познавательно
Что будет, если мягко положить Луну в Тихий океан?
Tony Hoffman wrote:
> Well, I'm relieved to hear that you're only going to try to *soft-land*
> the Moon in the Pacific--here I was afraid that you were going to hard-land
> it. ;-)
>
> P.S. Assuming that one could actually soft-land the Moon there--decelerate
> it and bring it to rest in the middle of the Pacific, what would happen,
> besides the mother of all tsunamis? I assume that the Earth-Moon would
> eventually revert to a sphere--how long would that take, and how would the
> process develop? I'd assume that a massive cracking of the Earth's and
> Moon's crust would be among the first things.
When I read this question, my immediate thought was "almost immediately, like in seconds to minutes", but I did some maths and got surprising results:
Let us imagine the Moon has almost magically been lowered gently to a kissing contact with the Earth, in the centre of the Pacific, on the Equator. Then let the controlling forces let go, and let's watch the results from a safe distance....
Loose material on the limbs of the Moon, like Moondust, rocks, Apollo spacecraft, and even whole mountains will experience a gravitational attraction sideways towards the Earth of 62% of g at Earth's surface. Meanwhile, Moon gravity remains at 1/6 g sideways into the Moon. The nett effect is that everything not glued down will fall off the sides and underneath of the Moon, but being gently focussed inwards as it drops. It will take over seven minutes just to fall from a height of 1738 km (the Moon's radius), dodging the occasional satellite in low orbit that will smack into the debris cloud. At impact, downwards velocity will be over 5km/sec. While not of totally cosmic proportions, this is a significant re-entry speed. Small debris will tend to burn up in the atmosphere, yet so much debris will be falling that the atmosphere will be compressed and displaced. Small primary impact craters will result from the impact of bodies larger than 10m or so.
Not just the loose material will fall. The sideways (shear) force will be enough to tear off the Moon's crust and Mantle. The strength of rock is almost irrelevant given the forces involved. If we imagine the Moon as a cylinder, for a moment (and it will rapidly flatten downwards towards the Earth into some sort of squashed pear-shaped blob), it has a cross sectional area of pi * radius^2 or 3.02 * 10^12 square metres. Its Mass is some 7.35*10^22 kg. Correcting for the decay of Earth's gravity field, there would be a force of 1.5*10^10 kgf per square metre (15 million tonnes), or 15 tonnes per square cm (atmospheric pressure is around 1kg per square cm). Hydraulic presses on Earth do wonderful things to steel at those sort of pressures.
It won't just be the Moon that deforms. The Earth will be indented by this mass, "locally". Outside of the dent, a bulge or bow wave will be thrust up. This wave will propogate out at seismic wave speeds in the Mantle of around 8 km/sec, forcing the seabed up to meet the falling debris, adding to the collision speed. The atmosphere and oceans will be blasted out sideways, as a hypersonic shock wave develops in a torus round the merge point.
Meanwhile, on the other side of the world, the oceans of the world will experience the Mother of all Tides. To first Order, the centre of gravity of the Earth-Moon system will be displaced towards the Moon by the ratio of masses i.e. 1/81 of their separation (8000 km) = 100km. The equipotential surface of the joint body will therefore be 100km above sea level in the Pacific, and 100km below it at the antipodal point (about the middle of Africa). The seas will pour bodily sideways in a cataclysmic tidal wave, mounting up into kilometres high tsunami that will rush bodily inland. Central America will be overwhelmed like a broken straw, and the Atlantic will surge across the USA all the way to the Rockies. Of course, this would take many minutes, or perhaps a few hours.
The tidal wave wouldn't get that far before the outgoing shock wave arrives from the Pacific, like a a huge "plop" as a stone falls into a pond. The surface wave (literally, a wave of the Earth's suface, and crust, and mantle....) would be tens to hundreds of km in amplitude, totally disrupting the crust. Every volcano on Earth would erupt cataclysmically as their magma chambers were torn open and exposed to the surface, but they would be the merest fart in comparison to the effect of the wave itself.
As the Moon was sucked into the Earth, a huge molten droplet could be thrown back upwards, like those beautiful "milkdrop in a cup of coffee" photos. This would "only" be 500km in radius and probably wouldn't make escape velocity, but it would splash nicely when it came back down again. Smaller pieces in the 1-100km range would be blasted sideways and upwards in ballistic trajectories to fall back in secondary impacts right around the globe. A debris cloud would orbit the Earth, but would rapidly be winnowed as lumps going one way met those going the other. Most of the debris would fall back to Earth on a time scale of years to centuries, but the Earth's dust rings would be a beautiful sight for any visiting spacefarers.
