3 ms·

Some time ago I had this thought: The original (single) continent was round, and about the diameter of the moon. Since the moon was much closer to the earth, it

• raattgift 3y ago
  > un-provable ... plausible Your idea's first hurdle is the length of day vs the length of month. How do you keep the moon over one bulged part of Earth at all times? (it's a very different story for the moon, whose 'day' is as long as its orbit, more at [1]) Additionally, there is a solar tide at work on the oceans that is visible daily, and especially at spring and neap tides. The sun's mass drives a maximum of almost a third of the total ocean tidal bulge. The second hurdle is viscosity (or if you like, elasticity): tides bulge the most readily-flowing parts of a body the strongest. That means the atmosphere (which is more strongly affected by daily heating <https://en.wikipedia.org/wiki/Atmospheric_tide https://en.wikipedia.org/wiki/Atmospheric_tide>), and the oceans are much more strongly affected than the "asthenosphere" (the rocky parts). In particular, tides bulge the ocean by about a metre. The solid earth tide is closer to 0.2 metres. Thus, the question is: how do you raise a tidal bulge in the rocky part of Earth and keep it from being flooded by ocean tides? > I am definitely not a geologist It's more gravitation vs the liquid flow of a stratified (layered) planet than geology that is are the big hurdles. For experts, tidal Love numbers are the important things for any round stratified body: <https://en.wikipedia.org/wiki/Love_number https://en.wikipedia.org/wiki/Love_number>, which describe the bumpiness (mass multipole) raised by tidal fores on a spherical body immersed in a tidal gravitational field. The mantle's rheology (elasticity, viscosity, rigidity), representing about 85% of Earth's total volume, is the primary driver of the Earth's Love numbers. There's more detail on that here: <https://geodesyworld.github.io/SOFTS/solid.htm#link3 https://geodesyworld.github.io/SOFTS/solid.htm#link3>. The crust is quite low-volume by comparison. So a third hurdle is straightforwardly: if there were a persistent bulge in the mantle, why wouldn't a relatively thick part of the crust (a giant continent in your idea) not just "roll downhill", or conversely, how do you get fractions of a round supercontinent to "roll uphill" to some lunar-attraction-induced meeting point? Finally, with present understanding of continental drift, the continents tend split apart and come together over millions of years, and there's no evidence for anything approaching circularity. The link at the top lets one step through almost a billion years of continental drift, showing this fairly clearly. - -- [1] Slide 8 ("Tides (2)") of the lecture notes at <https://websites.pmc.ucsc.edu/~fnimmo/eart162_10/Week8.pdf https://websites.pmc.ucsc.edu/~fnimmo/eart162_10/Week8.pdf> is pretty accessible, showing an Earth-centric diagram of tides and a moon-centric diagram of its tides, and throughout the rest of the slide deck there's lots of heavy stuff for people who like to grapple with mathematics.
  • mkl 3y ago
    I wondered if it was possible at some point in the past, but I don't think so. The moon's orbit has been increasing its whole existence. It started ~25,000 km away from Earth [0], and is now at ~384,000 km. That means at some point it was at the distance for a current geosynchronous orbit, ~36,000 km. Unfortunately, when the moon was formed, the Earth's day was only 6 hours long [0], so the geosynchronous orbit was much smaller at ~10,000km (based on [1]), and so I don't think they ever lined up. [0] https://physics.stackexchange.com/questions/31429/how-long-was-a-day-at-the-creation-of-earth#31435 https://physics.stackexchange.com/questions/31429/how-long-w... [1] https://www.dummies.com/article/academics-the-arts/science/physics/how-to-calculate-the-period-and-orbiting-radius-of-a-geosynchronous-satellite-174056/ https://www.dummies.com/article/academics-the-arts/science/p...
    • raattgift 3y ago
      It's really hard to break global hydrostatic equilibrium of a planet (in fact, that global roundness[1] is used in the IAU's definition of a planet). Any raising of bumps on the surface will tend to produce depressions elsewhere. For example, Mauna Kea depresses the level of the seabed/crust around it. Likewise, high tide at some points (in the solid earth, the oceans, or the atmosphere) are associated with low tides at other points. - -- [1] You could probably toy with models of Earth-moon as a pair of Jacobi ellipsoids or piriforms (pear-shaped, thin ends inwards) but I don't see that working without a much smaller mass ratio and higher spins. Piriform bodies (at least of homogenous self-gravitating fluid, which is a good representation of the mantle) are generally unstable. Maybe that's good if you can find a path that relaxes back to a Maclaurin (oblate) spheroid for the Earth mass that doesn't also relax the (whole of the) "bump", and relaxes the moon to its weak Jacobi (scalene) spheroid. Really speculating substantially away from what I know: maybe the "synestia" flavour(s) of the giant impact hypothes(e)s for the origin of the moon might be a path to some test simulation codes: coalesce an ellipsoidal (or as I said, piriform or even oviform) moon first and have that drive some aspects of Earth's planetary differentiation (which happens later in that (family of) model(s): <https://en.wikipedia.org/wiki/Synestia https://en.wikipedia.org/wiki/Synestia>). In particular, the driving should be away from homogeneity in an attempt to escape eventual hydrostatic equilibrium for the Earth-mass which otherwise leaves you stuck with encoding surface features on the (very) thin crust and then dealing with the Mauna Kea problem above. I don't know how you could approach this idea with realistic chemistry though, which I think melts & dissolves this line of thinking. ETA: Really wild speculation: with unrealistic chemistry, freeze out a long-term solid hourglass structure with the neck at the Earth's centre of mass, piling lots of rocks on the ends terminating just under the surface (but above the mantle) at the poles, and then have one pole always point to the moon Mass. Doesn't at all fit lots of lines of evidence in very old surface rocks, though. Also very hard to wash out tides raised by the sun.