Astronomers, led by St Anne’s Vice Principal and Fellow, Professor Patrick Irwin, have observed a large dark spot in Neptune’s atmosphere, with an unexpected smaller bright spot adjacent to it.
Professor Irwin and his group used a rich new dataset from the European Southern Observatory’s Multi Unit Spectroscopic Explorer (MUSE), part of the Very Large Telescope (VLT) to investigate what causes Neptune’s dark spots to be visible. The group used the data to rule out the possibility that dark spots appear simply due to a clearing in the clouds. The new observations indicate that instead, 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.
Until now, all that was known was that these short-lived and elusive dark spots, previously observed by Voyager 2 in 1989 and more recently by the Hubble Space Telescope, are dark at blue wavelengths and invisible at red and longer wavelengths. The most recent dark spot, NDS-2018, was first observed by the Hubble Space Telescope in 2018 and it is this spot that has been studied by Professor Irwin’s team. This is the first time a dark spot on Neptune has ever been observed from a ground-based telescope and also the first time its continuous, visible reflection spectrum has been measured. It is the measurement of this reflection spectrum that has allowed Professor Irwin’s team to rule out the cloud-clearing scenario. In the process of analysing the dark spot observation, the group also discovered a rare deep bright spot, or cloud type, near NDS-2018 that has never been previously detected – even from space.
MUSE’s adaptive optics
As a hyperspectral imager, the MUSE instrument was fundamental to this work – not least due to its adaptive optics (AO). Professor Irwin explains: ‘Neptune is very small as seen from the ground and we would never be able to see small features such as dark spots without the AO system which corrects for the blurring of the Earth’s atmosphere in real time.’
He continues: ‘The final processed MUSE data reveal detail at a scale almost as good as data from the Hubble Space Telescope but, unlike Hubble imaging data, the MUSE data give a spectrum at every pixel. This spectral data enabled us to pick out the reflection spectrum of the dark spot itself and quantify how it is different from other locations on the planet.’
Professor Irwin’s group first accessed the data from MUSE in 2019 and found that an additional tranche of work was required to interpret it – the spatial resolution, although high from MUSE, wasn’t high enough at short wavelengths to clearly detect the spot. They developed a spatial deconvolution algorithm to improve the spatial resolution and were then able to make the first ever measurement of the complete reflection spectrum of a dark spot.
Newly detected small bright spot
Professor Irwin’s group recently concluded that there are two distinct layers of cloud and haze on Neptune: one at the level where methane condenses (1-2 bar) and one at deeper levels (~5 bar) where hydrogen sulphide is believed to condense. From the shape of the dark spot spectrum, the group were able to show that it must be due to a spectrally-dependent darkening of the deeper layer of aerosol in Neptune’s atmosphere at about the 5-bar pressure level, which is assumed to be composed of a mixture of H2S ice and photochemically-produced haze that has been mixed down from above. The group also report the detection of a small bright spot near NDS-2018; this is shown to be caused by a spectrally-dependent brightening of the very same aerosol layer. This makes it very different from the usual ‘companion’ clouds that are often seen near dark spots, and which are thought to be methane ice clouds based much higher in the atmosphere (0.2-0.6 bar).
‘Now we know the pressure level at which dark spots – and the new deep bright companion – are based, and what makes them dark, we have much tighter constraints to place on dynamical models on Neptune’s atmospheric circulation,’ explains Professor Irwin. ‘This may help us to understand better how large anticyclonic vortices such as Neptune’s dark spots, but also more famous features, such as Jupiter’s Great Red Spot, form and are sustained.’
NDS-2018 has now vanished so the next step would be to observe a similar occurrence and compare datasets. ‘It is astonishing how far we have come,’ concludes Dr Mike Wong, one of the paper’s co-authors. ‘At first we could only detect these spots by sending a spacecraft up, then we were able to make them out remotely as with Hubble and now, technology has advanced to enable us to observe such phenomena from the ground.’
Spectral determination of the colour and vertical structure of dark spots in Neptune’s atmosphere, P Irwin et al, Nature Astronomy, 24 August 2023