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NASA’s Voyager 2’s post-encounter view of Neptune’s south pole as the spacecraft sped away on a southward trajectory.

NASA/JPL

I tend to feel sorry for the ice giants. When it comes to the outer solar system, its gas giants, from the colossal, comet-perturbing Jupiter and Saturn, inarguably the galaxy’s most beautiful seismometer, understandably soak up plenty of the media’s attention. Neptune, and to a lesser extent Uranus – largely thanks to its own name and convenient atmospheric composition – are comparatively underappreciated.

Fortunately, a new study serves to remind us just how enthralling these distant planets can be. As reported in today’s edition of Geophysical Research Letters, scientists from NASA’s Goddard Space Flight Center and the University of California Berkeley have used Hubble’s images to chronicle the formation of a new dark vortex on Neptune, which became fully visible in 2018.

On the right, you can see Voyager 2’s image of the original Great Dark Spot. On the left, as seen by Hubble, a brand-new one is forming at the top of the shot, accompanied by some bright clouds.

NASA/ESA/GSFC/JPL

Although Hubble has observed spots travelling around Neptune before, this is the first time that the space telescope has documented the birth and development of one, which began in 2016. This work will catalyse our comprehension of one of the least understood planets in our cosmic backwater, while also readying us for encountering exoplanets a little bit like it far out there in the dark, strange beyond.

Neptune, like its gas giant comrades, is no stranger to giant storms. When Voyager 2 flew past Neptune in 1989, it spotted an Earth-sized vortex in its southern hemisphere. The so-called Great Dark Spot was accompanied by a somewhat smaller vortex in the same part of the planet, which was dubbed Dark Spot 2.

The Great Dark Spot, with its characteristic bright-looking icy methane clouds (centre image), seen by Voyager 2.

NASA/JPL

In 1994, Hubble noticed that the two storms had disappeared, and another somewhat like the larger one had cropped up in the northern hemisphere. The first new Dark Spot of the 21st century, another southern hemispheric spinner, was spotted by Hubble in 2016. This one was about the width of the contiguous United States.

Although mathematical models and computer simulations have helped approximate what these dark spots might be like, the venerable space telescope has been the star player in this regard.

When a dark spot begins to form, it tends to be accompanied by bright-looking companion clouds, which appear to get brightest just before the vortex bursts into view. These are likely icy methane patches; they appear when air flows up over the perturbing storms, causing it to freeze out and become fairly reflective. They have been compared to the flattened-out-but-ripple-filled clouds that you sometimes see hanging out atop mountains back home on terra firma.

Although ground-based observatories can see these clouds, the vortex itself remains hidden from view. As explained in a 2016 NASA press release, these dark vortices tend to show up in the blue wavelength part of the spectrum, something plenty of telescopes aren’t designed for.

One of the more recently observed dark spots, seen by the Hubble Space Telescope in 2016.

NASA, ESA, and M.H. Wong and J. Tollefson (UC Berkeley)

Hubble, with its bespoke imaging tech, is uniquely able to see such storms in relatively high resolution. Once again, it is responsible for clocking the evolution of Neptune’s youngest Great Dark Spot, from 2016 to 2018, one that is much the same size and shape as the original Great Dark Spot seen by Voyager 2.

Like the other Neptunian spots, it’s another anticyclone, a high-pressure version of the low-pressure hurricanes and typhoons we get on Earth. (We also get anticyclones too, to be fair, which are associated with calm, warm weather.) It was also preceded by those familiar companion clouds like its predecessors.

Planetary scientists have spent decades trying to work out exactly how such storms form on Neptune. By catching one in the act of creation, Hubble’s Outer Planet Atmospheres Legacy (OPAL) program has provided the community with an unprecedented amount of new information.

Pre-existing models suggested that the brighter the companion clouds, the deeper the storm they are associated with. The clouds accompanying the new Great Dark Spot were not only bright, but they appeared 24 to 36 months prior to the storm itself. This implies that storms nucleate far deeper within Neptune’s hydrogen-helium-dominated, methane-infused atmosphere than previously suspected, although specific depth measurements remain elusive for the time being.

