Enormous Ring Of Dust And Ice Is Nearly Invisible
NASA's Spitzer Space Telescope has discovered an enormous ring
around Saturn -- by far the largest of the giant planet's many
rings.
The new belt lies at the far reaches of the Saturnian system,
with an orbit tilted 27 degrees from the main ring plane. The bulk
of its material starts about six million kilometers (3.7 million
miles) away from the planet and extends outward roughly another 12
million kilometers (7.4 million miles). One of Saturn's farthest
moons, Phoebe, circles within the newfound ring, and is likely the
source of its material.
Saturn's newest halo is thick, too -- its vertical height is
about 20 times the diameter of the planet. It would take about one
billion Earths stacked together to fill the ring.
"This is one supersized ring," said Anne Verbiscer, an
astronomer at the University of Virginia, Charlottesville. "If you
could see the ring, it would span the width of two full moons'
worth of sky, one on either side of Saturn." Verbiscer; Douglas
Hamilton of the University of Maryland, College Park; and Michael
Skrutskie, of the University of Virginia, Charlottesville, are
authors of a paper about the discovery to be published online
tomorrow by the journal Nature.
The ring itself is tenuous, made up of a thin array of ice and
dust particles. Spitzer's infrared eyes were able to spot the glow
of the band's cool dust. The telescope, launched in 2003, is
currently 107 million kilometers (66 million miles) from Earth in
orbit around the sun.
NASA Image
The discovery may help solve an age-old riddle of one of
Saturn's moons. Iapetus has a strange appearance -- one side is
bright and the other is really dark, in a pattern that resembles
the yin-yang symbol. The astronomer Giovanni Cassini first spotted
the moon in 1671, and years later figured out it has a dark side,
now named Cassini Regio in his honor.
Saturn's newest addition could explain how Cassini Regio came to
be. The ring is circling in the same direction as Phoebe, while
Iapetus, the other rings and most of Saturn's moons are all going
the opposite way. According to the scientists, some of the dark and
dusty material from the outer ring moves inward toward Iapetus,
slamming the icy moon like bugs on a windshield.
"Astronomers have long suspected that there is a connection
between Saturn's outer moon Phoebe and the dark material on
Iapetus," said Hamilton. "This new ring provides convincing
evidence of that relationship."
Verbiscer and her colleagues used Spitzer's longer-wavelength
infrared camera, called the multiband imaging photometer, to scan
through a patch of sky far from Saturn and a bit inside Phoebe's
orbit. The astronomers had a hunch that Phoebe might be circling
around in a belt of dust kicked up from its minor collisions with
comets -- a process similar to that around stars with dusty disks
of planetary debris. Sure enough, when the scientists took a first
look at their Spitzer data, a band of dust jumped out.
The ring would be difficult to see with visible-light
telescopes. Its particles are diffuse and may even extend beyond
the bulk of the ring material all the way in to Saturn and all the
way out to interplanetary space. The relatively small numbers of
particles in the ring wouldn't reflect much visible light,
especially out at Saturn where sunlight is weak.
Spitzer Telescope NASA Image
"The particles are so far apart that if you were to stand in the
ring, you wouldn't even know it," said Verbiscer.
Spitzer was able to sense the glow of the cool dust, which is
only about 80 Kelvin (minus 316 degrees Fahrenheit). Cool objects
shine with infrared, or thermal radiation; for example, even a cup
of ice cream is blazing with infrared light. "By focusing on the
glow of the ring's cool dust, Spitzer made it easy to find," said
Verbiscer.
These observations were made before Spitzer ran out of coolant
in May and began its "warm" mission.