By Space Telescope Science Institute December 14, 2024
Collected at: https://scitechdaily.com/a-tale-of-four-worlds-hubbles-ten-year-hunt-for-secrets-in-the-outer-solar-system/
From 2014 to 2024, NASA and ESA’s Hubble Space Telescope has been observing the outer planets through a program called OPAL (Outer Planet Atmospheres Legacy). This initiative focuses on tracking Jupiter, Saturn, Uranus, and Neptune to study their atmospheric dynamics and long-term evolution. Hubble’s unique ability to capture high-resolution, stable images allows scientists to monitor cloud patterns, storm activity, and atmospheric motion consistently. These observations provide crucial data to better understand the complex weather and climate systems of these distant worlds.
Hubble Space Telescope Keeps a Vigilant Eye on Weather on Other Worlds
Wandering across the heavens, the bright planets Jupiter and Saturn were named by the Romans after their most powerful gods. Centuries later, in the 1700s and 1800s, two more planets were uncovered, telescopically, far beyond Saturn. They were named after the Greek god Uranus and the Roman god Neptune.
Early skywatchers could never have imagined that robotic spacecraft would one day travel billions of miles to explore these distant worlds. In the 1980s, NASA’s Voyager missions provided breathtaking close-up images, captivating a generation. But these spacecraft could only gather brief snapshots of planetary data, like a tour bus barreling across a continent.
That’s where the Hubble Space Telescope came in to pick up where this interplanetary “Lewis and Clark” expedition left off. Astronomers were realizing that the outer planets were far more complex than imagined. There was a lot more left to be learned about their turbulent, colorful, and frigid atmospheres. Building on Voyager’s legacy, the Hubble program called OPAL (Outer Planet Atmospheres Legacy) has now obtained a full decade’s worth of observations of Jupiter, Saturn, Uranus, and Neptune. Hubble is essentially being used as an interplanetary weatherman to study meteorology on other worlds. And, this gives fresh insights into the behavior of complex weather on Earth, and other planets across our galaxy.
From upper-left toward center, the hazy white polar cap on the three teal-colored Uranus images appears more face-on as the planet approaches northern summer.
From center-right to far-center right, three images of the blue planet Neptune show the coming and going of clouds as the Sun’s radiation level changes. Several of Neptune’s mysterious dark spots have come and gone sequentially over OPAL’s decade of observations.
Seven views of yellow-brown Saturn stretch across the center of the mosaic in a triangle—one for each year of OPAL observations—showing the tilt of the ring plane relative to the view from Earth. Approximately every 15 years the relatively paper-thin rings (about one mile thick) can be seen edge-on. In 2018 they were near their maximum tilt toward Earth. Colorful changes in Saturn’s bands of clouds can be followed as the weather changes.
At bottom center, three images of Jupiter spanning nearly a decade, form a triangle. There are notable changes in Jupiter’s banded cloud structure of zones and belts. OPAL measured shrinking of the legendary Great Red Spot, while its rotation period shortens. Credit: NASA, ESA, Amy Simon (NASA-GSFC), Michael H. Wong (UC Berkeley), Joseph DePasquale (STScI)
Hubble Space Telescope Celebrates Decade of Tracking Outer Planets
In 1989, NASA’s Voyager mission completed humanity’s first close-up exploration of the solar system’s four giant outer planets when it encountered Neptune. Since their launch in 1977, Voyager 1 and Voyager 2 revealed that Jupiter, Saturn, Uranus, and Neptune were far more complex than scientists had anticipated. Their discoveries made it clear that there was much more to learn.
To continue this exploration, NASA’s Hubble Space Telescope launched the OPAL (Outer Planet Atmospheres Legacy) program. OPAL conducts long-term observations of the outer planets to study their atmospheric dynamics and track changes over time, providing deeper insights into how these distant worlds evolve.
“The Voyagers don’t tell you the full story,” explained Amy Simon of NASA’s Goddard Space Flight Center in Greenbelt, Maryland, who conducted giant planet observations with OPAL.
Hubble’s image sharpness is comparable to the Voyager views as they approached the outer planets, and Hubble spans wavelengths from ultraviolet to near-infrared light. Hubble is the only telescope that can provide high spatial resolution and image stability for global studies of cloud coloration, activity, and atmospheric motion on a consistent time basis to help constrain the underlying mechanics of weather and climate systems.
All four of the outer planets have deep atmospheres and no solid surfaces. Their churning atmospheres have their own unique weather systems, some with colorful bands of multicolored clouds, and with mysterious, large storms that pop up or linger for many years. Each also has seasons lasting many years as they revolve around the Sun. (The James Webb Space Telescope’s infrared capabilities will be used to probe deep into the atmospheres of the outer planets to complement the OPAL observations.)
Following the complex behavior is akin to understanding Earth’s dynamic weather as followed over many years, as well as the Sun’s influence on the solar system’s weather. The four distant worlds also serve as proxies for understanding the weather and climate on similar planets orbiting other stars.
Planetary scientists realized that any one year of data from Hubble, while interesting in its own right, doesn’t tell you the full story on the outer planets. Hubble’s OPAL program has routinely visited the planets once a year when they are closest to the Earth, an alignment called opposition. This has yielded a huge archive of data that has led to a string of remarkable discoveries to share with planetary astronomers around the world.
Highlights of the OPAL team’s decade of discovery is provided below.
