For the first time, scientists capture mysterious dark spot in Neptune's atmosphere from Earth

A team of researchers led by Patrick Irwin from the University of Oxford seized a remarkable opportunity to unlock the enigmatic nature of dark spots on Neptune. These perplexing formations first caught the attention of scientists in 2018 when an inky blemish emerged in the planet's atmosphere, prompting a determined quest for answers.

The saga of Neptune's dark spots traces back to 1989 when Voyager 2 ventured to the distant ice giant. The spacecraft unveiled a colossal anticyclonic vortex spanning thousands of miles in diameter, embellishing Neptune's northern hemisphere. However, this intriguing vortex vanished into obscurity over time, leaving astronomers baffled.

Over subsequent years, the Hubble Space Telescope witnessed a series of analogous dark spots emerging and dissipating across both hemispheres of Neptune. While superficially resembling Jupiter's famed Great Red Spot, these Neptunian counterparts exhibited a stark contrast. The Great Red Spot has endured for over three and a half centuries, adorning Jupiter's upper atmosphere, while Neptune's ephemeral dark spots lingered for just a few years, concealed deeper within the ice giant's gaseous cloak.

The pivotal moment arrived in 2018 when Hubble detected yet another enigmatic dark spot on Neptune's surface. Promptly, Irwin's team leaped into action, capitalizing on the European Southern Observatory's Very Large Telescope (VLT) equipped with the Multi Unit Spectroscopic Explorer (MUSE) instrument, positioned in the Chilean desert. This groundbreaking combination allowed them to not only spot the elusive entity but also conduct a three-dimensional spectroscopic analysis, unraveling the intricacies of its behavior at various atmospheric depths.

Published recently on August 24 in the journal Nature Astronomy, the team's findings shed light on the origin of Neptune's dark spots, which had hitherto eluded comprehensive understanding. "Since the first discovery of a dark spot, I’ve always wondered what these short-lived and elusive dark features are," remarked Irwin, encapsulating the team's collective curiosity.

The investigation relied on the principle that different wavelengths of light can probe distinct layers of Neptune's atmosphere, dictated by its composition and reflective properties. Employing MUSE, the team constructed a three-dimensional reflection spectrum, spotlighting how sunlight interacts with the planet's atmosphere at varying depths. Notably, the newly identified dark spot, named NDS-2018, nestled within an atmospheric zone exhibiting minimal light reflection. This finding debunked the notion that the dark spot merely unveiled hidden darker layers beneath Neptune's clouds.

Intriguingly, the team proposed that the presence of aerosol haze within the atmosphere played a pivotal role in the dark spot's appearance. Their observations positioned the dark spot around a depth of 5 bars, a layer where hydrogen sulfide can crystallize into ice. The researchers posited two plausible explanations for this phenomenon.

One proposition involves an upwelling at the vortex's center, transporting hydrogen sulfide from deeper layers to higher altitudes. Upon interaction with solar ultraviolet light, the gas reacts, causing the atmosphere to darken. Alternatively, the hydrogen sulfide might condense around darker particles, with anticyclonic conditions inducing local heating and subsequent sublimation, unveiling the hidden particles.

Unexpectedly, VLT's observations revealed an additional revelation—an adjacent bright spot, designated DBS-2019, positioned on the southwest edge of the dark spot. Michael Wong, a planetary scientist from the University of California, Berkeley, part of Irwin's team, expressed amazement at discovering a previously unidentified deep, bright cloud type.

Bright spots in Neptune's atmosphere are not entirely novel, but DBS-2019 distinguishes itself by sharing an altitude with the dark spot. This proximity suggests potential convective processes, possibly expelling brighter material from the icy depths below.

Regrettably, both the dark and bright spots vanished in 2022, leaving Neptune's atmosphere devoid of their fleeting presence. The abrupt disappearance may be attributed to the sun's solar cycle and a decline in photoionizing ultraviolet light during the solar minimum of 2019. This absence of crucial influences could lead to the gradual fading of Neptune's cloud coverage. As experts anticipate solar maximum in 2024-2025, they speculate a resurgence in the planet's atmospheric activity.

For now, astronomers must await the emergence of fresh dark spots to resume their investigations from Earth's vantage point. The tantalizing prospect of a future space mission akin to Voyager 2's odyssey beckons, holding the promise of unveiling Neptune's secrets once more. The research paper published on August 24 in the journal Nature Astronomy stands as a testament to humanity's unyielding pursuit of unraveling the mysteries of the cosmos.