Astronomers using ESO’s Very Large Telescope (VLT) have observed a large dark spot in Neptune’s atmosphere and an unexpectedly smaller bright spot next to it. This is the first time a dark spot on the planet has been observed using a terrestrial telescope. These random features on the blue background of Neptune’s atmosphere are a mystery to astronomers, and the new results provide additional clues about their nature and origin.

Large spots are typical of the atmospheres of giant planets, the most famous of which is Jupiter’s Great Red Spot. The dark spot on Neptune was first discovered by NASA’s Voyager 2 spacecraft in 1989 and disappeared a few years later. “Since the first discovery of the dark spot, I’ve always wondered what these ephemeral and elusive dark elements are,” says Patrick Irwin, professor at the University of Oxford in England and lead researcher on the study.

This research is presented in the article “The Cloud Structure of Dark Spots and Storms in Neptune’s Atmosphere”. Nature Astronomy .

Irwin and his team used ESO’s VLT data to rule out the possibility that dark spots are due to “illumination” in clouds. Instead, the new observations suggest that the dark spots are likely the result of darkening of air particles in a layer below the visible main fog layer as a mixture of ice and fog in Neptune’s atmosphere.

Coming to this conclusion was not easy because dark spots are not a permanent feature of Neptune’s atmosphere, and astronomers have never been able to study them in sufficient detail before. This possibility emerged after NASA/ESA’s Hubble Space Telescope discovered several dark spots in Neptune’s atmosphere, including one in the planet’s northern hemisphere, first detected in 2018.

Irwin and his team immediately set about probing it from the ground, using an instrument ideal for these difficult observations.

Using the VLT’s Multi-Unit Spectroscopic Explorer (MUSE), the researchers were able to separate sunlight reflected from Neptune and sunspots into its component colors, or wavelengths, and obtain a three-dimensional spectrum. This meant they could study the area in more detail than was previously possible. “I’m very happy to not only be able to detect a dark spot on the ground for the first time, but also to be able to record the reflection spectrum of such a detail for the first time,” says Irwin.

Because different wavelengths probe different depths in Neptune’s atmosphere, having the spectrum allowed astronomers to better determine the height at which the dark spot was located in the planet’s atmosphere. The spectrum also provided information on the chemical composition of the different layers of the atmosphere, giving the team clues as to why the spot appears dark.

The observations also yielded an unexpected result. “In the process, we discovered a rare, deep and bright type of cloud that has never been identified before, even from space,” says study co-author Michael Wong, a researcher at the University of California, Berkeley.

This rare type of cloud appeared as a bright spot next to a larger main dark spot; The VLT data shows that the new “deep bright cloud” is level with the main dark spot in the atmosphere. This means that compared to the small “companion” clouds of high-altitude methane ice previously observed, this is an entirely new type of object.

With ESO’s VLT, astronomers can now study objects similar to these points on Earth. “This is an extraordinary increase in humanity’s ability to observe space. At first we could only detect these points by sending a spacecraft like Voyager there. Later we were able to distinguish them remotely using Hubble. Eventually, technology advanced enough to make this possible from scratch,” concludes Wong. Source