Neptune’s First-Ever Detected Mysterious Dark Spot


Astronomers have observed a large dark spot in Neptune’s atmosphere, as well as an unexpected small bright spot right next to it.

If we know of the existence of these dark spots, this is the first time that we have observed one with a terrestrial telescope. These occasional structures against the blue background of Neptune’s atmosphere remain a mystery to astronomers for now, but the new results provide further clues to their nature and origin.

Spots are regularly observed on the major planets, the most famous being Jupiter’s Great Red Spot. But Neptune is no exception, a dark spot was first discovered there by NASA’s Voyager 2 in 1989… before disappearing a few years later.

“Since the first discovery of a dark spot, I have always wondered what these ephemeral and elusive dark structures were”, explains Patrick Irwin, professor at the University of Oxford (United Kingdom) and principal investigator of the study published today in the journal Nature Astronomy.

Find out what it isn’t before you know what it is

The mystery remains whole for the moment but the hypotheses are going well. One of them would be that the dark spots are caused by a “thinning” in the clouds. To counter this hypothesis, the team used ESO’s Very Large Telescope.

Thanks to the VLT’s Multi Unit Spectroscopic Explorer (MUSE) instrument, astronomers have been able to better determine the height at which the dark spot is located in the planet’s atmosphere. These observations then allowed them to launch a new hypothesis: the spots would be “the result of the darkening of air particles in a layer located below the main layer of visible haze, when ice and haze mix in the atmosphere of Neptune”. The spectrum also provided information about the chemical composition of different layers of the atmosphere, which helped the team understand why the spot appeared dark.

“I am absolutely thrilled to not only have been able to detect a dark spot from the ground for the first time, but also to have recorded a reflection spectrum of such a spot for the first time,” said Patrick Irwin.

A rare and unexpected cloud

The observations also led scientists to discover a rarity they hadn’t expected: “We discovered a rare type of deep, bright cloud that had never been identified before, even from space,” says Michael Wong, co-author of the study. This is a completely new type of cloud compared to the small high-altitude methane ice clouds previously observed.

The discoveries are expected to continue thanks to the VLT and its various analytical instruments, “this is a staggering 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 acquired the ability to distinguish them from a distance thanks to Hubble. Finally, the technology has advanced to allow us to do it from the ground”, concludes Michael Wong.

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