Jupiter at its best
Insights—and sightings—of the red-eyed planet
Throughout the month of May, astronomy buffs will have the best possible view of Jupiter, as the planet reaches “opposition”—the state in which Jupiter and the sun are opposite one another from our earthly perspective. This is an excellent time to think about how, 400 years ago, a glimpse of Jupiter’s surface changed our world, as well as about how recent research at the Weizmann Institute is transforming our understanding of the Solar System’s largest planet.
When Galileo peered into a telescope and saw moons passing above Jupiter’s surface in 1610, he eradicated the notion that all bodies in space must orbit Earth. Now, using gravitational measurements captured by a NASA satellite, Weizmann Institute planetary scientists have given us a more complete picture of the red-eyed gas giant, by revealing that Jupiter’s famous bands are cylindrical jet streams that extend from the outer surface of the planetary atmosphere to a depth of 3,000 kilometers. In doing so, the researchers confirmed their prediction that deep-penetrating Jovian winds would result in detectable changes in Jupiter’s gravitational field.
Long ago, telescopic observations and space missions alerted the astrophysics community to the existence of cloud-level winds on Jupiter, and to the fact that these winds—which blow fiercely in different directions—are what create the planet’s familiar colored bands. However, until now, no one has known how deep the bands penetrate, or what drives them.
Weizmann Institute planetary scientist Prof. Yohai Kaspi set his sights on demystifying Jupiter’s atmospheric dynamics long before NASA launched its Juno space probe from Cape Canaveral back in 2011. Working together with Dr. Eli Galanti, his colleague from the Department of Earth and Planetary Sciences, Prof. Kaspi predicted that these jet streams—which flow fiercely from east to west or west to east, and in asymmetrical distribution between Jupiter’s north and south poles—would disrupt the even distribution of mass on the planet, creating measureable gravitational changes. As the Juno probe approached Jupiter, the gravitational data sent back to Earth indicated that Prof. Kaspi’s prediction was correct. These findings were recently published in Nature.
By analyzing Jupiter’s gravitational changes, Prof. Kaspi and Dr. Galanti were able to calculate that the planet’s distinctive colored bands are actually the upper layer of powerful flows about 3,000 kilometers in depth—revealing that Jupiter’s atmosphere is many times larger than was previously known.
Prof. Kaspi and Dr. Galanti also developed a method of determining how wind flow changes with depth, revising science’s picture of Jupiter’s atmosphere all the way down. At depths below 3,000 kilometers, Jupiter rotates like a rigid body, without hints of the jet streams.
This revised model also made it possible for the scientists to quantify something never measured before: Jupiter’s atmosphere as a percentage of its total planetary mass. Calculations performed by Prof. Kaspi and Dr. Galanti revealed that Jupiter’s atmosphere is 1 percent of its total mass. This is a far larger than Earth—our home planet’s atmosphere contains less than a millionth of Earth’s total mass—and is also significantly greater than what is known from other planets in the Solar System.
In the future, the researchers plan further measurements to determine whether Jupiter has a solid core, and if so, to determine its mass. Using some of the same methods they developed to characterize jet streams, they also plan to examine Jupiter’s Great Red Spot—a giant wind storm that has remained stable for as long as telescopes have existed, but has been shrinking in recent years.
Prof. Kaspi is supported by the De Botton Center for Marine Science, the André Deloro Institute for Space and Optics Research, and the Sussman Family Center for the Study of Environmental Sciences.