In July of 2015, after a nine year journey through space, New Horizons flew past Pluto at a speed of more than 30,000 mph (13 km/s). Over the span of just a few hours, it took so much data with so many cameras and instruments it took a full 16 months to send it all back to Earth, a task that was just completed weeks ago. The data it sent back allowed us to construct a full map of one of Pluto’s hemispheres, plus a glorious backlit shot of its night side in eclipse. But scientifically, there was so much more than a slew of beautiful pictures, and that data enabled us to understand, for the first time, the interior of a Kuiper belt world.
Here on Earth, we have mountains, plateaus, plains and oceans covering the surface. But these surface variations correspond to different physical properties the farther down into the Earth’s interior you go. The Earth’s crust floats atop the mantle, which in turn floats above the outer and inner cores. Similarly, the ocean floats above the crust, and the atmosphere above them both. In general, the less dense layers of any world are found atop the denser layers, and that gives rise to what we see here on the surface. But just as water has to displace to stably support a ship submerged in it, a lower layer needs to displace so that mountains don’t tip over or so that upswells don’t destroy valleys or crustal troughs. In order for these surface variations to exist and be stable, we need the lower layers to compensate as well.
On Earth, that means that the highest mountain ranges also see the crust dip into the mantle beneath those ranges a significant amount, something we can detect my intricately measuring the Earth’s magnetic field. The ocean bottoms are where the crust is thinnest: only 2-5 km thick in some places. And similarly, plateaus, plains and continental shelves have identifiable features beneath the surface as well. Our active geology isn’t just about what happens on the surface, but deep in the planet’s interior as well.
Pluto may not officially be an astronomical planet, but as a world, it has its own complex, interesting and active geology. A combination of four types of molecules — nitrogen, methane, water and carbon monoxide — can all exist in the solid, liquid and gaseous phases on Pluto, and give rise to an incredible variety of terrain. The tall, water-ice mountains; the cracked, washboard-like terrain; the cellular ice plains with flowing streams; the dark-colored highlands and more display huge variations in crustal thickness, age and altitude. An ultra-high-resolution flyover showcases some of the greatest variations.
Now that the map of Pluto is complete and the varied terrain has been studied, scientists have determined regions of instability and have identified how the Plutonian interior must be behaving in order to deliver the Pluto that we see. The surface features we see are transient on significantly shorter timescales than the mountains and continents are on Earth, and faulting and mountainous reorientation must be common. Sputnik Planitia, a large, teardrop-shaped depression, represents a massive unit of actively convecting volatile ices several kilometers thick. The gravitational stresses resulting from this instability can lead to planet-wide faulting in the crust, further indicating how active Pluto is.
Despite having a less-dense ice surface that must be 3-4 kilometers thick, with a denser layer more similar to the rest of Pluto’s surface underneath it, this portion of Pluto exhibits a positive gravitational anomaly. Much like Earth’s oceans, where the crust is thinnest, can be explained by Earth’s sub-crustal mantle, Sputnik Planitia could be explained as a natural result if Pluto has a huge subsurface ocean. In particular, the New Horizons Geology, Geophysics & Imaging Theme Team indicates it:
would naturally result because of shell thinning and ocean uplift, followed by later modest nitrogen deposition
With a subsurface ocean, Pluto’s entire geology can be explained in one fell swoop.
Just as on Earth, we say “it’s only the tip of the iceberg” with the full knowledge that 90% of the mass of an iceberg is submerged beneath the surface, there should be a water-ice ocean beneath an icy crust, and that crust should be thinnest underneath the crater that Sputnik Planitia resides in. The left “lobe” of Pluto’s famous heart is the deepest depression in the Plutonian surface, and must have reoriented itself to align in a gravitationally favorable way with the Pluto-Charon axis. With this observation under our belts, we can now more accurately map the interior of Pluto than ever before.
