New images from NASA's New Horizons spacecraft reveal a bewildering array of geologic features — all carved by or into exotic ices — on the surface of Pluto.
For at least a decade, Alan Stern, the principal investigator and head cheerleader of NASA's New Horizons mission, has been extolling all the great results we'd get from the spacecraft's flyby of Pluto. Even after showing off eye-popping images from July 14th's long-awaited encounter, Stern constantly reminded us that the best was yet to come. "We've never been to a double-planet system before," he says, "and it's turning out to be a scientific wonderland."
Indeed, only about 5% of the images, spectra, and other observations made by the New Horizons spacecraft have reached the mission's scientists. More findings, especially results involving the solar wind and its interaction with Pluto, will be radioed to Earth in the weeks and months ahead.But if what mission scientists already have in hand is any indication of what the remaining 95% is like, it will be, as team member Bonnie Buratti says, "like opening up a birthday present every day from now until the end of the next year.”
I, for one, can't stop staring at the false-color image seen here. NASA released it on Friday, in conjunction with a televised press briefing to bring everyone up to date on the mission's findings. This view, actually a composite of four quadrants recorded through four visible and near-infrared filters, reveals features on the surface of Pluto as small as 1.4 miles (2.2 km) across. Click on the image to see all of its eye-popping detail.
Here's a quick orientation: Pluto's north pole is near the top, tipped a bit toward the camera. Its equator runs along the bottom, marked by a broad dark region at lower left (provisionally named Cthulhu) and another at lower right (Krun). In between is the bright, heart-shaped region first seen months ago (dubbed Tombaugh Regio).
Even in the early views returned by New Horizons last week, it was obvious that the heart's left half, called Sputnik Planum, is roughly the size of Texas and very, very smooth. This characteristic remains true even when the terrain is seen in higher resolution, as in the close-ups below. Along its lower-left margin are clusters of mountains called Hillary Montes and Norgay Montes, names to honor the first climbers to reach the summit of Mount Everest. (Check out this animated "flyover" of Sputnik Planum.)
The complete absence of impact craters on Sputnik Planum means it can't be older than a few tens of millions of years. Some as-yet unknown process must be continually renewing the surface, and this activity could be ongoing today. "To see evidence for recent geological activity is simply a dream come true," observes investigator William McKinnon (Washington University).
Maybe there aren't any craters because the surface is moving around. A close-up made public Friday, seen above, shows that icy flows are moving northward into a much older, eroded, heavily cratered, and generally beat-up-looking terrain. In one spot, ice has moved through a crater's breached rim and onto its floor.
These flows mimic the motions of glaciers on Earth, but they can't be moving slabs of water ice. Pluto's surface temperature (as measured by the spacecraft's radio experiment) is just 38 Kelvin or –391°F — at which water ice is about as mobile as the Rock of Gibraltar. Instead, the flows must consist of frozen nitrogen (N2), methane (CH4), and carbon monoxide (CO), all of which remain pliable even in the Pluto's extreme cold. In fact, spectra taken by the spacecraft show that CO is concentrated within Sputnik Planum and nowhere else on Pluto.
With each new wave of data, it's looking more and more like Sputnik Planum serves as a reservoir for the bright, icy material seen around its margins. Maybe all that ice flowed in from its surroundings or was somehow churned or percolated up from the interior. At this point, no one knows its origin. Eventually, the science team will have stereo imagery to show whether this region is higher than its surroundings — and if so by how much.
They'll also have highly diagnostic visible and near-infrared spectra spaced over the disk roughly every 5 km (3 miles). The spacecraft's observations have shown so far that, in addition to various ices, Pluto's surface has a coating of reddish material that's most likely a mixture of complex organic compounds. Notice the reddish tint in the darkest areas? Scientists suspect these dark spots, and others, are coated with deposits of tholins, a catchall term coined in the 1970s by Carl Sagan and Bishun Khare for complex, carbon-rich organic compounds.
A Hazy Halo
Ever since the discovery of Pluto's tenuous atmosphere in 1988, planetary scientists have debated whether a delicate haze is suspended above the frigid surface. But now there's no question. About 7 hours after passing closest to Pluto, New Horizons briefly ducked into its shadow and took an images looking back toward the Sun.
The result, as revealed on Friday, is a silhouetted globe surrounded by a bright, gauzy halo. According to team member Michael Summers (George Mason University), the haze extends to heights of roughly 160 km (100 miles), with hints of discrete layers — or maybe density enhancements due to waves — about 50 and 80 km (30 and 50 miles) up.
Here's the conundrum: models suggest that hazes shouldn't exist any higher than about 30 km, because higher up the wisps of gas are supposedly too warm for hazes to condense. So Pluto's hazes extend five times higher than those models predict. "We're having to start from scratch" to understand the atmosphere, notes a stunned Summers.
Pluto's haze has a lot in common with hydrocarbon smogs found on Earth. Even 30 astronomical units from the Sun, ultraviolet sunlight is intense enough to break down molecules of methane, which first recombine into ethylene (C2H4) and acetylene (C2H2) — both detected by New Horizons. With time, Summers explains, these clump together to form haze particles that grow bigger and eventually gently fall to ground. Long-term exposure to more sunlight and space radiation eventually converts these molecules to tholins.
The chemistry is straightforward, but what the team doesn't yet understand is why the haze floats so high. Nor is it clear why Pluto's atmosphere, after gradually increasing over the past two decades (as determined from Earth by occultation measurements), has suddenly collapsed. Based on occultation conducted with a radio beacon from Earth as the spacecraft briefly ducked behind Pluto, the surface pressure is no more than 10 microbars. Yet as recently as two years ago, ground-based occultations found it was nearly twice that value.
To give you a sense of just how tenuous Pluto's atmosphere is, 10 microbars is what you'd experience at an altitude of 96 km (54 miles) above Earth. The Kármán line, used by aerospace specialists to define the beginning of outer space, lies at 100 km. So little gas — and yet such an obvious haze!
Curious about the extent of Pluto's atmosphere, I asked mission scientist Randy Gladstone (Southwest Research Institute) to estimate its total mass. "The 10-microbar number is still pretty preliminary," he replied, because only a fragment of the occultation data is in hand. But, assuming that value is correct, and that it's entirely nitrogen (not quite, but close), he estimates a total mass of 30 billion metric tons. That's seems impressive — yet it's only enough to coat the nitrogen to coat Pluto's entire surface with a layer of nitrogen frost just 1½ mm thick!
A "Time Out" for New Images
Keep in mind that we've only seen a handful of the images and spectra stored amid the 50 gigabytes of results gathered by New Horizons. However, you won't see much at all over the next two months (except for an occasional frame of particular interest to mission scientists). Instead, through mid-September the spacecraft will be streaming back engineering data as well as real-time measurements made by its solar-wind and energetic-particle detectors, known by their respective acronyms SWAP and PEPSSI. We'll also get a peek at whether the craft's student-built dust detector picked up any particles as the spacecraft dashed through the orbital plane of Pluto's five moons.
After that, a steady stream of images and spectra will arrive on Earth. "The sky will be raining presents from the Pluto system," Stern notes. By mid-November, all the observations should be in hand — albeit in compressed form. Then the spacecraft will start the complete playback a second time, but without any compression. That effort will take most of 2016 to complete.
You can get more details at NASA's New Horizons website.
Alan Stern has written a series of blogs exclusively for SkyandTelescope.com. Here's the one describing the observations that New Horizons made during its historic flyby.