Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Earth period after moon created

  1. Feb 14, 2016 #1
    From http://www.scientificamerican.com/article/moon-life-tides/
    It all started some 4.5 billion years ago when, as theory has it, our nascent Earth was blindsided by a Mars-size planetary embryo, believed to have spun Earth into its initial fast rotation of roughly 12 hours per day.​

    Can someone give a value for the moon's distance and orbital period at the time the moon was formed, and whether of not the mass of the Earth and/or Moon have significantly changed since then.
     
  2. jcsd
  3. Feb 14, 2016 #2

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    Considering you are looking at the collision model for the formation of the Moon, the distance at formation would have been between 0 and 4000km (direct impact or glancing blow). But you can look up how the Moon is retreating from the earth and extrapolate backwards, which is pretty much what any other source would have to do.

    Mass of the Earth and Moon is not usually considered to have changed significantly enough to affect models of the orbit dynamics, not since formation.
    However, what counts as "significant" depends on what you need the information for.
     
  4. Feb 15, 2016 #3
    Hi Simon:

    Thanks for your reply to my question.

    I interpret your quote as an absence of any particular theory to justify the assumption that the current rate of change to the moon's orbit (and earth daily period) can be extrapolated back to the moon origin time. This seems like a big unjustified assumption. The current mechanism for change is the tidal friction of the ocean on Earth. There were no oceans at the time of the moon's origin. Any changes at that time would have to be based on a different mechanism.

    I would like to think about the range of possible tidal patterns at the time of abiogenesis in the context of the possibility those tidal patterns may have influenced abiogenesis.
    I am guessing "significant" means greater than 10%.

    Regards,
    Buzz
     
  5. Feb 15, 2016 #4

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    I would also say that the Earth, in this context, is not usually considered to be flat.
    "significant" means "big enough to be measured" or have an important impact.
    Consider. CO2 levels are less than 10% ... are they insignificant? What would happen to you if your bodybtemperature changed by 8%?

    Tidal friction via oceans is one of the contributing mechanisms, compared to it, the others are usually insignificant. Tides affect everything, not just water.

    Note: At the time the moon formed, the Earth and Moon would have been completely molten, and sterile.
     
    Last edited: Feb 15, 2016
  6. Feb 15, 2016 #5
    Hi Simon:

    Sorry, I lost you. Where does the consideration of a flat Earth come from?

    Regards,
    Buzz
     
  7. Feb 15, 2016 #6

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    Im afraid "think about" is too vague.

    From the link, you seem to want to know the period of the Moon's orbit at the time when the Earth had cooled enough to be covered in water with rna in some of it? About the circumstances in the example there. This would give you a timescale for that sort of abiogenisis. Is this correct?
     
  8. Feb 15, 2016 #7

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    See post #3, where you boldfaced some of what Id said in a quote and then made an interpretation.
     
  9. Feb 15, 2016 #8
    Hi Simon.

    Thanks for your response. I am completely OK with accepting your definition of response. I thought you were asking me what value I thought might be "significant" in the context of scenarios about abiogenesis. My uninformed guess was about 10%.

    I agree with this also. However, It seems likely that the nature of the material that the tides are working on would effect the amount of tidal friction. I think that the changes in shape of the oceans might well also affect the amount of tidal friction. My point, mostly one of disappointment, was that extrapolating backwards based only on the current rate of change in the moon's orbit seems unreliable. I get that this may be the only option because of the limits of current knowledge to use of the basis of a better model.

    I understand this also. I was hoping that some one might have a better estimate of the moon's orbital parameters at the time of the moon's origin based on capture models. I could then extrapolate forward from then as well as backwards from now to get the best estimate available for the moon's orbit at time of pre-cell abiogenesis.

    Regards,
    Buzz
     
  10. Feb 15, 2016 #9
    Hi Simon:

    I apologize for my denseness. I get that the "flat Earth" is some kind of subtle metaphor related to what I quoted and your response, but I don't get the connection.

