Volume: 03, Issue: 10 05/18/2005 
The small moon that orbits within the Keeler gap in Saturn's rings is resolved for the first time. Image courtesy NASA/JPL/Space Science Institute.
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Other Articles in This Issue:
Chandra Observatory Catches X-ray Super-flares
Titan Atmosphere Decoded
Opportunity Takes Tentative Step Forward
Explore Distance Learning Courses
 

New Saturn Moon Makes Waves

As prime conditions for viewing Saturnís rings continue, the Cassini spacecraft has confirmed earlier suspicions of an unseen moon hidden in a gap in Saturn's outer A ring. The new moon has been shown to raise waves in the surrounding ring material.

The moon, provisionally named S/2005 S1, was first seen in a time-lapse sequence of images taken May 1, 2005, as Cassini began its climb to higher inclinations in orbit around Saturn. A day later, an even closer view was obtained, allowing the moon's size and brightness to be measured.

The images show the tiny object in the center of the Keeler gap and the wavy patterns in the gap edges, generated by the moon's gravitational influence. The Keeler gap is located about 155 miles inside the outer edge of the A ring, which is also the outer edge of the bright main rings. The new object is about 4 miles across and reflects about half the light falling on it -- a brightness that is typical of the particles in the nearby rings.

"It's too early to make out the shape of the orbit, but what we've seen so far of its motion suggests that it is very near the exact center of the gap, just as we had surmised," said Dr. Joseph Spitale, imaging team associate and planetary scientist at the Space Science Institute.

The new moonlet orbits approximately 84,820 miles from the center of Saturn. More Cassini observations will be needed to determine whether the moon's orbit around Saturn is circular or eccentric.

S/2005 S1 is the second known moon to exist within Saturn's rings. The other is Pan, measuring 16 miles across and orbiting in the Encke gap. Atlas and other moons exist outside the main ring system, as do the two F ring shepherd moons, Prometheus and Pandora.

Imaging scientists had predicted the new moon's presence and its orbital distance from Saturn after last July's sighting of a set of peculiar spiky and wispy features in the Keeler gap's outer edge. The similarities of the Keeler gap features to those noted in Saturn's F ring and the Encke gap led imaging scientists to conclude that a small body was lurking in the center of the Keeler gap, awaiting discovery.

"The obvious effect of this moon on the surrounding ring material will allow us to determine its mass and test our understanding of how rings and moons affect one another," said Dr. Carl Murray, imaging team member from Queen Mary, University of London.

An estimate of the moon's mass, along with a measure of its size, yields information on its physical makeup. For instance, the new moonlet might be quite porous, like an orbiting icy rubble pile. Other moons near the outer edge of Saturn's rings -- like Atlas, Prometheus and Pandora -- are also porous. Whether a moon is porous or dense says something about how it was formed and its subsequent collision history.

The Keeler gap edges also bear similarities to the scalloped edges of the 200-mile-wide Encke gap, where Pan resides. From the size of the waves seen in the Encke gap, imaging scientists were able to estimate the moonís mass. They expect to do the same eventually with this new moon.

Additional closer observations of the new body may take place in the next several months, as Cassini continues its intensive survey of Saturn's beautiful and mysterious rings.

"We anticipate that many of the gaps in Saturn's rings have embedded moons, and we'll be in search of them from here on," said Dr. Carolyn Porco, imaging team leader at the Space Science Institute.

The new images can be seen at the following websites:
http://saturn.jpl.nasa.gov
http://www.nasa.gov/cassini
http://ciclops.org

    
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