Juno Delivers Stunning New Views of Great Red Spot

Scientists and the public are dazzled by images from the spacecraft’s close encounter with Jupiter’s largest—and the Solar System’s most famous—storm

An artistic rendition of the NASA Juno spacecraft flying over Jupiter’s Great Red Spot. Credit: NASA/JPL/Björn Jónsson/Seán Doran

 

Humanity has just gained its best-ever views of Jupiter’s Great Red Spot, a storm large enough to swallow Earth whole that has raged for centuries in the gas-giant planet’s atmosphere.

Snapped earlier this week [week of June 9, 2017] by NASA’s basketball court–size solar-powered Juno spacecraft, the new images from just 9,000 kilometers above Jupiter reveal never-before-seen details of the Great Red Spot and its turbulent surroundings that raise just as many questions as they answer. Scientists know the feature is generally anticyclonic—spinning counterclockwise. They also know it is much higher and colder than most of Jupiter’s upper atmosphere; it might better be called the Great Cold Spot because it rises far above the surrounding clouds, expanding and cooling as it goes. Like most of Jupiter’s cloud deck, it is rich in ammonia. What they still do not know is how the storm has endured for so long, or how deep it swirls into Jupiter. In recent decades it has slowly become more circular than oval, for reasons unknown, leading some researchers to suspect it is on the verge of dissipating. No one fully understands the origins of its reddish color, either.

Although studied for centuries through small ground-based telescopes, the Spot only received its first close-ups in the latter half of the 20th century through a progressive series of close encounters with NASA’s Pioneer, Voyager and Galileo spacecraft—as well as through detailed remote monitoring by the Hubble Space Telescope and other observatories. With each additional observation, researchers have gradually gained a deeper understanding of the storm’s—and Jupiter’s—dynamic nature, and the latest are no exception.

“This week’s images of the Great Red Spot are the best yet, surpassing those from Voyager,” says John Rogers, a veteran observer of the giant planet and director of the Jupiter section of the British Astronomical Association. “Although the improvement in resolution and quality is incremental, not a big leap, it has crossed a threshold to reveal small-scale waves and tiny shadow-casting clouds in the Great Red Spot which were never seen before.”

The new observations are “going to enable great comparisons with what we saw from the Galileo spacecraft 20 years ago—and Voyager 20 years before that,” says Amy Simon, an expert in planetary atmospheres at NASA Goddard Space Flight Center. “We can certainly say it has evolved a lot in 20 years—it’s much rounder, of course, but the changes to the shear in the internal cloud structures catch my eye; more swirls and turbulence in areas that used to be completely stretched apart into long streaks. We’ll have to analyze that to fully understand how the Great Red Spot is changing over time.”

Already, some researchers are speculating about subtle shadows and color gradations in the Spot, which the new images suggest are most intense near its towering center. That would help validate a recent theory for its coloration, which posits that the red color is a sunburn of sorts. Arising from ultraviolet light bombarding ammonia and trace hydrocarbons lofting high into Jupiter’s stratosphere, the Spot’s hues would thus be most intense where it reaches highest above the surrounding clouds.

According to Michael Wong, a planetary scientist at the University of California in Berkeley, the overarching takeaway from these new images is how relatively blinkered most of our earlier views have been. Wong used the Hubble Space Telescope to monitor Jupiter concurrently with Juno’s close encounter with the Great Red Spot. “When you go to Juno’s images, the areas with the finest structure are the areas that seemed plain in the Hubble image,” Wong says. “Like a fractal, we see detail at the limit of the resolution no matter what scale we observe.”

In a broad sense, whereas these new vistas come courtesy of Juno, which launched in 2011 and swooped into polar orbit around Jupiter just over a year ago, their true source is the spacecraft’s most modest instrument—a camera called JunoCam considered so scientifically unremarkable that it was given a barebones operational budget and officially included only for “public outreach.”

Those limited resources mean that JunoCam’s scientists rely on a small army of volunteer “citizen scientists” using backyard telescopes to flag transient features in the Jovian atmosphere as “points of interest” for the instrument to observe. Each feature is given a fanciful name, such as Mortyland, Hotspot Tail and Carl Sagan’s Jawbreaker. Because of Juno’s swooping polar orbit that takes it breathtakingly close to the planet, most of JunoCam’s images of these features are distorted into an hourglass shape due to foreshortened horizons; the colors are pale, the outlines of clouds hazy. A second group of amateurs then extracts awe-inspiring details from these raw images after Juno beams them home. Steadily streaming onto JunoCam’s Web site, their best-processed images correct for distortion, enhance colors and sharpen contrast in a way that leaves professionals spellbound.

