Ground-based telescope captures Neptune's 'Great Dark Spot'

For reference, this azure world is approximately 2.7 billion miles away from Earth. 
Mrigakshi Dixit
Bird’s eye view of the Very Large Telescope
Bird’s eye view of the Very Large Telescope

J.L. Dauvergne & G. Hüdepohl/ESO 

Scientists have pushed the limits of one of Earth's most powerful ground telescopes: the Very Large Telescope (VLT). 

Located in the isolated Atacama Desert of northern Chile, it was able to observe in unprecedented detail the “Great Dark Spot” in Neptune’s atmosphere. Not only that, but the VLT managed to image the other smaller bright spots close to the larger one as well. 

For reference, this azure world is approximately 2.7 billion miles (4.3 billion km) away from Earth. 

According to the European Southern Observatory (ESO), which runs this telescope, this is the first time a dark spot on a faraway planet has been detected with an Earth-based telescope. 

“This is an astounding increase in humanity’s ability to observe the cosmos. At first, we could only detect these spots by sending a spacecraft there, like Voyager. Then we gained the ability to make them out remotely with Hubble. Finally, technology has advanced to enable this from the ground,” said co-author Michael Wong, a researcher at the University of California, Berkeley, in an official release. 

The origin of this mysterious large dark spot  

Large vortices of this sort are said to be common in the atmospheres of giant planets. Among them, the most well-known is Jupiter's Great Red Spot. 

Neptune’s oval-shaped, counterclockwise-spinning spot appears to be large enough to store the entire Earth. 

It was first discovered in 1989 by NASA's Voyager 2 during its Neptune flyby. However, when Hubble set out to image this storm a few years later, in 1994, it had vanished. This demonstrated that Great Dark Spots do not endure long and are short-lived as well as appear occasionally. 

Since then, these sporadic atmospheric phenomena have been one of Neptune's most enigmatical mysteries. 

“Since the first discovery of a dark spot, I’ve always wondered what these short-lived and elusive dark features are,” said Patrick Irwin, Professor at the University of Oxford in the UK and lead investigator of the study, in an official release. 

The researchers used VLT data to find evidence of the nature and genesis of Neptune's colossal storm.

The researchers ruled out the notion that the black patches were generated by a “clearing in the clouds” based on their observations. 

“The new observations indicate instead that dark spots are likely the result of air particles darkening in a layer below the main visible haze layer, as ices and hazes mix in Neptune’s atmosphere,” mentioned the release.  

Ground-based telescope captures Neptune's 'Great Dark Spot'
Dark spot on Neptune observed with MUSE at ESO’s Very Large Telescope

The 3D spectrum of the spot

Due to the brief occurrences of these large storms, reaching this conclusion was not a simple process. 

The team decided to explore these from the ground after the Hubble telescope identified multiple dark spots in Neptune's atmosphere, including one in the planet's northern hemisphere discovered in 2018. 

The observations of this faraway world were obtained using the VLT's Multi Unit Spectroscopic Explorer (MUSE). 

The researchers were able to create a 3D spectrum by splitting reflected sunlight from Neptune and its spot into various wavelengths. 

The spectrum data revealed the altitude of the large spot in the planet's atmosphere. Furthermore, it offered information on the chemical makeup of the various layers of the atmosphere, thereby shedding light on the dark appearance of the spot. 

“I’m absolutely thrilled to have been able to not only make the first detection of a dark spot from the ground but also record for the very first time a reflection spectrum of such a feature,” said Irwin.

The results have been reported in the journal Nature Astronomy. 

Study abstract:

We present the first determination of the visible reflection spectrum of a dark spot in Neptune’s atmosphere, ‘NDS-2018’, analogous to the ‘Great Dark Spot’, discovered by Voyager 2 in 1989. Observations were made in 2019 with the MUSE Integral Field Unit spectrometer at the Very Large Telescope. We show that the feature is caused by a darkening at wavelengths shorter than 700 nm of a deep layer of aerosol in Neptune’s atmosphere at a pressure of ∼5 bar, which we suggest is coincident with the main H2S condensation layer. In addition, we have detected an apparently short-lived companion deep bright spot feature, ‘DBS-2019’, on the south-west edge of NDS-2018, with a spectral signature consistent with a brightening of the same 5-bar layer at wavelengths longer than 700 nm. This feature is fundamentally different from previously studied companion clouds of the Voyager-2 Great Dark Spot, which were located much higher in the atmosphere at 0.6–0.2 bar.

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