Imagine an Earth-like world in all respects except that the Moon is much much closer.

I think I’m right to say if the Moon were very close that tidal forces would slowly rob it of momentum and it would probably eventually hit the Earth destroying both. Although theoretically the Moon should be disrupted before it hit the Earth, (Earths radius 6371km, Moon radius 1737km, Moon’s Roche limit 9492km) given its elliptical orbit and the extreme proximity I would have thought a collision was more or less unavoidable.

My question is this in the last one thousand years before impact (or disruption) is there any permanently dry land on Earth? And the related question do the oceans even behave like oceans as we would know them under these extreme conditions?


One point about this; we believe it's already happened.

Our current theories about how a moon as large as ours (as a proportion to the Earth's mass) formed and why it's so close is that it was formed from a planetary collision.

The thinking is that a planet around the size of Mars collided with the proto-Earth early in the formation of both. This caused a massive amount of debris to be flung into space, although the net mass of the Earth increased. Gravity being what it is, the debris forms a ring around the Earth, which in the space of a few thousand years, forms the Moon.

Thing is, the early Moon was very close to the Earth. We know this because the Moon is actually drifting away from us ever so slightly. It's believed that the early Moon would have orbited the Earth every 35 hrs or so, and caused MASSIVE tides and storm fronts (assuming that the water fell back down to Earth or was condensed back after flash evaporation) because of being so close.

It's not until the Moon recedes a little that things become sufficiently stable on Earth to sustain the first life.

Based on our projections, it's believed that the moon will eventually free itself from Earth orbit and drift away as a rogue. This will cause problems for the habitability of the Earth as many of the environmental cycles we take for granted (including our stable rotation) are based on having the moon in orbit. Th the time this happens though (in about a billion years), the Earth is pretty much uninhabitable anyway because of the Sun and its increasing temperature and diameter.

The real point here is that there's information out there about the effects that the Earth would have likely experienced during the early days of the moon which might help you extrapolate this out in terms of tides and the like.

  • $\begingroup$ Some sources would be nice, but +1 anyway. $\endgroup$ – Mołot Dec 5 '17 at 22:17
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    $\begingroup$ Ask and you shall receive, @Molot. :) $\endgroup$ – Tim B II Dec 5 '17 at 22:26
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    $\begingroup$ The moon isn't going to escape Earth before the sun turns into a red giant and swallows the Earth, Moon and the rest of the inner solar system. For various reasons the moon is actually moving away much faster now than it has done before, or will do in the future (there is a resonance in the tides that won't exist for long) $\endgroup$ – James K Dec 5 '17 at 22:53
  • $\begingroup$ All very interesting but it doesn't realy answer the question... $\endgroup$ – Slarty Dec 5 '17 at 23:18
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    $\begingroup$ @JamesK The Moon gets further away by slowing down the Earth's rotation. If the Earth-Moon system survived long enough, the distance between the Earth and the Moon would eventually stabilise, i.e. there is not enough rotational energy in the Earth to allow the Moon to escape. The Earth would then be tidally locked to the Moon, the same way the Moon is already tidally locked to the Earth. However, I believe the Sun will expand and destroy both before that happens. $\endgroup$ – CJ Dennis Mar 27 at 1:42

Fairly unpredictable,

A moon that close would likely be tearing the Earth's atmosphere away let alone causing extreme geological stress.

Likely the Earth would be a heaping mass of lava with all surface water vaporized and dispersed into the atmosphere now shared with the moon and vulnerable to be dispersed by solar winds.

I would think there would be a good chance for a tidal lock to form causing a great big volcano to appear.


If the moon orbits the Earth as posited, the orbit will fairly rapidly get higher due to momentum transfer (unless the moon is in a retrograde orbit, it which case the orbit will decay).

If you assume a retrograde orbit, how big are the tides on Earth at the Roche Limit. The solid body Roche limit for Earth/Moon is 9492 km vs 384000 km today. This is a ratio of 40.58:1, and tidal stresses are about 66,400 times as large as today.

This is causing major damage to Earth (and the moon is breaking up). Earthquakes and Volcanoes are widespread and severe, tides wash over major portions of the continents, etc.

I would not consider Earth, much less the oceans, familiar under such extreme conditions.

When the moon breaks up, I would expect quite a few large chunks to impact Earth too.


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