Cassini Spies Wave Rattling Jet Stream on Jupiter

PASADENA,  Calif.–(ENEWSPF)– New movies of Jupiter are the first to catch an invisible wave  shaking up one of the giant planet’s jet streams, an interaction that also  takes place in Earth’s atmosphere and influences the weather. The movies, made  from images taken by NASA’s Cassini spacecraft when it flew by Jupiter in 2000,  are part of an in-depth study conducted by a team of scientists and amateur  astronomers led by Amy Simon-Miller at NASA’s Goddard Space Flight Center in  Greenbelt, Md., and published in the April 2012 issue of Icarus.

“This  is the first time anyone has actually seen direct wave motion in one of  Jupiter’s jet streams,” says Simon-Miller, the paper’s lead author.  “And by comparing this type of interaction in Earth’s atmosphere to what  happens on a planet as radically different as Jupiter, we can learn a lot about  both planets.”

Like  Earth, Jupiter has several fast-moving jet streams that circle the globe.  Earth’s strongest and best known jet streams are those near the North and South  Poles; as these winds blow west to east, they take the scenic route, wandering  north and south. What sets these jet streams on their meandering paths-and  sometimes makes them blast Florida and other warm places with frigid air-are  their encounters with slow-moving waves in Earth’s atmosphere, called Rossby  waves.

In  contrast, Jupiter’s jet streams “have always appeared to be straight and  narrow,” says co-author John Rogers, who is the Jupiter Section Director  of the British Astronomical Association, London, U.K., and one of the amateur  astronomers involved in this study.

Rossby  waves were identified on Jupiter about 20 years ago, in the northern  hemisphere. Even so, the expected meandering  winds could not be traced directly, and no  evidence of them had been found in the southern hemisphere, which puzzled  planetary scientists.

To  get a more complete view, the team analyzed images taken by NASA’s Voyager  spacecraft, NASA’s Hubble Space Telescope, and Cassini, as well as a decade’s  worth of observations made by amateur astronomers and compiled by the JUPOS  project.

The  movies zoom in on a single jet stream in Jupiter’s southern hemisphere. A line  of small, dark, v-shaped “chevrons” has formed along one edge of the  jet stream and zips along west to east with the wind. Later, the well-ordered  line starts to ripple, with each chevron moving up and down (north and south)  in turn. And for the first time, it’s clear that Jupiter’s jet streams, like  Earth’s, wander off course.

“That’s  the signature of the Rossby wave,” says David Choi, the postdoctoral  fellow at NASA Goddard who strung together about a hundred Cassini images to  make each time-lapse movie. “The chevrons in the fast-moving jet stream  interact with the slower-moving Rossby wave, and that’s when we see the  chevrons oscillate.”

The  team’s analysis also reveals that the chevrons are tied to a different type of  wave in Jupiter’s atmosphere, called a gravity inertia wave. Earth also has  gravity inertia waves, and under proper conditions, these can be seen in  repeating cloud patterns.

“A  planet’s atmosphere is a lot like the string of an instrument,” says  co-author Michael D. Allison of the NASA Goddard Institute for Space Studies in  New York. “If you pluck the string, it can resonate at different  frequencies, which we hear as different notes. In the same way, an atmosphere  can resonate with different modes, which is why we find different kinds of  waves.”

Characterizing  these waves should offer important clues to the layering of the deep atmosphere  of Jupiter, which has so far been inaccessible to remote sensing, Allison adds.

Crucial  to the study was the complementary information that the team was able to  retrieve from the detailed spacecraft images and the more complete visual  record provided by amateur astronomers. For example, the high resolution of the  spacecraft images made it possible to establish the top speed of the jet  stream’s wind, and then the amateur astronomers involved in the study looked  through the ground-based images to find variations in the wind speed.
The  team also relied on images that amateur astronomers had been gathering of a  large, transient storm called the South Equatorial Disturbance. This visual  record dates back to 1999, when members of the community spotted the most  recent recurrence of the storm just south of Jupiter’s equator. Analysis of  these images revealed the dynamics of this storm and its impact on the  chevrons. The team now thinks this storm, together with the Great Red Spot,  accounts for many of the differences noted between the jet streams and Rossby  waves on the two sides of Jupiter’s equator.

“We  are just starting to investigate the long-term behavior of this alien  atmosphere,” says co-author Gianluigi Adamoli, an amateur astronomer in  Italy. “Understanding the emerging analogies between Earth and Jupiter, as  well as the obviously profound differences, helps us learn fundamentally what  an atmosphere is and how it can behave.”

The  Cassini-Huygens mission is a cooperative project of NASA, the European Space  Agency, and the Italian Space Agency. NASA’s Jet Propulsion Laboratory,  Pasadena, Calif., manages the mission for NASA’s Science Mission Directorate,  Washington, D.C. JPL is a division of the California Institute of Technology,  Pasadena.