The Earth itself would be in turmoil. It would wobble like a jelly for days, and ring like a bell for months or years. The Oceans would boil, and the atmosphere could be significantly lost to space. Life would be wiped clean from the planet.
Perhaps (if there is any justice) the last human to be blasted with incandescent gas would be our friend AP who might possibly say "It wasn't supposed to do that!"
On a longer timescale, the site of the merging would be a boiling hell of lava 5000km across for millenia. The entire tectonic and convection system of the Earth would be disrupted. Remember that the Moon is made of relatively low density rock, like Anorthosite (rich in feldspars, like Earthly granite is, but much more so). The Moon is probably made of much of the primordial crust of the Earth, blasted off in a major impact billenia ago. Finally it would have returned. The Moon has enough volume to spread out in a layer around 50km thick across the whole Earth - More than double the current volume of the Earth's crust.
Like Venus, some 500Ma past, the entire surface of the Earth would be recycled. The plate tectonic process would start again almost from scratch and begin to sort mixed up crust from mantle, aided by the energy input to the Earth that would raise its global temperate a fair bit towards the liquidus (total melting point). There would be enough energy liberated by this collision to raise the temperature of the entire volume of the Earth by over 100 degrees Centigrade. Fluid magmas like the precambrian komatiites would gush out of every crack and fissure in the shattered Earth, flooding its surface with basalts. Gradually, the remnants of the crust would clot together like scum on a boiling pot and the continents would begin again as felsic islands surrounded by ferociously active greenstone belts and mobile zones.
Who knows, perhaps in about 2 or 3 billion years that organic soup could start again and give rise to new generations of geologists and astronomers to wonder at the heavens and discover the History of the Earth, but it would be a different planet than now:- Slightly larger, with no Moon, no Tides, Thick crust like Mars, but active volcanism like Venus.
Nick Hoffman
Geophysicist Extraordinaire
http://www.xent.com/FoRK-archive/july98/0041.html
Tony Hoffman wrote:
> Well, I'm relieved to hear that you're only going to try to *soft-land*
> the Moon in the Pacific--here I was afraid that you were going to hard-land
> it. ;-)
>
> P.S. Assuming that one could actually soft-land the Moon there--decelerate
> it and bring it to rest in the middle of the Pacific, what would happen,
> besides the mother of all tsunamis? I assume that the Earth-Moon would
> eventually revert to a sphere--how long would that take, and how would the
> process develop? I'd assume that a massive cracking of the Earth's and
> Moon's crust would be among the first things.
When I read this question, my immediate thought was "almost immediately, like in seconds to minutes", but I did some maths and got surprising results:
Let us imagine the Moon has almost magically been lowered gently to a kissing contact with the Earth, in the centre of the Pacific, on the Equator. Then let the controlling forces let go, and let's watch the results from a safe distance....
Loose material on the limbs of the Moon, like Moondust, rocks, Apollo spacecraft, and even whole mountains will experience a gravitational attraction sideways towards the Earth of 62% of g at Earth's surface. Meanwhile, Moon gravity remains at 1/6 g sideways into the Moon. The nett effect is that everything not glued down will fall off the sides and underneath of the Moon, but being gently focussed inwards as it drops. It will take over seven minutes just to fall from a height of 1738 km (the Moon's radius), dodging the occasional satellite in low orbit that will smack into the debris cloud. At impact, downwards velocity will be over 5km/sec. While not of totally cosmic proportions, this is a significant re-entry speed. Small debris will tend to burn up in the atmosphere, yet so much debris will be falling that the atmosphere will be compressed and displaced. Small primary impact craters will result from the impact of bodies larger than 10m or so.
Not just the loose material will fall. The sideways (shear) force will be enough to tear off the Moon's crust and Mantle. The strength of rock is almost irrelevant given the forces involved. If we imagine the Moon as a cylinder, for a moment (and it will rapidly flatten downwards towards the Earth into some sort of squashed pear-shaped blob), it has a cross sectional area of pi * radius^2 or 3.02 * 10^12 square metres. Its Mass is some 7.35*10^22 kg. Correcting for the decay of Earth's gravity field, there would be a force of 1.5*10^10 kgf per square metre (15 million tonnes), or 15 tonnes per square cm (atmospheric pressure is around 1kg per square cm). Hydraulic presses on Earth do wonderful things to steel at those sort of pressures.
