Mysterious dark spot on Neptune detected from Earth for the first time

In a remarkable discovery, astronomers employing the European Southern Observatory’s Very Large Telescope (VLT) have made an unexpected observation of Neptune’s atmospheric enigma: a large dark spot, accompanied by an unforeseen smaller bright spot adjacent to it.

Remarkably, this marks the first instance of a dark spot on Neptune being observed with an Earth-bound telescope.

The serene blue background of Neptune’s atmosphere occasionally reveals mysterious features which, until now, remained largely unexplained. This discovery offers new insights into the nature and origin of these atmospheric anomalies. 

Elusive dark features

While large spots are relatively common in the atmospheres of giant planets, such as Jupiter’s renowned Great Red Spot, Neptune’s own history with dark spots dates back to NASA’s Voyager 2 mission in 1989, which observed one such spot. However, it vanished a few years subsequent to its discovery.

“Since the first discovery of a dark spot, I’ve always wondered what these short-lived and elusive dark features are,” said Patrick Irwin, a professor at the University of Oxford and lead investigator of the study published today in Nature Astronomy.

Focus of the study 

Through the data acquired from the VLT, Professor Irwin and his colleagues dismissed the notion that these dark spots were simply openings in the clouds. 

Instead, the observations imply that the spots manifest as a result of air particles becoming darkened in a layer beneath Neptune’s primary visible haze layer, caused by the interaction of ices and hazes in the atmosphere.

The transient nature of these spots presented challenges in comprehending them. The turning point arrived when the NASA/ESA Hubble Space Telescope identified multiple dark spots on Neptune, including a prominent one in the northern hemisphere in 2018. 

How the research was conducted 

Seizing this opportunity, Professor Irwin and his team embarked on a comprehensive analysis utilizing the VLT’s Multi Unit Spectroscopic Explorer (MUSE), which fractionates reflected sunlight from Neptune into its individual colors or wavelengths, culminating in a 3D spectrum.

“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 Professor Irwin.

What the study revealed

The spectrum analysis, probing different depths of Neptune’s atmosphere, enabled the researchers to accurately pinpoint the altitude at which the dark spot resides. 

Furthermore, it yielded details regarding the atmospheric layers’ chemical compositions, shedding light on the causes of the spot’s darkness.

“In the process we discovered a rare deep bright cloud type that had never been identified before, even from space,” said study co-author Michael Wong, a researcher at the University of California, Berkeley.

Study implications 

The discovery of this “deep bright cloud” adjacent to the main dark spot, per VLT data, suggests that this is a novel atmospheric feature, differing from previously observed high-altitude methane ice clouds.

“This is an astounding increase in humanity’s ability to observe the cosmos,” said Wong. “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. This could put me out of work as a Hubble observer!”

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