Saturnian Wheels Turn, Without Spokes
Cassini’s approach to Saturn has begun. The Cassini image team has noted that new details in the atmosphere and rings are becoming visible, and scientists are already puzzling over the noticeable absence of the ghostly spoke-like dark markings in the rings first discovered during Voyager’s approach to the planet 23 years ago.
"I feel like a kid on a road trip at the beginning of our tour," said Dr. Dennis Matson, project scientist for the Cassini-Huygens mission to Saturn and its largest moon Titan. "We’ve been driving this car for nearly 3.5 billion kilometers (2.2 billion miles) and it’s time to get off and explore this ringed world and its many moons. I can hardly wait, but in the meantime, these weekly color images offer a glimpse of our final destination," Matson said.
|Scientists would like to know more about the origin of the ring ‘spokes’ imaged on Saturn by Voyager, but not so far by the current Cassini images. Image Credit: Cassini Imaging Team/Boulder/ ciclops.org|
As the size of the planet grows in the frame of their camera, the image team will begin conducting searches for new moons between the rings and the orbit of the chaotically rotating moon, Hyperion. In April, image scans of hazy Titan in a variety of wavelengths will allow searches for moving clouds high in its atmosphere and large-scale features on its surface.
A cold, dusky Saturn looms in the distance in this striking, natural color view of the ringed planet and five of its icy satellites. This image was composed of exposures taken by Cassini’s narrow angle camera on November 9, 2003 at 08:54 UTC (spacecraft event time) from a distance of 111.4 million km (69.2 million mi) — about three-fourths the distance of the Earth from the Sun — and 235 days from insertion into Saturn orbit.
The smallest features visible here are about 668 km (415 mi) across, which is a marked improvement over the last Cassini Saturn image released on November 1, 2002. New features such as intricate cloud patterns and small moons near the rings should become visible over the next several months as the spacecraft speeds toward its destination.
Some details within Saturn’s massive ring system are already visible. Structure is evident in the B ring, the middle and brightest of Saturn’s three main rings. The 4800 km (2980 mi)-wide Cassini Division is the distinctive dark, central band that separates the outermost A ring from the brighter B ring. Interestingly, the outer edge of the B ring is maintained by a strong gravitational resonance with the moon Mimas, also visible in this image. The 325 km (200 mi)-wide Encke gap in the A ring, near the outer edge of the ring system, is also visible, as is the fainter C ring, interior to the B ring.
With a thickness of only a few tens of meters or less, the main rings span 274,000 km (171,000 mi) from one end to the other about three-quarters of the distance between the Earth and the Moon.
Saturn’s multi-banded, multi-hued atmosphere is also apparent at this distance. In this composite made of images taken through broadband blue, green, and red spectral filters, the color is very close to what the human eye would see. The different hues of yellow, brown and red seen in the illuminated southern hemisphere are more delicate and subtle than the colors on Jupiter. Coloration on both Jupiter and Saturn is caused by small colored particles mixed with the white ammonia clouds. The ammonia clouds on Saturn are deeper and thicker than those on Jupiter because ammonia gas condenses at a deeper level in Saturn’s colder atmosphere. The composition of the colored particles is not known but is thought to include sulfur and nitrogen as key constituents at middle and low latitudes.
|Click image for high-resolution. Hubble image of Saturn rings. Credit: Hubble|
In the southern polar region, a dusky haze is visible, more gray than the light-brown at middle latitudes. This polar haze may be produced by energetic electrons and protons in the aurorae which destroy methane gas, leading to the formation of a haze of complex hydrocarbons.
Most of Saturn’s northern hemisphere is in shadow of the rings, with the exception of a small sliver visible on the limb. (Light passing through the Cassini Division illuminates the higher altitudes in the atmosphere.) This sliver appears bluer than the visible southern hemisphere, probably due to molecular scattering by hydrogen at these altitudes above the haze and clouds. As the Cassini tour unfolds over the next five years and beyond, we will have an opportunity to see how the colors change with time, whether due to changing seasonal heating or to some other mechanism.
|The Huygens probe descends through Titan’s murky, brownish-orange atmosphere of nitrogen and carbon-based molecules, beaming its findings to the distant Cassini orbiter. The probe is equipped with a variety of scientific sensors to measure the physical properties of the moon’s atmosphere; it also carries an imaging device to return pictures of Titan’s possibly hydrocarbon-lake-dotted surface.
Five Saturnian satellites can also be seen in the latest image. The brightnesses of these bodies have been increased three- to ten-fold to enhance visibility. The satellites are, on the left, from brightest to faintest, Rhea (1530 km, 951 mi across), Dione (1120 km, 696 mi), and Enceladus (520 km, 323 mi); and on the right, from brightest to faintest, Tethys (1060 km, 659 mi) and Mimas (392 km, 244 mi).
From the Voyager encounters in 1980 and 1981, we know that each of Saturn’s icy moons possesses intriguing features. Enceladus is the most reflective body in the solar system; both Mimas and Tethys exhibit large craters on their surfaces; Dione and Rhea have curious streaks of bright, wispy material. Cassini will make very close approaches to Rhea, Dione and Enceladus, returning images in which features as small as 50 meters or less will be detectable. Images with details finer than those seen by Voyager (~ 2 km, 1.3 mi) will be returned from all five moons.
On May 18, Cassini officially enters the Saturn planetary system.
On that day, the gravitational pull of Saturn begins to overtake the influence of the Sun and the probe crosses the outer limits of the most distant group of Saturnian moons, only weakly bound to Saturn and located tens of millions of kilometers from the planet. A few days later, a 3.5 week long campaign of Titan movie sequences commences to measure atmospheric winds and begin mapping its surface as Titan rotates. On June 11, Cassini makes a close, 2000-kilometer approach to Phoebe, collecting detailed images during the encounter. At 220 kilometers across, Phoebe is the largest of Saturn’s outer moons and believed to be a captured asteroid.
The seven year voyage will end when Cassini’s main engine is fired, the spacecraft is slowed, and the probe enters Saturn orbit on July 1, 2004.
"We very much want everyone to enjoy Cassini’s tour of this magnificent planetary system," said Dr. Carolyn Porco, leader of the Cassini imaging science team at the Space Science Institute in Boulder, Colo. "And I can say right now the views out the window will be stunning," Porco said.
The Cassini-Huygens mission is a cooperative mission of NASA, the European Space Agency and the Italian Space Agency. JPL, a division of the California Institute of Technology in Pasadena, manages the mission for NASA’s Office of Space Science, Washington, D.C.