Przerwa Keelera

Przerwa Keelera w okolicy księżyca Daphnis, wyraźnie widoczne są tworzone przez niego fale.
Zdjęcie wykonane podczas równonocy ukazuje cienie, rzucane przez księżyc i fale na krawędziach przerwy.

Przerwa Keelera – szczelina usytuowana w pobliżu zewnętrznej krawędzi pierścienia A Saturna. Jest to druga pod względem wielkości szczelina w tym pierścieniu po przerwie Enckego. Została ona odkryta przez sondę Voyager i nazwana na cześć amerykańskiego astronoma Jamesa E. Keelera, który w 1888 roku zaobserwował przerwę Enckego (choć niekoniecznie jako pierwszy[1]).

Charakterystyka

Przerwa Keelera ma 35–42 kilometry szerokości, jej centrum znajduje się w odległości 136 530 kilometrów od środka planety[2]. Sonda Cassini ukazała fale gęstości po dwóch stronach przerwy, których obserwacja poprzedziła odkrycie małego księżyca. Satelita ten, nazwany Daphnis, krąży po orbicie lekko nachylonej do płaszczyzny pierścieni, wskutek czego pierścień A w pobliżu przerwy nie jest płaski. Zdjęcia wykonane w pobliżu równonocy na Saturnie pokazują, że fale na krawędziach przerwy Keelera wznoszą się nawet ok. 4 km ponad płaszczyznę pierścienia[3].

Zobacz też

Przypisy

  1. ↑ Jeff Medkeff: This Rings of Confusion!. 10 lutego 1998.
  2. ↑ D.R. Williams: Saturnian Rings Fact Sheet. [dostęp 2010-10-27].
  3. ↑ Surprising, Huge Peaks Discovered in Saturn's Rings. space.com, 21.09.2009. [dostęp 2010-10-27].

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Daphnis edge wave shadows.jpg
Never-before-seen looming vertical structures created by the tiny moon Daphnis cast long shadows across the rings in this startling image taken as Saturn approaches its mid-August 2009 equinox.

Daphnis, 8 kilometers (5 miles) across, occupies an inclined orbit within the 42-kilometer (26-mile) wide Keeler Gap in Saturn's outer A ring. Recent analyses by imaging scientists published in the Astronomical Journal illustrate how the moon's gravitational pull perturbs the orbits of the particles forming the gap's edge and sculpts the edge into waves having both horizontal and vertical components.

Measurements of the shadows in this and other images indicate that the vertical structures range between one-half to 1.5 kilometers tall (about one-third to one mile), making them as much as 150 times as high as the ring is thick. The main A, B and C rings are only about 10 meters (about 30 feet) thick. Daphnis itself can be seen casting a shadow onto the nearby ring.

This image of shadows on the rings and others like it (see PIA11656 and PIA11655) are only possible around the time of Saturn's equinox which occurs every half-Saturn-year (equivalent to about 15 Earth years). The illumination geometry that accompanies equinox lowers the sun's angle to the ringplane and causes out-of-plane structures to cast long shadows across the rings.

This view looks toward the sunlit side of the rings from about 57 degrees below the ringplane. The image was taken in visible light with the Cassini spacecraft narrow-angle camera on May 24, 2009. The view was acquired at a distance of approximately 826,000 kilometers (513,000 miles) from Daphnis and at a Sun-Daphnis-spacecraft, or phase, angle of 121 degrees. Image scale is 5 kilometers (3 miles) per pixel.
PIA08319 Daphnis in Keeler Gap.jpg

Uploader's notes: the original NASA image has been modified by stretching by a factor of 2 in both dimensions, sharpening and converting from TIFF to JPEG format.

Original caption released with image: Daphnis drifts through the Keeler gap, at the center of its entourage of waves.

The little moon (7 kilometers, or 4.3 miles across) draws material in the Keeler gap (42 kilometers, or 26 miles wide) into these now familiar edge waves as it orbits Saturn.

This view looks toward the lit side of the rings from about 25 degrees below the ringplane.

The image was taken in visible light with the Cassini spacecraft narrow-angle camera on Oct. 27, 2006 at a distance of approximately 325,000 kilometers (202,000 miles) from Daphnis and at a Sun-Daphnis-spacecraft, or phase, angle of 36 degrees. Image scale is 2 kilometers (1 mile) per pixel.

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. The Cassini orbiter and its two onboard cameras were designed, developed and assembled at JPL. The imaging operations center is based at the Space Science Institute in Boulder, Colo.

For more information about the Cassini-Huygens mission visit http://saturn.jpl.nasa.gov/home/index.cfm. The Cassini imaging team homepage is at http://ciclops.org.