Astronomers using the Multi Unit Spectroscopic Explorer (MUSE) instrument on ESO’s Very Large Telescope (VLT) have observed a large dark spot in Neptune’s atmosphere, with an unexpected smaller bright spot adjacent to it. This is the first time a Neptunian dark spot has ever been observed with a ground-based telescope.
Large spots are common features in the atmospheres of giant planets, the most famous being the Great Red Spot on Jupiter.
On Neptune, a dark spot was first discovered by NASA’s Voyager 2 in 1989, before disappearing a few years later.
“Since the first discovery of a dark spot, I’ve always wondered what these short-lived and elusive dark features are,” said University of Oxford’s Professor Patrick Irwin.
In the new research, Professor Irwin and colleagues ruled out the possibility that dark spots are caused by a ‘clearing’ in the clouds.
Their 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.
Coming to this conclusion was no easy feat because dark spots are not permanent features of Neptune’s atmosphere and astronomers had never before been able to study them in sufficient detail.
The opportunity came after 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.
Using data from the MUSE instrument on VLT, the astronomers were able to split reflected sunlight from Neptune and its spot into its component colors, or wavelengths, and obtain a 3D spectrum. This meant they could study the spot in more detail than was possible 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,” Professor Irwin said.
Since different wavelengths…
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