    Regards,
    Buzz
     
  11. Feb 15, 2016 #10

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    Its not a metaphore. Its an example of the use of the bolded words in another context.
    Concentrate on the bolded words... Ill spell it out: your interpretation is incorrect.

    I am trying to get you to realise what you are asking in the hopes you will clarify your question.
    At the time of the Moons origin, the Moon did not orbit the Earth. This is a collision. There are a number of models for possible collisions, you can pick your favorite.
    I dont think it will help you though... dont you want to know something of the tidal patterns about when life was just about to kick off?

    I also think you need to revisit the timescales involved... the effect the exact shape of the oceans has on tidal drag would be small compared with the overall effect over the timescales involved. In the time of potential abiogenisis, there may have been a resonance where a particular approach is particularly favored by the timing... but over the whole period, the Moon is retreating, so there will be a long time when the month is close to the resonance. Fine grain models of tidal drag could tell you the resonance hit a year (or whatever) later than the course grain model suggests but so what? What timescale is significant here?

    It may help to know your education level.
     
    Last edited: Feb 15, 2016
  12. Feb 15, 2016 #11
    Hi Simon:

    My formal educated ended many decades ago. It did not include anything about orbital dynamics except Newton's Laws. It did not include anything about abiogenesis. I became interested in abiogenesis in 1968 when I read the English translation of Oparin's The Origin of Life.

    POST IN PROGRESS
     
  13. Feb 15, 2016 #12
    Hi Simon:

    My formal educated ended many decades ago. It did not include anything about orbital dynamics except Newton's Laws. It did not include anything about abiogenesis. I first became interested in abiogenesis in 1968 when I read the English translation of Oparin's The Origin of Life.

    You obviously know a great deal more than I do about orbits in general and the origin of the origin of the moon in particular. I much appreciate you patience in continuing this dialog.

    I apologize for misunderstanding the quote in post #3. Now that I have exhibited my confusion, would you please rephrase the quote so I might have a better chance of interpreting it correctly? I think the basis of the misunderstanding are the bolded words. Would you please explain what you intended "not usually considered" to mean?

    I still don't get the "flat Earth" concept in this context. I have tried to combine the phrases "flat Earth" and "not usually considered" into a meaningful sentence with respect to the context of my question, and I am unable to do this.

    I am beginning to get some ideas about how I might clarify my question. I hope to be able to post a rephrasing in a few days.

    Regards,
    Buzz
     
  14. Feb 15, 2016 #13

    davenn

    User Avatar
    Science Advisor
    Gold Member

    point the question back at you .... What do you think could have significantly changed the final mass of the earth or moon since the collision with the other object that formed the moon ?


    Dave
     
  15. Feb 15, 2016 #14
    Hi dave:

    Thanks for y our post.

    My thought was there might have been a lot of additional stuff falling onto the Earth or the Moon. I don't know enough about this topic to make a guess about how much there actually was since the Moon formed. Therefore I can't guess whether the amount exceeds my guess of 10% as sufficient to make a significant difference in abiogenesis.

    Regards,
    Buzz
     
  16. Feb 15, 2016 #15

    davenn

    User Avatar
    Science Advisor
    Gold Member

    Meteorites as rock or dust averages around 50 tons a year (+ - 20 tons) ( not sure how long that goes back for in time, couldn't find a reference)
    but even 50 tons a year compared to the mass of the earth is a tiny amount, less than 1 %
    So, significant, no, unless as Simon was trying to get you to define what YOU considered was a significant amount

    Abiogenesis doesn't add to the earths mass as that biology CAME from the earths raw materials originally, it has just changed its form


    Dave
     
  17. Feb 15, 2016 #16

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    As did mine ... you have said what it does not include, but not where your current understanding is at. Did your formal education include tertiary level courses in science? Maybe a degree?
    As long as you are unwilling to answer questions with specifics, I don't think anyone can help you. But you did provide a clue:
    Newton's Laws are normally included at the secondary-school level but as part of the earliest physics speciality but often as part of a general science course. Most western countries hold a science education at this level to be part of the mandated public education but not everyone remembers it. Unless you want to say otherwise, I think we have to use an early secondary understanding as a starting point.

