Astronomers at the University of California, Berkeley, have discovered large, Earth-sized spots at Jupiter’s north and south poles, which appear and disappear unpredictably. These dark ovals, visible only in ultraviolet light, are located beneath the auroral zones at the poles, similar to Earth’s northern and southern lights.
The spots absorb more UV than the surrounding areas, making them appear dark in NASA’s Hubble Space Telescope images. Between 2015 and 2022, dark ovals were observed 75% of the time at the south pole, but only in one out of eight images of the north pole.
Dark UV ovals observed at Jupiter’s poles suggest unusual magnetic processes occurring deep in the planet’s atmosphere, far beyond the auroras seen on Earth. First detected by Hubble in the late 1990s and later by the Cassini spacecraft, these spots were overlooked mainly until UC Berkeley undergraduate Troy Tsubota conducted a systematic study of Hubble’s recent images.
Tsubota found that dark UV ovals were common at the south pole, with eight observed between 1994 and 2022. In contrast, only two dark UV ovals were found at the north pole across 25 global maps of Jupiter.
Most Hubble images were captured in the Outer Planet Atmospheres Legacy (OPAL) program. This initiative involves yearly observations of Jupiter, Saturn, Uranus, and Neptune to study their atmospheric dynamics and how they evolve. By continuously monitoring these planets, OPAL astronomers gain valuable insights into their weather systems, storm patterns, and long-term atmospheric changes.
Tsubota said, “In the first two months, we realized these OPAL images were like a gold mine, in some sense, and I very quickly was able to construct this analysis pipeline and send all the images through to see what we get. That’s when we realized we could do some good science and real data analysis and start talking with collaborators about why these show up.”
The team consulted experts Tom Stallard and Xi Zhang to understand the cause of areas of dense haze observed on a planet. Stallard suggested that the dark oval might be caused by a vortex formed when the planet’s magnetic field lines experience friction in two distant locations: the ionosphere and a plasma sheet around the Earth from the volcanic moon Io.
This vortex spins fastest in the ionosphere and weakens as it moves deeper. Like a tornado stirring dust, the vortex may either dredge up haze from below or generate additional haze, creating the observed dense spots.
Based on their observations, the team believes the dark ovals form over about a month and dissipate in a few weeks. Xi Zhang noted that the haze in these ovals is 50 times thicker than usual, indicating that it likely results from swirling vortex dynamics rather than chemical reactions triggered by high-energy particles from the upper atmosphere.
The team’s data also showed that the timing and location of these energetic particles do not align with the appearance of the dark ovals, supporting the vortex hypothesis.
The findings align with the goals of the OPAL project, which aims to explore how atmospheric dynamics on the solar system’s giant planets differ from Earth’s. According to Wong, studying the connections between various atmospheric layers is crucial for understanding all planets, including exoplanets Jupiter and Earth.
The team observed evidence of a process linking all parts of the Jupiter system, from its interior dynamo and satellites to their plasma torii, ionosphere, and stratospheric hazes. These insights help us understand the planet as an interconnected whole.
Journal Reference:
- Tsubota, T.K., Wong, M.H., Stallard, T. et al. UV-dark polar ovals on Jupiter as tracers of magnetosphere–atmosphere connections. Nat Astron (2024). DOI: 10.1038/s41550-024-02419-0
