Cassini images bizarre hexagon on Saturn
Thu Mar 29, 2007 at 20:35 UTC
An odd, six-sided, honeycomb-shaped feature circling the entire north pole of Saturn has captured the interest of scientists with NASA's Cassini mission.
NASA's Voyager 1 and 2 spacecraft imaged the feature over two decades ago. The fact that it has appeared in Cassini images indicates that it is a long-lived feature. A second hexagon, significantly darker than the brighter historical feature, is also visible in the Cassini pictures. The spacecraft's visual and infrared mapping spectrometer is the first instrument to capture the entire hexagon feature in one image.
"This is a very strange feature, lying in a precise geometric fashion with six nearly equally straight sides," said Kevin Baines, atmospheric expert and member of Cassini's visual and infrared mapping spectrometer team at NASA's Jet Propulsion Laboratory, Pasadena, Calif. "We've never seen anything like this on any other planet. Indeed, Saturn's thick atmosphere where circularly-shaped waves and convective cells dominate is perhaps the last place you'd expect to see such a six-sided geometric figure, yet there it is."
The hexagon is similar to Earth's polar vortex, which has winds blowing in a circular pattern around the polar region. On Saturn, the vortex has a hexagonal rather than circular shape. The hexagon is nearly 25,000 kilometers (15,000 miles) across. Nearly four Earths could fit inside it.
The new images taken in thermal-infrared light show the hexagon extends much deeper down into the atmosphere than previously expected, some 100 kilometers (60 miles) below the cloud tops. A system of clouds lies within the hexagon. The clouds appear to be whipping around the hexagon like cars on a racetrack.
"It's amazing to see such striking differences on opposite ends of Saturn's poles," said Bob Brown, team leader of the Cassini visual and infrared mapping spectrometer, University of Arizona, Tucson. "At the south pole we have what appears to be a hurricane with a giant eye, and at the north pole of Saturn we have this geometric feature, which is completely different."
Image Credit: NASA/JPL/University of Arizona
This nighttime view of Saturn's north pole by the visual and infrared mapping spectrometer on NASA's Cassini orbiter reveals a dynamic, active planet at least 75 kilometers (47 miles) below the normal cloud tops seen in visible light. Clearly revealed is the bizarre six-sided hexagon feature present at the north pole.
This image is one of the first clear images of the north polar region ever acquired from a unique polar perspective. In this image, the blue color shows high-altitude emissions from atmospheric molecules excited by charged particles smashing into the atmosphere along Saturn's powerful magnetic field lines, producing the aurora at very high altitudes in Saturn's atmosphere. The red color indicates the amount of 5-micron wavelength radiation, or heat, generated in the depths of the warm interior of Saturn that escapes the planet. Clouds blocking this light are revealed as silhouettes against the background thermal glow of the planet.
This image is among the first to capture the entire hexagonal feature and north polar region in one shot. It is also one of the first polar views using Saturn's thermal glow at 5 microns (seven times the wavelength visible to the human eye) as the light source. This allows polar cloud features to be revealed during the persistent nighttime conditions under way during north polar winter.
The hexagonal feature was originally discovered by NASA's Voyager spacecraft in 1980, but those images and subsequent ground-based telescope images suffered from poor viewing perspectives, which placed the feature and the north pole at the extreme northern limb (edge) in those images.
The strong brightness of the hexagon feature indicates that it is primarily a clearing in the clouds, which extends deep into the atmosphere, at least down to the 3-bar (3-Earth atmospheres pressure) level, about 75 kilometers (47 miles) below the clouds and hazes seen in visible wavelengths. Thick clouds border both sides of the narrow feature, as indicated by the adjacent dark lanes paralleling the bright hexagon. This image and other images acquired over a 12-day period between Oct. 30 and Nov. 11, 2006, show that the feature is nearly stationary, and likely is an unusually strong pole-encircling planetary wave that extends deep into the atmosphere.
This image was acquired by the Cassini visual and infrared mapping spectrometer on Oct. 29, 2006, from an average distance of 905,000 kilometers (562,340 miles) above the clouds.
Image Credit: NASA/JPL/University of Arizona
High resolution image
This image was acquired with the Cassini visual and infrared mapping spectrometer on Oct. 30, 2006, from an average distance of 1.3 million kilometers (807,782 miles).
Image Credit: NASA/JPL/University of Arizona
High resolution image (6.0 MB)
To see the deep atmosphere at night, the infrared instrument images the thermal glow radiating from Saturn's depths. Clouds at depths about 75 kilometers (47 miles) lower than the clouds seen at visible wavelengths block this light, appearing dark in silhouette. To show clouds as features that are bright or white rather than dark, the original image has been contrast reversed to produce the image shown here. The nested set of alternating white and dark hexagons indicates that the hexagonal complex extends deep into the atmosphere, at least down to the 3-Earth-atmosphere pressure level, some 75 kilometers (47 miles) underneath the clouds seen by Voyager. Multiple images acquired over a 12-day period between Oct. 30 and Nov. 11, 2006, show that the feature is nearly stationary, and likely is an unusually strong pole-encircling planetary wave that extends deep into the atmosphere.
This image was acquired on Oct. 29, 2006, from an average distance of 902,000 kilometers (560,400 miles) above the cloud tops of Saturn.
The Saturn north pole hexagon has not been visible to Cassini's visual cameras, because it's winter in that area, so the hexagon is under the cover of the long polar night, which lasts about 15 years. The infrared mapping spectrometer can image Saturn in both daytime and nighttime conditions and see deep inside. It imaged the feature with thermal wavelengths near 5 microns (seven times the wavelength visible to the human eye) during a 12-day period beginning on Oct. 30, 2006. As winter wanes over the next two years, the feature may become visible to the visual cameras.
Based on the new images and more information on the depth of the feature, scientists think it is not linked to Saturn's radio emissions or to auroral activity, as once contemplated, even though Saturn's northern aurora lies nearly overhead.
The hexagon appears to have remained fixed with Saturn's rotation rate and axis since first glimpsed by Voyager 26 years ago. The actual rotation rate of Saturn is still uncertain.
"Once we understand its dynamical nature, this long-lived, deep-seated polar hexagon may give us a clue to the true rotation rate of the deep atmosphere and perhaps the interior," added Baines.
Jet Propulsion Laboratory News Release