It won't just be the Moon that deforms. The Earth will be indented by this mass, "locally". Outside of the dent, a bulge or bow wave will be thrust up. This wave will propogate out at seismic wave speeds in the Mantle of around 8 km/sec, forcing the seabed up to meet the falling debris, adding to the collision speed. The atmosphere and oceans will be blasted out sideways, as a hypersonic shock wave develops in a torus round the merge point.
Meanwhile, on the other side of the world, the oceans of the world will experience the Mother of all Tides. To first Order, the centre of gravity of the Earth-Moon system will be displaced towards the Moon by the ratio of masses i.e. 1/81 of their separation (8000 km) = 100km. The equipotential surface of the joint body will therefore be 100km above sea level in the Pacific, and 100km below it at the antipodal point (about the middle of Africa). The seas will pour bodily sideways in a cataclysmic tidal wave, mounting up into kilometres high tsunami that will rush bodily inland. Central America will be overwhelmed like a broken straw, and the Atlantic will surge across the USA all the way to the Rockies. Of course, this would take many minutes, or perhaps a few hours.
The tidal wave wouldn't get that far before the outgoing shock wave arrives from the Pacific, like a a huge "plop" as a stone falls into a pond. The surface wave (literally, a wave of the Earth's suface, and crust, and mantle....) would be tens to hundreds of km in amplitude, totally disrupting the crust. Every volcano on Earth would erupt cataclysmically as their magma chambers were torn open and exposed to the surface, but they would be the merest fart in comparison to the effect of the wave itself.
As the Moon was sucked into the Earth, a huge molten droplet could be thrown back upwards, like those beautiful "milkdrop in a cup of coffee" photos. This would "only" be 500km in radius and probably wouldn't make escape velocity, but it would splash nicely when it came back down again. Smaller pieces in the 1-100km range would be blasted sideways and upwards in ballistic trajectories to fall back in secondary impacts right around the globe. A debris cloud would orbit the Earth, but would rapidly be winnowed as lumps going one way met those going the other. Most of the debris would fall back to Earth on a time scale of years to centuries, but the Earth's dust rings would be a beautiful sight for any visiting spacefarers.
The Earth itself would be in turmoil. It would wobble like a jelly for days, and ring like a bell for months or years. The Oceans would boil, and the atmosphere could be significantly lost to space. Life would be wiped clean from the planet.
Perhaps (if there is any justice) the last human to be blasted with incandescent gas would be our friend AP who might possibly say "It wasn't supposed to do that!"
On a longer timescale, the site of the merging would be a boiling hell of lava 5000km across for millenia. The entire tectonic and convection system of the Earth would be disrupted. Remember that the Moon is made of relatively low density rock, like Anorthosite (rich in feldspars, like Earthly granite is, but much more so). The Moon is probably made of much of the primordial crust of the Earth, blasted off in a major impact billenia ago. Finally it would have returned. The Moon has enough volume to spread out in a layer around 50km thick across the whole Earth - More than double the current volume of the Earth's crust.
Like Venus, some 500Ma past, the entire surface of the Earth would be recycled. The plate tectonic process would start again almost from scratch and begin to sort mixed up crust from mantle, aided by the energy input to the Earth that would raise its global temperate a fair bit towards the liquidus (total melting point). There would be enough energy liberated by this collision to raise the temperature of the entire volume of the Earth by over 100 degrees Centigrade. Fluid magmas like the precambrian komatiites would gush out of every crack and fissure in the shattered Earth, flooding its surface with basalts. Gradually, the remnants of the crust would clot together like scum on a boiling pot and the continents would begin again as felsic islands surrounded by ferociously active greenstone belts and mobile zones.
Who knows, perhaps in about 2 or 3 billion years that organic soup could start again and give rise to new generations of geologists and astronomers to wonder at the heavens and discover the History of the Earth, but it would be a different planet than now:- Slightly larger, with no Moon, no Tides, Thick crust like Mars, but active volcanism like Venus.
Nick Hoffman
Geophysicist Extraordinaire
http://www.xent.com/FoRK-archive/july98/0041.html