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Did the Moon make life possible on Earth ?

  1. Feb 29, 2004 #1
    Would life have started on Earth without the Moon being around ?

    Our planet is unique in that it has such a sizable moon with which it has interacted for aeons.

    Many people assume that the Moons' gravitational force is merely limited to effecting the ocean tides, but the tidal effect goes to the very core of the Earth, causing our planet to ocillate in its orbit.

    On top of that the Moon has probably acted like a relatively last line of defence against in falling meteors and comets. ( much like Jupiter but on a smaller and more intimate level ).

    So my question is, would life as we know it have started, continued and evolved if there wasn't a moon orbiting the Earth ?

  2. jcsd
  3. Mar 3, 2004 #2


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    I don't think there's anyway to know with any certainty at this point since we don't know the details of how life started. But as you suggest, the Moon offers some possible benefits. One good possibility is the Moon's creation of significant ocean tides (hypothesis that life might have started in tide pools). The last line of defense against asteroid/comet impacts does help, but it is certainly a small shield.

    Well, so does Pluto. And we have yet to find any other Earth-sized planets in the universe, so there's no way to compare.

    Started? Good question.
    Continued and evolved? Yes.
  4. Mar 3, 2004 #3


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    Ward and Brownlee in their book "Rare Earth" devote quite a few pages to a similar discussion.

    They conclude, IIRC, that the main contribution of the Moon to life on Earth was (and still is) stabilising the spin axis so there have been no 'snowball Earth's' (Earth covered in glaciers; all surface water frozen) since the pre-Cambrian. This allowed complex life (multi-cellular organisms) to become established on land, and evolve into the diversity we obserse today.

    The Moon's role in shielding the Earth from bombardment was non-existant for the first several hundred million years; the surface of the Earth was repeatedly reduced to a magma ocean several (hundred?) km deep in this period of late heavy bombardment.

    Subsequently, the Moon may have had a modest effect in reducing KT-type events, but Jupiter (and to a lesser extent Saturn) were - and still are - far more important in this regard.

    Does the Moon play a role in keeping plate-tectonics going? Maybe, but it's a minor one. (Plate tectonics are essential to keep the carbon cycle going, and for creating a reasonable amount of land).

    Once we know about life on Mars - when it began, is it still there, how complex it got before the surface water disappeared, etc - we'll have a better handle on the ubiquity of life, the rarity (or otherwise) of complex life, and what conditions may be necessary for both.
  5. Mar 4, 2004 #4
    I fail to see that logic. A highly oblique spin axis would only aggravate seasons, colder winters (but consequently less precipitation) and hotter summers. The exteme 90 degrees would give a days year for the whole planet. No change for ice sheets to build, since they would melt and dry completely in the all day summer.

    A perfectly perpendicular spin axis in relation to the plane of orbit would never bring the poles into brighter sunlight. It would always be winter and ice sheets could grow.
  6. Mar 4, 2004 #5


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    I don't have my copy of "Rare Earth" to hand, so I'm going by memory ... the greater the tilt, the more extreme summers and winters are, and extreme winters allow ice to build up (which doesn't all get melted in the summers) - it's not symmetrical.

    Anyone got the book?
  7. Mar 4, 2004 #6
    Thanks folks. Sounds like the jury is still out.
  8. Mar 5, 2004 #7


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    This is science; the jury is always out.
  9. Mar 5, 2004 #8
    Well I found some judge, Milutin Milankovitch:

    There is your snowball world. For the record, the Milankovitch-Croll forcing ideas were sound in themselves so the jury has found the theory guilty for the cause of the ice ages. However, some scientists have appealed since the harsh reality showed that much evidence was unaccounted for. So the science suppreme court of Appeals will be busy for a few more decades to come.

