October 14, 2019

Scientists find signs of potential geysers on Pluto


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This week's New Horizons flyby of Pluto revealed unexpected terrain, such as these blocky patches of ice at a region dubbed Sputnik Planum. New Horizons' LORRI camera took this image from a distance of 48,000 miles (77,00 kilometers) on July 14. The resolution is about 1 kilometer (0.6 miles). Credit: NASA/JHUAPL/SWRI
This week’s New Horizons flyby of Pluto revealed unexpected terrain, such as these blocky patches of ice at a region dubbed Sputnik Planum. New Horizons’ LORRI camera took this image from a distance of 48,000 miles (77,00 kilometers) on July 14. The resolution is about 1 kilometer (0.6 miles). The large pixelated areas are caused by the compression of the image. A later version of the image will be downlinked without noise. Credit: NASA/JHUAPL/SWRI

New results from NASA’s New Horizons mission, which zipped about 7,700 miles from faraway Pluto on Tuesday, show puzzling icy landforms covered in frozen carbon monoxide and mysterious dark smudges, which may be evidence for plumes erupting from Pluto’s warmer interior, scientists said Friday.

“I’m a little biased, but I think the solar system saved the best for last,” said Alan Stern, the mission’s principal investigator from the Southwest Research Institute.

Imagery from the probe’s telescopic camera captured icy terrain broken into shapes, with vein-like features, some filled with unknown dark material, separating the frozen blocks across a region about the size of the Netherlands.

“I’m still having to remind myself to take deep breaths,” said Jeffrey Moore, a New Horizons co-investigator from NASA’s Ames Research Center in California. “The landscape is just astoundingly amazing.”

The images from New Horizons, which was more than 2 million miles outbound from Pluto on Friday, were culled from a small dataset downlinked to Earth since Tuesday’s flyby. Only about 1 gigabit of the encounter’s total data haul of around 50 gigabits has been downlinked to Earth, according to Stern.

While the data is just a sampling of what is to come, the results so far are rich.

“The surface is broken up into … irregularly-shaped segments that are roughly 12 to 20 miles across,” Moore said. “They are bordered by what appear to be shallow troughs. Some of these troughs have some darker material … I don’t know if that’s material that’s collected there or erupted there. I don’t know.”

The craterless ice field revealed Friday has been dubbed Sputnik Planum. It is part of a larger bright, heart-shaped feature on Pluto informally named Tombaugh Regio after Clyde Tombaugh, who discovered the dwarf planet lurking at the solar system’s frontier in 1930.

Sputnik Planum is part of a swath of Pluto detected by New Horizons to be made of frozen carbon monoxide. Earth-based measurements found carbon monoxide ice on Pluto in 1992, but New Horizons pinpointed its location.

The initial data from New Horizons, which made the first ever visit to Pluto, does not tell whether the carbon monoxide detection is from a thin layer covering another type of ice, or a more significant part of a mixture of methane and nitrogen ice scientists say dominate Pluto’s crust.

The Ralph instrument on New Horizons has discovered the signature of frozen carbon monoxide concentrated in the western half of Pluto's bright heart-shaped feature, informally named Tombaugh Regio. Credit: NASA/JHUAPL/SWRI
The Ralph instrument on New Horizons has discovered the signature of frozen carbon monoxide concentrated in the western half of Pluto’s bright heart-shaped feature, informally named Tombaugh Regio. Credit: NASA/JHUAPL/SWRI

“You only need a centimeter (0.4 inches) or something to produce an absorption of that depth,” said Will Grundy, head of the composition science team on New Horizons. “So we know that there’s a lid that includes a lot of carbon monoxide, but how that interacts is potentially quite subtle.”

At Pluto’s orbit 3 billion miles from the sun, the temperature in the ice field is around minus 400 degrees Fahrenheit.

“Judging from the absence of impact craters, it’s clear that Sputnik Planum couldn’t possibly be more than 100 million years old, and possibly is still being shaped to this day by geologic processes,” Moore said. “This could be only a week old for all we know.”

The ice blocks at Sputnik Planum, which Moore calls polygons, could be produced in several ways.

“One possibility, out of many, is that the polygons are signs of convection occuring within the surface layer of carbon monoxide, methane and nitrogen ice driven by the modest heat from the interior of Pluto itself, creating the same sorts of patterns you see when you look at the surface of a boiling pot of oatmeal, or like the blobs in a lava lamp,” Moore said.

“Alternatively, these polygons could be analogous to mud cracks, and could be caused by contraction of surface materials,” Moore said.

