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Cassini Finds Saturn Moon May Have Tipped Over

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Enceladus the Storyteller

A masterpiece of deep time and wrenching gravity, the tortured surface of Saturn’s moon Enceladus and its fascinating ongoing geologic activity tell the story of the ancient and present struggles of one tiny world. This is a story that is recounted by imaging scientists in a paper published in the journal Science on March 10, 2006.

The enhanced color view of Enceladus seen here is largely of the southern hemisphere and includes the south polar terrain at the bottom of the image.

Ancient craters remain somewhat pristine in some locales, but have clearly relaxed in others. Northward-trending fractures, likely caused by a change in the moon’s rate of rotation and the consequent flattening of the moon’s shape, rip across the southern hemisphere. The south polar terrain is marked by a striking set of `blue’ fractures and encircled by a conspicuous and continuous chain of folds and ridges, testament to the forces within Enceladus that have yet to be silenced.

The mosaic was created from 21 false-color frames taken during the Cassini spacecraft’s close approaches to Enceladus on March 9 and July 14, 2005. Images taken using filters sensitive to ultraviolet, visible and infrared light (spanning wavelengths from 338 to 930 nanometers) were combined to create the individual frames.

The mosaic is an orthographic projection centered at 46.8 degrees south latitude, 188 degrees west longitude, and has an image scale of 67 meters (220 feet) per pixel. The original images ranged in resolution from 67 meters per pixel to 350 meters (1,150 feet) per pixel and were taken at distances ranging from 11,100 to 61,300 kilometers (6,900 to miles) from Enceladus.
Credit: NASA/JPL-Caltech/ Space Science Institute

 

Cassini Finds Saturn Moon May Have Tipped Over

Saturn’s icy, ocean-bearing moon Enceladus may have tipped over in the distant past, according to recent research from NASA’s Cassini mission. Researchers with the mission found evidence that the moon’s spin axis — the line through the north and south poles — has reoriented, possibly due to a collision with a smaller body, such as an asteroid.

Examining the moon’s features, the team showed that Enceladus appears to have tipped away from its original axis by about 55 degrees — more than halfway toward rolling completely onto its side. “We found a chain of low areas, or basins, that trace a belt across the moon’s surface that we believe are the fossil remnants of an earlier, previous equator and poles,” said Radwan Tajeddine, a Cassini imaging team associate at Cornell University, Ithaca, New York, and lead author of the paper.

The area around the icy moon’s current south pole is a geologically active region where long, linear fractures referred to as tiger stripes slice across the surface. Tajeddine and colleagues speculate that an asteroid may have struck the region in the past when it was closer to the equator. “The geological activity in this terrain is unlikely to have been initiated by internal processes,” he said. “We think that, in order to drive such a large reorientation of the moon, it’s possible that an impact was behind the formation of this anomalous terrain.”

In 2005, Cassini discovered that jets of water vapor and icy particles spray from the tiger stripe fractures — evidence that an underground ocean is venting directly into space from beneath the active south polar terrain.

Whether it was caused by an impact or some other process, Tajeddine and colleagues think the disruption and creation of the tiger-stripe terrain caused some of Enceladus’ mass to be redistributed, making the moon’s rotation unsteady and wobbly. The rotation would have eventually stabilized, likely taking more than a million years. By the time the rotation settled down, the north-south axis would have reoriented to pass through different points on the surface — a mechanism researchers call “true polar wander.”

The polar wander idea helps to explain why Enceladus’ modern-day north and south poles appear quite different. The south is active and geologically young, while the north is covered in craters and appears much older. The moon’s original poles would have looked more alike before the event that caused Enceladus to tip over and relocate the disrupted tiger-stripe terrain to the moon’s south polar region.

The results were published in the online edition of the journal Icarus on April 30, 2017.

The Cassini-Huygens mission is a cooperative project of NASA, ESA (European Space Agency) and the Italian Space Agency. NASA’s Jet Propulsion Laboratory, a division of Caltech in Pasadena, California, manages the mission for NASA’s Science Mission Directorate, Washington. JPL designed, developed and assembled the Cassini orbiter.

source: NASA – Jet Propulsion Laboratory – California Institute of Technology

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