Most impressively, this research raises a tantalizing possibility: that as Sputnik Planitia continues to accumulate ices, Pluto may yet reorient itself again, as subsurface changes continue to ensue. This is possible because nitrogen becomes an atmospheric gas during the “day” side, but then when Pluto continues in its orbit and the nitrogen heads to the “night” side, it precipitates, and some of that lands in Pluto’s heart. According to researcher James Keane,
Each time Pluto goes around the sun, a bit of nitrogen accumulates in the heart. And once enough ice has piled up, maybe a hundred meters thick, it starts to overwhelm the planet’s shape, which dictates the planet’s orientation. And if you have an excess of mass in one spot on the planet, it wants to go to the equator. Eventually, over millions of years, it will drag the whole planet over.
The biggest implications are for a massive subsurface ocean on Pluto, but this also indicates a world that continues to change, evolve, tip, crack and even reorient itself as time goes on. The most distant worlds in our Solar System are still active. Being frozen was never such a hot topic as it is today.
This article was based on the research in the Nature Letters “Reorientation of Sputnik Planitia implies a subsurface ocean on Pluto” by F. Nimmo et al., Nature (2016) and “Reorientation and faulting of Pluto due to volatile loading within Sputnik Planitia” by J. Keane et al., Nature (2016).
Pluto's Heavy Heart Shaped Dwarf Planet's Landscape
Pluto's famous heart may not have been born in violence after all.
Researchers have generally assumed that the heart's left "lobe" — a 600-mile-wide (1,000 kilometers) plain called Sputnik Planitia (formerly Sputnik Planum) — is an enormous impact crater that subsequently filled with frozen nitrogen and other exotic ices.
But a new study suggests that the ice buildup came first and the accumulated material eventually pushed the underlying landscape down, much as Greenland's enormous ice sheet has done here on Earth.
"Pluto's big heart weighs heavily on the small planet, leading inevitably to depression," study lead author Douglas Hamilton, a professor of astronomy at the University of Maryland, said in a statement.
It's no accident that Sputnik Planitia — which was discovered by NASA's New Horizons probe during its epic Pluto flyby in July 2015 — is centered at 25 degrees north, Hamilton said. His team's computer models predicted that ice would accumulate at about 30 degrees north or south latitude, because these are the coldest regions on Pluto. (The dwarf planet is tilted roughly 120 degrees relative to its orbital plane, compared with 23 degrees for Earth.)
"The poles on Pluto, averaged over a year, are actually the hottest parts of the planet, not the coldest," Hamilton told Space.com, referring to one Pluto year, which lasts 248 Earth years. "That's alien to us and sounds wrong, but math doesn't lie. That's how it works out."
In addition, the group's simulations indicated that ice wouldn't build up in a band at 30 degrees north and/or south. Rather, a single ice cap would form, as the result of a (nearly literal) snowball effect: As ices started to accumulate in one spot, that area would reflect more sunlight and thus become colder than surrounding regions, leading to greater ice deposition, and so on.
Previous studies based on New Horizons data suggest that Sputnik Planitia's ice is at least 1.2 to 1.8 miles (2 to 3 km) thick (and possibly much thicker). So, at a minimum, 0.03 percent of Pluto's entire mass is concentrated in the ice cap, Hamilton said. That's enough material to push the landscape down and form a huge basin, he added.
The team's models further indicate that ice accumulation in Sputnik Planitia happened quickly. The cap likely formed within a million years of the giant impact on Pluto that formed the dwarf planet's largest moon, Charon, and has been relatively stable since, researchers said. (Exactly when the Charon-forming impact occurred is unknown, but it was probably quite early in the solar system's history.)
After Charon's birth, Pluto's rotation slowed until the two bodies were "tidally locked," showing each other only one face, as Earth's moon shows just one side to us. According to the researchers' simulations, Charon's gravitational tug then pulled Sputnik Planitia into its current alignment opposite the moon.
In short, the team's modeling results explain pretty well what New Horizons saw on July 14, 2015, Hamilton said.
