Detection of Enigmatic Dark Spot on Neptune Using Earth-Based Telescope for the First Occurrence

An illustration depicts Neptune as observed by the MUSE instrument at ESO’s Very Large Telescope (VLT). With the VLT, at each pixel of Neptune, MUSE divides the incoming light into its essential colors or wavelengths. This process equates to acquiring images across thousands of varying wavelengths simultaneously, offering astronomers an abundance of valuable information. Credit: ESO/P. Irwin et al.

Astronomers utilizing the European Southern Observatory’s Very Large Telescope (VLT) have pinpointed a considerable dark spot in Neptune’s atmosphere, accompanied by a smaller bright spot nearby. This constitutes the first instance of such a discovery made using a telescope stationed on Earth.

By employing the VLT, astronomers have managed to detect a substantial dark spot on Neptune, coupled with a surprising adjacent bright spot. This observation, marking the first time a dark spot has been identified on the planet via an Earth-bound telescope, has perplexed astronomers. The newly gleaned insights could yield further understanding of these mysterious phenomena’s nature and origin.

Large atmospheric spots are prevalent features on giant planets, the most renowned being Jupiter’s Great Red Spot. On Neptune, a dark spot was first found by NASA’s Voyager 2 in 1989 but vanished a few years later. “The initial discovery of a dark spot has always left me pondering over these ephemeral and elusive dark characteristics,” says the study’s lead investigator, Professor Patrick Irwin of the University of Oxford, UK. The study was unveiled on August 24 in Nature Astronomy.

Through the utilization of ESO’s VLT, the observation of a substantial dark spot on Neptune was made possible, revealing an unexpected smaller bright spot close by. A brief video summarizes this groundbreaking discovery. Credit: ESO

Observational Findings

Irwin’s team, using data from ESO’s VLT, dismissed the likelihood that dark spots result from cloud ‘clearing.’ Rather, the fresh observations indicate that the dark spots are more probably a consequence of air particles darkening beneath the primary visible haze layer due to the mingling of ices and hazes in Neptune’s atmosphere.

The investigation was intricate, as dark spots are not lasting features on Neptune, and prior scrutiny has not been thorough enough. However, the opportunity arose after the NASA/ESA Hubble Space Telescope spotted several dark spots on Neptune, including one initially detected in 2018. Irwin’s team promptly began ground-based observations using an ideally suited instrument.

With the aid of the VLT’s Multi Unit Spectroscopic Explorer (MUSE), the scientists managed to dissect reflected sunlight from Neptune and its spot into its constituent colors or wavelengths, thereby obtaining a 3D spectrum.[1] This facilitated a more detailed analysis of the spot. “I am utterly elated to have not only accomplished the inaugural detection of a dark spot from the ground but to have also captured a reflection spectrum of such a feature for the first time,” declares Irwin.

A composite image showcases Neptune as surveyed with MUSE at ESO’s Very Large Telescope. The resulting view of Neptune reveals a dark spot to the upper right. Credit: ESO/P. Irwin et al.

Spectrum Analysis Significance

Since varying wavelengths explore different depths of Neptune’s atmosphere, possessing a spectrum allowed astronomers to more precisely ascertain the altitude of the dark spot within the atmosphere. The spectrum analysis also imparted information regarding the chemical makeup of the atmosphere’s various layers, shedding light on why the spot appeared dark.

The observations also brought forth an unanticipated outcome. “Along the way, we uncovered a previously unknown deep bright cloud type,” mentions study co-author Michael Wong, a researcher at the University of California, Berkeley, USA. This unusual cloud formation was displayed as a bright spot adjacent to the main large dark spot, with VLT data revealing that this ‘deep bright cloud’ was at an identical level in the atmosphere as the primary dark spot, indicating it as an entirely novel feature.

Future Observational Implications

Thanks to ESO’s VLT, studying features such as these spots from Earth is now feasible. “Humanity’s capacity to observe the universe has witnessed a remarkable augmentation. Initially, we could only discern these spots by dispatching a spacecraft like Voyager. Subsequently, we achieved remote detection through Hubble. At last, technological advancement has permitted this from the ground,” concludes Wong, half-jokingly remarking, “This could render me jobless as a Hubble observer!”

A 3D spectrograph known as MUSE allows astronomers to survey an entire astronomical entity like Neptune in one effort. At each pixel, the intensity of light is measured as a function of color or wavelength. The resulting data forms a 3D set where each image pixel has a complete light spectrum. MUSE measures over 3500 colors in total. The instrument’s design capitalizes on adaptive optics, which corrects for Earth’s atmospheric turbulence, leading to crisper images. Without this amalgamation of features, the ground-based study of a Neptune dark spot would have been unachievable.

Reference: “Spectral determination of the colour and vertical structure of dark spots in Neptune’s atmosphere” by Patrick G. J. Irwin, et al., 24 August 2023, Nature Astronomy.
DOI: 10.1038/s41550-023-02047-0

The research team includes various members from institutions like the University of Oxford, University of California, University of Leicester, University of Bristol, Instituto Nacional de Técnica Aeroespacial in Spain, Jet Propulsion Laboratory, University of the Basque Country in Spain, University of Wisconsin, NASA Goddard Space Flight Center, New Mexico Institute of Technology, and Columbia University in the USA.

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• European Southern Observatory (ESO)
• Very Large Telescope (VLT)
• NASA’s Voyager 2
• Hubble Space Telescope
• University of Oxford
• Nature Astronomy