The Cassini spacecraft watches a pair of Saturn's moons, showing the hazy orb of giant Titan beyond smaller Tethys. This view looks toward the Saturn-facing sides of Titan (3,200 miles, or 5,150 kilometers across) and Tethys (660 miles, or 1,062 kilometers across).
The image was taken in visible green light with the Cassini spacecraft narrow-angle camera on Oct. 18, 2010
A recent study has provided new insights into how the warm interior of Saturn's geologically active moon Enceladus could be sustained for billions of years.
Credits: NASA/JPL/Space Science Institute
Heat from friction could power hydrothermal activity on Saturn's moon Enceladus for billions of years if the moon has a highly porous core, according to a new modeling study by European and U.S. researchers working on NASA's Cassini mission.
The study, published today in the journal Nature Astronomy, helps resolve a question scientists have grappled with for a decade: Where does the energy to power the extraordinary geologic activity on Enceladus come from?
Cassini found that Enceladus sprays towering, geyser-like jets of water vapor and icy particles, including simple organics, from warm fractures near its south pole. Additional investigation revealed the moon has a global ocean beneath its icy crust, from which the jets are venting into space. Multiple lines of evidence from Cassini indicate that hydrothermal activity -- hot water interacting chemically with rock -- is taking place on the seafloor.
One of those lines was the detection of tiny rock grains inferred to be the product of hydrothermal chemistry taking place at temperatures of at least 194 degrees Fahrenheit (90 degrees Celsius). The amount of energy required to produce these temperatures is more than scientists think could be provided by decay of radioactive elements in the interior.
Titan
Discovered in 1655 by the Dutch astronomer Christiaan Huygens, Titan is the biggest of the 31 known moons orbiting Saturn. It is a cold world enclosed by a thick, hazy atmosphere impenetrable by optical instruments.
Image right: Titan taken by Voyager 2
Image credit: NASA/JPL
With a diameter of 5,150 kilometers (3,200 miles), Titan is the second largest moon in the solar system. It's bigger than our own moon and even bigger than the planet Mercury. Only Jupiter's moon Ganymede is larger than Titan, with a diameter barely 112 kilometers (62 miles) greater. Temperature at the surface is about minus 178 Celsius (minus 289 Fahrenheit).
Titan orbits Saturn at a distance of about 1.2 million kilometers (745,000 miles), taking almost 16 days to complete a full orbit -- 15.94 days to be exact.
Titan is of great interest to scientists because it is the only moon in the solar system known to have clouds and a mysterious, thick, planet-like atmosphere. In 1980, during a flyby of Titan, NASA's Voyager 1 tried to take close up images of the natural features of the landscape but it wasn't able to penetrate the thick clouds. The only results netted were slight color and brightness variations within the atmosphere. Titan's atmospheric pressure is about 60 percent greater than Earth's -- roughly the same pressure found at the bottom of a swimming pool.
In 1994, NASA's Hubble Space Telescope recorded pictures of Titan, which suggested that a huge bright "continent" exists on its hemisphere that faces forward in orbit. These Hubble results don't prove that liquid "seas" exist, however; only that Titan has large bright and dark regions on its surface.
The instruments aboard the Cassini-Huygens spacecraft are designed to uncover many of Titan's characteristics. During dozens of flybys, the Cassini orbiter will map Titan with cloud-penetrating radar and collect atmospheric data. The Huygens probe will dive through Titan's dense atmosphere with instruments capable of analyzing its components.
Image above: Some of Saturn's most interesting moons.
Combined with the "big picture" information that the orbiter Cassini will collect during Titan flybys, data from the Huygens probe will provide scientists with critical information that may shed light on ancient questions, such as "Where did we come from?" And, "How did the planets form?"
Because of the extremely cold temperatures typical of celestial bodies that are that far away from the Sun, the structure of Titan's chemical atmosphere is in a state of deep freeze. It is this chemical composition that interests scientists a great deal because Titan's atmosphere might consist of compounds similar to those present in the primordial days of the Earth's atmosphere. Titan's thick cloudy atmosphere is mostly nitrogen, like Earth's, but may contain much higher percentages of "smog-like" chemicals such as methane and ethane. The smog may be so thick that it actually rains "gasoline-like" liquids. The organic nature of some of the chemicals found in Titan's atmosphere might indicate that this fascinating moon could harbor some form of life.