Indeed, given any starting conditions on Pluto, those simulations produce an ice cap sitting in a basin in one of four spots, Hamilton said — at about 30 degrees north or south, and either facing toward or away from Charon.
This scenario provides a "simpler" explanation for Sputnik Planitia than the prevailing view, which posits that the basin formed after a cosmic impact, study team members said.
"This interpretation has the advantage of providing an explanation for why the basin is coincident with the ice cap and why both are located at the coldest latitude on Pluto and at a longitude that is directly opposite Charon," Hamilton and his colleagues wrote in the new study, which was published online today (Nov. 30) in the journal Nature.
Other researchers have modeled the formation and evolution of Sputnik Planitia as well, but these efforts have tended to assume that the basin is an ancient impact feature. One such team, led by Francis Nimmo of the University of California, Santa Cruz, recently concluded that the position of Sputnik Planitia hints at the presence of a subsurface ocean on Pluto, a possibility raised by several other lines of evidence as well.
The new study has little bearing on this question, said Hamilton, who was also a co-author on the Nimmo-led paper.
"My modeling doesn't care too much one way or the other about whether there's an ocean," Hamilton told Space.com.
Definitively figuring out whether Pluto harbors a buried ocean may require launching an orbiter to the dwarf planet, he added.
As 2016 ends, I can’t help but point out an interesting symmetry in where the mission has recently been and where we are going. Exactly two years ago we had just taken New Horizons out of cruise hibernation to begin preparations for the Pluto flyby. And exactly two years from now we will be on final approach to our next flyby, which will culminate with a very close approach to a small Kuiper Belt object (KBO) called 2014 MU69 – a billion miles farther out than Pluto – on Jan. 1, 2019. Just now, as 2016 ends, we are at the halfway point between those two milestones.
During this phase between flyby operations, all of the systems and scientific instruments aboard New Horizons are healthy. In October, we completed the 16-month-long transmission of all Pluto flyby data to Earth. Our science team is now steadily analyzing those data, making new discoveries and writing reports to research journals like Science, Nature, Icarus, the Journal of Geophysical Research and the Astronomical Journal. Almost 50 scientific papers reporting new results about Pluto and its system of moons were submitted this year!
Additionally, our science and science operations teams have made two major Pluto submissions to NASA’s archive of all planetary mission data, the Planetary Data System (PDS). Two final submissions to the PDS will be made in 2017, wrapping up the archiving of Pluto data for others in the scientific community to use. Those upcoming submissions will include better-calibrated datasets resulting from the intensive, post-Pluto flyby calibration campaign we conducted this summer using all seven payload instruments aboard New Horizons and a series of “meta-products” like maps and atmospheric profiles created from New Horizons data.
The year ahead will begin with observations of a half-dozen KBOs by our LORRI telescope/imager in January. Those observations, like the ones we made in 2016 of another half-dozen KBOs, are designed to better understand the orbits, surface properties, shapes, satellite systems and frequency of rings around these objects. These observations can’t be done from any groundbased telescope, the Hubble Space Telescope, or any other spacecraft – because all of those other resources are either too far away or viewing from the wrong angles to accomplish this science. So this work is something that only New Horizons can accomplish.
New Horizons prepares for New Year’s Day 2019 Kuiper Belt Object encounter
A year and a half after its historic flyby of dwarf planet Pluto, NASA’s New Horizons spacecraft is preparing for its encounter with a second Kuiper Belt Object. Now just two years away from the planned 1 January 2019 encounter with 2014 MU69, New Horizons is in a healthy state as it sails toward the small, rocky, classical Kuiper Belt Object.
2014 MU69 – a new target for New Horizons:
Kuiper Belt Object (KBO) 2014 MU69 – located approximately 1.6 billion km (1 billion mi) beyond Pluto – was discovered by the Hubble Space Telescope on 26 June 2014 during a dedicated survey of the sky and portion of the Kuiper Belt along New Horizons’ post-Pluto trajectory to find potential targets for the craft to encounter after Pluto.