    So much for formal education, did you manage to get some informal education in physics? I'm trying to work out how best to talk to you.

    Oparin advanced the primordial soup model - standard models for abiogenisis today are variations on this, usually focussing on submarine volcanic vents.
    You may find the following an accessible overview.
    http://www.universetoday.com/41024/abiogenesis/

    It means only that there are some circumstances where the mass of the earth may be considered to have changed just as there are circumstances to consider that the earth is flat (i.e. in gravity experiments conducted in a classroom.) Those circumstances are not important to your question.

    But don't worry, I'll endeavour to be clearer in future... on the understanding this may mean I have to be less diplomatic.

    The Earth may, in some contexts, be considered to have changed mass ... i.e. increased by infalling matter from space (rocks, cosmic rays etc) and decreased by outgassing and people sending spacecraft away. So we want to know if this is likely to affect our calculations concerning tides and orbital mechanics... how big is this change likely to be?

    After planet formation, expect most of the available matter in the orbit to have been deflected away or hoovered up and formed into the earth-Moon system.
    We can figure out the average infalling mass rate and 50T/yr seems reasonable ... over 4 billion years this averages out to 200 billion tons. If we pessimistically allow a 100% margin of error here then the mass gain is a number between 0 and 400 billion T ... is that a lot? (rhetorical, see below)

    Well the current mass of the Earth is measured at about 5.6 thousand billion billion tons... so the mass increase is about 8 billionths of a percent of it's current mass. This means that the error introduced by assuming the Earth+Moon has always had the mass it does now will only show up in the 12th significant figure. If you have a way to measure something to 12 or more significant figures, then this may be a problem - but it is much more likely that any instrument capable of that accuracy will also pick up variations in the cosmic ray flux and outgassing and tremors from someone breathing nearby ... i.e. background fluctuations are much bigger.

    For the kind of "I just want to think about" stuff you are after, you probably don't even need to account for the mass of the moon when working out the Earth's orbit and the moons orbit can be reasonably approximated as a circle with the Earth at the center. This model can be adjusted later jic.

    For the same "think about" level, you can treat the Earth as a sphere with a uniform shallow ocean over the entire surface (only very minor bumps to allow for a shoreline). We would treat this as a "back of envelope" or "order of magnitude" approach ... to be treated in more detail if warrented.

    You'll notice I didn't worry about the size and mass of the moon in the "infalling mass" calculation - the Moon adds more surface area to catch rocks with, but also blocks some of the Earth's surface from doing the same: the rocks caught by the moon are not caught by the Earth so the calculation is the same. Technically a few rocks could have slipped between the Earth and moon to strike the near-side of either, witness the craters, but they would be easily accounted for in the 100% margin for error.
    You can see for yourself if including the mass of the moon makes a big difference to the percentage calculation...

    You said
    ... so are you interested in the tidal patterns around the abiogenesis time or not?

    Lets start you on some accessible resources -
    Earth/Moon system misunderstandings: http://www.talkorigins.org/faqs/moonrec.html
    Abiogenisis misunderstandings: http://www.talkorigins.org/faqs/abioprob/
    ... these may go into the kind of detail you are after without swamping you.
    You are correct to worry about the variation of the recession over time ... I'm hunting an accessible reference for you, meantime see:
    http://jkas.kas.org/journals/2012v45n2/v45n2p049_naa.pdf

    The estimate in your won link (12 day orbit) could be used to work out the patterns by conservation of angular momentum ... at least as a back-of-envelope approach suitable to just "thinking about" things at an early secondary education level. Hope this helps.
     