He said he lends more weight to the theory that the icy plain was formed by heat bubbling up from underneath, but thermal contraction, which is responsible for polygon features at the icy northern latitudes of Mars, could also be the root.

The youthful appearance of Pluto’s frozen plains, coupled with the discovery of huge 11,000-foot-tall mountains announced Wednesday, point to tectonic activity on the frozen body, a surprise to many scientists.

Other parts of Pluto have impact craters, evidence that those surfaces likely formed billions of years ago.

“Some of the craters appear partially destroyed, perhaps by erosion, and there are also hints of parts of Pluto’s crust that have been fractured,” Moore said. “That probably indicates some form of tectonics. Now that we’ve seen mountains, I think there are pretty obviously mountain-building forces operating on Pluto.”

Exactly what drives Pluto’s resurfacing is unclear. Scientists thought the faraway icy dwarf was far too small to support geologic activity in modern times, and Pluto does not orbit a massive mother planet.

Researchers surmised the gravity from a huge nearby planet may be to power geologic activity on small bodies in the outer solar system. Scientists believe the pull of gravity from planets like Jupiter, Saturn, Uranus and Neptune stretch the rock in their moons, softening the interiors of the icy satellites and triggering melting, volcanoes and geysers.

The icy plains of Sputnik Planum are dotted with hills and small pits. Moore said stereo imagery to be sent back to Earth in the coming weeks will tell scientists the size of the hills, and high-fidelity, uncompressed imagery will confirm the pits are not actually image artifacts.

The origin of the hills is still up for debate, but either theory is mind-boggling, Moore said.

Scientists created this composite of Pluto and its largest moon Charon using the latest full-frame images of the objects from New Horizons on approach. The relative reflectivity, size, separation, and orientations of Pluto and Charon are approximated in this composite image, and they are shown in approximate true color. Credit: NASA/JHUAPL/SWRI
Scientists created this composite of Pluto and its largest moon Charon using the latest full-frame images of the objects from New Horizons on approach. The relative reflectivity, size, separation, and orientations of Pluto and Charon are approximated in this composite image, and they are shown in approximate true color. Credit: NASA/JHUAPL/SWRI

“We suspect the hills may have either been pushed up from underneath along the cracks, but alternatively, a completely different explanation is that they are erosion-resistant knobs that are standing out as the surface is being massively eroded and lowered,” Moore said. “We don’t know which of those two explanations is correct.”

Moore calls the competing theories the “bottom up” and the “top down” hypotheses.

“I like the scenario of it upwelling from below, but I dont think we’re anywhere near proving that’s what’s happening,” Grundy said.

Either way, evidence is mounting for Pluto being an evolving world.

Besides the intriguing mystery material lining the borders of some of Sputnik Planum’s frozen blocks, which could be a sign of eruptions from underneath, Moore highlighted another finding that may point to the presence of geysers or cryo-volcanism on Pluto.

A line of dark smudges to the northwest of Sputnik Planum has caught the attention of scientists.

On Earth and Mars, such features are often caused by material deposited by prevailing winds against hills and other protrusions.

While careful to tamp down expectations, Moore said one “speculative” source for the smudges could be cryo-volcanism.

“They may be plume deposits associated with glaciers like those seen on Neptune’s icy satellite Triton. The plumes themselves, if they exist on Pluto, have not been spotted yet,” Moore said. “But we will be looking for them in images yet to be received from the spacecraft.”

On Triton, one of Pluto’s closest known analogs in the outer solar system, such markings are left by volcanoes periodically throwing subsurface material into the sky.

Moore cautioned he wants to see “unambiguous evidence” for geysers and plumes before declaring Pluto has modern-day eruptions.

What is clear — and surprising — is Pluto is a living planet, with forces reshaping its topography in recent geologic timescales.

“People have for many years, since the 1970s at least, wondered whether these very evolved young terrains you see on the icy moons of the gas giants were made that way because of the process we call tidal heating, where the moons interact with themselves and with the body they’re orbiting around to basicaly heat up their interiors through friction,” Moore said. “People see (Jupiter’s moon) Io’s volvanoes erupting, (and) they attribute it to this process called tidal torque heating.”

