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Thread: Planet Venus, retrogard rotation, why?

  1. #1 Planet Venus, retrogard rotation, why? 
    The Enchanter westwind's Avatar
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    The Planet Venus rotates on it's axis so that the Sun rises in the West and sets in the East. ( Google information. Planet Venus ). Many years ago I was browsing amongst Magazines and came upon an article stating the Theory that Venus was a captured Planet in our Solar System. Having entered within the Gravitational influence of mass within our Solar System it was drawn into its present Orbit around the Sun, almost circular. Can any member update this Theory? westwind.


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    Quote Originally Posted by westwind View Post
    The Planet Venus rotates on it's axis so that the Sun rises in the West and sets in the East. ( Google information. Planet Venus ). Many years ago I was browsing amongst Magazines and came upon an article stating the Theory that Venus was a captured Planet in our Solar System. Having entered within the Gravitational influence of mass within our Solar System it was drawn into its present Orbit around the Sun, almost circular. Can any member update this Theory? westwind.
    That's pretty unlikely. Every object captured within the inner solar system (such as long period, Oort Cloud comets, which become Halley type comets, then Jupiter family comets before hitting something or being ejected) have very eccentric orbits. It's hard to reconcile that with the almost perfectly circular Venusian orbit.

    There are two predominant theories. The more popular is that, much as earth was hit by a large impactor, creating the debris that became our Moon, Venus was also clobbered. The object hit at a different angle, so rather than speeding up the planet's rotation as happened to us, it slowed it down enough to reverse it. Because of the angle, and solar effects, no ring was formed to create a moon.

    The more complicated theory holds that with the lack of a moon, gravitational effects from all the other planets, combined with a slow initial rotation rate, and solar heating of it's atmosphere, it actually flipped from one stable state (prograde rotation) to the other almost equally stable state (retrograde rotation). I recall reading an interesting paper when that theory was first proposed a few years ago.

    Do we know for sure? Not yet, it's still a mystery of the solar system until we have more data (evidence) and perhaps better mathematical models.


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    Administrator KALSTER's Avatar
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    If I am not mistaken, I don't think it is impossible for a planetary disc to spin in a retrograde direction from the beginning. An impact might simply have slowed it down, instead of reversing the direction in that case.
    Disclaimer: I do not declare myself to be an expert on ANY subject. If I state something as fact that is obviously wrong, please don't hesitate to correct me. I welcome such corrections in an attempt to be as truthful and accurate as possible.

    "Gullibility kills" - Carl Sagan
    "All people know the same truth. Our lives consist of how we chose to distort it." - Harry Block
    "It is the mark of an educated mind to be able to entertain a thought without accepting it." - Aristotle
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    Universal Mind John Galt's Avatar
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    I lean strongly to the impact hypothesis. Mercury was probably hit by a large proto-planet that stripped away much of its mantle, making it unusually dense.
    Earth, as mentioned, was probably struck by Theia and from the debris the moon formed.
    The Martian crustal dichotomy was possibly due to a massive impactor in the northern hemisphere.

    Impacts seem the norm. The early solar system was a violent place.
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    The Enchanter westwind's Avatar
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    For John Galt. Looking at your observation concerning the Martian crustal dichotomy, ( I looked in my Grong Grong dictionary for your location description but came away none the wiser ), still, I'll take a chance that you are referring to the northern hemisphere of Planet Mars, land surface. When I studied the images returned from the Mars robotic cameras, I couldn't help but wonder about the shattered basalt rocks. To me, they appeared as rock of a similar nature would if blasted by explosive in a quarry here on Earth. I live close to an old Quarry, Basaltic Lava Flow, that has been quarried for much of victorian Melbourne and its early roads. The resultant debris that remains could be on the Martian Surface. westwind.
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  7. #6  
    Universal Mind John Galt's Avatar
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    You inventive attempt at understanding my shorthand description is regretably flawed.

    The Martian crustal dichotomy refers to a broad difference in character between(roughly) the northern and southern hemispheres of Mars. The north is young, consists of lowlands, has a thin crust and few craters. The south is old, with highlands, has a thick crust and many craters. It has been suggested that this was a consequence of the impact of a proto-planet in the north, though other explanations exist. The minor debris you refer to is not connected with this in any significant way.

    Here are some comments I prepared earlier:

    The Martian Dichotomy
    The Martian topography falls into two quite distinct regions. “The dichotomy is expressed three ways: as a change in elevation, a change in crustal thickness, and a change in crater density.”[i] The difference in crater density is a reflection of differing ages, a concept and dating technique established during the pre-Apollo lunar mapping[ii].

    (As with all generalizations we can nitpick here if we wish. For example, topographic and gravity data determined by the Mars Global Surveyor revealed two distinct crustal types that did not exactly match the dichotomy.[iii])The southern highlands are exactly that – high. They occupy about 60% of the planet. They have an uneven crust that thins progressively northwards. They are heavily cratered and they are old. The northern plains also live up to their name. They are low; they have a thin, uniform crust; they have few craters; they are young. (But see the discussion in Northern Hemisphere Plate Tectonics below).

