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Thread: The gas giants.

  1. #1 The gas giants. 
    Forum Ph.D. Cat1981(England)'s Avatar
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    I have two questions about the gas giants if someone would be kind enough to answer them.

    1) Why didn't the gas's hydrogen and helium move into the centre of the solar system, or at least get evenly distributed among the other planets ?

    2) Why is Saturn's rings larger than the others ?

    Thank you.


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  3. #2 Re: The gas giants. 
    Universal Mind John Galt's Avatar
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    Quote Originally Posted by Cat1981(England)
    1) Why didn't the gas's hydrogen and helium move into the centre of the solar system, or at least get evenly distributed among the other planets ?
    I think you will be familiar with the currently accepted theory for the formation of planetary systems. It is modified from LaPlace's Nebular Hypothesis. In this, a collapsing cloud of dust and gas forms a proto-star whose heat and light are derived from the kinetic energy of the collapsing cloud. Much of the dust and gas forms an acccretion disc about the star. This disc is composed initially of the same materials, in the same proportion, as in the proto-star.
    The heat generated by the proto-star, and to a lesser extent by the collapse of cloud to disc, raise the temperature dramatically. Since the proto-star is the primary source of heat there is a temperatures gradient across the width of the disc. Close in it is a couple of thousand degrees, so that no solid material can condense, but gradually the temperature drops and grains of material begin to form. (Chondritic meteorites are formed at this stage, for example.) Further out, beyond the 'ice line' water and ammonia and other ices can condense.
    The particles that are condensing anywhere in the nebula begin to clump together, into progressively larger lumps, then in to planetesimals and finally planets.
    In the region where the giant planets form a point is reached where the mass of the planet is sufficient to create a runaway accretion effect, sweeping the 'feeding zone' clean of gas. This is how Jupiter likely formed.
    Meanwhile the proto-sun has now condensed enough for nuclear fusion to begin at its heart. At this stage the sun blows off large quantities of material as a solar wind, that sweep the inner solar system free of gas and most of the dust. So we are left with small, hard solid bodies in the inner system and large, gaseous or icy bodies in the outer system.

    Quote Originally Posted by Cat1981(England)
    2) Why is Saturn's rings larger than the others ?
    Rings are produced when a satellite breaks apart because of excessive tidal forces. The material in the rings dissipates over periods of millions of years. Saturn's rings are currently large because they have formed comparatively recently in an astronomical sense.


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    Forum Ph.D. Cat1981(England)'s Avatar
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    Brilliant. That makes the whole thing clearer, thank you.
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  5. #4  
    Time Lord
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    A simpler reason is that higher gravity makes it possible to hold onto their light gases better. The main reason Mars has hardly any atmosphere is because it's gravity is too weak to keep the solar wind from blowing it out into space.

    The Earth has a magnetic field that helps in this regard, but I'm sure we'd more atmosphere too if our gravity were higher.
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    Brilliant.
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  7. #6  
    Universal Mind John Galt's Avatar
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    Quote Originally Posted by kojax
    A simpler reason is that higher gravity makes it possible to hold onto their light gases better. The main reason Mars has hardly any atmosphere is because it's gravity is too weak to keep the solar wind from blowing it out into space.
    This is a valid point. Current theory suggests that runaway accreitonof the gas envelopes of the giant planets will begin when the core mass is around 5- 10 times the mass of the Earth.
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