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Astronomers Solve Puzzle of the Mountains That Fell From Space

Posted by Unknown on from the just-a-crashed-ring-station dept.

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."

42 of 51 comments (clear)

  1. How about... by fyngyrz · · Score: 1

    ...a collapsed ring system?

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    1. Re:How about... by fyngyrz · · Score: 1

      Guess I should have been a little more explicit. I meant, as distinguished from one that required another object impact. Just an original ring system.

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    2. Re:How about... by Anonymous Coward · · Score: 3, Informative

      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'.

    3. Re:How about... by neonv · · Score: 1

      The wikipedia article discusses different theories of the mountain range formation. A collapsed ring system is one of them. The article here is introducing a new theory.

    4. Re:How about... by wonkey_monkey · · Score: 2

      See a doctor.

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    5. Re:How about... by Jarik+C-Bol · · Score: 4, Insightful

      No its not, the article talks about the idea that a collision with another body both caused the inclined orbit of Lapetus, and created enough ejected materials to form a unstable ring system that fell back to the surface, causing the fancy walnut shaped moon we all know and love. The article is more about how they gathered and interpreted the data that re-enforces this hypothesis. If saturn where solid, I'm sure it would have a similar mountain range, or at least an equatorial band of impact craters from parts of its ring system that have fallen over the millennia.

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    6. Re:How about... by icebike · · Score: 2

      Exactly, this article essentially is an elaboration on the Wiki Article's theory # 3.

      They make much of the angle of repose, but the angle of repose is not a constant. Gravity of the planet/moon affects this angle, (which I am sure they accounted for), but so does the water content, or any other potentially binding agent (frozen CO2, etc) of the material. Even the shape of the grains of sand can affect the Angle of Repose.

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  2. Er... by Anonymous Coward · · Score: 1

    "That's not proof but it certainly consistent with this exotic formation theory."

    So they didn't solve the puzzle.

    1. Re:Er... by Shatrat · · Score: 1

      They solved the puzzle, but didn't compare their solution to the one printed upside down on the bottom of the page yet.

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  3. Doesn't Gravity Affect Angle of Repose? by Toad-san · · Score: 1, Interesting

    Iapetus has only a fraction of Earth's gravity (Iapetus radius 735 KM, Earth radius 6371 KM, you do the math, after figuring out the relative density for yourself). Wouldn't a hugely smaller gravity significantly affect the angle of repose they carry on about in that referenced scientific paper? I doubt you can compare the angle of repose of rounded particles (or snow and hail) on Earth with that of a very small _and airless!_ moon.

    But I'll leave that to the astrophysicists to work out.

    1. Re:Doesn't Gravity Affect Angle of Repose? by RavenousRhesus · · Score: 5, Insightful

      Iapetus has only a fraction of Earth's gravity (Iapetus radius 735 KM, Earth radius 6371 KM, you do the math, after figuring out the relative density for yourself). Wouldn't a hugely smaller gravity significantly affect the angle of repose they carry on about in that referenced scientific paper? I doubt you can compare the angle of repose of rounded particles (or snow and hail) on Earth with that of a very small _and airless!_ moon.

      But I'll leave that to the astrophysicists to work out.

      From the references in that exact article you criticize (but clearly didn't read):

      "Kleinhans, M. G., Markies, H., de Vet, S. J., in 't Veld, A. C., Postema, F. N., 2011. Static and dynamic angles of repose in loose granular materials under reduced gravity. Journal of Geophysical Research 116, E11004."

      So, yes, I'd say they did take into account the low gravity.

    2. Re:Doesn't Gravity Affect Angle of Repose? by danbert8 · · Score: 3, Informative

      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/...

      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.

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    3. Re:Doesn't Gravity Affect Angle of Repose? by wonkey_monkey · · Score: 1

      But I'll leave that to the astrophysicists to work out.

      Why do you presume they didn't take it into account already?

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    4. Re:Doesn't Gravity Affect Angle of Repose? by Jarik+C-Bol · · Score: 1

      probably, but at the same time, the low gravity does allow for the fallen ring system to have built up into a mountain, instead of pounding a long valley around the equator in the form of impact craters.

