Neptune dark and bright spots are a subject of VLT research

Astronomers using the Very Large Telescope (VLT) at the European Southern Observatory have observed a large dark spot and an unexpectedly small bright spot in Neptune’s atmosphere. This is the first time Neptune’s dark spots have been observed with Earth-based telescopes. These occasional structures in Neptune’s blue atmosphere have puzzled astronomers, but the new results provide more insight into their nature and origin.

Large spots are common on gaseous planets, such as the famous large red spots on Jupiter. Dark spots on Neptune were first observed by NASA’s Voyager 2 probe in 1989, but disappeared a few years later.

“Since that first discovery, I have always wondered what these fragile and elusive dark spots really are,” said Patrick Irwin, professor at the University of Oxford (UK) and lead researcher on the study presented today.

Irwin and his team used data from ESO’s VLT to rule out the possibility that the dark spots were caused by cloud layer ‘clearing’. The new observations show that they are most likely caused by a deep, darkened layer of mixed ice and vapor aerosol under the most prominent vapor layer.

Dark spots are not always visible in Neptune’s atmosphere, as astronomers have not previously had the opportunity to study them in detail. This changed in 2018 when the Hubble Space Telescope discovered several dark spots in Neptune’s atmosphere, including one in Neptune’s northern hemisphere for the first time.

Researchers have now used the VLT’s Multi-Unit Spectroscopic Explorer (MUSE) to separate sunlight reflected from Neptune and its dark spots into their component colors and wavelengths. Because the depth of Neptune’s atmosphere varies by wavelength, the spectrum allowed the team to more accurately determine at what height in Neptune’s atmosphere the dark spot is located. The spectra also provided information about the chemical composition of different layers of the atmosphere, giving clues as to why the dark spots are visible.

The observations also brought surprises. During the observations, astronomers discovered a deep and bright cloud that was previously unnoticed even from space: The VLT data showed that this new ‘deep and bright cloud’ is at the same atmospheric level as the main dark spot. This makes it quite different from the previously observed high ‘incidental’ methane ice clouds.