The post-launch search for another flyby target for New Horizons was designed to maximize the science return from the mission in the far and difficult to resolve region of our solar system and to satisfy a Decadal Survey recommendation that any mission to Pluto not only investigate that world but as many others as possible in the Kuiper Belt.
To this end, the Hubble Space Telescope was employed in the search for a new Kuiper Belt flyby target because of its high precision observation capability, ability to resolve objects with high apparent magnitudes that are not visible to Earth-based telescopes, and its ability to provide reliable orbit determinations of any objects discovered.
When 2014 MU69 was discovered, it was initially labeled 1110113Y, with a nickname “11”, and was announced by NASA as a potential post-Pluto target for New Horizons in October 2014 – receiving the internal label PT1 (Potential Target 1) by NASA.
The object did not receive its official designation of 2014 MU69by the Minor Planet Center until March 2015 after sufficient orbital information had been computed via multiple observations of the body.
The designation of 2014 MU69 is, in fact, itself a provisional designation and indicates that the object was the 1,745th object discovered between 16 and 30 June 2014.
Based on current observations, 2014 MU69 is understood to be in a 295 Earth-years orbit with an aphelion of 45.86 AU and a perihelion of 42.69 ±0.04 AU – with a semi-major axis of 44.28 AU and an orbital eccentricity of 0.0358 ±0.001.
The object’s orbit is inclined 2.4532° ±0.0002° to the ecliptic plane (Earth’s orbital plane as defined by the path of the Sun on the sky) and carries a longitude of ascending node (the angular position of a celestial body as it travels across the reference plane as measured from a reference point on the ecliptic plane) of 158.933° ±0.007.
Moreover, 2014 MU69 has an argument of perihelion (the angular distance from the ascending node to the perihelion measured in the orbit plane) of 179.9° ±1° and is currently understood to be between 25 – 45 km (15.5 – 28 mi) in diameter and is observed to have an apparent magnitude of 26.8 and an absolute magnitude of 10.9 – with ranges for its assumed albedo being 0.04 – 0.15.
Moreover, the low inclination and low eccentricity of the object’s orbit indicate that 2014 MU69 is a cold, classical KBO that is unlikely to have experienced significant perturbations over the course of its existence.
New Horizons preparations and observations:
Following the historic flyby of Pluto, the New Horizons team began preparing the spacecraft for a potential encounter with another KBO.
At the time of the Pluto encounter, two potential KBOs for a post-Pluto flyby remained of the initial four potential targets.
Those two remaining targets, PT1 and PT3, were both easily accessible given New Horizons’ remaining trajectory-adjustment fuel supply, though PT3 was actually the more preferred object from a scientific perspective.
Nonetheless, PT3 would have required nearly all of the trajectory-adjustment fuel supply aboard New Horizons – which would have left little fuel for adjustment maneuvers based on then unforeseen events.
Therefore, on 28 August 2015, NASA and the New Horizons team officially selected PT1 – 2014 MU69 – as the primary flyby target, while also allowing New Horizons to study nearly 20 KBOs from greater distances.
“2014 MU69 is a great choice because it is just the kind of ancient KBO, formed where it orbits now, that the Decadal Survey desired us to fly by,” said New Horizons Principal Investigator Alan Stern.
“Moreover, this KBO costs less fuel to reach, leaving more fuel for the flyby, for ancillary science, and greater fuel reserves to protect against the unforeseen.”
With the target selected, a series of four trajectory-adjustment burns began on 22 October 2015. The second burn occurred on 25 October, with the third following on 28 October. The maneuver series culminated on 4 November 2015 with a 20-minute burn from New Horizons’ hydrazine thrusters.
In total, the four burns pushed New Horizons sideways from its then-trajectory – totaling a 57 mps (128 mph) nudge that put New Horizons on the proper intercept course for 2014 MU69 .
Each successive burn was the farthest course correction ever performed by a spacecraft and also represented the largest, longest, and quickest-in-succession targeting maneuvers of the New Horizons mission.
