Study on Neptune dark spot reveals curious smaller bright cloud nearby

(CN) — More than a decade after it was launched, NASA’s Voyager 2 space probe flew by the planet Neptune where it observed a giant dark spot on the aqua blue planet. Years later, when photographed from a telescope on Earth, the spot disappeared.

According to a scientific study published Thursday in the journal Nature Astronomy, researchers were not only able to observe the spot for the first time from Earth, but also discovered a smaller bright spot, and have begun to figure out both what happens to it and what it is.

“What causes them, and exactly where they are located in the atmosphere wasn’t known. This was partly because previous observations have only been made with cameras that view in a few channels covering a wide range of wavelengths, e.g., UV, blue, green, etc. Such observations can’t give us much leverage on the vertical structure,” Patrick Irwin, a professor of planetary physics at the University of Oxford in the U.K. and one of the authors of the study, explained in an email.

Other giant planets like Jupiter and Saturn also have gargantuan spots on them. The most famous is probably Jupiter’s Great Red Spot, an anticyclonic storm that’s been observed for hundreds of years. But while Jupiter’s enormous spot might be more famous, Neptune’s is deeper, Irwin said.

Irwin explained that further research conducted via the Hubble Space Telescope discovered more dark spots in Neptune’s atmosphere. All of them had similar characteristics: they were only visible at particular wavelengths, they appeared at mid-latitudes and then slowly drifted towards the equator of the planet, finally disappearing after roughly the equivalent of one year on Earth.

Using the European Southern Observatory’s Very Large Telescope’s MUSE instrument, or Multi Unit Spectroscopic Explorer, in Chile’s Atacama Desert, Irwin and a team of researchers were able to detect, for the first time ever, a dark spot from a telescope on Earth. They were able to see into the different layers of Neptune’s atmosphere using the MUSE instrument’s ability to split reflected sunlight from the planet into different wavelengths, which gave them a three-dimensional spectrum of Neptune.

After analyzing that data, researchers determined that the dark spot is caused by a darkening of the particles in a layer of aerosols believed to be composed of a mixture of photochemical haze and methane ice. They also observed, for the first time, a smaller bright cloud beside the larger dark spot.

“Its close proximity to the dark spot suggests some sort of link, but what this might be is impossible to say at the moment,” Irwin wrote.

Further research is required to determine what caused the larger spot to darken.

“To further probe these differences requires thermal measurements to be made at multiple depths in the atmosphere, ideally from a future proposed space mission to either Neptune or Uranus, combined with better laboratory determinations of the refractive index spectra of potential aerosols and more detailed numerical dynamical simulations,” the authors say in the paper.