If we had to rely solely on spacecraft to learn about the outer planets, we wouldn’t be making great progress. It takes a massive effort to get a spacecraft to the outer Solar System. But thanks to the Hubble Space Telescope, we can keep tabs on the gas giants without leaving Earth’s orbit.

NASA’s Outer Planet Atmospheres Legacy (OPAL) program keeps an eye on the outer planets to monitor changes in their atmospheres. Changes on Jupiter, Saturn, Uranus, and Neptune take place on timescales of years and decades—or longer—so Hubble checks in yearly to see how the gas/ice giants are doing. It gives scientists a long baseline of data. Each year OPAL captures images of the outer planets, and OPAL is slated to continue until either Hubble itself is no longer operational, or until Hubble’s WFC3/UVIS camera is no longer functioning.

Jupiter

Even people who don’t follow astronomy closely know that Jupiter is a big ball of gas with a visually stunning and ever-changing atmosphere. (Don’t they?) Jupiter’s Great Red Spot (GRS) even has its own Wikipedia page. This year’s Hubble image of Jupiter puts the planet’s mesmerizing atmosphere on full display.

Jupiter as seen by Hubble on September 4th, 2021. Image Credit: NASA, ESA, A. Simon (Goddard Space Flight Center), and M.H. Wong (University of California, Berkeley) and the OPAL team.
Jupiter as seen by Hubble on September 4th, 2021. Image Credit: NASA, ESA, A. Simon (Goddard Space Flight Center), and M.H. Wong (University of California, Berkeley) and the OPAL team.

In this year’s image, the colour of the planet’s equatorial zone caught the OPAL team’s interest. It now has a deep orange hue which is unusual. The region is usually white or yellowish, and though it’s departed from that scheme recently, the deep orange is still a surprise.

Most of what’s visible in the upper atmosphere is ammonia. Dark bands in Jupiter’s atmosphere are called belts, while lighter bands are called zones. The zones are colder than the belts and are associated with upwellings. Belts are descending gas. The lighter colour of the zones is because of ammonia ice, while the cause of the darker belts is uncertain.

Two Hubble OPAL images of Jupiter. On the left is the image from 2020, and on the right is the image from 2021. Note the small white spot below the GRS in 2020 which is gone in 2021. Also note the deepening orange hue of the equatorial zone, something that surprised scientists. The moon in the 2020 image is Europa. Image Credit: NASA, ESA, A. Simon (Goddard Space Flight Center), and M. H. Wong (University of California, Berkeley) and the OPAL team.
Two Hubble OPAL images of Jupiter. On the left is the image from 2020, and on the right is the image from 2021. Note the small white spot below the GRS in 2020 which is gone in 2021. Also note the deepening orange hue of the equatorial zone, something that surprised scientists. The moon in the 2020 image is Europa. Image Credit: NASA, ESA, A. Simon (Goddard Space Flight Center), and M. H. Wong (University of California, Berkeley) and the OPAL team.

The 2021 image shows several elongated reddish storm cells that scientists call “barges.” They’re actually cyclonic vortices, which means they’re rotating counterclockwise. There are
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