    It is true of course that the more tilt, the more the total radiation is evened out over the the planets surface. But the physical properties of the atmosphere would most likely prevent ice sheets from building up. During the long polar winter night it is too cold to snow whilst during the long tropical summer day there is ample energy to melt all of it and probably turn the area into a simmering inferno.
    Last edited: Mar 5, 2004
  10. Mar 5, 2004 #9


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    not snowball, just extreme

    I did find one webpage which purports to summarise the key ideas in Ward and Brownlee's "Rare Earth"

    "An important consequence of our giant Moon is that it stabilizes the 23º tilt of the Earth's rotational axis with respect to its orbital plane. Geological evidence indicates that over many millions of years the tilt of the Earth's axis has stayed within a few degrees of its present value. Recent calculations have shown that without the Moon, the gravitational effects of Jupiter and the Sun would have caused the Earth's tilt to wander chaotically over a wide range, producing enormous changes in climate and a hostile environment for the development of complex life."

    So, the Moon is important for establishing and maintaining an environment in which complex life can exist (according to Ward and Brownlee).
  11. Mar 6, 2004 #10
    I don't know. Most planets except Uranus and Pluto, I believe, have inclinations less -often much less- than 30 degrees but none are stabilized by a sizable moon compared to the planet.

    A spinning planet acts as a gyroscope, determined to stay upright. So is the orbit of the planet stabilized as well by the same effect regardless of the orbit of the sun in the galaxy and the movement of the galaxy itself. In classical Newtonian, the only change to disturb that stabilization is exerting torque forces. As gravity acts differential on the equatorial bulge, precession is the logical result. In that respect the moon is trying to destabilize the Earth, and not stabilizing.

    So to answer the original question of the thread. I don't think so.
    Last edited: Mar 6, 2004
  12. Mar 6, 2004 #11


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    The phrase "recent calculations have shown ..." likely means some team put an awful lot of effort into accurately modelling the effects on the Earth's spin axis (and orbit, no doubt) of the Sun, Moon, and major planets (in this case, Venus, Mars, Jupiter and maybe Saturn). If they found that, without the Moon, the Earth's spin axis wasn't stable - over 10? 100 million? 1 billion? years - it's likely that Mars' isn't at all. Jupiter and Saturn may well be too massive (or the thermal atmospheric tides too weak), Mercury is locked, and we're already discussing Venus in another thread.

    Can anyone find a paper reporting the 'recent calculations'?
  13. Mar 6, 2004 #12
    Two points:

    (One) No-one has described or measured the axial/rotation displacement of the Earth by the Moon ? As the Moon Orbits the Earth, it pulls the Earth slightly off it's centre of rotation, inducing a little wobble. Not to be confused with precession.

    (Two ) In connection to this effect, the Moon has a tidal / friction effect on the Earth. This friction/tidal effect must consequently generate tremendous amounts of heat which possibly contribute to the heating of the centre of the Earth.

    To best picture the effect, imaging a glass half filled with water. Using a circular motion to swill the glass around in your hand, causes the water to climb up the side of the glass.

    In the same way, the rotational displacement of the Earths' molten interior in combination with the tidal effect of the moon may be part of mystery as to how the Earth sustains so much interior heat relative to its age.
  14. Mar 6, 2004 #13


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    Would you like to give a rough OOM (order of magnitude) estimate of the amount of heat generated by the effect? You may wish to make two estimates - one which is surely below the amount of heat generated, and the other very likely above. If you need some input parameters - e.g. density of the Earth's core, deviation of the geoid from a perfect sphere - I'm sure Andre and I (at least) will be only too pleased to help find them for you.
  15. Mar 7, 2004 #14
    Nice of you to ask but I am not trained in these matters.

    Nevertheless my curiosity is genuinely aroused. Has any scientist worked out how much of an axial tug the Moon has on the Earth ?

    Is it several miles / kilometers ? Is it less ?

    Anyone have a figure..?

    Surely someone has it at their fingertips.



    I clearly remember astronomers discussing how the Earth is continually pulled somewhat off it's line of orbit as it moves around the Sun; and that this phenomenon was caused by the Moon.

