Since its inception in 1990, NASA’s Hubble Space Telescope has been monitoring the ever-changing atmospheres of the outer planets, making it an invaluable interplanetary weather observer. While spacecraft missions to the outer planets have provided close-up views of these atmospheres, Hubble’s unparalleled sharpness and sensitivity allow for continuous and comprehensive observations of the complex activities over time.
The telescope’s observations complement those of other spacecraft, such as the Juno mission currently orbiting Jupiter, the retired Cassini mission to Saturn, and the Voyager 1 and 2 probes that flew by all four giant planets between 1979 and 1989. Inaugurated in 2014, the Outer Planet Atmospheres Legacy (OPAL) Program of the Hubble Space Telescope provides yearly views of the giant planets.
On November 12, 2022, the left image captured the stormy weather forecasted for Jupiter’s low northern latitudes. A distinct “vortex street,” composed of a series of alternating storms, was visible. Planetary astronomers refer to this phenomenon as a wave pattern of nested anticyclones and cyclones, resembling the gears of a machine moving both clockwise and counterclockwise.
Despite the possibility of an even larger storm forming through the merger of nearby storms, the staggered arrangement of anticyclones and cyclones prevents such an occurrence. Within these storms, activity can be observed, with thunderstorms present in both cyclones and anticyclones, a phenomenon that was not seen by the Hubble telescope in the 1990s but has emerged in the last decade. Hubble’s ability to detect differences in cloud heights and depths is evident from the strong color variations observed in the image.
This view of Jupiter’s vibrant cloud tops is interrupted by the photobombing orange moon, Io, which casts a shadow over the planet’s western edge. Thanks to the exceptional sharpness of the Hubble telescope, Io’s surface features can be clearly observed, including its mottled-orange appearance resulting from its numerous active volcanoes.
These volcanoes were first discovered during the Voyager 1 spacecraft’s flyby in 1979, and are a result of Io’s molten interior overlaid with a thin crust through which materials are ejected. The presence of sulfur in different hues at varying temperatures contributes to the moon’s colorful appearance.
On January 6, 2023, an image was captured of Jupiter’s famous Great Red Spot, which takes center stage in the frame. Despite being large enough to engulf the Earth, the vortex has actually shrunk to its smallest size in recorded history dating back 150 years.
In the lower right corner of the image, Jupiter’s largest moon, Ganymede, can be seen transiting the planet. With a surface area slightly larger than that of Mercury, Ganymede is the largest moon in the solar system.
The moon’s primarily water-ice surface is heavily cratered, and apparent glacial flows are driven by its internal heat. It’s worth noting that the photo is smaller in size because Jupiter was situated 81,000 miles farther from Earth when it was taken.
Uranus, a peculiar planet in our solar system, follows an unusual 84-year orbit around the sun while rolling on its side instead of spinning in a more upright position like Earth. This is due to its abnormally titled “horizontal” rotation axis, angled just eight degrees off the plane of the planet’s orbit.
One prevailing theory suggests that Uranus was destabilized by a massive moon and subsequently collided with it. Other potential explanations include significant impacts during planetary formation or the impact of giant planets exerting resonant torques on each other over time.
The consequences of Uranus’ tilt are intriguing, with stretches of time lasting up to 42 years where certain hemispheres experience total darkness. During the 1980s when the Voyager 2 spacecraft visited, Uranus’ south pole was nearly directly facing the sun.
However, Hubble’s latest observations indicate that the northern pole is now tilting towards the sun.
In 2014, the Hubble telescope captured an image of Uranus during the OPAL program, seven years after the northern spring equinox. The photo reveals multiple storms with methane ice-crystal clouds in mid-northern latitudes, above the planet’s cyan-tinted lower atmosphere.
The rings, which were previously photographed edge-on in 2007, began to open up in this image. During this time, the planet also had several small storms and faint cloud bands.
Fast-forward to 2022, and a new image shows Uranus’s north pole covered in a thick photochemical haze, similar to smog over cities. There are also several small storms near the edge of the polar haze boundary. Hubble has been tracking the size and brightness of the north polar cap, which continues to grow brighter each year.
Astronomers are working to understand the various factors that control how the atmospheric polar cap changes with the seasons, including atmospheric circulation, particle properties, and chemical processes.
As the northern summer solstice approaches in 2028, the cap may grow even brighter, and the ring system will be face-on when aimed directly toward Earth, allowing for better views of the rings and north pole. The image was captured on November 10, 2022.