Ever since Voyager 2 conducted its historic flyby of Neptune in 1989, scientists have been intrigued by the enigmatic dark patches that manifest in the distant planet’s atmosphere.
Now, for the very first time, researchers have managed to observe these baffling features in exceptional detail using Earth-based instruments. This breakthrough is shedding light on why these patches appear exceptionally dark and differ significantly from similar phenomena on other celestial bodies.
Astronomer Patrick Irwin from the University of Oxford in the UK expressed his excitement, saying, “Ever since the first discovery of a dark spot, I’ve been curious about the nature of these transient and mysterious dark features.”
Irwin’s team has not only achieved the first-ever detection of a dark spot from terrestrial observations but has also recorded a reflection spectrum of such a feature for the very first time.
While initially mistaken for voids in Neptune’s atmosphere, it turns out that Neptune’s dark vortices are, in fact, anticyclonic storms akin to Jupiter’s Great Red Spot.
However, they exhibit intriguing differences, including their relatively short lifespan, appearing and disappearing every few years. These vortices also have an unusual lack of substantial cloud cover at their cores, unlike storm vortices on Saturn and Jupiter.
Instead, they display fluffy white clouds around their perimeters, likely formed as gases freeze into methane ice crystals upon ascent from lower altitudes.
The study encountered significant challenges due to Neptune’s considerable distance from Earth and the fleeting nature of these vortices. Until now, the Hubble Space Telescope has been the primary tool for observing and tracking them.
However, when a large storm vortex emerged in 2018, Irwin and his team turned to the Very Large Telescope’s Multi Unit Spectroscopic Explorer (MUSE). This instrument detected sunlight reflecting off Neptune, enabling researchers to construct a 3D spectrum by analyzing various wavelengths corresponding to different altitudes within Neptune’s atmosphere.
The astonishing revelation was that these dark spots weren’t “holes” in Neptune’s atmosphere but resulted from the darkening of particles within the hydrogen sulfide layer beneath the top aerosol haze layer.
This darkening may be caused by localized heating in the core of the anticyclonic vortex, leading to the vaporization of hydrogen sulfide ice and revealing a darker core. Additionally, the aerosol particles above were found to have reduced in size, resulting in decreased opacity.
Another unexpected discovery was the presence of a bright cloud accompanying the vortex, distinct from the typical methane clouds often associated with Neptune’s vortices. Remarkably, this cloud appeared at the same altitude as the dark vortex, introducing a new and intriguing aspect for researchers to investigate.
Further research is required to validate these findings and explore the proposed mechanisms behind Neptune’s atmospheric darkening. Nevertheless, the ability to conduct ground-based observations of Neptune signifies a significant leap forward in humanity’s capacity to explore the cosmos.