BOSTON — Alan Stern, principal investigator of the New Horizons mission to Pluto, began his talk at the meeting of the American Association for the Advancement of Science by showing off the Hubble Space Telescope’s best image of Pluto. It was greeted with laughter as it took the audience only seconds to count the dozens of pixels that contained actual data. “You can laugh again,” said Stern. “We’ve written countless articles based on this image.”
He now has a lot more data to work with, although it took him a lot of patience to get it. Not only did it take months to get all of the data from New Horizons back to Earth, but it also took decades for the probe to be approved in the first place. Stern shared that story with his audience in Boston.
The astronomy community meets periodically to do so-called Decadal Surveys, which help NASA prioritize future missions. But as Stern put it, these surveys take into account “far more good ideas than there is budget to implement.” So five Pluto missions had appeared in it without being approved. But a variety of data about Pluto trickled in even without a visit, and this gradually built up the rationale for sending hardware there.
It may surprise some people to learn that Charon, Pluto’s largest moon, was only discovered in 1978 – not long before the Voyagers visited Jupiter. Charon’s large size relative to Pluto is shared by only one other planet-moon system in the solar system: ours. Like Earth’s moon, the relatively small difference in size means that Charon was almost certainly formed by a giant impact and thus offers the only other opportunity to study this that we will soon have access to.
Subsequent infrared imaging showed that the surface was complex and contained carbon monoxide, methane and nitrogen. All three of these compounds could switch between gaseous and solid forms under the conditions expected to prevail on Pluto’s surface, offering the potential for a number of complex surface-atmosphere interactions. And imaging of a star as Pluto passed from Earth’s perspective confirmed that Pluto must have an atmosphere.
Finally, astronomers began to identify a large collection of small, distant objects beyond Pluto. While these eventually relegated Pluto to dwarf planet status, they turned it from a Solar System eccentric into the primary member of an entire class of bodies called Kuiper Belt Objects. Studying Pluto could possibly tell us more about the rest of this.
All this put an intellectual weight behind the idea of sending hardware to the distant body, making it a high priority in the Decadal Survey. After five failed proposals, New Horizons was approved. By the time that happened, however, it was facing a very severe constraint: There was only one window into the next decade when Jupiter would be well positioned to gravitationally assist Pluto. The New Horizons team would have just four years to design and build New Horizons to meet NASA’s science goals.
Despite the limitations, Stern says the spacecraft “has more scientific firepower in it” for initial exploration than anything we’ve sent out before.
Long before New Horizons got closer, it was able to provide better images of Pluto than Hubble ever could. And this was essential; Pluto’s orbit was not known precisely enough to navigate the probe through its flyby. After a few months of imaging, final adjustments could be made.
Stern described the system as working from the outside in. Pluto’s four small moons, Hydra, Kerberos, Nix and Styx, are all irregularly shaped and quite reflective, indicating that their surfaces are likely mostly clean water ice. Craters on the surfaces of the two larger ones indicate that they are about four billion years old and thus likely formed along with the rest of the solar system. The moons are arranged in an alternating order of small-large-small-large, and Stern says this could tell us something about their formation, but we don’t have any other examples like this, so it could also be a coincidence.
Charon, Pluto’s largest and closest companion, is also heavily cratered and likely formed very early in the solar system’s history as well. All of its geography, such as an equatorial belt of ridges, is also very old, suggesting that Charon has been inert for a long time. The surface is mostly water ice with outcrops rich in ammonia, but there are many pits suggesting more volatile materials have boiled off and escaped. This would have given Charon a temporary vibe at some point in the past.
Now its only atmosphere comes from minuscule amounts of gases lost by Pluto and captured by Charon. This is what gives Charon’s poles a darker color (after the gases undergo reactions catalyzed by radiation), and Stern says this is the only example we know of, other than stars, where there is a transfer of atmosphere between two bodies .
The main act
The materials we knew existed on Pluto’s surface aren’t rigid enough to support much in terms of geology, so it wasn’t clear what to expect on the dwarf planet. But for most areas, these materials were just a thin layer on top of water ice, which is a very tough material at plutonic temperatures. At the surface, nitrogen is more common at mid-latitudes and methane at the poles; this, Stern said, could be explained by their volatility at local temperatures. Pluto’s atmosphere turned out to be lower and denser than expected, limiting the rate at which it is lost to space by a factor of about 1000.
The age of the surface varies, with some heavily cratered areas appearing to date from the formation of the dwarf planet. There are a few areas of intermediate craters (the East Tombaugh Reggio), and then there’s the Sputnik Planum. It is a nitrogen glacier where there are hundreds of thousands of square kilometers where they cannot find a single significant crater. Estimates are that the surface is less than 30 million years old. Visually, it appears that there are currents moving through some parts of the glacier and other places where smaller glaciers drain into it.
During the flyby, “My science team didn’t have a single glaciologist on it,” Stern said. He now has two.
Stern also suggested that the New Horizons images contain hints of liquids on Pluto’s surface, almost certainly nitrogen. While Pluto is currently too cold for liquid nitrogen, it experiences the same kind of wobbles that drive Earth’s ice cycles, except that its axis tilt changes even more. During one of the dwarf planet’s warmer phases, Stern says, calculations suggest the existence of liquid nitrogen should be possible.
Finally, there’s a grab bag of other features we don’t currently understand. Stern showed a photo of a mountain comparable in scale to Hawaii’s Mauna Loa: 100 km wide and 4000 m high. He said it is recent (based on craters) and looks like a volcano, but it was not possible to confirm that yet.
All of New Horizons’ Pluto data has now been safely transmitted to Earth and it has fired up the engine needed to send it to its next target, 2014 MU.69. At 44 times the distance from the Earth to the Sun, 2014 MU69 is part of the “cold classic” region of the Kuiper belt; with a diameter of 20-40 km, its size lies intriguingly between minor planets and comets. So there’s a chance that New Horizons will have some more big news during the 2019 flyby, or some other point, before power runs out in the 2030s.
However, while waiting for the next rendezvous, Stern will not be bored. He has already taken up a position in the science team of a planned Europe mission.