Seven Years to Saturn

As Cassini nears its rendezvous with Saturn, new detail in the banded clouds of the planet’s atmosphere are becoming visible.

High, thick clouds and Saturn’s rotating dark spot. Image Credit: Cassini Imaging Team/Boulder/

Cassini began the journey to the ringed world of Saturn nearly seven years ago and is now less than two months away from orbit insertion on June 30. Cassini’s narrow-angle camera took this image on April 16, 2004, when the spacecraft was 38.5 million kilometers (23.9 million miles) from Saturn.

Dark regions are generally areas free of high clouds, and bright areas are places with high, thick clouds which shield the view of the darker areas below. A dark spot is visible at the south pole, which is remarkable to scientists because it is so small and centered.

The spot could be affected by Saturn’s magnetic field, which is nearly aligned with the planet’s rotation axis, unlike the magnetic fields of Jupiter and Earth. From south to north, other notable features are the two white spots just above the dark spot toward the right, and the large dark oblong-shaped feature that extends across the middle. The darker band beneath the oblong-shaped feature has begun to show a lacy pattern of lighter-colored, high altitude clouds, indicative of turbulent atmospheric conditions.

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/

The cloud bands move at different speeds, and their irregularities may be due to either the different motions between them or to disturbances below the visible cloud layer. Such disturbances might be powered by the planet’s internal heat; Saturn radiates more energy than it receives from the Sun.

The moon Mimas (396 kilometers, 245 miles across) is visible to the left of the south pole. Saturn currently has 31 known moons. Since launch, 13 new moons have been discovered by ground-based telescopes. Cassini will get a closer look and may discover new moons, perhaps embedded within the planet’s magnificent rings.

This image was taken using a filter sensitive to light near 727 nanometers, one of the near-infrared absorption bands of methane gas, which is one of the ingredients in Saturn’s atmosphere. The image scale is approximately 231 kilometers (144 miles) per pixel. Contrast has been enhanced to aid visibility of features in the atmosphere.

On May 18, Cassini officially entered the Saturn planetary system. This event marks when the gravitational pull of Saturn began 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.

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.

Early next year, ESA’s Huygens spaceprobe will be descending through the atmosphere of Saturn’s largest moon (Titan), becoming the first spacecraft to land on a body in the outer Solar System. Titan is the only moon with a thick atmosphere.

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.
Credit: ESA

Astronomers think this atmosphere might closely match the one Earth possessed millions of years ago, before life began. Certainly Titan’s atmosphere is rich in carbon, the chemical necessary for life on Earth. What is more, this is all stored in ‘deep freeze’, ten times further from the Sun than the Earth. The haze is much thicker than Earth’s worst city smog. It was impenetrable to cameras aboard the Pioneer and Voyager spacecraft that flew by the Saturn system in the late 1970s and early 1980s. The big mystery is Titan’s surface, which is hidden by a cloud layer. This is why ESA built Huygens, to probe through this layer which is impenetrable by Earth-based observations. Huygens’ battery of instruments will return over 1000 images as it floats down and samples the chemistry of this exotic place.

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.