Space Science ---------Enceladus: Saturn's Tiny, Shiny Moon

Enceladus: Saturn's Tiny, Shiny Moon

Enceladus is the sixth-largest moon of Saturn. It was discovered in 1789 by William Herschel, but little was known about Enceladus until the two Voyager spacecraft passed near it in the early 1980s. The Voyagers showed that the diameter of Enceladus is only 500 kilometers (310 mi),about a tenth of that of Saturn's largest moon, Titan, and that it reflects almost all the sunlight that strikes it. Voyager 1 found that Enceladus orbits in the densest part of Saturn's diffuse E ring, indicating a possible association between the two. Voyager 2 revealed that, despite Enceladus's small size, it has a wide range of surfaces ranging from old, heavily cratered regions to young, tectonically deformed terrains that formed as recently as 100 million years ago.

  
                                                               Voyager 2 view (August 26, 1981) of Enceladus's Saturn-facing hemisphere, showing a young surface.

In 2005, the Cassini spacecraft started multiple close flybys of Enceladus, revealing its surface and environment in greater detail. In particular, Cassini discovered a water-rich plume venting from Enceladus's south polar region. Cryovolcanoes near the south pole shoot geyser-like jets of water vapor, other volatiles, and solid material including sodium chloride crystals and ice particles into space, totaling approximately 200 kilograms (440 lb) per second. Over 100 geysers have been identified.Some of the water vapor falls back as "snow" and the rest escapes. This discovery revealed the mechanism by which Enceladus has released most of the material making up Saturn's E ring.


                                       View of Enceladus's orbit (highlighted in red) from above Saturn's north pole

These observations, along with the finding of escaping internal heat and very few (if any) impact craters in the south polar region, show that Enceladus is geologically active today. Moons in the extensive satellite systems of gas giants often become trapped in orbital resonances that lead to forced libration or orbital eccentricity. Enceladus is in such a resonance with Saturn's fourth largest moon, Dione; proximity to Saturn leads to tidal heating of Enceladus's interior, offering a possible explanation for the activity. In 2014, NASA reported that evidence for a large south polar subsurface ocean of liquid water within Enceladus with a thickness of around 10 km had been found by Cassini.

Cassini has provided strong evidence that Enceladus has an ocean with an energy source, nutrients and organic molecules, making Enceladus one of the best places for the study of potentially habitable environments for extraterrestrial life. By contrast, the water thought to be on Jupiter's moon Europa is locked under a very thick layer of surface ice, though recent evidence may show that Europa also erupts water plumes.



Enceladus' icy breath


Enceladus has at least five different types of terrain on its surface. Craters mar the landscape, reaching sizes no larger than 22 miles (35 km) across. Other regions are smooth and free from signs of impact, indicating recent resurfacing. Plains, fissures, and breaks in the crust also fill the surface.

Although small, the moon shines brightly. Its icy surface reflects more than 90 percent of the sunlight that falls on it, making it one of the brightest objects in the solar system. Because the planet reflects sunlight rather than absorbing it, it reaches temperatures as low as minus 201 degrees C (minus 330 degrees F).


Enceladus is only one-seventh the diameter of Earth's moon. It is the sixth largest and most massive moon of Saturn. Like most spherical bodies, it takes the shape of an oblate spheroid, bulging out slightly around its equator due to the effects of gravity as it spins.

Because the moon is not very massive, and feels only the smallest tug of gravity at its surface, scientists were surprised to discover that it has an atmosphere, which dominates at its warmer south pole. Further observations by NASA's Cassini mission revealed cracks in the surface, known as 'tiger stripes', which periodically vented material into space. While some of the icy eruptions fed the moon's atmosphere, much of the material escaped into space. The icy discharge contributes to Saturn's massive E-ring, the largest planetary ring in the solar system, spanning over 600,000 miles (about a million kilometers). Unlike other rings, the gossamer E-ring is made up of tiny particles rather than large chunks of rock.
  
                         This photo of water geysers spouting from Saturn's moon Enceladus was taken by NASA's Cassini orbiter in October 2007

At least 70 Yellowstone-like geysers vent icy material from beneath the moon's into space as the moon interacted with its parent planet. Gravitational forces open and close the cracks as Enceladus travels closer to and farther from Saturn over the course of its elliptical 1.37-Earth-day orbit.

Although the freezing moon should be too cold for liquid water, the presence of ammonia in the material streaming from Enceladus could act as antifreeze to keep water beneath the surface from freezing. Cassini detected other complex chemicals and organics in the vaporous plumes, which could make the moon a bright spot in the solar system when it comes to hosting life, making it an excellent target for further exploration.

Cassini


The answers to many remaining mysteries of Enceladus had to wait until the arrival of the Cassini spacecraft on July 1, 2004, when it entered orbit around Saturn. Given the results from the Voyager 2 images, Enceladus was considered a priority target by the Cassini mission planners, and several targeted flybys within 1,500 km of the surface were planned as well as numerous, "non-targeted" opportunities within 100,000 km of Enceladus.




 These encounters are listed on the left. The flybys have yielded significant information concerning Enceladus's surface, as well as the discovery of water vapor with traces of simple hydrocarbons venting from the geologically active south polar region. These discoveries prompted the adjustment of Cassini's flight plan to allow closer flybys of Enceladus, including an encounter in March 2008 which took the probe to within 52 km of the moon's surface.The extended mission for Cassini included seven close flybys of Enceladus between July 2008 and July 2010, including two passes at only 50 km in the later half of 2008.

No comments:

Post a Comment