Planet Four Talk

Planetary Migration - An explanation for Mars' early climate?

  • pete-j by pete-j

    'In addition to estimate the composition of ancient Mars’ atmosphere, modelers also needed to consider that the 4 billion years ago, our sun was only three-quarters as bright as it is today. This means there was less sunlight available to warm Mars’ surface, which Bob explained would have resulted in an average surface temperature on an atmosphere-less Mars of a chilly -77˚ C (recall, without the addition of any salts or thin films, water needs a temperature of 0˚ C to be liquid). Models have shown that it is possible to add an atmosphere to warm Mars to just above above freezing even with a faint young sun, but it’s very difficult to do without invoking special processes that may or may not be physically realistic'.

    8th Mars Report: Was Ancient Mars Warm and Wet or Cold and Icy?, 2014.

    In this respect, I had a few thoughts about the ancient Martian climate particularly when reading reports here from which the above text is sourced. Could Mars, and therefore all the other terrestrial planets have all been slightly closer-in to the sun (however this may sound) and then all migrated outwards (the orbits gradually expanding)? After Jupiter migrated out to its present position? That is according to a model is known as the Grand Tack.

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  • Kitharode by Kitharode moderator

    This article in New Scientist says that the Moon-Earth distance is increasing by about 4cm per year, and the Sun-Earth distance is increasing by about 15cm per year. I can't find any figures for Sun-Mars increase, if indeed it is increasing.

    Any thoughts on how much closer in Mars would need to be to get 'warmed up' enough? If it has indeed been moving away from the Sun, at say 10cm per year, then 4 billion years ago it would have been 4 million kms closer. (You might want to check my sums) 😉 ...
    (Mars' mean distance is currently about 228 million km).

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  • pete-j by pete-j

    I have heard of the Earth-Moon distance increasing but that’s the first time I have read about the Earth-Sun distance increasing being quoted directly. My first thought on reflection was that perhaps it shouldn’t be such a surprise, after all the sun is converting ~ 4.3 x 10^9kg of mass to energy per second via fusion and so it is becoming ‘lighter’ and losing its gravitational grip? But then there are the tidal effects to factor in.

    Energy drops off as per the inverse square law:

    I = [150/228]^2 = 0.43; that is Mars receives ~ 43% at 228mln km compared to the flux the Earth receives in its present position at ~150mln km.
    Moving Mars 4million km closer in to the Sun at 224mln km we have I = [150/224]^2 = 0.45 = 45% of the flux the Earth receives. This is a 'back of the envelope' calculation considering all wavelengths. Basically, a planet twice as far as Earth is from the Sun would receive [1/2]^2 = ¼ = 0.25 of the energy of the Earth. The above is therefore playing about with fractions.
    At the beginning of the Solar System, if the Sun was 75% as bright as it is now, at its present (average) distance, then Mars would have received 0.75 x 0.43 = 0.32 = 32% of the energy it receives today from the sun.

    I think this needs a ‘planetary models’ answer rather than just quoting a distance that Mars would have to be brought in closer to the Sun due to varying levels of greenhouse effects. What I meant was that more compact orbits (closer to a dimmer Sol) of the terrestrial planets in the distant past could have been an part of the answer and helped Mars. Unfortunately(!) I don’ t work on planetary models in my present work just my background is Astrophysics.

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  • mschwamb by mschwamb scientist, translator in response to pete-j's comment.

    Hi,

    Interesting question and discussion. The short answer is that based on the current understanding of how our Solar System formed, the theory doesn't allow for significant migration of the fully formed terrestrial planets. The giant planets did move, Jupiter and Saturn first during the grand tack and then all 4 (or maybe 5) giant places through planetesimal driven migration. Evidence in the small body populations in the asteroid belt, kuiper belt, and irregular satellites tend to support this idea.

    The grand tack invokes gas driven migration, where Jupiter and Saturn migrate in towards the inner Solar System and back out, cutting of Mars' feeding zone and stirring up the different populations in the asteroid belt. Work is still being done on the model. When the Grand Tack is happening the terrestrial planets are still forming through accretion of planetesimals and at the end stages of gas in the circumstellar disk, so I think by the time the terrestrial planets are fully formed, there shouldn't be much gas lift in the disk, so I'm not sure you can kick off. Also the trick to getting Jupiter to turn back around is that it has smaller Saturn locked in a resonance moving with it. Mars wouldn't have had that, so even if you did check off gas migration, it shouldn't have stopped and turned back around. If the terrestrial planets were in a more compact configuration, I'm not sure there is a mechanism to spread them out.

    The Moon-Earth distance increases for a different reason, transfer of angular momentum. Because the Moon is tidally locked to the Earth (it's rotation rate and orbital period are the same), but the earth is still rotating faster, this causes the Moon to slowly pull on the Earth slowing it's day down ever so slightly on and on. That system must conserve angular momentum, so when the Earth slows down that angular momentum gets imparted to the Moon's orbit and the Moon moves further out.

    The faint young Sun issue is still an open question, but Mars also likely had a thicker atmosphere in the past which it's now lost. How all this interplayed and Mars' history is still an open question. One of the reasons MAVEN is in orbit around Mars is to better understand how Mars lost its atmosphere.

    I hope this (rather rambling) response is helpful for the discussion here.

