The planet Uranus is finally revealing some of its best-hidden phenomena, thanks to a completely new mapping of its upper atmosphere. Created for the first time in three dimensions, this overview provides access to the active mechanisms of this icy world.
This unprecedented view shows bright auroras near the magnetic poles, as well as notable temperature changes across several thousand kilometers (several thousand miles) of altitude. The imprint of Uranus' tilted magnetic field clearly appears in these measurements, indicating how energy diffuses in the outer layers and interacts with charged particles, which generates unique atmospheric structures.
Two bright auroral bands were detected near Uranus' magnetic poles, with a reduction in emissions and ion density in the region between the bands, likely related to transitions in magnetic field lines.
Credit: ESA/Webb, NASA, CSA, STScI, P. Tiranti, H. Melin, M. Zamani (ESA/Webb) To achieve these results, the James Webb Space Telescope was pointed at Uranus for nearly 17 hours, thus recording a complete rotation of the planet. Its NIRSpec instrument allowed for the quantification of the faint emissions from molecules present in the upper atmosphere, offering unprecedented precision regarding the distribution of ions and temperature at different altitudes, taking advantage of its stable position at the L2 point.
Another notable point concerns the continuous cooling of Uranus' upper atmosphere since the 1990s. Temperatures peak between 3000 and 4000 kilometers (approx. 1,860 to 2,485 miles) above the clouds, with an average of about 426 kelvins (306.3°F or 152.9°C), which is lower than previous estimates. This long-term evolution indicates that the planet is gradually releasing stored heat, which has consequences for its atmospheric circulation and energy balance.
As for Uranus' auroras, they manifest as two bright bands near the magnetic poles, separated by a dark zone associated with the particular structure of the magnetic field. This phenomenon, reminiscent of what is observed on Jupiter, shows how charged particles interact with the atmosphere, generating distinct patterns that change with longitude and altitude, related to the off-center and tilted nature of the Uranian magnetosphere.
Understanding Uranus' energy balance helps astronomers better comprehend ice giant planets, which are common in other stellar systems. These discoveries thus pave the way for the study of distant exoplanets by providing models to interpret data concerning similar bodies beyond our Solar System.