Ever since Voyager 2 flew past Neptune in 1989, the giant dark spots that appear in the distant planet's atmosphere have posed a strange puzzle.

Now, for the first time, we have observed one at unprecedented resolution with Earth-based instruments, helping scientists understand why these spots appear so dark and why they are so different from spots on other planets.

"Since the first discovery of a dark spot, I have always wondered what these short-lived and elusive dark features are," says astronomer Patrick Irwin of the University of Oxford in the UK.

"I am absolutely thrilled to have not only made the first detection of a dark spot from the ground, but also to be able to record the reflectance spectrum of such a feature for the first time."

Neptune's dark vortices are actually anticyclonic storms like the Great Red Spot on Jupiter, but they differ in several important and mysterious ways. First, they are relatively short-lived, appearing and dissipating every few years.

They are also thought to be relatively cloud-free at their centres compared to the storm vortices on Saturn and Jupiter. The clouds we have been able to detect are fluffy white clouds that appear at the edges, possibly as a result of gases freezing into methane ice crystals as they are lifted up from lower altitudes.

However, due to Neptune's distance and the short-lived nature of the vortices, it has been difficult to learn more. The Hubble Space Telescope is the only instrument that has been able to observe and track these vortices since 1994, limiting the wavelength range in which the planet can be seen.

But when a large storm vortex appeared in 2018, Irwin and his team began working with another instrument: The Very Large Telescope's Multi-Unit Spectroscopic Explorer (MUSE). MUSE was able to detect sunlight reflected from Neptune and split it into its constituent wavelengths to reconstruct the planet's 3D spectrum.

Different wavelengths are associated with different heights in Neptune's atmosphere, so the researchers were able to calculate the height of the dark spot. And they found something surprising: it didn't look like a "hole" in Neptune's atmosphere after all.

Rather, the deeper colour appears to be the result of the darkening of particles in the hydrogen sulphide layer, which lies beneath the top layer of Neptune's atmospheric aerosol haze. The researchers believe this could be the result of local heating in the deep part of an anticyclonic vortex that vaporises the hydrogen sulfide ice, revealing a darker vortex core. The researchers' observations are consistent with particles in the aerosol layer above shrinking, reducing the opacity.

The researchers found another surprise: a bright cloud accompanying the vortex. This was not one of the methane clouds that usually accompany Neptune's vortices, but a type of cloud that had never been seen before. Instead of lingering higher in the atmosphere, it seemed to be at the same height as the dark vortex.

What this is, and whether any of the team's proposed mechanisms for Neptune's atmospheric darkening are correct, will need to be investigated further. However, with ground-based observations of Neptune now possible, we seem much closer to answers.

"This is an astonishing increase in humanity's ability to observe the universe," says astronomer Michael Wong of the University of California, Berkeley.

"At first, we could only detect these points by sending a spacecraft like Voyager there. Then with Hubble, we gained the ability to detect them remotely. Finally, technology has advanced enough to make it possible to do it from the ground."

 

Source: https://www.sciencealert.com/