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Saturn moon Iapetus' huge landslides stir intrigue

Saturn's moon Iapetus frequently plays host to a huge type of landslide or avalanche that is rare elsewhere in the Solar System, scientists report.
Sturzstroms or "long-runout landslides" move faster and farther than geological models predict they should.
They have been seen on Earth and Mars, but there is debate about their causes.

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Ice avalanches on Saturn moon could unlock secrets of Earth landslides

Giant avalanches on an icy moon of Saturn may provide clues about devastating landslides on Earth, say scientists.
Images from Nasa's Cassini spacecraft revealed 30 massive ice falls on Iapetus, a walnut-shaped moon girdled by steep 12-mile high mountains.
In 17 cases the avalanches plunged down crater walls, while 13 swept down the sides of the equatorial mountain range.
Scientists identified a strange feature of the avalanches. At high speeds, the falling ice began to behave like a liquid, travelling many miles before finally coming to rest. Experts are trying to explain a similar phenomenon seen in landslides and earthquakes on Earth.

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Discovery of Saturns Moon Iapetus in 1671

Saturn's Moon: Iapetus Rotation



Iapetus's equatorial ridge modelled from real Cassini imaging data



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Title: Spectro-polarimetry of the bright side of Saturn's moon Iapetus
Authors: C. Ejeta, H. Boehnhardt, S. Bagnulo, G.P. Tozzi

Measurements of the polarised reflected sunlight from atmosphereless solar system bodies, over a range of phase angles, provide information about the surface structure and composition. With this work, we provide analysis of the polarimetric observations of the bright side of Iapetus at five different phase angles, and over the full useful wavelength range (400-800nm), so as to assess the light scattering behaviour of a typical surface water ice. Using FORS2 of the ESO VLT, we have performed linear spectro-polarimetric observations of Iapetus' bright side from 2009 to 2011 at five different phase angles, in the range from 0.80-5.20^{0}, along with one circular spectro-polarimetric observation at one phase angle. By measuring, with high accuracy (~0.1% per spectral bin for each Stokes parameter), the spectral polarisation of the bright trailing hemisphere of Saturn's moon Iapetus, we have identified the polarimetric characteristics of water ice, and found that its linear degree of negative polarisation decreases with increasing phase angle of observation (varying from -0.9% to -0.3%), with a clear dependence on wavelengths of observation.

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Iapetus moon's mighty ridge stirs debate

The mountainous ridge that circles the equator on the Saturnian moon Iapetus is both weird and spectacular.
Discovered in 2004, the icy rim is as much as 20km high and runs fully 1,600km from end to end.
No explanation for its existence has yet won total support; it is a puzzle.
Dr Andrew Dombard and colleagues have now made a compelling case for the ridge being the remains of a huge ring of debris that once orbited Iapetus but which eventually fell on to the moon.

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Icy Debris Formed Iapetus' Ridge

As space-based probes and telescopes continue to reveal new and seemingly unimaginable features of our universe, a geological landmark on Saturn's moon Iapetus ranks high on the list of things particularly peculiar.
Images provided by NASA's Cassini spacecraft in 2005 reveal an almost straight-line mountain range that towers higher than 12 miles and spreads as wide as 60 miles, spanning more than 75 percent of the equatorial belt on Iapetus - the ringed planet's third-largest moon at 900 miles in diameter. It is shaped like a walnut, which has a ridge between the halves of its shell, but the Iapetus ridge is higher than Mount Everest and extends for thousands of miles - almost completely around the moon's equator.
While other scientists have hypothesized that Iapetus' mountains were formed by internal forces such as volcanism, Dombard, along with Andrew Cheng, chief scientist in the Space Department at the Johns Hopkins University Applied Physics Laboratory in Laurel, Md., William McKinnon, professor of earth and planetary sciences at Washington University in St. Louis and Jonathan Kay, a UIC graduate student studying with Dombard, think the mountains resulted from icy debris raining down from a sub-satellite, or mini-moon orbiting Iapetus that burst into bits under tidal forces of the larger moon.



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Two scientists propose an explanation for the bizarre ridge belting Saturn's moon Iapetus at the equator. At one time Iapetus itself may have had a satellite, created by a giant impact with another body. The satellite's orbit, would have decayed because of tidal interactions with Iapetus, and at some point it would have been ripped apart, forming a ring of debris around Iapetus that would eventually slam into the moon near its equator.
William B. McKinnon, PhD, Washington University professor of earth and planetary sciences in Arts & Sciences, and his former doctoral student, Andrew Dombard, PhD, associate professor of earth and environmental sciences at the University of Illinois Chicago (UIC), propose that at one time Iapetus itself had a satellite, or moon, created by a giant impact with another big body. The sub-satellite's orbit, they say, would have decayed because of tidal interactions with Iapetus, and it would have gradually migrated towards Iapetus. At some point, the researchers say, the tidal forces would have torn the sub-satellite apart, forming a ring of debris around Iapetus that would eventually slam into the moon near its equator.
Dombard will make a presentation on the preliminary findings Wed., Dec. 15, 2010, at the fall meeting of the American Geophysical Union in San Francisco. The team also included Andrew F. Cheng of the Johns Hopkins Applied Physics Laboratory, and Jonathan P. Kay, a graduate student at UIC.
Dombard says that Iapetus's Hill sphere - the zone close to an astronomical body where the body's gravity dominates satellites - is far bigger than that of any other major satellite in the outer solar system, accounting for why Iapetus is the only body known to have such a ridge.

University of Illinois at Chicago

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iap180210b.jpg
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Credit: NASA/JPL/Space Science Institute


This image of Iapetus was taken by the Cassini spaceprobe on the 18th February, 2010 when it was approximately 1,694,538 kilometres away.
The image was taken using the P120 and GRN filters.

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These false-colour images of Saturn's moon Iapetus were taken by the Cassini spacecraft narrow-angle camera on the 15th October, 2004, when the spacecraft was approximately 1.2 million kilometres away.
The images, obtained with infrared, green and ultraviolet spectral filters (centred at 953, 563 and 338 nanometers), were combined to create the false colour views.

iap150504b.jpg
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Credit:    NASA/JPL/Space Science Institute

The different images show the boundary of the global "colour dichotomy" on the hemisphere of this moon facing away from Saturn. The "colour dichotomy," is a second global pattern found on Iapetus besides the well-known global brightness dichotomy.
North on Iapetus is approximately up in these images.
Image scale is 7 kilometres per pixel.

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New data from NASA's Cassini spacecraft at Saturn helps explain the bizarre yin-yang appearance of the ringed planet's odd moon Iapetus, where one side is dark and the other is bright.
The images and heat-mapping data collected by Cassini support the leading explanation of the moon's strange appearance, which suggests that migrating ice makes half the moon reflective and bright, while the other half is dust-covered and dark.

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