect.
This would eventually lead to a single dominating ice cap, just like Sputnik Planitia.
As billions of years of ice cap development occurred, the ice cap may have then become so heavy that it could sink a few kilometers (miles) into Pluto’s crust, explaining why Sputnik Planitia is lower than the surrounding terrain.
“Sputnik Planitia is one of Pluto’s crown jewels, and understanding its origin is a puzzle,” said Alan Stern. “Whatever caused it 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.”
(Part 5 – Planet Nine – of NASASpaceflight.com’s five part Year In Review will be published in the coming days)
(Images: NASA, APL, SWRI, and the American Geophysical Union)