"lowered gently to a kissing contact"

Joe Barrera (joebar@MICROSOFT.com)
Wed, 1 Jul 1998 11:33:54 -0700

Kinda puts Y2K in perspective...

-----Original Message-----
To: nev@bostic.com
Subject: alt.pave-the-earth
Forwarded-by: Lawrence Kesteloot <lk@pdi.com>
Forwarded-by: Grue <grue@pdi.com>

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
> 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

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

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

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