Other Moons
Besides Titan, there are at least 30 other moons orbiting Saturn. Some carry exotic names such as Prometheus, Pandora, Enceladus and Hyperion, others are known by plain letter and digits codes, as in S/2000 S4 and S/2000 S5.
These moons vary considerably in shape and size. Some appear to be porous, icy bodies with craters, ridges and valleys -- others show corrugated, irregular terrain. Some appear to have formed billions of years ago, while others appear to be part of a bigger, fragmented body. Some moons appear to have rocky surfaces, and might be covered by organic material similar to the complex substances found in the most primitive meteorites. For example, the closest large moon of Saturn, Enceladus, appears to have a very bright, white surface, as it reflects almost 100 percent of the sunlight that hits it. Because Enceladus reflects so much sunlight, the surface temperature is only minus 201 Celsius (minus 330 Fahrenheit).
Image left: Artist's rendition of icy Enceladus. Credit: NASA/David Seal.
According to images recorded by Voyager 2 in 1981, Enceladus has fissures, plains, corrugated terrain and other crustal deformation. All of this indicates that the interior of the moon may be liquid today, even though it should have frozen eons ago.
For the most part, very little is known about Saturn's moons, except through data gathered that measures their brightness. These moons' estimated sizes are based on assumptions of their reflectivity.
These 31 known moons vary greatly in their orbital positions within the Saturn system -- some are as close as 133,600 kilometers (83,015 miles) to the planet, others as far as almost 13 million kilometers (about 8 million miles). During its four-year mission in this immense region, the Cassini spacecraft will extensively photograph these moons, and collect data that will increase our understanding of their composition
While the larger moons are spherical, others are shaped like a sweet potato (Prometheus), a regular potato (Pandora), a meatball (Janus), and even a sponge (Hyperion). Some have a gnarled, irregular shape and texture like a dirty ice-ball (Epimetheus). One object observed in the rings (and unofficially called Peggy) may be a moon forming or disintegrating, or it might not truly be a moon at all.
Four spacecraft have visited the Saturn system, but Cassini alone actually orbited the ringed planet. Doing so bought Cassini time—more than a decade—to linger and watch Saturn’s exotic zoo of 60-plus moons like no spacecraft before. Cassini looked, listened, sniffed and even tasted Saturn’s moons, and what it learned about them is nothing less than extraordinary.
Habitability Outside the Habitability Zone
Haze-enshrouded Titan is Saturn’s largest moon and the solar system’s second-largest, and an all-around exceptional place. Titan is the only known world other than Earth where any sort of liquid collects on its surface.
Earth has liquid water oceans because it orbits within the solar system’s habitable zone, the range of distance from our sun that is neither so hot that oceans boil away nor so cold that they’re frozen. The Saturn system is far outside of the habitable zone, and Titan’s surface temperature is so cold that the moon’s surface is largely water ice instead of rock. But the temperature is just right for liquid methane and ethane.
Cassini scientists found not only that liquid methane and ethane exist on Titan, but that they rain from the sky and fill liquid lakes, which are as wide as several hundred miles (or kilometers) and as deep as hundreds of feet (or meters).
Despite Titan’s distance from the sun, Cassini also found evidence of a global (or at least regional) liquid water ocean below its frigid surface, but Titan isn’t the only Saturnian moon that boasts of a subsurface sea.
Ocean Moon
Cassini scientists were astonished when they found a plume spraying water vapor, icy particles and simple organic materials into space from the south polar region of Enceladus. Scientists determined that the jets are the source of material for Saturn’s E-Ring. The jets spew out of great fissures, or cracks, known as “tiger stripes,” which Cassini scientists found grow wider or narrower depending on where Enceladus is in its orbit around Saturn.
Observations of its gravity and the contents of the plume suggested that Enceladus almost certainly contains a subsurface liquid water ocean near its southern polar region. Now more and more Cassini data indicates that Enceladus is home to a global (not just regional) ocean, and the jets near the south polar region are powered by hotspots, where heat from the interior ocean escapes.
On Earth, hydrothermal vents at the ocean floor can produce habitats where organisms can flourish, completely cut off from sunlight. Many scientists suspect that life on Earth might have begun around these undersea environments. The evidence is beginning to pile up that Enceladus might have hydrothermal vents too.
But vents or no vents, the Cassini mission showed that Enceladus appears to have all the components necessary for life. The food component is methane, which has been observed in surprising abundance in the plume, said Bonnie Buratti, a senior research scientist at NASA’s Jet Propulsion Laboratory. “Water, food, and heat—everything is there for life as we know it,” she said. “It’s all there.”