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    5. Re:Doesn't Gravity Affect Angle of Repose? by almitydave · · Score: 1

      In case someone were wondering, Iapetus' equatorial surface gravity is about 2.3% that of Earth's, or 1/43rd as strong. https://en.wikipedia.org/wiki/Iapetus_(moon)

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    6. Re:Doesn't Gravity Affect Angle of Repose? by icebike · · Score: 1

      Yes, but his experimental platform is far from perfect, wouldn't you agree?

      He's talking parabolic flights in powered aircraft, which lasts, what, maybe 30 seconds, and could not easily be shielded from all sorts of vibrations.

      So the good geologist's work probably can't account for a moon-sized platform, or a mixed particle size, or the inclusion of water ice, etc. The angles do vary with gravity, grain size, grain polishing, binding agent inclusion, etc.

      Still the Subject study uses a wide definition of the Angle of Repose ("anything between about 25 to 40 depending on size and type of particles involved."), and they suggest that all (or most) of these observed ridge shapes fit within that definition. While they mention water ice in their theory they don't seem to acknowledge its ability to drastically increase the Angle of Repose, easily up to 60 degrees in terrestrial gravity).

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    7. Re:Doesn't Gravity Affect Angle of Repose? by Toad-san · · Score: 1

      The article I read. The references I read, but didn't look up (since I don't subscribe to the Journal of Geophysical Research). If they were using reduced gravity data, they should've said so in the body of the article. They didn't. That's my point: we don't know WHAT they took into account.

    8. Re:Doesn't Gravity Affect Angle of Repose? by RavenousRhesus · · Score: 1

      I didn't read the reference material either. All I did is read the reference section and see that they are references a paper with "granular materials under reduced gravity" in the title. It's by no means a stretch of the imagination to assume that this scientific paper actually uses the results of a referenced scientific paper in its own analysis. That's kind of how the process works.

  4. Nope by RavenousRhesus · · Score: 1

    Erroneous. It was our space overlords running an experiment.

    1. Re:Nope by Anonymous Coward · · Score: 1

      Looks more like our space overlords were using a mold and didn't clean up the seam.

  5. Solved? by Arker · · Score: 1

    Does not sound like they solved it. Headline should be "Astronomers Ponder Puzzle..." perhaps?

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    1. Re:Solved? by radtea · · Score: 4, Insightful

      Does not sound like they solved it. Headline should be "Astronomers Ponder Puzzle..." perhaps?

      No, it should be, "Astronomers Increase Plausibility of Exotic Formation for Iapetus Mountain Range".

      "Proof" is not something science does. Nor does it do "disproof", despite Karl Popper's well-marketed myth of method.

      Science is the discipline of publicly testing ideas by systematic observation, controlled experiment and Bayesian inference, and the only thing Bayesian inference can ever do is increase or decrease the plausibility of some proposition or propositions. Plausibilities range between epsilon and omega = 1 - epsilon (0 and 1 are epistemic errors, the term for which is "faith").

      So in this case they have done more than "pondering the puzzle": they have contributed to knowledge (which is by its nature uncertain) by increasing the plausibility of the proposition that these mountains "fell from space".

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  6. Iapetus by rossdee · · Score: 1

    "Iapetus, Saturn's third largest moon, was first photographed by the Cassini spacecraft on 31 December 2004."

    Really? First photographed in 2004? Didn't any of the earlier probes like Voyager take pictures of it?

    I seem to remember something about strange light and dark patterns on that moon from a book i read many years ago.
    I think it was written by a "distinguished but elderly scientist"

    1. Re:Iapetus by tyme · · Score: 4, Funny

      Yes, Iapetus was photographed by Voyager 2 in 1981 (link to NASA image with metadata listed), and I would suspect that there were earth based images taken well before that (but none that would show any detail).

      Who would have expected a summary on Slashdot to be carelessly wrong about something factual and easily verified?

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    2. Re:Iapetus by cusco · · Score: 2

      New year's eve 2004 was the first time that Cassini imaged Iapetus. At least that was how I read the sentence.

