2 decades of Hubble Space Telescope data track changing seasons on Uranus

An analysis of two decades of data from NASA's Hubble Space Telescope has provided fresh insights into the complex atmospheric changes on Uranus that are largely driven by the effects of the sun's radiation.

Uranus, the seventh planet from the sun, is unique for its extreme axial tilt, with its equator and orbit making nearly at a right angle — likely the result of a collision with an Earth-size object long ago. This tilt causes the planet's poles to experience prolonged, dark winters and bright summers, leading to dramatic seasonal shifts, especially at the northern and southern poles. Despite these extreme traits, however, Uranus remains one of the least understood planets in our solar system, largely because it was only visited by a single spacecraft nearly 40 years ago, Voyager 2 — and that sole encounter coincided with an exceptional solar event, further complicating our understanding of the distant ice giant.

Over the past two decades, a team led by astronomer Erich Karkoschka of the University of Arizona used the Space Telescope Imaging Spectrograph on the Hubble Space Telescope to track seasonal changes on Uranus. Because the ice giant takes just over 84 Earth years to complete one orbit around the sun, the researchers mainly got to observe the planet's northern spring as the sun moved from shining directly over the planet's equator to almost directly over its north pole by 2030.

The series of Hubble images above, seen from left to right, reveal the south polar region darkening as it enters winter shadow, while the north polar region brightens as northern summer approaches, according to a NASA statement.

Uranus's atmosphere is primarily composed of hydrogen and helium, with a small amount of methane, which gives the planet its characteristic blue-green hue by absorbing red light from the sun and reflecting blue light.

From 2002 to 2022, Karkoschka and his colleagues observed the ice giant four times — in 2002, 2012, 2015 and 2022 — documenting a richer picture of the planet's atmospheric structure than was gathered by the single Voyager 2 flyby. The recent observations suggest complex atmospheric circulation patterns on Uranus during this period, with the data most sensitive to the methane distribution indicating a downwelling in the polar regions and upwelling in other areas, according to the NASA statement.

Notably, scientists found that methane is not uniformly distributed across Uranus but is rather strongly depleted near its poles, with this depletion remaining consistent over the years.

The observations also revealed changes in aerosol concentrations, allowing scientists to map out the planet's atmospheric structure. While methane depletion and aerosol patterns remained relatively stable in the middle and low latitudes over the observed two decades, the polar regions showed more dramatic shifts.

Notably, aerosols near the north pole became brighter, especially in recent years as the planet approached its northern summer, according to the statement. These long-term observations have provided scientists with a deeper understanding of how the ice giant's atmosphere functions and reacts to changing sunlight.

They can also "serve as a proxy for studying exoplanets of similar size and composition," the Hubble team said in the statement.