    Last edited: Feb 15, 2016
  18. Feb 16, 2016 #17

    ogg

    User Avatar

    Some random shots here: The Theia models of Earth-Moon formation suggest that Theia collided with the planet called Earth. I'd argue that that planet would be better called something else, that Earth didn't exist before the collision had "settled down" to the point where Earth had a crust AND had cleared its orbit of most of the debris from that and any other early solar system debris. Keep in mind we have no evidence whether abiogenesis happened once or a thousand times. We think we know that a collision with a sufficiently large body would sterilize the planet for thousands of years (which is *nothing* on a billion year time scale). So, if we assume that the abiogenesis you're talking about is the last one, and that this occurred after the Late Heavy Bombardment (~ 4 Gya) then we should consider what the Earth looked like at that time. Vaalbara, the "oldest" postulated supercontinent is dated to about 3 to 3.3 Gya. What Earth looked like as comets (asteroids) rained down bringing our ocean's water with it during the LHB and for about the next billion years is anyone's guess. Not coincidentially, plate tectonics is believed to have begun at about that time (3 Gya) (Gya = giga years ago). Temperature. I am not even partially educated on the surface temperature profile of Earth between LHB and the Great Oxidation Event, but it would be critical in any discussion of abiogenesis, it seems to me. Earth was (and is) cooling both from formation and from radioactive decay (we've got about a Gy more of plate tectonics (we hope) before it stops). You gotta model atmosphere, weather, water, erosion, and of course the Earth-Moon tides (as already explained, tides include the movement (stress) of the crust as well as the water). You have to postulate a chemical composition and you have to allow that to vary over the entire surface of the planet. I wish you luck, but what ever you do, won't be science. We just don't know enough.
     
  19. Feb 16, 2016 #18
    Yes.

    I have used your posts to help me search for some information I had difficulty finding previously. In particular:
    My search skills are admittedly mediocre.

    To gain a better idea about where I am coming from WRT this thread, please see

    Some key dates:
    1. Formation of the earth: 4550 MA
    2. Formation of the moon: 4527 MA (23 million years after 1.) Is this time when "the hit" occurred, or when a spheroid moon achieved a more-or-less stable orbit? Or are these two times less than one million years apart and can with the million year granularity of dating, they are about at the same time?
    a. Beginning of Late Heavy Bombardment: ??
    b. Formation of permanent oceans : ??
    c. Oceans becoming "organic soup": ??
    d. First DNA chains that reproduce: ?? This the date I would use for the begining of "the abiogenesis time".
    7. Fist cell = "first life": 4000 MA 473 million years after 2.

    Question: What are the best estimates for dates for a, b, c, and d?
    a, b, c, and d are all between 2 and 7.
    b < c < d
    Can a's timing be placed somewhere WRT b, c, and d?

    Question: How much did the LHB change the mass, rotation period, and angular momentum of the Earth, and the parameters of the Moon's orbit?

    I understand that what I am doing is not science. I am not a scientist. However, my goal in seeking answers from the PF is to help me in my speculations about an area of science that science had not yet explored very much: plausible details about the processes leading to the first cell.

    My thanks to both Simon and ogg for their posts.

    Regards,
    Buzz
     
  20. Feb 19, 2016 #19

    Chronos

    User Avatar
    Science Advisor
    Gold Member

    Here are some references that might aid in answering your moon questions: http://arxiv.org/abs/1504.01421, Highly Siderophile Elements in the Earth's Mantle as a Clock for the Moon-forming Impact; and http://arxiv.org/abs/1508.01467, The Tethered Moon. We have less satisfactory answers to some of the other questions you pose. Here are some sources that may be helpful: http://arxiv.org/abs/astro-ph/0602008, From Protoplanets to Protolife: The Emergence and Maintenance of Life; http://arxiv.org/abs/1309.4729, Astrobiology: An Astronomer's Perspective; and http://arxiv.org/abs/0907.3552, Origin of Life.
     
  21. Feb 20, 2016 #20
    Hi Chronos:

    I much appreciate all your links. It will probably take me a while before I can read them all.

    Regards,
    Buzz
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Earth period after moon created
  1. If Earth was a moon? (Replies: 7)

  2. Earth tilt and moon (Replies: 3)

Loading...