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    3. Re:Iapetus by mbone · · Score: 1

      Yeah, me too. It could be ambiguous, but you kinda have to work at it.

    4. Re:Iapetus by radarskiy · · Score: 1

      Your errors in parsing the date clause notwithstanding, which has been covered by others, the latter part of your comment is also off point: Clarke was not working from pictures of Iapetus. He completely made up his description, based on zero evidence, which just happened to turn out to me similar to the actual two-tone coloring. No monolith, though.

    5. Re:Iapetus by R3d+M3rcury · · Score: 1

      It was first photographed by the Cassini spacecraft on 31 December 2004. It was first photographed by any spacecraft in 1981.

    6. Re:Iapetus by blue+trane · · Score: 1

      Clarke was probably aware of the following (from wikipedia's article on Iapetus):

      In the 17th century, Giovanni Cassini observed that he could see Iapetus only on the west side of Saturn and never on the east. He correctly deduced that Iapetus is locked in synchronous rotation about Saturn and that one side of Iapetus is darker than the other, conclusions later confirmed by larger telescopes.

  7. Like 1999 KW4 by mbone · · Score: 1

    You mean, like the asteroid 1999 KW4 ? I'd say that the source of the Iapetus ridge has been pretty obvious since the Science papers on that body.

  8. Re:medium.com by mbone · · Score: 1

    Just read the original paper.

    (Yes, I know it was one of the links in the OP, but...)

  9. Re:Simple answer? by mbone · · Score: 3, Insightful

    Saturn's ring material falling onto the Iapetus. This "mountain range" is technically an equatorial ridge, but as anyone who's seen an hour glass it's not hard to imagine (-- disclaimer) the same thing is happening on the moon of a planet with it's own ring system.

    No. In that theory, the satellites interior to Iapetus, i.e., Mimas, Enceladas,Tethys, Dione, Rhea and (maybe) Titan would all have similar equatorial ridges, which they do not.

  10. Quite interesting by 140Mandak262Jamuna · · Score: 4, Informative
    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. ). 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".

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    1. Re:Quite interesting by dpiven · · Score: 1

      we normally do not see 1300 km long objects in space that are just 10 to 15 km in diameter.

      Space-whale turds?

    2. Re:Quite interesting by Anonymous Coward · · Score: 1

      Hmm...

      You are explicitly assuming large objects "1300 km long objects" - whereas they are discussing much smaller objects, most may be much smaller than a metre, probably smaller than sand grains.

      The exact size is not too crucial, but at any rate much much smaller than 1300 km long.

      They are definitely not saying each mountain is a separate large object, but built up of many much smaller objects.

    3. Re:Quite interesting by 140Mandak262Jamuna · · Score: 1
      That moon is too small to have strong gravitational field gradient to stretch any object into a long string.

      The best source of large number of smaller meter sized rocks aligned in a long line is the rings of Saturn nearby. Since the moon is tidally locked to Saturn, and its orbit is oblique, if it passes the rings it would possibly pass at the same angle and same orientation every time. If it keeps picking up stuff from the rings, it could provide the source rocky rain drops all meter size or smaller that all will accumulate at the same place. It gels with their theory. Need to go back and read to see if they were speculating the rings to be the "source" of mountain that fell from the sky.

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  11. Re:Medium.com total fail by wonkey_monkey · · Score: 1

    If only.

    It seems to be catching on around teh interwebs.

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  12. Roche limit by micahraleigh · · Score: 1

    The summary seems to reflect the way 2 bodies in space interact when they pass the Roche limit and the smaller body dissolves due to tidal forces.

  13. Or by koan · · Score: 1

    Since it's composed of ice something from the center, (water) ejected leaving a cavity and the 2 halves "pinched" and extruded a ridge.

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  14. Plea to God by Tablizer · · Score: 1

    Do it again, do it again!

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  15. have a nice day www.blossomsquare.com by myblossomsquare · · Score: 1

    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.

  16. "Iapetus, Saturn's third largest moon, was first p by myblossomsquare · · Score: 1

    "Iapetus, Saturn's third largest moon, was first photographed by the Cassini spacecraft on 31 December 2004. The images created something of a stir. good job blossomsquare.com