Last May amateur astronomers alerted the world to the fact that the gas giant planet Jupiter had lost a belt.
Normally the stormy world is encircled by two dark, rusty bands of clouds created by fast-moving jet streams. The features are easy to spot with a backyard telescope (and even easier with pro ‘scopes, such as Hubble or Cassini).
Jupiter in visible light, as seen by the Cassini space probe.
—Picture courtesy NASA
But seemingly out of the blue, one of the bands—called the south equatorial belt (SEB)—up and vanished in the spring, leaving only a zone of whiteness in its place.
“This is the most obvious change on Jupiter that I can recall,” Alan MacRobert, senior editor of Sky & Telescope magazine, told National Geographic News at the time.
Now, astronomers had seen this kind of thing before: Jupiter had lost the SEB in the 1970s and the 1990s, and both times it reappeared within one to three years.
Sure enough, by November the SEB was showing signs of a comeback, and scientists who had been watching the region were starting to paint a more detailed picture of what drives Jupiter’s colorful cloud bands.
Normally downwelling winds keep the SEB free of higher-level clouds, allowing the lower, darker clouds to dominate in visible light.
But sometimes the downwelling winds die down, and high clouds of bright white ammonia ice cover the belt. A few months to years later, the ammonia clouds dissipate and the belt fades back into view.
It’s still unclear what makes the winds subside. But using a combination of visible and infrared images of Jupiter, astronomers were able for the first time to see the atmospheric explosions that trigger the belt’s comeback.
“At infrared wavelengths, images in reflected sunlight show that the spot is a tremendously energetic ‘outburst,’ a vigorous storm that reaches extreme high altitudes,” UC Berkeley astronomer Imke de Pater said in a November press release.
“The storms are surrounded by darker areas, bluish-grey in the visible, indicative of ‘clearings’ in the cloud deck.”
The newest pictures of Jupiter from the W.M. Keck Observatory in Hawaii show how infrared readings can reveal what’s happening inside the re-emerging belt in another way.
First off, it’s key to know that what we see when we look at an object changes based on the wavelength of light. Here’s a nifty NASA illustration of wavelengths:
Each type of light—visible, infrared, microwave, etc.—spans a set of wavelengths. An exact light band is based on measuring a given wave from peak to peak or trough to trough.
Combining infrared images with wavelengths of 1.65, 1.58, and 1.21 microns gives us a false-color picture of Jupiter that looks like this:
—Picture courtesy UC Berkeley, University of Toronto, University of San Carlos, Philippines
But the infrared spectrum spans from 0.75 microns up to 1,000 microns!
The shorter wavelengths—called near-infrared and short-wave infrared—show light that’s being reflected by an object. In Jupiter’s case, it’s reflected sunlight.
Mid-range to long wavelengths are also called thermal infrared, because they show light from heat being radiated by a given object.
Looking at Jupiter in five-micron thermal infrared, astronomers can peer below the high covering of cold, white clouds and see the structure of the cloud layers silhouetted against Jupiter’s hot insides.
—Picture courtesy Mike Wong, Franck Marchis & W.M. Keck Observatory
“The thermal IR senses breaks in the [white] cloud cover,” UC Berkeley astronomer Mike Wong said in a statement released this week.
“The data show that the change from zone-like to belt-like appearance is a complex process that takes place at different speeds in each layer of Jupiter’s atmosphere.”
As a bit of background, getting this shot—one of the highest resolution thermal images of Jupiter so far—required some careful timing by the Keck team.
After all, the Keck telescope is in Hawaii, and even though it’s high on the Mauna Kea volcano, there’s enough of Earth’s shifting atmosphere between the ‘scope and Jupiter to distort the images.
Keck scientists normally get around this issue with a technique called adaptive optics. Shine a laser near the target object, and a computer attached to the telescope can use the bright dot as a guide to cancel out atmospheric distortions, adjusting its mirrors up to 2,000 times a second.
In this case, however, Jupiter is so bright that the computer has trouble finding its guide.
To get the shot, astronomers needed an object near Jupiter that would be even brighter than the gassy world.
Enter Europa, one of Jupiter’s four largest moons. It’s bright, icy surface was just the ticket for the Keck team, and the moon scooted into the perfect alignment on November 30.
Thanks, Europa, we owe you one!
—Picture courtesy NASA/JPL/DLR