Ground-Based Observations Shed Light on Neptune’s Mysterious Dark Spots
Since Voyager 2’s flyby of Neptune in 1989, scientists have been perplexed by the appearance of giant dark smudges in the planet’s atmosphere. However, for the first time, researchers have observed one of these dark spots using Earth-based instruments, providing unprecedented resolution and valuable insights into their nature.
Astronomer Patrick Irwin from the University of Oxford in the UK expresses his excitement, stating, “Since the first discovery of a dark spot, I’ve always wondered what these short-lived and elusive dark features are. 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.”
While Neptune’s dark vortices resemble anticyclonic storms, like Jupiter’s Great Red Spot, they differ in several mysterious ways. Notably, they have shorter lifespans, appearing and dissipating every few years. Additionally, these vortices are believed to have relatively little cloud activity at their centers compared to storm vortices on Saturn and Jupiter. The clouds that can be detected are fluffy cumulus-like formations that appear around the edges, likely caused by gasses freezing into methane ice crystals as they rise from lower altitudes.
Unfortunately, studying these phenomena has been challenging due to the planet’s distance and the short-lived nature of the vortices. Up until now, the Hubble Space Telescope has been the sole instrument capable of observing and tracking them, imposing limitations on the range of wavelengths in which Neptune can be observed.
However, in 2018, a large storm vortex provided an opportunity for Irwin and his team to employ another instrument: the Very Large Telescope’s Multi Unit Spectroscopic Explorer (MUSE). MUSE allowed the researchers to detect sunlight reflecting off Neptune and analyze its wavelengths to reconstruct a 3D spectrum of the planet.
By associating different wavelengths with different altitudes in Neptune’s atmosphere, the team was able to determine the altitude of the dark spot and made a surprising discovery. Contrary to previous assumptions, the darker color of the spot does not indicate a “hole” in Neptune’s atmosphere. Instead, it seems to result from a darkening of particles in the layer of hydrogen sulfide beneath the top layer of atmospheric aerosol haze. The researchers propose that local heating in the deep part of an anticyclonic vortex vaporizes the hydrogen sulfide ice, revealing a darker vortex core. Additionally, the team observed a bright cloud accompanying the vortex, unlike the methane clouds often found alongside Neptune vortices. This new cloud appears to be at the same altitude as the dark vortex.
Further investigations will be required to understand the nature of this newly discovered cloud and to confirm the team’s proposed mechanisms for the atmospheric darkening of Neptune. Nonetheless, the ability to conduct ground-based observations of the planet marks a significant advancement in humanity’s ability to observe the cosmos.
Astronomer Michael Wong from the University of California, Berkeley emphasizes the importance of this progress, saying, “This is an astounding 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 gained the ability to make them out remotely with Hubble. Finally, technology has advanced to enable this from the ground.”
The research detailing these findings has been published in the journal Nature Astronomy.