The gigantic storm on Jupiter known as the 'Great Red Spot' continues to shrink mysteriously and become more circular, new images from NASA's Hubble Space Telescope have found.
Scientists have produced new maps of Jupiter - the first in a series of annual portraits of the solar system's outer planets.
The observations are designed to capture a broad range of features, including winds, clouds, storms and atmospheric chemistry.
Already, the Jupiter images have showed a rare wave just north of the planet's equator and a unique filamentary feature in the core of the Great Red Spot not seen previously.
"Every time we look at Jupiter, we get tantalising hints that something really exciting is going on," said Amy Simon, a planetary scientist at NASA's Goddard Space Flight Centre in Maryland.
Simon and her colleagues produced two global maps of Jupiter from observations made using Hubble's high-performance Wide Field Camera 3.
The new images confirm that the Great Red Spot continues to shrink and become more circular, as it has been doing for years.
The long axis of this characteristic storm is about 240 kilometres shorter now than it was in 2014. Recently, the storm had been shrinking at a faster-than-usual rate, but the latest change is consistent with the long-term trend.
The Great Red Spot remains more orange than red these days, and its core, which typically has more intense colour, is less distinct than it used to be.
An unusual wispy filament is seen, spanning almost the entire width of the vortex.
This filamentary streamer rotates and twists throughout the 10-hour span of the Great Red Spot image sequence, getting distorted by winds blowing at 150 meters per second or even greater speeds.
In Jupiter's North Equatorial Belt, the researchers found an elusive wave that had been spotted on the planet only once before, decades earlier, by Voyager 2.
In those images, the wave is barely visible, and nothing like it was seen again, until the current wave was found travelling at about 16 degrees north latitude, in a region dotted with cyclones and anticyclones.
Similar waves - called baroclinic waves - sometimes appear in Earth's atmosphere where cyclones are forming.
"Until now, we thought the wave seen by Voyager 2 might have been a fluke. As it turns out, it's just rare!" said co-author Glenn Orton of NASA's Jet Propulsion Laboratory in Pasadena, California.
The findings are published in the Astrophysical Journal.