Jupiter
Jupiter’s bands of clouds present an ever-changing kaleidoscope of shapes and colors. There is always stormy weather on Jupiter: cyclones, anticyclones, wind shear, and the largest storm in the solar system, the Great Red Spot (GRS). Jupiter is covered with largely ammonia ice-crystal clouds on top of an atmosphere that’s tens of thousands of miles deep.
Hubble’s sharp images track clouds and measure the winds, storms, and vortices, in addition to monitoring the size, shape, and behavior of the GRS. Hubble follows as the GRS continues shrinking in size, but is still large enough to swallow Earth. OPAL data recently measured how often mysterious dark ovals—visible only at ultraviolet wavelengths—appeared in the “polar hoods” of stratospheric haze. Unlike Earth, Jupiter is only inclined three degrees on its axis (Earth is 23.5 degrees). Seasonal changes might not be expected, except that Jupiter’s distance from the Sun varies by roughly 64 million kilometers over its 12-year-long orbit, and so OPAL closely monitors the atmosphere for seasonal effects. Another Hubble advantage is that ground-based observatories can’t continuously view Jupiter for two Jupiter rotations, because that adds up to 20 hours. During that time, an observatory on the ground would have gone into daytime and Jupiter would no longer be visible until the next evening.
OPAL’s findings may also support ESA’s Jupiter Icy Moons Explorer, Juice, which was launched on April 14, 2023. Juice will make detailed observations of Jupiter and its three large ocean-bearing moons — Ganymede, Callisto, and Europa — with a suite of remote sensing, geophysical, and in situ instruments. The mission will characterize these moons as both planetary objects and possible habitats, explore Jupiter’s complex environment in-depth, and study the wider Jupiter system as an archetype for gas giants across the Universe.
Saturn
Saturn takes more than 29 years to orbit the Sun, and so OPAL has followed it for approximately one-quarter of a Saturnian year (picking up in 2018, after the end of the Cassini mission). Because Saturn is tilted 26.7 degrees, it goes through more profound seasonal changes than Jupiter. Saturnian seasons last approximately seven years. This also means Hubble can view the spectacular ring system from an oblique angle of almost 30 degrees to see the rings tilted edge-on. Edge-on, the rings nearly vanish because they are relatively paper-thin. This will happen again in 2025.
OPAL has followed changes in the colors of Saturn’s atmosphere. The varying color was first detected by the Cassini orbiter, but Hubble provides a longer baseline. Hubble revealed slight changes from year-to-year in color, possibly caused by cloud height and winds. The observed changes are subtle because OPAL has covered only a fraction of a Saturnian year. Major changes happen when Saturn progresses into the next season.
Saturn’s mysteriously dark ring spokes, which slice across the ring plane, are transient features that rotate along with the rings. Their ghostly appearance only persists for two or three rotations around Saturn. During active periods, freshly formed spokes continuously add to the pattern. They were first seen in 1981 by Voyager 2. Cassini also saw the spokes during its 13-year-long mission, which ended in 2017. Hubble shows that the frequency of spoke apparitions is seasonally driven, first appearing in OPAL data in 2021. Long-term monitoring shows that both the number and contrast of the spokes vary with Saturn’s seasons.
Uranus
Uranus is tilted on its side so that its spin axis almost lies in the plane of the planet’s orbit. This results in the planet going through radical seasonal changes along its 84-year-long trek around the Sun. The consequence of the planet’s tilt means part of one hemisphere is completely without sunlight, for stretches of time lasting up to 42 years. OPAL has followed the northern pole now tipping toward the Sun.
With OPAL, Hubble first imaged Uranus after the spring equinox, when the Sun was last shining directly over the planet’s equator. Hubble resolved multiple storms with methane ice-crystal clouds appearing at mid-northern latitudes as summer approaches the north pole. Uranus’ north pole now has a thickened photochemical haze with several little storms near the edge of the boundary. Hubble has been tracking the size of the north polar cap and it continues to get brighter year after year. As the 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.
Neptune
When Voyager 2 flew by Neptune in 1989, astronomers were mystified by a great dark spot the size of the Atlantic Ocean looming in the atmosphere. Was it long-lived like Jupiter’s Great Red Spot? The question remained unanswered until Hubble was able to show in 1994 that such dark storms were transitory, cropping up and then disappearing over a duration of two to six years each. During the OPAL program, Hubble saw the end of one dark spot and the full life cycle of a second one – both of them migrating toward the equator before dissipating. The OPAL program ensures that astronomers won’t miss another one.
Hubble observations uncovered a link between Neptune’s shifting cloud abundance and the 11-year solar cycle. The connection between Neptune and solar activity is surprising to planetary scientists because Neptune is our solar system’s farthest major planet. It receives sunlight with about 0.1% of the intensity Earth receives. Yet Neptune’s global cloudy weather seems to be influenced by solar activity. Do the planet’s four seasons (each lasting approximately 40 years) also play a role? We may find out, if the OPAL program continues running on Hubble until the year 2179!
The Hubble Space Telescope has been operating for over three decades and continues to make ground-breaking discoveries that shape our fundamental understanding of the universe. Hubble 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 and mission operations. Lockheed Martin Space, based in Denver, also supports mission operations at Goddard. The Space Telescope Science Institute in Baltimore, which is operated by the Association of Universities for Research in Astronomy, conducts Hubble science operations for NASA.
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