    It has always stuck in my mind as a little bit of a curiousity that just wouldn't go away. :smile:
    Last edited: Mar 7, 2004
  16. Mar 7, 2004 #15

    Your "recent calculations" are not that recent:


    Well those guys studied the case. I merely gave my unsophisticated guess :wink:


    Calculating (or rather guestimating) the mutual centre of gravity of the Earth-Moon system should not be that difficult. High school math, I think. Consider Earth and moon point masses on the opposite ends of a weightless rod. The balance point of that rod can be calculated: mass1*dist1=mass2*dist2. We know the masses and the total distance. You can take it from here. The answer should be some thousand miles below the Earth surface.
  17. Mar 7, 2004 #16
    I assue that this question can be answered by using both conservation of (angular) momentum and conservation of energy.

    Take two situations, the present one and a hypothetical one, a couple of thousand years ago. We know either the receeding speed of the moon to be about an inch or so per year and/or the loss in rotational speed of Earth together with the usual numbers for masses angular speeds and distances.

    We can come up with the total angular momentum (sums of Iw)that should be equal in both situations. We can also calculate the total energy (sums of 1/2Iw^2) in both situations and those are most likely to be different. The difference being the friction that is generated due to the combined effect of the tidal friction and the heat generated in the core. It should bigger than the average heat loss of the Earth lithosphere in that same period.

    Not difficult, I guess, but a lot of sorting out to do.

    But good thinking Auqafire. Have been there too a couple of years ago and used it to crackpot my RTPW hypothesis as discussed here.
  18. Mar 7, 2004 #17


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    Chaotic spin-orbit resonances

    Looking around for papers similar to the Laskar, Joutel & Robutel one (which doesn't seem to be online :frown: ), I found Murray and Holman Role of Chaotic Resonances in the Solar System. An extract (I've deleted the references; my emphasis):

    "Because solar tides are so weak, dissipative effects tend to be less important for planetary spins, allowing for richer dynamics. Ward showed that the angle between the spin and orbital axes of Mars (the obliquity) varies by +/-13.6o around its average of 24o over millions of years. These variations are a result of a resonance between the precession of the spin axis and a combination of orbital precession frequencies; improvements in orbital models resulted in an increased variation of +/-20o (ref. 9). Numerical integrations then showed that the obliquity of Mars is evolving chaotically, and varies over an even larger range. Such a variation has profound, but as yet poorly understood, implications for climate variation. It is likely that the spin axes of Mercury and Venus underwent chaotic variations in the past.

    The tilt of the Earth, currently 23o, will also increase in the future; the Moon will evolve outward under the infuence of the tides, resulting in a decrease in the precession rate of the Earth. Eventually the precession rate will become resonant with yet another combination of orbital precession rates. Once again, numerical integrations show that the Earth's obliquity will vary chaotically. The tilt of the Earth's axis may increase to 90o. The effect on our climate is hard to estimate, but the result is unlikely to be pleasant."

    [Edit: second para of extract added]
    Last edited: Mar 7, 2004
  19. May 19, 2004 #18
    u ppl have too much time on ur hands. i mean this sounds very interesting andn everything, but seriously, what does it matter? :confused:
    Last edited: May 19, 2004
  20. May 20, 2004 #19
    Welcome, Horsedancer

    What it seriously matters? A very philosofical question. Well most people like to find out. Find out everything there is to find out. Several methods have been develloped as guidelines to find out. There is religion that is explaining the unexplainable, there is pseudo science and science fiction that has no problem with the unknown. And there is science that tries methodically to explain everything, omitting/rejecting unphysical phenomena, using basic analytic models. But there is a lot to sort out.
  21. Jun 16, 2004 #20


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    Another thing to keep in mind is that it might not always be possible for mankind to carry out investigations of this sort, some of which imply that the habitability of our planet, or at least large surface regions thereof, may not remain as climatically stable as it has been for the relatively brief span of recorded history. The work done now in gathering information on asteroid impact probabilities and effects, changes in Earth's obliquity, resumptions of Ice Age glaciation, and the like, can at least help people in the post-industrial age (the future following the end of fossil fuel energy) to understand some of the cosmic threats that they might potentially face.

    Jerry Abbott
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