Astronomers at the University of California, Berkeley, have uncovered an intriguing phenomenon on Jupiter: large, Earth-sized spots that periodically appear and disappear at the planet’s north and south poles. These mysterious dark ovals, visible only in ultraviolet (UV) light, were first detected in the late 1990s using NASA’s Hubble Space Telescope but had not been thoroughly studied until recently. The newly published findings, appearing in Nature Astronomy on November 26, 2024, reveal that these spots—referred to as UV-dark ovals—are linked to unusual atmospheric processes occurring within Jupiter’s powerful magnetic field, suggesting complex interactions between the planet’s atmosphere, ionosphere, and magnetic forces.
The discovery was made possible by the systematic analysis of Hubble’s global maps of Jupiter, specifically from the Outer Planet Atmospheres Legacy (OPAL) project. This project, which monitors the atmospheres of Jupiter, Saturn, Uranus, and Neptune, provides yearly observations aimed at understanding the atmospheric dynamics and evolution of these distant worlds. The team, led by UC Berkeley undergraduate Troy Tsubota and senior researcher Michael Wong, found that the dark UV ovals are common at Jupiter’s south pole, appearing in about 75% of the images taken between 2015 and 2022. In contrast, the spots appear much less frequently at the north pole, only in 2 out of 25 images, marking a striking asymmetry between the poles.
These UV-dark ovals are embedded in layers of stratospheric haze that cap Jupiter’s poles. The dark spots absorb more UV light than the surrounding areas, making them visible on Hubble’s ultraviolet images. They typically appear just below Jupiter’s auroral zones—bright, glowing bands of light similar to the auroras seen at Earth’s poles. The formation and behavior of these spots suggest the presence of dynamic atmospheric processes deep within Jupiter’s atmosphere, potentially linked to its powerful magnetic field.
The first clues about the nature of these spots were found by Tsubota, who conducted a thorough review of Hubble’s images of Jupiter. Between 1994 and 2022, he counted eight southern UV-dark ovals (SUDOs), and in the process, discovered a pattern of their formation and dissipation. These ovals, he noted, are often temporary, lasting only a few weeks before fading away. “The spots are not permanent features like Jupiter’s Great Red Spot, but instead seem to be dynamic and ephemeral,” Tsubota said.
The Role of Jupiter’s Magnetic Field
The dark ovals seem to be related to complex interactions between Jupiter’s magnetic field and its atmosphere. The prevailing theory, supported by experts like Tom Stallard at Northumbria University, is that the UV-dark ovals are the result of vortex-like motions that develop when magnetic field lines experience friction in two distant regions: the ionosphere (the uppermost layer of the atmosphere) and the sheet of hot, ionized plasma surrounding the planet, which is created by the volcanic activity of Jupiter’s moon Io.
Stallard hypothesizes that these magnetic field interactions create a vortex that spins in the ionosphere, with the strongest activity occurring at higher altitudes and progressively weakening as the vortex extends downward into the stratosphere. Much like a tornado stirring up dust on the ground, this vortex might stir up the stratospheric haze and concentrate it into dense ovals. This process could either dredge up more haze from below or generate additional haze, resulting in the observed dark spots. The mixing of atmospheric layers in this manner could explain why the UV-dark ovals are found in the upper atmospheric layers, where the haze is thickest.
Understanding the Timing and Formation of the Ovals
The team’s research suggests that these dark ovals likely form over a span of about a month and dissipate over a few weeks. The haze in the dark ovals is approximately 50 times thicker than the typical haze found in Jupiter’s atmosphere, which further supports the idea that these features are a result of dynamic vortex processes rather than chemical reactions triggered by high-energy particles from Jupiter’s auroras. According to Xi Zhang, a co-author of the study from UC Santa Cruz, the timing and location of high-energy particles do not correlate with the appearance of the dark ovals, ruling out the hypothesis that they are caused by auroral activity.
While the exact cause of the dark UV ovals remains uncertain, their periodic formation and disappearance suggest they are part of a larger atmospheric cycle, likely connected to the planet’s magnetic field and its interactions with external factors such as solar winds and Io’s volcanic emissions. These findings suggest that Jupiter’s atmospheric dynamics are more complex than previously understood, with different atmospheric layers interacting in ways that are not yet fully explained.
Implications for Planetary Science
The discovery of these dark ovals on Jupiter has broader implications for understanding planetary atmospheres, both within our solar system and on exoplanets in distant systems. Michael Wong, senior author of the study, emphasized that the OPAL project is helping to bridge the gap in our understanding of how planetary atmospheres behave under extreme conditions. “These findings are part of a larger effort to understand how atmospheric dynamics work across the solar system’s giant planets,” Wong explained. “By studying Jupiter’s atmosphere in such detail, we gain insights into the fundamental processes that govern atmospheric circulation and the behavior of gases at different altitudes.”
The study of Jupiter’s poles and their atmospheric dynamics also sheds light on the interactions between different atmospheric layers, a phenomenon that is not unique to Jupiter but can also apply to other planetary bodies, including exoplanets. By understanding how layers of atmosphere and magnetic fields interact, scientists hope to build more accurate models of planetary systems, from the gas giants in our own solar system to the exoplanets orbiting distant stars.