Thanks to both OPAL and Voyager 2’s data, we now know that Neptune’s tempests seem to last several years or so, unlike the decades-to-centuries-long storms on Jupiter and the days-to-week-long storms on Earth. Only a few Neptunian spots have been seen so far, but using this small sample size it seems that they last for around two years apiece, on average, and appear every four to six years.

False-colour cylindrical maps of Neptune taken by Hubble, showing the development of the new dark spot, NDS-2018, while SDS-2015, a prior dark spot, fades away.

Simon et al./GRL, 2019

Jupiter also has thin, powerful jet streams – fast-moving atmospheric rivers – that act as speed lanes for its giant storms. These barriers don’t just prevent chaotic wind shear from tearing the storm’s structures to shreds, but it also stops them from migrating north or south. Neptune lacks such narrow rivers of air, meaning its own storms can change latitude with a fair degree of freedom.

Novel though they may be, they do have something in common with Jupiter’s record-breaking storms. As noted in a press release, the researchers estimate that the wind strength of such spots on Neptune are comparable with Jupiter’s Great Red Spot. Although direct wind measurements haven’t been from Neptune’s dark spots, they are estimated to be around 360 kilometres (224 miles) per hour. This, by the way, is far below the estimates made for the original Great Dark Spot, whose 2,410 kilometres (1,500 miles) per hour gusts were about ten times that of a Category 5 hurricane on Earth.

Despite this revised understanding of Neptune’s anticyclones, plenty remains unknown at this stage, including how these storms manage to take shape in the first place. Saying that, the authors of the study write that, at this stage, “Neptune may be unique among the giant planets,” in that its storms have visible precursors in the shape of those icy methane patches. On Jupiter, the storms simply burst through the dense clouds, just as they do through the thick hazes on Saturn and Neptune.

For now though, scientists have to rely on Hubble and computer simulations to make these sorts of speculative conclusions. A closer look would be ideal, but it’ll certainly be many years before another spacecraft pays a personal visit to the cyan-hued ice giant.

Future missions to Neptune really do matter, because its structure, even more so that its storms, is hugely enigmatic. It may be four times the size of Earth, but it is also around 4.5 billion kilometres (2.8 billion miles) away from us, so getting a detailed look at it is pretty tricky. In fact, it is so far from us that the planet’s existence was the first in the solar system to be determined not through observational work but mathematically, all the way back in 1846.

With limited observational data compared to the inner rocky planets and gas giants, what we know of its internal structure comes largely from the manner in which it orbits the Sun, which it accomplishes once every 165 Earth-years, tilted at 28 degrees and moving along an oval-shaped path. Scientists can speculate, but little can be said with any certainty, and right now there are more questions than answers.

For example, the small amount of methane in its atmosphere should give Neptune a similar blue-green tint as Uranus. Instead, it’s a far richer blue colour, suggesting there’s some atmospheric chemistry happening there, or perhaps another chemical compound, that remains undiscovered.

Its inner layers are even more puzzling. According to NASA, scientists reckon that the hydrogen-helium-methane atmosphere goes fair deep into the planet, whereupon it likely meets and interacts with a sea of various melted water, methane and ammonia ices. This sea is probably incredibly hot, but the super thick atmosphere puts immense pressure on it. This prevents bubbles from forming, which means it cannot boil away.

Beneath this strange sea is probably a rocky core, one that has about the same mass as Earth. It’s not clear at all what that core is made of, though.

If we are to understand how our outer solar system took shape billions of years ago, and if we wish to learn how exoplanets like the ice giants came to be, the path forward is clear: we need spacecraft to swim through Neptune’s clouds, weather its storms, and dive towards its heart.

This will only be possible if we know how its tumultuous winds work. With a firm grasp on the formation and behaviour of its dark spots, space agencies will know how to prevent their spacefaring tech being torn asunder by Neptune’s furious storms – and this study represents nothing less than a vital stepping stone toward that goal.

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