However, while these burns set up New Horizons to physically fly by its new KBO target, funding and a specific proposal had not yet been approved or submitted to NASA.
A formal plan to study 2014 MU69 was submitted in early 2016, and NASA officially approved the extended mission on 1 July 2016.
Under the approved plan, the encounter with 2014 MU69 carries six primary mission objectives, including: mapping the surface geology to learn how it formed and has evolved; measuring the surface temperature; mapping the 3D surface topography and surface composition to learn how it is similar to and how it is different from comets like 67P and dwarf planets such as Pluto.
Moreover, New Horizons will also search 2014 MU69 for any signs of activity (such as a cloud-like coma), any satellites or rings, and measure and constrain the body’s mass.
Then, in October 2016, newly analyzed data gathered by the Hubble Space Telescope revealed that the surface of 2014 MU69 is likely red, if not redder, than Pluto.
The observations made 2014 MU69the smallest KBO to have its color measured and confirmed that the object is part of the cold classical region of the Kuiper Belt, which is believed to contain some of the oldest, most prehistoric material in the solar system.
“The reddish color tells us the type of Kuiper Belt object 2014 MU69 is,” said Amanda Zangari, a New Horizons post-doctoral researcher from the Southwest Research Institute.
“The data confirms that on New Year’s Day 2019, New Horizons will be looking at one of the ancient building blocks of the planets.”
With all course correction maneuvers complete and funding in place, New Horizons is expected to fly by 2014 MU69 on 1 January 2019.
At this time, the object will be 43.4 AU from the Sun and located – as viewed from Earth – in the constellation Sagittarius.
While the precise altitude of closest approach has not yet been determined, New Horizons’ scientists have said they will get as close to the object as the navigation team will allow – which is expected to be far closer than the 12,500 km (7,767 mi) close approach New Horizons perform with Pluto in July 2015.
In all, 2014 MU69 will be the second KBO explored via close-up observations – following the exploration of Pluto – and will also make history as it will become the first object to under go up-close investigation by a spacecraft that was launched prior to the object’s discovery.
(Images: NASA, Johns Hopkins University, Applied Physics Laboratory, Southwest Research Institute, Alex Parker, ESA, and the New Horizons KBO Search Team)
Scientists Offer Sharper Insight into Pluto’s Bladed Terrain
Using a model similar to what meteorologists use to forecast weather and a computer simulation of the physics of evaporating ices, scientists have found evidence of snow and ice features on Pluto that, until now, had only been seen on Earth.
Formed by erosion, the features, known as “penitentes,” are bowl-shaped depressions with blade-like spires around the edge that rise several hundreds of feet.
The research, led by John Moores of York University, Toronto, and done in collaboration with scientists at the Johns Hopkins University Applied Physics Laboratory and NASA Goddard Space Flight Center, indicates that these icy features may also exist on other planets where environmental conditions are similar.
The identification of these ridges in Pluto’s informally named Tartarus Dorsa area suggests that the presence of an atmosphere is necessary for the formation of penitentes – which Moores says would explain why they have not previously been seen on other airless icy satellites or dwarf planets. “But exotic differences in the environment give rise to features with very different scales,” he adds. “This test of our terrestrial models for penitentes suggests that we may find these features elsewhere in the solar system, and in other solar systems, where the conditions are right."
The research team, which also includes York’s Christina Smith, Anthony Toigo of APL and Scott Guzewich of Goddard Space Flight Center, compared its model to ridges on Pluto imaged by NASA’s New Horizons spacecraft in 2015. Pluto’s ridges are much larger – more than 1,600 feet (about 500 meters) tall and separated by two to three miles (about three to five kilometers) – than their Earthly counterparts.
“This gargantuan size is predicted by the same theory that explains the formation of these features on Earth,” says Moores. “In fact, we were able to match the size and separation, the direction of the ridges, as well as their age: three pieces of evidence that support our identification of these ridges as penitentes.”