In the aftermath of a raw-image dump, “about every five minutes I refresh my screen, and each time I find there are more of these beautiful, hugely valuable products,” says Candice Hansen, lead scientist for NASA’s JunoCam team. “We have a tiny, little team because this is an outreach instrument, so the public is really our team—we are relying heavily on our amateur cadre.” To demonstrate, she pulls up a zoomed-in image of the Spot incrementally processed by two of JunoCam’s star volunteers—a first pass by Gerald Eichstädt, a mathematician in Stuttgart, Germany, and a second pass by Seán Doran, a visual artist in London.

“These guys in particular—Gerald and Seán—gave us products from Juno’s earlier encounters with Jupiter that showed all these ‘little’ storms just 25, 50 kilometers wide popping up from the cloud tops of the planet’s south tropical zone,” Hansen says. “The storms sort of remind me of squall lines. And here they are, on top of the Great Red Spot! It almost looks frothy…. These are the kinds of details you get when you suddenly have high enough resolution. I can’t tell you what this means for atmospheric dynamics but I’m sure it’s important, and I’m sure that once we get this sorted out it will be a real science result.”

“We really use ‘amateur’ in air quotes for them,” says Glenn Orton, a JunoCam co-investigator at the NASA Jet Propulsion Laboratory. “They really know what they’re doing—and they work for free.”

Eichstädt began working with JunoCam in 2013, when he processed some images of Earth the instrument captured as it looped around our planet to pick up speed en route to Jupiter. Ever since, he has been embroiled in developing a proprietary software “pipeline” for enhancing JunoCam’s images, which requires careful calibration to remove noise from faulty detector pixels as well as modeling variables such as the angle of illumination from the Sun, the absorption of light by the planet and the trajectory of the spacecraft. All together, he says, the routine takes a few hours to produce one processed image—giving him plenty of time and insight to ponder finer details that most others might fail to notice.

Among other things, he has noticed small numbers of individual bright pixels scattered in and around his processed images of the Spot. “Those have been left over by my patching algorithm, and are therefore likely to be no camera artifacts but instead energetic particle hits [from Jupiter’s intense radiation environment] or, with a lot of luck, lightning.” First witnessed in Jupiter’s clouds as rare flashes of scattered light by Voyager 1 in its 1979 flyby and observed decades later by the Galileo orbiter, Jupiter’s lightning is thought to be an indirect tracer of the planet’s water content. Voyager’s and Galileo’s observations suggest each thunderbolt emerges from deep in the atmosphere below Jupiter’s high ammonia clouds, in regions where temperatures and pressures reach the triple point of water and whirling maelstroms of vapor, rain and hail build up immense electric charges. Lightning, however, has never been seen before in the Spot—and Eichstädt is first to say his preliminary comments are only tentative speculations that require much more detailed follow-up.

That will come later, with further close observations by Juno—not only with JunoCam but also its eight other instruments that can measure the planet’s temperature, its magnetic and gravitational fields, microwave emissions from its deep interior, and more. The gravitational and microwave measurements in particular could soon reveal just how far the Spot extends into Jupiter—whether it floats like an iceberg near the top of the atmosphere, or instead bores deep into the planet’s innards.

The spacecraft will in coming years be plunged into Jupiter’s atmosphere, bringing the mission to a fiery end designed to avoid contaminating any of the planet’s astrobiologically interesting icy moons. JunoCam itself may expire much sooner, as early as this fall, due to the intense radiation around the planet, mission planners say. But its legacy will endure—Eichstädt, Doran and other intensive image-processors say their best work is yet to come.

“Two years ago I wasn’t sure if this would work at all,” Hansen says. “I would just tell people, ‘we don’t have a backup imaging team waiting in the wings, so we are just going to put the images out there and see if we get any takers!’ It’s wonderful to see this has actually succeeded, really beyond my wildest dreams.”

[1]Lee Billings on July 13, 2017 in Scientific American