lundi 29 juin 2015

Saturn - Spirals in the D Ring












NASA & ESA - Cassini-Huygens Mission to Saturn & Titan patch.

June 29, 2015

 Spirals in the D Ring

Image above: Although the D ring of Saturn is so thin that it's barely noticeable compared to the rest of the ring system, it still displays structures seen in other Saturnian rings. Here the spiral structures in the D ring are on display.

The D ring spirals, discovered in Cassini images, are believed to be due to a warp in the ring created in the early 1980s. The precise mechanism remains the subject of scientific debate.  Over the course of the Cassini mission, scientists have been able to observe the spiral winding ever more tightly as it evolves. For more about the spiral.

(The bright specks and faint vertical streaks are merely image artifacts.  The processes typically employed to remove these artifacts would also have degraded the exquisite details of the D ring which are visible here.)

This view looks toward the sunlit side of the rings from about 22 degrees above the ringplane. The image was taken in visible light with the Cassini spacecraft narrow-angle camera on July 6, 2013.

The view was acquired at a distance of approximately 350,000 miles (563,000 kilometers) from Saturn and at a Sun-Saturn-spacecraft, or phase, angle of 131  degrees. Image scale is 2.1 miles (3.4 kilometers) per pixel.

Tilting Saturn's Rings

Animation above: This animated graphic shows in a series of three images how Saturn's rings, after they became tilted relative to Saturn's equatorial plane, would have transformed into a corrugated ring.

Images taken after Saturn's August 2009 equinox from NASA's Cassini spacecraft revealed alternating light and dark bands extending from Saturn's D ring, completely across the C ring, and right up to the inner B ring edge. These brightness variations are almost certainly caused by the changing slopes in the rippled ring plane, much like the corrugations of a tin roof.

This series of images shows how such a vertical corrugation can be produced from an initially inclined ring by the natural tendency for inclined orbits to wobble systematically and slowly at different rates, depending on their distance from Saturn. The top image shows a simple inclined ring (the central planet is omitted for clarity), while the lower two images show the same ring at two later times, where the ring particles' wobbling orbits have sheared this inclined sheet into an increasingly tightly-wound spiral corrugation.

Cassini images show the corrugation extends for 19,000 kilometers (12,000 miles). Based on detailed studies of this structure, scientists conclude that a broad swath of the rings became suddenly tilted in the early 1980s, likely because cometary debris crashed into the rings. The corrugation's radial extent implies that the impacting material was a dispersed cloud of debris instead of a single object. The corrugation's amplitude of 2 to 20 meters (7 to 60 feet) indicates that the debris' total mass was around 1 trillion kilograms (or one billion metric tons).

A Twisted Tale

Original Caption Released with Image:

Figure 1

Images above: Saturn's D ring--the innermost of the planet's rings -- sports an intriguing structure that appears to be a wavy, or "vertically corrugated," spiral. This continuously changing ring structure provides circumstantial evidence for a possible recent collision event in the rings.

Support for this idea comes from the appearance of a structure in the outer D-ring that looks, upon close examination, like a series of bright ringlets with a regularly spaced interval of about 30 kilometers (19 miles). When viewed along a line of sight nearly in the ringplane, a pattern of brightness reversals is observed: a part of the ring that appears bright on the far side of the rings appears dark on the near side of the rings, and vice versa (see Image bellow).

D Ring Sight Lines

Image above: A vertical corrugation in Saturn's almost transparent D ring can give rise to brightness variations.

In this graphic representation, the diagonal lines (or arrows) correspond to lines of sight through the corrugated ring. The grayscale plot along the bottom shows the ring's total optical depth (a measure of opacity) and brightness as seen by the observer.

This phenomenon would occur if the region contains a sheet of fine material that is vertically corrugated, like a tin roof. In this case, variations in brightness would correspond to changing slopes in the rippled ring material (see figure 1).

An observation made with NASA's Hubble Space Telescope in 1995 also saw a periodic structure in the outer D ring, but its wavelength was then 60 kilometers (37 miles). There were insufficient observations to discern the spiral nature of the feature. Thus, it appears the wavelength of the wavy structure has been decreasing: that is, this feature has been winding up like a spring over time.

The rate at which the pattern appears to be winding up is quite close to the rate scientists would expect for a vertically corrugated spiraling sheet of material at this location in the rings that is responding to gravitational forcing from Saturn.

Cassini passing over the Saturn rings

As Cassini imaging scientists extrapolated the spiraling trend backward in time, they found that it completely unwound in 1984, leaving only an inclined, or tilted, sheet of material. The researchers speculate such an inclined sheet may have been produced around that time by the impact of a comet or meteoroid into the D ring which kicked out a cloud of fine particles that ultimately inherited some of the tilt of the impactor's trajectory as it slammed into the rings. Another possibility is that the impactor struck an already inclined moonlet, shattered it to bits and the debris remained in an inclined orbit.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the mission for NASA's Science Mission Directorate, Washington, D.C.

For more information about the Cassini-Huygens mission visit http://saturn.jpl.nasa.gov or http://www.nasa.gov/cassini. The Cassini imaging team homepage is at http://ciclops.org and http://www.esa.int/Our_Activities/Space_Science/Cassini-Huygens

Images, Text, Credits: NASA/JPL-Caltech/Space Science Institute/Cornell.

Best regards, Orbiter.ch

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