Not Pluto, but Triton! NASA's Voyager 2 spacecraft flew by Neptune and its largest moon Triton in August 1989. Credit: NASA/JPL/USGS
Not Pluto, but Triton! NASA’s Voyager 2 spacecraft flew by Neptune and its largest moon Triton in August 1989. Triton is believed to be a large Kuiper Belt Object like Pluto, which was captured into orbit by Neptune. Credit: NASA/JPL/USGS

“The question was could icy worlds minding their own business — not orbiting some giant planet — also be geologically active, and the answer is obviously yes,” Moore said. “Pluto is every bit as geologically active as any place we’ve seen in the solar system. This really answers a fundamental question about are icy worlds able to do their thing in their own right, or are they dependent upon the help of the big planets they orbit around.”

New Horizons scientists announced other findings in press briefing Friday at NASA Headquarters in Washington.

Observations of Pluto’s tenuous atmosphere, about one hundred thousandth the thickness of Earth’s, have also revealed a surprising uniformity and a thin veneer of gas mostly packed within a mile of the icy world’s surface.

New Horizons also detected Pluto’s atmosphere extending out to 1,000 miles as particles spread out due to the dwarf planet’s low gravity. Before the encounter, Earth-based views only showed the atmosphere out to 170 miles.

Randy Gladstone, head of the New Horizons atmospheric science team, said spectroscopic measurements made by the probe’s Alice instrument show the atmosphere to be mostly stagnant.

“We still don’t have a good measure of the lowest atmosphere, where it’s very complicated,” Gladstone said. “We think all the atmosphere on Pluto is sort of compressed into a very thin layer near the surface, where the winds could be up to a few meters per second (5 to 8 mph) easily.”

The frigid, tenuous atmosphere is mostly nitrogen — the primary constituent of Earth’s atmosphere — and pressures at Pluto’s surface are roughly equivalent to conditions about 50 miles above Earth, according to Gladstone.

Better data on the atmosphere is coming down in the coming days.

NASA blasted 80 kilowatts of radio waves toward Pluto as New Horizons passed behind the far side of the distant dwarf, and a receiver aboard the spacecraft measured how the signals were distorted in an attempt to learn about the atmosphere’s profile.

And the craft’s long-range camera snapped photos of Pluto’s sunlit crescent as it sped away at nearly 31,000 mph in search of haze and clouds. Those images should be back on Earth next week, Gladstone said.

A diagram of Pluto's ionized tail. Credit: NASA/JHUAPL/SWRI
A diagram of Pluto’s ionized tail. Credit: NASA/JHUAPL/SWRI

Scientists knew Pluto’s atmospheric particles were escaping into space, and New Horizons confirmed that the atmosphere is slowly disintegrating.

All the data is not in, but scientists suspected before the flyby that Pluto lacks a magnetosphere, which is driven on other planets by internal dynamos. Instead, Pluto’s escaping atmosphere creates a bubble encompassing all five of the world’s moons, and the solar wind smashes into atmospheric particles on the side of Pluto facing the sun.

On the downwind side, Pluto has a tail of ionized gas at least tens of thousands of miles long, New Horizons has observed.

“What we see behind Pluto is a tail, an ion tail, of this ionized escaping atmosphere that’s been pulled away and carried away in the solar wind,” said Fran Bagenal, a New Horizons co-investigator from the University of Colorado at Boulder.

Information from the flyby still stored aboard New Horizons will help researchers quantify the rate at which the atmosphere is escaping.

“What we think it is, based on models and a pretty good guess, is about 500 tons per hour of material that is escaping,” Bagenal said. “For comparison, we know that the escaping atmosphere from Mars, which has been studied by NASA’s MAVEN mission, is about one ton per hour. This is substantially more because of the weaker gravity on Pluto.”

With so much of Pluto’s nitrogen-rich atmosphere clearing out, scientists are asking what replenishes it during the dwarf planet’s 248-year orbit around the sun.

The results so far are just the “tip of the iceberg,” said Jim Green, director of NASA’s planetary science division.

“For nearly 10 years, the New Horizons team was always talking about each day we’re closer to Pluto,” Green said. “Now each day we’re farther away from Pluto.”

It will take 16 months for all the data from the flyby to flow back to Earth at a few kilobits per second. New Horizons is initially sending down a first look with compressed images prone to artifacts. The “lossless” data will come down later.

“It’s important to remember that it’s during this time that we’re going to be able to obtain the data from the flyby,” Green said. “Right now, we’ve only received 1 to 2 percent of that data on the ground. By next week, we’ll have perhaps as much as 5 to 6 percent.”

Beginning later this month through September, New Horizons will halt the downlinking of high-resolution imagery to send down data from other instruments.

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Follow Stephen Clark on Twitter: @StephenClark1.


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