Hubble’s Grand Tour of the Outer Solar System

From its vantage point high above Earth’s atmosphere, NASA’s Hubble Space Telescope has completed this year's grand tour of the outer solar system – returning crisp images that complement current and past observations from interplanetary spacecraft. This is the realm of the giant planets – Jupiter, Saturn, Uranus, and Neptune – extending as far as 30 times the distance between Earth and the Sun.

Unlike the rocky terrestrial planets like Earth and Mars that huddle close to the Sun’s warmth, these far-flung worlds are mostly composed of chilly gaseous soups of hydrogen, helium, ammonia, methane, and deep water around a packed, intensely hot, compact core.

Though robotic spacecraft have sent back snapshots of their visits to these four enormous planets over the past 50 years, their swirling, colorful atmospheres are constantly changing. While robotic spacecraft that fly close to the planets can take sharper images, Hubble frequently revisits these distant worlds to reveal new surprises, offering fresh insights into their wild weather, driven by still largely unknown dynamic forces working under the cloud tops.

Hubble’s snapshots of the outer planets reveal both extreme and subtle changes rapidly taking place in these distant worlds. Hubble’s sharp view gleans insights into the fascinating, dynamic weather patterns and seasons on these gas giants and allows astronomers to investigate the very similar – and very different – causes of their changing atmospheres.

These Hubble images are part of yearly maps of each planet taken as part of the Outer Planets Atmospheres Legacy program, or OPAL. The program provides annual, global views of the outer planets to look for changes in their storms, winds, and clouds. Hubble’s longevity, and unique vantage point, has given astronomers a unique chance to check in on the outer planets on a yearly basis. Knowledge from the OPAL program can also be extended far beyond our own solar system in the study of atmospheres of planets that orbit stars other than our Sun.

This year’s Hubble images of Jupiter track the ever-changing landscape of its turbulent atmosphere, where several new storms are making their mark, and the pace of color changes near the planet’s equator is continuing to surprise researchers.

Hubble’s September 4^th photo puts the giant planet’s tumultuous atmosphere on full display.

The planet’s equatorial zone has remained a deep orange hue for a much longer time, compared to previous darkening episodes. While the equator has changed from its traditional white or beige appearance for a few years now, scientists were surprised to find the deeper orange color to persist in Hubble’s recent imaging, instead expecting the zone to lose its reddish haze layer.

Just above the equator, researchers note the appearance of several new storms, nicknamed “barges” during the Voyager era. These elongated red cells can be defined as cyclonic vortexes, which vary in appearance. While some of the storms are sharply defined and clear, others are fuzzy and hazy. This difference in appearance is caused by the properties within the clouds of the vortexes.

“Every time we get new data down, the image quality and detail in the cloud features always blow me away,” said Amy Simon of the Goddard Space Flight Center in Greenbelt, Maryland. “It strikes me when I look at Jupiter, in the barges or in the red band right below, you can see cloud structures that are clearly much deeper. We’re seeing a lot of structure here and vertical depth variation.”

Researchers also note that a feature dubbed “Red Spot Jr.” (Oval BA), below the Great Red Spot, where Hubble just discovered winds are speeding up, is still a darker beige color, and is joined by a string of white, anticyclonic storms to the south.

Hubble’s new look at Saturn on September 12^th shows rapid and extreme color changes of the bands in the planet’s northern hemisphere, where it is now early autumn. The bands have varied throughout Hubble observations in both 2019 and 2020. Notably, Saturn’s iconic hexagonal storm, first discovered in 1981 by the Voyager 2 spacecraft, was difficult to distinguish in 2020, but it is again clearly evident in 2021. Hubble’s Saturn image catches the planet following the southern hemisphere’s winter, evident in the lingering blue-ish hue of the south pole. In the past, Hubble has allowed researchers to closely track the northern hemisphere's seasonal changes.

“This is something we can best do with Hubble. With Hubble’s high resolution, we can narrow things down to which band is actually changing,” said Michael Wong of the University of California, Berkeley. “If you were to look at this through a ground-based telescope, there’s some blurring with our atmosphere, and you’ll lose some of those color variations. Nothing from the ground will get visible-light images as sharp as Hubble’s.”

Hubble’s October 25^th view of Uranus puts the planet’s bright northern polar hood in the spotlight. It’s springtime in the northern hemisphere and the increase in ultraviolet radiation absorbed from the Sun seems to be causing the polar region to brighten. Researchers are studying how the brightening polar hood results from changes in the concentration of atmospheric methane gas and the characteristics of haze particles, as well as the atmospheric flow patterns. Curiously, even as the atmospheric hood gets brighter, the sharp southernmost boundary remains fixed at the same latitude. This has been constant over the past several years of OPAL observations, perhaps because a jet stream is setting up a barrier at that latitude of 43 degrees.

In observations taken on September 7^th, researchers found that Neptune’s dark spot, which recently was found to have reversed course from moving toward the equator, is still visible in this image, along with a darkened northern hemisphere. There is also a notable dark, elongated circle encompassing Neptune’s south pole. Neptune's and Uranus’ blue color is a result of the absorption of red light by the planets’ methane-rich atmosphere, combined with the same Rayleigh-scattering process that makes the Earth's sky blue. In 2021, there are few bright clouds on Neptune, and its distinct blue with a singular large dark spot is very reminiscent of what Voyager 2 saw in 1989.

The Hubble Space Telescope is a project of international cooperation between NASA and ESA (European Space Agency). NASA’s Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore, Maryland, conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy in Washington, D.C.

Media Contacts:

Claire Andreoli
NASA's Goddard Space Flight Center
301-286-1940

Hannah Braun
Space Telescope Science Institute, Baltimore, Maryland

Ray Villard
Space Telescope Science Institute, Baltimore, Maryland

Science Contacts:

Amy Simon
NASA Goddard Space Flight Center, Greenbelt, Maryland

Michael H. Wong
University of California, Berkeley, Berkeley, California