Mysterious ‘dark spot’ on Neptune detected from Earth for first time

By Stephen Beech via SWNS

A mysterious "dark spot" on Neptune has been detected from Earth for the first time.

Using the European Southern Observatory’s Very Large Telescope (VLT) in Chile, astronomers observed a large dark spot in Neptune’s atmosphere - with an unexpectedly smaller bright spot next to it.

The occasional features in the blue background of Neptune’s atmosphere remain a mystery to astronomers.

But they say the new results, published in the journal Nature Astronomy, provide further clues as to their nature and origin.

Large spots are common features in the atmospheres of giant planets, the most well-known being Jupiter’s Great Red Spot.

On Neptune, a dark spot was first discovered by NASA’s Voyager 2 in 1989, before disappearing a few years later.

Study lead investigator Professor Patrick Irwin, of Oxford University, said: “Since the first discovery of a dark spot, I’ve always wondered what these short-lived and elusive dark features are."

Irwin and his team used data from ESO’s VLT to rule out the possibility that dark spots are caused by a "clearing" in the clouds.

The 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.

Irwin explained that coming to that conclusion was not easy because dark spots are not permanent features of Neptune’s atmosphere and astronomers had never before been able to study them in sufficient detail.

He said the chance came when the NASA/ESA Hubble Space Telescope discovered several dark spots in Neptune's atmosphere, including one in the planet’s northern hemisphere first noticed in 2018.

Irwin and his team immediately got to work studying it from the ground - with an instrument that is ideally suited to these challenging observations.

Using the VLT’s Multi Unit Spectroscopic Explorer (MUSE), the research team were able to split reflected sunlight from Neptune and its spot into its component colours, or wavelengths, and obtain a 3D spectrum.

That meant they could study the spot in more detail than was possible before.

Irwin said: “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."

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.

Dr Irwin explained that 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.

Study co-author Dr. Michael Wong, a researcher at the University of California, Berkeley, explained that the observations also offered up a "surprise" result.

He said: “In the process, we discovered a rare deep bright cloud type that had never been identified before, even from space."

Dr Wong said the rare cloud type appeared as a bright spot right beside the larger main dark spot, the VLT data showing that the new "deep bright cloud" was at the same level in the atmosphere as the main dark spot.

He says it means it is a completely new type of feature compared to the small "companion" clouds of high-altitude methane ice that have been previously observed.

With the help of ESO’s VLT, it is now possible for astronomers to study features such as the spots from Earth.

Dr Wong said: “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."

He added: "This could put me out of work as a Hubble observer.”