    Martian researchers recognize that the biggest challenge facing them is to adequately explain this dichotomy. Numerous hypotheses have been put forward, both exogenic and endogenic.

    Collisional Excavation:
    This is the same mechanism that created the lunar highlands and the low-lying Mare, subsequently infilled by flood basalts
    [iv]. During the latter stage of major planetary accretion, possibly during the Heavy Bombardment Phase, chance (or some presently unknown mechanism) concentrated the impacts on the northern half of Mars, thinning the crust by removal of substantial volumes of crustal ejecta, either by a single giant impact[v], or multiple smaller impacts[vi]. The single giant impactor hypothesis has recently been revisited[vii].

    Mantle Convection
    A number of researchers have proposed that the Martian crust was originally a similar thickness over the entire planet. Thinning of the northern hemisphere occurred as a result of a) a long wave length mantle convective planform
    [viii]; b)erosion of the base of the crust by convection[ix],[x]; c) crustal subduction during an early phase of plate tectonics[xi]; d) post accretion core formation[xii]. Options a) and b) appear to me to remain valid possibilites. c) is discussed in more detail below. d) is rejected since it postulates a lengthy (1 billion year) core formation process which is at odds with the observed residual magnetic fields (see next section) and the U/Pb isotopic ratios of SNC meteorites[xiii].

    Possible Plate Tectonics on Mars


    Southern Hemisphere Plate Tectonics
    During the aero-braking maneuver to place it in orbit, Mars Global Surveyor detected linear magnetic anomalies in the southern hemisphere of Mars[xiv]. Researchers, using the data from two full Mars years of polar orbital data and with an improved technique to eliminate the effects of external fields, constructed a global map of remnant crustal magnetization[xv]. (I note in passing that the strength of this magnetism is more than an order of magnitude greater than comparable magnetism on the Earth.)

    The similarity between these parallel patterns of magnetic reversal on Mars and those flanking mid-ocean ridges on Earth, immediately suggested a Martian equivalent to sea floor spreading. The patterns are absent from large impact basins, such as Hellas and Argyre, and from the volcanic terrain of the Tharsis bulge. This is consistent with the loss of the Martian dynamo early in history, before the final Heavy Bombardment Phase and Tharsis volcanism, but after crustal formation.


    The researchers provisionally identified two major faults from offsets in the magnetic patterns. The character of these faults shows them to be transform faults, rather than simple strike slip faults. “The great faults in Meridiani are consistent with the properties of transform faults and define an axis of rotation (23°S and 80.5°E) describing the relative motion of two plates, north and south of the equator. The separation of the faults (1,200km) and offset of the putative ridge axis (+/-240km) in Meridiani are comparable with what is observed along ocean ridges on Earth.”


    (Continued in next post.....)


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  8. #7  
    Universal Mind John Galt's Avatar
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    The Sound of One Hand Clapping

    Zhong’s
    [1] explanation for the dichotomy is that uneven convection in a young Mars thinned the northern crust. In the south, hot mantle material rose, cooled, and then sank in the north.

    ·
    In 2001 Zhong reported that conventional models of mantle convection could not account for an internal explanation of the dichotomy. However, a thicker and weaker asthenosphere would allow a single convection cell, which in turn might allow the apparent thinning of the northern crust by mantle convection currents, or build up the crust in the southern hemisphere.

    ·
    In 2004, working with Roberts[2], he is struggling to generate the required degree-1 convection cell within a stagnant lid setting. They broach the possibility that “The mobile lid regime may be appropriate if the crust and mantle are sufficiently warm as to decouple the crust and the mantle, as one may expect for the early Mars”.
    · In 2006, again with Roberts[3], Zhong reached the conclusion, based on further FEA modeling, that “degree-1 mantle convection induced by a layered viscosity structure may be responsible for the formation of the crustal dichotomy.”
    · Also in 2006, in another paper, they offered a description of the process “Degree-1 mantle convection develops within the first few hundred Ma. The one-plume structure drives a TPW event that places the plume near the equator. Melt associated with the plume is erupted onto the surface above it, thickening the crust in that hemisphere. This melt cools in the ancient global magnetic field and produces remnant magnetism, consistent with suggested paleopole positions near the present-day equator. As the planet cools the lithosphere thickens, reducing the dynamic topography. When Te exceeds about 30km, the geoid above the plume becomes negative and the plume rotates the planet such that it is near the south pole.”

    ·
    Roberts and Zhong[4] continue this line in 2007, declaring “that the crustal thickness variations associated with the dichotomy may have driven true polar wander, establishing the north south orientation of the dichotomy very early in Martian history.” In other words, the entire crust was moving relative to the mantle.
    · Also in 2007 Zhong[5] argues the case for degree-1 convection, over the alternate endogenic hypotheses (magma ocean[6]; ‘conventional’ plate tectonics[7]).

    (Note: I have referenced only about half of the papers Roberts and Zhong wrote on the subject over this period. A search of the ADS data base will reveal the rest.)