Moores says though Pluto's environment is very different from Earth’s -- it is much colder, the air much thinner, the sun much dimmer and the snow and ice on the surface are made from methane and nitrogen instead of water -- the same laws of nature apply. He adds that both NASA and APL were instrumental in the collaboration that led to this new finding; both provided background information on Pluto's atmosphere using a model similar to what meteorologists use to forecast weather on Earth. This was one of the key ingredients in Moores’ own models of the penitentes, without which this discovery would not have been made.
The findings appear this week in the journal Nature.
Charon Is Pluto's First Line of Defense Against Solar Wind Onslaught
Lacking a strong magnetic shield, Pluto's thin atmosphere is being eroded into space — but Charon is doing its bit to protect its dwarf planet buddy.
Space weather can be a nightmare for planetary atmospheres, particularly for ones that don't have a magnetic field to protect them — unlike Earth's, which has a powerful magnetosphere acting as a shield. It might therefore be strange to hear that dwarf planet Pluto, which isn't known for its powerful global magnetic field, is able to possess an atmosphere at all. But like other planets in the solar system, the sun erodes Pluto's atmosphere — albeit at a slower rate than expected.
Although astronomical measurements detected the presence of an atmosphere at Pluto long before the NASA New Horizons flyby in July 2015, very little was known about how much was being eroded into space by the continuous stream of solar wind particles. New Horizons measurements, however, proved that the rate of atmospheric loss was 100 times less than expected and, in new research published this week in the journal Icarus, researchers think they know what might be protecting Pluto's tenuous atmospheric gases.
Researchers from Georgia Institute of Technology have shown that when Charon orbits between Pluto and the sun, its presence can modify the dwarf planet's bow shock — a standing shock wave that appears "upstream" of Pluto as the solar wind particles encounter Pluto's thin atmosphere, like the wave that roils in front of a boat's bow when it powers through water — thereby shielding Pluto's atmosphere for a short time. Charon maximizes this protection should it also have an atmosphere, but its protective impact is minimal when it either doesn't have an atmosphere or when it is positioned "downstream" of Pluto.
As Pluto and Charon orbit so close to one another, the pair are believed to share atmospheric gases and when Charon passes behind Pluto particles originating from Pluto are deposited at the moon's poles, appearing as a dark brown deposit in New Horizons observations.
As Pluto is located so far away from the sun in the Kuiper Belt, the impact of the solar wind is much lower than its impact on planets closer to the sun. The space weather impact has been reduced even further with the help of Charon.
"As a result, Pluto still has more of its volatile elements, which have long since been blown off the inner planets by solar wind," said Georgia Tech student John Hale. "Even at its great distance from the sun, Pluto is slowly losing its atmosphere. Knowing the rate at which Pluto's atmosphere is being lost can tell us how much atmosphere it had to begin with, and therefore what it looked like originally. From there, we can get an idea of what the solar system was made of during its formation."
As Pluto and Charon orbit so close, and Charon is roughly half the size of its dwarf planet buddy, the pair orbit a common point in space known as the "barycenter." This orbital oddity added fuel to the debate as to whether Pluto should be called a dwarf planet, or whether Pluto and Charon should be designated a "binary planet." Now, with more findings about the pair's atmospheric interactions, it could be argued that the case for calling Pluto a binary planet is as valid as ever.
Pluto Global Color Map
This new, detailed global mosaic color map of Pluto is based on a series of three color filter images obtained by the Ralph/Multispectral Visual Imaging Camera aboard New Horizons during the NASA spacecraft’s close flyby of Pluto in July 2015. The mosaic shows how Pluto’s large-scale color patterns extend beyond the hemisphere facing New Horizons at closest approach, which were imaged at the highest resolution. North is up; Pluto’s equator roughly bisects the band of dark red terrains running across the lower third of the map. Pluto’s giant, informally named Sputnik Planitia glacier – the left half of Pluto’s signature “heart” feature – is at the center of this map. Note: Click on the image to view in the highest resolution.