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Mysterious Neptune dark spot detected from Earth for the first time
Using ESO’s Very Large Telescope (VLT), astronomers have observed a large dark spot in Neptune’s atmosphere, with an unexpected smaller bright spot adjacent to it. This is the first time a dark spot on the planet has ever been observed with a telescope on Earth. These occasional features in the blue background of Neptune’s atmosphere are a mystery to astronomers, and the new results provide further clues as to their nature and origin.
Dark spot on Neptune observed with MUSE at ESO’s Very Large Telescope. Photo: ESO
Large spots are common features in the atmospheres of giant planets, the most famous being Jupiter’s Great Red Spot. On Neptune, a dark spot was first discovered by NASA’s Voyager 2 in 1989, before disappearing a few years later.
“Since the first discovery of a dark spot, I’ve always wondered what these short-lived and elusive dark features are,” says Patrick Irwin, Professor at the University of Oxford in the UK and lead investigator of the study.
Irwin and his team used data from ESO’s VLT to rule out the possibility that dark spots are caused by a ‘clearing’ in the clouds. 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.
Coming to this conclusion was no easy feat because dark spots are not permanent features of Neptune’s atmosphere and astronomers had never before been able to study them in sufficient detail.
The opportunity came after the NASA/ESA Hubble Space Telescope discovered several dark spots in Neptune's atmosphere, including one in the planet’s northern hemisphere first noticed in 2018.
Using the VLT’s Multi Unit Spectroscopic Explorer (MUSE), the researchers were able to split reflected sunlight from Neptune and its spot into its component colours, or wavelengths, and obtain a 3D spectrum.
Since different wavelengths probe different depths in Neptune’s atmosphere, having a spectrum enabled astronomers to better determine the height at which the dark spot sits in the planet's atmosphere.
The spectrum also provided information on the chemical composition of the different layers of the atmosphere, which gave the team clues as to why the spot appeared dark.
The observations also offered up a surprise result. “In the process we discovered a rare deep bright cloud type that had never been identified before, even from space,” says study co-author Michael Wong, a researcher at the University of California, Berkeley, USA.
This rare cloud type appeared as a bright spot right beside the larger main dark spot, the VLT data showing that the new ‘deep bright cloud’ was at the same level in the atmosphere as the main dark spot.
This means it is a completely new type of feature compared to the small ‘companion’ clouds of high-altitude methane ice that have been previously observed. (ESO)
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