Credit: SCIENCE: NASA, ESA, Amy Simon (NASA-GSFC), Michael H. Wong (UC Berkeley) IMAGE PROCESSING: Joseph DePasquale (STScI)

Ever since its launch in 1990, NASA’s Hubble Space Telescope has been an interplanetary weather observer for keeping an eye on the largely gaseous outer planets, which all have ever-changing atmospheres. NASA spacecraft missions to the outer planets have given us a close-up look at these atmospheres, but Hubble’s sharpness and sensitivity keeps an unblinking eye on an ever-changing kaleidoscope of complex activities long after those missions have ended. Inaugurated in 2014, the telescope’s Outer Planet Atmospheres Legacy (OPAL) Program has been providing us with yearly views of the giant planets. URANUS

Planetary oddball Uranus rolls on its side around the Sun as it follows an 84-year orbit, rather than spinning in a more-vertical position as Earth does. Uranus has a weirdly tipped “horizontal” rotation axis angled just eight degrees off the plane of the planet’s orbit. A recent idea is that Uranus once had a massive moon that gravitationally destabilized it and then crashed into it. Other possibilities include giant impacts during planetary formation, or even giant planets exerting resonant torques on each other over time. The consequences of the planet’s tilt are that for stretches of time lasting up to 42 years, parts of one hemisphere are completely without sunlight. When the Voyager spacecraft visited during the 1980s the planet’s south pole was pointed almost directly at the Sun. Hubble’s latest view shows the northern pole now tipping toward the Sun.

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This is a Hubble view of Uranus taken in 2014, seven years after northern spring equinox when the Sun was shining directly over the planet’s equator, and shows one of the first images from the OPAL program. Multiple storms with methane ice-crystal clouds appear at mid-northern latitudes above the planet’s cyan-tinted lower atmosphere. Hubble photographed the ring system edge-on in 2007, but the rings are seen at an oblique angle seven years later in this view. At this time, the planet had multiple small storms and even some faint cloud bands.

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As seen in 2022, Uranus’ north pole is now capped by low methane humidity and a thickening photochemical haze that looks similar to the smog over cities. Several little storms can be seen near the edge of the polar haze boundary. Hubble has been tracking the size and brightness of the north polar cap and it continues to get brighter year after year. Astronomers are disentangling multiple 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 northern summer solstice approaches in 2028 the cap may grow brighter still, and will be aimed directly toward Earth, allowing good views of the rings and north pole; the ring system will then appear face-on. This photo was taken on November 10, 2022.

Provider: Space Telescope Science Institute

Image Source: https://hubblesite.org/contents/news-releases/2023/news-2023-007

Curator: STScI, Baltimore, MD, USA

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