Since Voyager 2’s historic flyby of Neptune in 1989, scientists have been intrigued by mysterious dark spots appearing in the planet’s atmosphere. These peculiar spots are still a mystery because of their unique characteristics and fleeting nature. However, for the first time, researchers have observed one of these dark vortices with Earth-based instruments, providing unprecedented resolution and valuable insights into their formation and composition. The study was published in the journal Nature Astronomy
Astronomer Patrick Irwin of the University of Oxford, UK, expressed his excitement about the discovery, stating, “Since first discovering the dark spot, I have always wondered what these short-lived and elusive dark objects represent. I am absolutely thrilled that I have been able not only to detect a dark spot from Earth for the first time, but also to record the reflection spectrum of such an object for the first time.”
Neptune’s dark vortices are actually anticyclonic storms similar to Jupiter’s Great Red Spot. However, they differ from it in several intriguing and mysterious ways. First, they are relatively short-lived – appearing and disappearing every few years. Also, compared to vortices on Saturn and Jupiter, there are no cloud formations at their centers. Instead, fluffy white clouds are observed at the edges, which are probably formed by the freezing of gases into methane ice crystals as they rise from lower altitudes.
Understanding these phenomena has been difficult because of Neptune’s distance from Earth and the transience of the eddies. Until now, the only instrument capable of observing and tracking them has been the Hubble Space Telescope, limiting the range of wavelengths in which the planet can be studied.
In 2018, when a major storm vortex developed, Irwin and his team took the opportunity to use the Very Large Telescope’s Multi-Unit Spectroscopic Explorer (MUSE). MUSE was able to detect the reflection of sunlight from Neptune and analyze its wavelength components, constructing a three-dimensional spectrum of the planet.
By correlating the different wavelengths with different altitudes in Neptune’s atmosphere, the researchers were able to determine the height of the dark spot’s location. To their surprise, they found that it was not a “hole” in Neptune’s atmosphere, as previously thought. Rather, the deeper color appears to be the result of particle darkening in the layer of hydrogen sulfide beneath the top layer of atmospheric aerosol haze. The authors hypothesized that local heating within the anticyclonic vortex leads to evaporation of the hydrogen sulfide ice, resulting in a darker color in the vortex core. In addition, observations show that particles in the overlying aerosol layer become smaller, reducing opacity.
In addition to this groundbreaking discovery, the researchers found another unexpected finding: a bright cloud accompanying the vortex. This cloud was different from the methane clouds typically associated with Neptune vortices and appeared to be at the same altitude as the dark vortex.
Although these results provide valuable insight into the darkening of Neptune’s atmosphere and the formation of its vortices, further research is needed to fully understand these phenomena. Nevertheless, by being able to make ground-based observations of Neptune, scientists are one step closer to unraveling the mysteries surrounding this distant planet.