    Cheers,

    ~Meg

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  • wassock by wassock moderator

    There are a few things that strike me as inconsistent about this grand tack
    Why does the sun form first and then the gas giants? Surely they would all be coalescing from the intial gas cloud at the same time.
    why do the gassy bitsodies which are made of teeny tiny bits formfaster than the rocky bits which are madeof bigger stuff? Other than "because they have to"
    If the initial cloud of debris is a cloud and gets strecthed out into a disc by rotation why are the ort lumps still in a sphere?

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  • mschwamb by mschwamb scientist, translator in response to wassock's comment.

    HI Wassock,

    The process you describe does happen. It forms brown dwarfs and low mass stars. Brown dwarfs are Jupiter-sized, but they have cores of hydrogen and deuterium and are massive enough to kick of burning of deuterium but not hydrogen, so once the deuterium runs out they just simply cool off. It's the cores of the giant planets that are the issue. They have massive rocky and icy cores (10 or more times the mass of the Earth). Simple jeans collapse doesn't appear to get enough material to make a core, but a disk of material rotating around the star after it forms will have solid material eventually settled in the midplane and then grow to larger sizes and then kick of oligarchic growth. The cores for the giant planets get massive enough that they then start accreating large amounts of gas from the disk.

    This talk by dynamicist and planetary theorist Hal Levison has a nice overview and also discusses some of the most recent theories on giant plant formation. The file requires adobe acrobat reader for pdfs in order for the animations to play correctly.

    Cheers,

    ~Meg

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  • wassock by wassock moderator

    Thanks for that Meg - I thought that the jury was still out on the giants cores?

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  • mschwamb by mschwamb scientist, translator in response to wassock's comment.

    For Jupiter and Saturn we know they have cores, because we've had spacecraft measure and map the gravity field. With that we can understand how the mass is distributed, so based on that, there is strong evidence for cores. Giant planets in other solar systems may have different core sizes if any. For only some of those planets do we have bulk densities and look a bit at internal structure.

    Cheers,

    ~Meg

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  • wassock by wassock moderator

    OK that makes a lot m8re sense then, was thinking they were all gas which is whyit wasnt working for me

    So all the rocky planets are just the bits that jupiter and saturn didnt get hold of?

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  • pete-j by pete-j

    The moment of inertia factor here is an important factor in giving a feel for the distribution of mass within a planetary body. It is defined as: C/MR^2 where C is the polar moment of inertia of the body, M is the mass of the body, and R is the mean radius of the body.
    ‘For a sphere with uniform density, C/MR^2 = 0.4. For a differentiated planet or satellite, where there is an increase of density with depth, C/MR^2 < 0.4. The quantity is a useful indicator of the presence and extent of a planetary core, because a greater departure from the uniform-density value of 0.4 conveys a greater degree of concentration of dense materials towards the centre’.
    Different bodies in our SS have different MI values considering the terrestrial planets e.g. Mercury 0.346 ± 0.014, Venus is unknown, Earth 0.3307, Moon 0.3929 ± 0.0009, Mars 0.3662 ± 0.0017.
    I sourced Wiki (2015) for the above.

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  • pete-j by pete-j

    My comments seem to be limited to a few sentences as this post has been cut down from ~7 sentences. This is the second time this has happened, I tried to post something here the day before and got the same result.

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  • mschwamb by mschwamb scientist, translator in response to wassock's comment.

    Kind of... In a way you could say that. jupiter and saturn were forming much faster and evolved much faster so while the terrestrial planets were still forming from planetesimal material. the asteroid belt and kuiper belt are the material/debris leftover after this planetary formation processes

    Cheers,

    ~Meg

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  • mschwamb by mschwamb scientist, translator in response to pete-j's comment.

    odd... I definitely have posted previously with more than 7 lines. What browser are you using? if it happens again can you let me know?

    Cheers,

    ~Meg

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  • pete-j by pete-j

    Hi Meg

    I seem to be having the same problem as before. I have always used the same browser (Internet Explorer) in all my time with P4. initially I thought that it was because I occasionally like to write something in word and then copy and paste it here. This time I both typed everything in and pasted and the text became truncated again after a few lines... I removed the post and have typed this in.

    In 'Show Preview' this effect is visible as you type - but when typing in posts it seems to allow longer posts and then the sentence stops mid-way even though the post is longer. Weird.

    PJ

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  • mschwamb by mschwamb scientist, translator in response to pete-j's comment.

    Hi Pete,

    I'm sorry that's happening.
    Can you give the version of IE and operating system? I'll put in a support ticket.

    This might be hard to fix if the development team can't readily replicate it. The Zooniverse is in the middle of rebuilding the Talk platform. Planet Four may or may not switch to that new platform, but the developers may not have time to spend patching the over version of Talk but instead fix the issue in the new version. Also if they do decide to look into it, this will be low on the queue since the development team is busy giving attention to new Zooniverse projects (we got the same kind of attention when we launched). In any case once you get me those other details I'll make a support ticket.

    Cheers,

    ~Meg

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  • pete-j by pete-j

    Hi Meg,

    Thanks, my operating system is Windows 7 Professional, 64 bit, service pack 1. I am unable to ascertain which version of IE, even when looking through the properties and the six tabs that are with it. There is no version number stated (i.e....6,7,8...).

    Cheers
    PJ

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  • mschwamb by mschwamb scientist, translator

    Hi Pete,

    Thanks. The support ticket is in.

    Cheers,

    ~Meg

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  • pete-j by pete-j

    Hi Meg

    My version of Internet Explorer is 11 - found it completely by chance when looking for something else!

    Cheers

    PJ

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