Jupiter and Saturn: A Meeting of Giants

In Roman mythology, Jupiter (on the right) is the king of gods while Saturn (left) is the god of time. With their slow march across the sky (a cycle of roughly 12 years for Jupiter and 29 years for Saturn), and with Jupiter being one the brightest celestial objects visible to the naked eye, the two planets caused awe and wonder since ancient times and across cultures. In Western astrology, the conjunction of the two planets, or Great Conjunction, was regarded as an omen signaling both good and bad. Shown here are two pages from the first printed edition of an influential treatise on this topic titled The Great Conjunctions, originally written in the 9th century by the Persian astrologer Albumasar.  

This illustration from Christiaan Huygens’s "Systema Saturnium", published in 1659, presents different telescopic depictions of Saturn recorded by several observers over the preceding decades. Various hypotheses were put forward to explain Saturn’s intriguing “anses” or “handles”—the terms given to the features around the planet’s disc seen through the telescope. Huygens came up with an explanation that is essentially correct: Saturn is surrounded by a ring that produces—combined with the tilt of its axis and its orbital motion—varying appearances as seen from Earth.

The Great Red Spot is one of Jupiter’s most distinctive and iconic features. It corresponds to a high-pressure region in the planet’s atmosphere that produces an enduring anticyclonic storm. It shows prominently in this watercolor painting, which is based on observations of Jupiter carried out at Chicago’s first Dearborn Observatory in 1880. Though spots on Jupiter have been reported over the last 350 years or so, it was only in the late 1870s that systematic observations of the Great Red Spot began, showing that this feature has persisted at least since that time to this day.  Also noticeable in the image (which has the south at the top to show the planet as seen through a refracting telescope) are Jupiter’s typical pattern of alternating dark and light bands (respectively called “belts” and “zones”), plus the shadows of two of its satellites, one of which falls within the Great Red Spot. 

This scene showing a spacecraft approaching Jupiter was painted by Chesley Bonestell to illustrate Arthur C. Clarke’s book Beyond Jupiter: Worlds of Tomorrow, published in 1972. In this book Clarke addresses what was known about the outer planets (Jupiter, Saturn, Uranus, Neptune) at the time, and explores what might be found by space probes sent to those distant worlds. In the same year that Beyond Jupiter was published, the Pioneer 10 probe was launched. It became the first spacecraft to cross the asteroid belt and to obtain close-up images of Jupiter.

Saturn, the second largest planet in our solar system, is most well known for its spectacular rings. While it isn’t the only planet with rings - Jupiter, Uranus, and Neptune also have rings - Saturn’s are by far the easiest to see. While Saturn’s rings look like a single solid disc from a distance, they’re formed of thousands of separate rings and made of particles that range from the size of a grain of sand to the size of a mountain. They extend 175,000 miles from the planet, but are relatively thin by comparison: they’re between just 30 to 300 feet in height.

Having studied the surfaces of Jupiter's moons and the nature of Jupiter rings as a graduate student, Dr. Brooks came to JPL to work on the Cassini mission to Saturn.  His work on Cassini afforded him the opportunity to help plan, design and analyze hundreds observations of Saturn's rings and moons. “Looking forward, I am very excited to be a part of NASA's return to Jupiter's moon Europa.  The data that will be obtained will be the first close-up look at Europa that we have had since the Galileo mission to Jupiter.  It will answer questions raised while I was a graduate student and certainly pose many new mysteries.”  In addition, Dr. Brooks has been serving the planetary science community as a member of the Planetary Culture and Climate subcommittee of the Division for Planetary Sciences, a committee that advises the Division for Planetary Science committee and the broader community on issues related to equity, diversity and inclusion.

Dr. Martin is most interested in how the surfaces of these icy moons fracture and crack which can tell us a lot about the oceans on the inside. Places like Saturn’s moon Enceladus are extra exciting because some of that ocean is erupting out of Enceladus’s south pole draping the surface with a snow-like material and feeding Saturn’s E-ring. Dr. Martin also explores the surfaces of icy moons by creating geological maps. She is currently leading a project to create a map of the surface of Triton, Neptune’s largest moon. 

“On Earth, we’re used to volcanic eruptions, during which molten rock and ash erupt. The icy moons of Jupiter and Saturn have cryovolcanism, during which briny mixtures of water and ice erupt instead. We can think of cryovolcanism as an eruption of icy slush. Studying the process of cryovolcanism, how it works, where it happens, and how it differs from classical volcanism really excites me. It’s the coolest way I can think of (no pun intended!) to apply my physics and math background to learn about Jupiter and Saturn’s moons and icy worlds around other stars.” - Dr. Lynnae Quick

Jupiter and Saturn are the farthest planets from Earth than can be easily observed with just the naked eye. While the conjunction is an exciting time to observe them, they’re easy to find in the sky for many months a year. Because they move around the sun at different speeds than the Earth does, their visibility in our sky changes from year to year. In 2020, these planets are still visible in the early evening through the end of December. In 2021, they’ll be visible in the evening from August through to the end of January of 2022. In later 2022, expect to see them even later in the year, from September 2022 to January 2023 with Jupiter staying up a bit later until February 2023. 

The Great Conjunction of Jupiter and Saturn will be visible right after sunset on December 21st, 2021. If you’re in or near Chicago, take a look close to the horizon in your southwest sky. You’ll see two bright objects that look like stars, close enough to each other that you could cover them both with your thumb outstretched at arm’s length. If you are able to observe them with a telescope or binoculars, you can capture them both in the same telescope or binocular view at moderate magnification.