One of Neptune's Dark Spots Finally Seen From Earth

There’s no getting around it: our Solar System’s gas giants all have big, conspicuous spots on their faces. These include Jupiter’s Great Red Spot, Saturn’s Great White Spot, Uranus’ Great Dark Spot, and Neptune’s Great Dark Spot. Far from blemishes or features that tarnish the planets’ natural beauty, these “spots” are caused by massive storms or other processes in the planets’ atmospheres. While they are extremely large by Earth standards, they are difficult to study by anything other than robotic probes that can get close to the planet.

Neptune’s Great Dark Spot was not discovered until NASA’s Voyager 2 probe flew past the planet in 1989 on its way to the edge of the Solar System. Decades later, scientists are still unsure how this storm originated or what mechanisms drive it today. Using the ESO’s Very Large Telescope (VLT), a team of astronomers was able to observe the Great Dark Spot for the first time using a ground-based telescope. Their results provided the most detailed data on the spot to date and some interesting insights into the nature and origin of this mysterious feature.

The team was led by astronomers from the University of Oxford and included researchers from the Center for Integrative Planetary Science (CIPS), the Instituto Nacional de Técnica Aeroespacial (INTA), the University of the Basque Country (UPV/EHU), the New Mexico Institute of Mining and Technology, the American Museum of Natural History, NASA’s Jet Propulsion Laboratory, and the Solar System Exploration Division at NASA Goddard. Their paper, “Spectral determination of the color and vertical structure of dark spots in Neptune’s atmosphere,” recently appeared in Nature Astronomy.

Since the Voyager 2 probe passed through the system, astronomers did not get another look at Neptune’s Great Dark Spot for twenty-nine years. By 2018, astronomers observed several more dark spots using the Hubble Space Telescope in Neptune’s atmosphere, including one in the northern hemisphere no one had ever seen. These observations encouraged Patrick Irwin – a professor of physics at the University of Oxford and the lead investigator – and an international team of astronomers to investigate the dark spot more closely.

To get a more detailed look, Irwin and his colleagues relied on the VLT’s Multi Unit Spectroscopic Explorer (MUSE) to provide a 3D spectrum, allowing them to study Neptune’s atmosphere in greater detail than was ever possible. On the one hand, this spectrum provided a breakdown of the chemical composition of the different layers in Neptune’s atmosphere. Since chemicals absorb light at different wavelengths, the spectra also revealed data on the depth of each layer, allowing the team to constrain where the dark spot rests in the atmosphere. As Irwin explained in a recent ESO press release:

“Since the first discovery of a dark spot, I’ve always wondered what these short-lived and elusive dark features are. 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.”

Irwin and his team also used the VLT data to rule out the possibility that these dark spots result from clearings in the cloud layers. Instead, their observations indicate that they are likely caused by ices and hazes mixing in Neptune’s atmosphere beneath the top hazy layer, causing air particles to darken. The process was rather difficult since dark spots are not permanent features in Neptune’s atmosphere, and the distance makes detailed studies of this planet very difficult.

In addition, they were surprised to see a smaller bright cloud adjacent to the Great Dark Spot. This feature appeared as a bright spot next to the larger dark spot. The VLT spectra also revealed that this “bright spot” is at the same altitude as the main dark spot. This means it is a completely new type of feature compared to the small dark clouds composed of methane ice in the upper atmosphere that have been previously observed. These findings show that by using modern-day spectrometers, it is now possible for astronomers to study features like these spots from Earth. Said study co-author Michael Wong, a CIPS researcher at UC Berkely:

“In the process we discovered a rare deep bright cloud type that had never been identified before, even from space. This is an astounding increase in humanity’s ability to observe the cosmos. 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!”

Further Reading: ESO, Nature Astronomy