    References for 1st Post

    [i] McFadden. L-A, et al (eds) (2007) Encyclopedia of the Solar System Academic Press p.319
    [ii] Schumaker, G. et al (1960) The Moon
    [iii] Zuber, M.T. et al (2000) Internal Structure and Early Thermal Evolution of Mars from Mars Global Surveyor Topography and Gravity. Science, Volume 287, Issue 5459, pp. 1788-1793.
    [iv] Basaltic Volcanism Study Project (1981) Basaltic Volcanism on the Terrestrial Planets. Lunar and Planetary Institute, p236.
    [v] Wilhelms, D.E. & Squyres, S.W. (1984). The Martian hemispheric dichotomy may be due to a giant impact. Nature 309, 138–40.
    [vi] Frey, H. & Schultz, R.A. (1988). Large impact basins and the mega-impact origin for the crustal dichotomy on Mars. Geophys. Res. Lett. 15, 229–32.
    [vii] Nimmo, F. et al (2008) Implications of an impact origin for the martian hemispheric dichotomy Nature, 453, Issue 7199, pp. 1220-1223.
    [viii] Schubert, G. & Lingenfelter, R.E. (1973) Martian centre of mass – centre of figure offset. Nature 242, 251–2.
    [ix] McGill, G.E. & Dimitriou, A.M. (1990) Origin of the Martian global dichotomy by crustal thinning in the late Noachian or early Hesperian. J.Geophys.Res. 95, 12595–605
    [x] Smith, D.E. (1999) The global topography of Mars and implications for surface evolution. Science 284, 1495 – 503.
    [xi] Sleep, N.H. (1994) Martian plate tectonics. J.Geophys.Res. 99, 5639–55.
    [xii] Davies, G.F. & Arvidson, R.E. (1981) Martian thermal history, core segregation, and tectonics. Icarus 45, 339–46.
    [xiii] Chen, J.H. & Wasserburg, G.J. (1986) Formation ages and evolution of Shergotty and its parent planet from U–Th–Pb systematics. Geochim.Cosmochim.Acta 50, 955–68.
    [xiv] Connerney, J.E.P., et al. (1999) Global distribution of crustal magnetization discovered by the Mars Global Surveyor MAG/ER experiment. Science 284, 794–798.
    [xv] Connerney, J.E.P., et al (2005) Tectonic implications of Mars crustal magnetism. P.N.A.S. , 102, 14970–14975

    References for 2nd Post
    [1] Zhong, S. et al (2000) Degree-1 mantle convection and the crustal dichotomy on Mars. Earth and Planetary Science Letters, Volume 189, Issue 1-2, p. 75-84.

    [2] Roberts, J.H. & Zhong, S. (2004) Degree-1 mantle convection as a process for generating Martian hemispheric dichotomy. Workshop on Martian Hemispheres

    [3] Roberts, J.H. & Zhong, S. (2006) Degree-1 convection in the Martian mantle and the origin of the hemispheric dichotomy. Journal of Geophysical Research, Volume 111, Issue E6,

    [4] Roberts, J.H. & Zhong, S. (2007) The cause for the north south orientation of the crustal dichotomy and the equatorial location of Tharsis on Mars. Icarus, Volume 190, Issue 1, p. 24-31.

    [5] Zhong, S. (2007) Understanding the Early Evolution of Mars and the Formation of Crustal Dichotomy. Lunar and Planetary Science XXXVIII.

    [6] Elkins-Tanton et al., (2005) Earth and Planetary Science Letters, 236, 1 –12.

    [7] Sleep, N.H. (1994) Martian plate tectonics. Journal of Geophysical Research (ISSN 0148-0227), vol. 99, no. E3, p. 5,639-5,655
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  9. #8  
    The Enchanter westwind's Avatar
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    For John Galt. OMG. Not an expression of mine, but what else can I say? I call on all other Members to take up the challenge of increasing their understanding and knowledge of the expositions exposed by this Thread. Because of my age time does not allow me to consume this barium meal of contempory knowledge concerning the geo=physics of the Martian surface and atmosphere. You win John Galt. I'll be very wary, in fact, I will tip toe around this particular Forum ( Cosmology ) with such learned Members having the hands on approach and in--depth study of our Solar System. One nervous question if I may, If impacted by a large mass, would this have been sufficient to remove the Martian Atmosphere? westwind.
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  10. #9  
    Universal Mind John Galt's Avatar
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    Quote Originally Posted by westwind View Post
    One nervous question if I may, If impacted by a large mass, would this have been sufficient to remove the Martian Atmosphere? westwind.
    This is an excellent question and I am confident that the answer is a resounding yes. However, there are caveats: there always are. I believe there is still active debate over exactly when and how planets derive their atmospheres. Certainly some is from degassing, some is brought in by cometary and asteroidal impact, some may just be accreted gas. But what are the proportions, and does any of the primeval atmosphere survive the rigours of the accretion process? These are all intersting thoughts. Let me do some literature searches and come back to you in a day, or year, or so.
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