Astronomers Solve Puzzle of the Mountains That Fell From Space

KentuckyFC (1144503) writes "Iapetus, Saturn's third largest moon, was first photographed by the Cassini spacecraft on 31 December 2004. The images created something of a stir. Clearly visible was a narrow, steep ridge of mountains that stretch almost halfway around the moon's equator. The question that has since puzzled astronomers is how this mountain range got there. Now evidence is mounting that this mountain range is not the result of tectonic or volcanic activity, like mountain ranges on other planets. Instead, astronomers are increasingly convinced that this mountain range fell from space. The latest evidence is a study of the shape of the mountains using 3-D images generated from Cassini data. They show that the angle of the mountainsides is close to the angle of repose, that's the greatest angle that a granular material can form before it landslides. That's not proof but it certainly consistent with this exotic formation theory. So how might this have happened?

Astronomers think that early in its life, Iapetus must have been hit by another moon, sending huge volumes of ejecta into orbit. Some of this condensed into a new moon that escaped into space. However, the rest formed an unstable ring that gradually spiraled in towards the moon, eventually depositing the material in a narrow ridge around the equator. Cassini's next encounter with Iapetus will be in 2015 which should give astronomers another chance to study the strangest mountain range in the Solar System."

Re:Doesn't Gravity Affect Angle of Repose?

[danbert8]

Astrophysicists my ass... Geologists have this covered! From "Static and dynamic angles of repose in loose granular materials under reduced gravity"
http://adsabs.harvard.edu/abs/... [harvard.edu]

Until now it has been assumed that the angles of repose are independent of gravitational acceleration. The objective of this work is to experimentally determine whether the angles of repose depend on gravity. In 33 parabolic flights in a well-controlled research aircraft we recorded avalanching granular materials in rotating drums at effective gravitational accelerations of 0.1, 0.38 and 1.0 times the terrestrial value. The granular materials varied in particle size and rounding and had air or water as interstitial fluid. Materials with angular grains had time-averaged angles of about 40 degrees and with rounded grains about 25 degrees for all effective gravitational accelerations, except the finest glass beads in air, which was explained by static electricity. For all materials, the static angle of repose increases about 5 degrees with reduced gravity, whereas the dynamic angle decreases with about 10 degrees. Consequently, the avalanche size increases with reduced gravity.

Re:How about...

[Anonymous Coward]

It is generally accepted that planetary ring systems are not stable, permanent features but are a result of collisions, gravitational disintegration, or other disruption of an existing satellite. The rings, over time, will eventually re-form into a new satellite, or be lost to space or to the parent object. So there is really no such thing as an 'original ring system'.

Quite interesting

[140Mandak262Jamuna]

The explanation is interesting. The moon is half the diameter of our moon, which means 8 times smaller in volume, and possibly mass. Tidally locked to a much bigger planet Saturn, compared to earth. The only thing against "mountain range fell from the sky scenario" is that, we normally do not see 1300 km long objects in space that are just 10 to 15 km in diameter. One possibility is that a loosely accreted comet was pulled into a long string by the gravity of Saturn, (Remember? the Schoemaker - Levy comet colliding with Jupiter was pulled in to a string of rocks. [google.com] ). And this moon got in the way and got whacked in the process. May be the accretion of matter into a spherical moon did not quite achieve completion.

Till we see 1300km long and 10 to 10 km diameter asteroids in space, we just have to file it under, "it is the best we could do, under these circumstances".