Astronomers detect Neptune's dark spot from Earth

Before this detection, astronomers were able to study such spots only by sending a spacecraft into interstellar space.

Yasmin Tinwala

Dark spot on Neptune observed with MUSE at ESO’s Very Large Telescope / Image –

In a first, astronomers were able to detect a very large dark spot in Neptune’s atmosphere, with the help of the  European Southern Observatory's Very Large Telescope (VLT) from Earth. They were also able to study the spot in great detail, with observations including the unexpected detection of a cloud that had never been seen before. The complete study was published in Nature Astronomy recently.

According to an official release, the recent detection of the dark spot and its detailed study was initiated after the NASA/ESA Hubble Space Telescope discovered several dark spots in Neptune’s atmosphere in 2018.

Patrick Irwin, a professor at the University of Oxford in the UK, and lead investigator of the study, along with his team got to work studying the dark spots on Neptune and ruled out the possibility that these dark spots are caused by a “clearing” in the clouds.

The new observations indicated that these dark spots are likely formed because of the darkening of air particles in a layer below the main visible haze layer, as ices and hazes mix in Neptune’s bone-freezing atmosphere where the average temperature is believed to be -364 degrees Fahrenheit (-220 degree Celsius).

Additionally, Irwin and his team used the VLT’s multi-unit spectroscopic explorer (MUSE) to split reflected sunlight from Neptune and its dark spot -- that was observed from the earth -- into its component colours, or wavelengths, and obtained a 3D spectrum, to study the spot in more detail.

“I’m 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,” Irwin said. Having a spectrum helped researchers to better determine the height at which the dark spot sits on Neptune’s atmosphere since different wavelengths probe different depths in the planet’s atmosphere.

The observations yielded the discovery of a “rare deep bright cloud type that had never been identified before, even from space,” said Michael Wong, co-author of the study and a researcher at the University of California. This rare cloud type appeared as a bright spot beside the larger main dark spot. VLT data showed that the “deep bright cloud” was at the same level in the atmosphere as the main dark spot.


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