Since its launch in 1990, NASA’s Hubble Space Telescope has been an interplanetary weather observer that keeps an eye on the mostly gaseous outer planets and their ever-changing atmospheres. NASA spacecraft missions to the outer planets have given us a detailed view of these atmospheres, but the sharpness and sensitivity of Hubble makes it possible to watch the kaleidoscope of complex activity over time without blinking an eye.
In this way, HST complements observations from other spacecraft such as Juno, which is currently orbiting Jupiter; the retired Cassini mission to Saturn and the Voyager 1 and 2 probes, which together flew by all four giant planets between 1979 and 1989.
[left] — The forecast for Jupiter is stormy weather in low northern latitudes. A distinct chain of alternating storms can be seen, forming a “vortex street,” as some planetary astronomers call it. It is a wave pattern of nested anticyclones and cyclones that are linked to each other as in a machine with alternating gears moving clockwise and counterclockwise.
If the storms come close enough to each other, in the highly unlikely event of mergers, they could create an even larger storm, potentially rivaling the current size of the Great Red Spot. The alternating anticyclone and cyclone pattern prevents individual storms from merging.
Activity is also seen inside these storms; in the 1990s, HST saw no cyclones or anticyclones with embedded storms, but these storms have appeared in the last decade. The strong color differences indicate that HST is also seeing different cloud heights and depths.
Orange moon Io photobombs this view of Jupiter’s multicolored cloud tops, casting a shadow toward the planet’s western side. Hubble’s resolution is so sharp that it can see Io’s mottled orange appearance associated with its many active volcanoes.
These volcanoes were first discovered when the Voyager 1 spacecraft flew by in 1979. The Moon’s molten interior is covered by a thin crust that the volcanoes eject material from. Sulfur takes on different hues at different temperatures, which is why Io’s surface is so colorful. This picture was taken on November 12, 2022.
[right] — Jupiter’s legendary Great Red Spot is in focus in this view. Although this vortex is large enough to engulf the Earth, it has actually shrunk to the smallest size it has ever been over 150 years of observational records. Jupiter’s icy moon Ganymede can be seen passing the giant planet in the lower right. Slightly larger than the planet Mercury, Ganymede is the largest moon in the Solar System.
It is a cratered world with a mostly water ice surface with apparent glacial flows driven by internal heat. (This image is smaller because Jupiter was 81,000 miles away from Earth when the photo was taken). This picture was taken on January 6, 2023.
Uranium
The planetary oddball Uranus rotates on its side around the Sun as it follows an 84-year orbit, rather than rotating in a more vertical position like Earth. Uranus has a curiously bent “horizontal” spin axis tilted just eight degrees from the plane of the planet’s orbit. One recent theory proposes that Uranus once had a massive moon that gravitationally destabilized it and then crashed into it.
Other possibilities include giant impacts during planet formation, or even giant planets exerting resonant torques on each other over time. As a result of the planet’s tilt, parts of one hemisphere are completely without sunlight for periods of up to 42 years. When Voyager 2 visited in the 1980s, the planet’s south pole was pointing almost directly at the Sun. The latest view from Hubble shows that the North Pole is now tilting towards the Sun.
[left] — This is Hubble’s view of Uranus taken in 2014, seven years after the northern vernal equinox, when the Sun was shining directly above the planet’s equator, and shows one of the first images from the OPAL program. In the mid-northern latitudes, numerous storms with clouds of methane ice crystals appear above the planet’s azure-colored lower atmosphere. Hubble took a side-on image of the ring system in 2007, but the rings are starting to open in this view seven years later. At this time, the planet had several small storms and even a few faint cloud bands.
[right] — As seen in 2022, Uranus’ north pole shows an intensified photochemical haze that looks similar to smog over cities. A few small thunderstorms can be seen near the edge of the polar haze. Hubble tracks the size and brightness of the northern polar cap, and it gets brighter every year. Astronomers tease apart the various effects from atmospheric circulation, particle properties, and chemical processes that control how the atmospheric polar cap changes with the seasons.
At the Uranian equinox in 2007, neither pole was particularly bright. As the northern summer solstice approaches in 2028, the cap may become even brighter and will be pointed directly at Earth, allowing good views of the rings and the North Pole; the ring system then appears face down. This picture was taken on November 10, 2022.
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