Even if Enceladus isn’t home to even simple organisms, the presence of the necessary conditions is itself a remarkable finding.
Moon Mysteries Managed
Enceladus and Titan get a lot of attention, but the Cassini mission also provided countless insights about dozens of Saturn’s other moons.
The moon Mimas, whose Herschel Crater makes it look like the Death Star from "Star Wars," was thought to be an old, dead world, according to Cassini project scientist Linda Spilker. But Cassini scientists found evidence that it too may have a subsurface ocean, or at least a football-shaped water-ice core. “It’s curious. Mimas and Enceladus are the same size,” Spilker said, but Enceladus has water jets while Mimas appears not to. The reason for their differences is unknown.
Cassini also found thin atmospheres around the moons Dione and Rhea—but we’re talking thin, around 5 trillion times less dense than at Earth’s surface.
Saturn and its Largest Moon Reflect Their True Colors
08.29.12
A giant of a moon appears before a giant of a planet undergoing seasonal changes in this natural color view of Titan and Saturn from NASA's Cassini spacecraft. Image credit: NASA/JPL-Caltech/SSI › Full image and caption
Saturn's rings obscure part of Titan's colorful visage in this image from NASA's Cassini spacecraft. Image credit: NASA/JPL-Caltech/SSI› Full image and caption
NASA's Cassini spacecraft looks toward the night side of Saturn's largest moon and sees sunlight scattering through the periphery of Titan's atmosphere and forming a ring of color. Image credit: NASA/JPL-Caltech/SSI› Full image and caption
The recently formed south polar vortex stands out in the color-swaddled atmosphere of Saturn's largest moon, Titan, in this natural color view from NASA's Cassini spacecraft. Image Credit: NASA/JPL-Caltech/SSI› Full image and caption
PASADENA, Calif. -- Posing for portraits for NASA's Cassini spacecraft, Saturn and its largest moon, Titan, show spectacular colors in a quartet of images being released today. One image captures the changing hues of Saturn's northern and southern hemispheres as they pass from one season to the next.
This spectacular image shows Saturn's moons Prometheus (at right) and Pandora (at left), with their flock of icy ring particles (the F Ring) between them. Pandora is exterior to the ring, and closer to the Cassini spacecraft here. Each of the shepherd satellites has an unusual shape, with a few craters clearly visible.
The effect of Prometheus (102 kilometers, or 63 miles across) on the F ring is visible as it pulls material out of the ring when it is farthest from Saturn in its orbit. Pandora is 84 kilometers (52 miles) across.
How many moons does Saturn have? So far 62 have been discovered, the smallest only a fraction of a kilometer across. Six of its largest satellites can be seen here, though, in a sharp Saturnian family portrait taken on March 9. Larger than Earth's Moon and even slightly larger than Mercury, Titan has a diameter of 5,150 kilometers and starts the line-up at the lower left. Continuing to the right across the frame are Mimas, Tethys, [Saturn], Enceladus, Dione, and Rhea at far right. Saturn's first known natural satellite, Titan was discovered in 1655 by Dutch astronomer Christiaan Huygens, while most recently the satellite provisionally designated S/2009 S1 was found by the Cassini Imaging Science Team in 2009. Tonight, Saturn reaches opposition in planet Earth's sky, offering the best telescopic views of the ringed planet and moons.
The effect of Prometheus (102 kilometers, or 63 miles across) on the F ring is visible as it pulls material out of the ring when it is farthest from Saturn in its orbit. Pandora is 84 kilometers (52 miles) across.
How many moons does Saturn have? So far 62 have been discovered, the smallest only a fraction of a kilometer across. Six of its largest satellites can be seen here, though, in a sharp Saturnian family portrait taken on March 9. Larger than Earth's Moon and even slightly larger than Mercury, Titan has a diameter of 5,150 kilometers and starts the line-up at the lower left. Continuing to the right across the frame are Mimas, Tethys, [Saturn], Enceladus, Dione, and Rhea at far right. Saturn's first known natural satellite, Titan was discovered in 1655 by Dutch astronomer Christiaan Huygens, while most recently the satellite provisionally designated S/2009 S1 was found by the Cassini Imaging Science Team in 2009. Tonight, Saturn reaches opposition in planet Earth's sky, offering the best telescopic views of the ringed planet and moons.
No comments:
Post a Comment