Astronomers muse over dark spot discovered on Neptune

24 Aug 2023

Neptune observed with the MUSE instrument at VLT. Image: ESO/P. Irwin et al

Using the MUSE instrument on the ESO’s Very Large Telescope in Chile, scientists now have data to better explain ephemeral dark spots on the gaseous giant.

Astronomers have for the first time been able to study a mysterious dark spot in Neptune’s atmosphere using a ground-based telescope from Earth.

While dark spots are not uncommon in the atmosphere of giant plants in our solar system – the most notable of which is the giant red dot on Jupiter – the circumstances that have led to the dark spot on Neptune have long eluded scientists.

But now, using data from observations made by the Very Large Telescope (VLT) operated by the European Southern Observatory (ESO) in Chile, a team of astronomers has made progress in understanding the phenomenon better.

“Since the first discovery of a dark spot, I’ve always wondered what these short-lived and elusive dark features are,” said Patrick Irwin, a professor at the University of Oxford and lead investigator of the study published today (24 August) in Nature Astronomy.

What makes it even harder to study this spot in the atmosphere of Neptune is its ephemerality. First observed by NASA’s Voyager 2 in 1989, the dark spot on our solar system’s most distant planet from the sun disappeared a few years later.

Years later, the Hubble Space Telescope discovered several dark spots in Neptune’s atmosphere, including one in the planet’s northern hemisphere first noticed in 2018. This is when Irwin and his team jumped on the opportunity using the VLT.

Data from the telescope, located in the Atacama Desert of northern Chile, ruled out the possibility of the dark spots being caused by “clearing” in the clouds, as was initially supposed.

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.

ESO switches on the largest ever optical telescope in Chile.

The Very Large Telescope during one of its rotations to observe the cosmos. Image: ESO/G.Hüdepohl

Using the VLT’s multi-unit spectroscopic explorer, called MUSE, Irwin and his team were able to split reflected sunlight from Neptune and its spot into its component colours, or wavelengths, and obtain a 3D spectrum. This meant they could study the spot in more detail than ever before.

“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 said.

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.

But, as it turns out, there was more than just a silver lining to this dark cloud.

“In the process we discovered a rare deep bright cloud type that had never been identified before, even from space,” said study co-author Michael Wong, a researcher at the University of California, Berkeley.

Data from the VLT showed that this rare “deep bright cloud” was right beside and at the same level as the larger dark spot on Neptune, revealing a previously unseen feature compared to the small “companion” clouds of high-altitude methane ice that have been previously observed.

“This is an astounding increase in humanity’s ability to observe the cosmos,” Wong remarked of the team’s use of the VLT.

“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!”

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Vish Gain is a journalist with Silicon Republic