

The Lunar Craters

The question is: Why do the craters on the Moon, which show so conspicuously along the sunrise and sunset terminators,
look as though they were made by the impacts of objects that fell straight into the Moon? Why don't they show any trace of
the angle of entry? And why are the floors of the larger craters smooth and dark, and lacking the central "splashmountains"
so characteristic of the floors of the smaller craters? And what makes the bright streaks, radially from the
crater Tycho which are so conspicuous when the Moon is full, and which are known in the trade as "rays"? And finally, what
is the Gegenschein?
First we need to know how we got our Moon, because a large moon this close to the Sun is not expectable. Large moons are
expectable around Jupiter and Saturn, but not here. It is now thought that when the Earth was forming we were collided by
something about as big as Mars. It is thought that the iron core of that one joined the iron core of the Earth, and that
that's why the Earth is denser than any of our other planets. Our Moon is presumed to have been made from some of the less
dense material that was exploded off both objects by the impact.
Now both the Earth and the Moon are big enough so that the denser materials like iron would have sunk to the center,
overlaid by the rocks of the mantle and the crust. And the mantle lava on both the Earth and the Moon is still hot
enough to melt if the overlying rocks are removed. So if the impacting comet or asteroid is large enough or hits hard
enough to go through the crust, then the molten mantle lava floods the crater made by the impact explosion.That's why the
very large craters on the Moon are smooth and dark. They are flood basaltes. And that's why there are no splash-mountains
in the very large craters.
Since the comets and asteroids colliding with the Earth and the Moon come in typically at some twenty or thirty miles per
second, they do not stop when they hit the surface, but continue on for miles. Now although the explosion starts when
they hit the surface, or on the Earth, the atmosphere, it continues on into the rocks for perhaps several miles and
leaves a crater perhaps many miles wide. And it is the width of the resulting crater that makes it impossible for us to
tell from which direction the impact came. If an asteroid hits the Moon at an angle of forty-five degrees to the
vertical and goes ten miles into the Moon, the crater resulting from the consequent explosion would exceed sixty
miles in diameter and would obliterate all evidence of the direction from which the asteroid came.
Now the vaporized stone resulting from such impacts on the Moon tends to condense to spherical glass beads because the
Moon is virtually without an atmosphere. And the smaller beads are sufficiently transparent to reflect the sunlight
back toward the Sun. That is what forms the "rays" on the Moon. One third of the surface material on the Moon is glass
(obsidian). And the glass bead streamers from the crater Tycho would reach from San Francisco to Denver. They are a
thousand miles long.
There are two reasons why the full Moon is much brighter than two quarters would be. First, because the Sun "sees no
shadows," so that when we see the Moon from the Sun's direction, we too see no shadows. The shadows of the rocks
and mountains which are exposed to our view when the Moon is at first or third quarter, are not exposed to our view when
the Moon is full. Second, the young glass beads scattered by the more recent impacts on the Moon reflect the sunlight back
in our direction when the Moon is full.
But since even the dust grains come into the Moon at some twenty or thirty miles per second, they leave little glass
lined sockets in the stones, and tend to spoil the reflectivity of the glass beads that formed there earlier.
It's the younger beads, less than a million years old, that appear bright when the Moon is full. And there is a curious
thing that you can see through a good telescope on a clear night when the Moon is full. You don't see the mountains
nicely because they are not lit on one side and shady on the other. But you see the glass beads. And the crater rims of
the smaller craters are bright because when the Moon gets hit by an asteroid it shakes the mountains of the crater rims and
exposes unspoiled glass beads. So the crater rims look like little bright circles all over the Moon.
Now we know, from the military, that from every impact site something leaves that site at a speed in excess of the speed
of the impacting missile. And since the Moon cannot be hit at less than the escape velocity, because the Moon's own gravity
would bring it in that fast, something must always get away. Now part of what gets away must surely be vaporized stone
condensing to spherical glass beads which will thenceforth orbit the Sun. So when we look down the shadow of the Earth,
in the direction opposite the Sun, we see these glass beads reflecting the sunlight back toward the Sun. That is what's
known as the Gegenschein. It appears as a slightly bright patch in very dark sky, midway between the zodiacal lights
from east and west.
Although the Moon appears covered with craters, and the Earth, much less so, it must not be supposed that we get hit
less often. We don't. But we have oceans, volcanoes and continental drift, and an atmosphere with rain and snow. And
we have living organisms that can push up more than a mile of soil in a million years. And we have forests, the acids of
whose rotting leaves corrode the stones. Even the asteroids help to obliterate the earlier craters.
There is a great deal of evidence that what we see as continental drift is driven partly by asteroid impacts that
go through the crust to the mantle. The basalt of the mantle then melts and rises. It has been suggested that the North
Atlantic seafloor spreading may have been started by an asteroid impact at Iceland, while the South Atlantic seafloor
spreading may have been started by an impact at the south end of Africa. 
Some two hundred and fifty million years ago the great mass extinction at the end of the Permian, may have followed two
such impacts at the southern tip of Africa. One is thought to have separated Africa from Antarctica, and the other is
thought to have separated Africa from South America. 
The more recent mass extinction, associated with the demise of the dinosaurs, seems to have followed a similar pair of
impacts, one in the Yucatan and the other in India. The lower layer of the iridium clay at the Cretaceous-Tertiary boundary
is without shocked quartz. That must be from the Yucatan impact. The upper layer, with shocked quartz, must be from
India which is largely granite. That impact went through the crust and left the Deccan Plateau, and may have been
responsible for the Indian Ocean seafloor spreading. 
Galileo was probably the first person to study the Moon through a telescope, and he called the dark patches maria, or
seas. We know they're not seas, and we know that the boiling point of water on the Moon is colder than the freezing point.
But we still use Galileo's terminology when we refer to the astronaut's first landing site on the Moon, in 1969, as the
Sea of Tranquility. Now the Sea of Tranquility is about the size of the State of Oregon where, but for erosion and the
like, and continental drift, we might have a similar lava sea now. Some seventeen million years ago an asteroid went
through the crust in what is now Southeast Oregon, and the flood basaltes spread over Oregon, and north into Washington,
and south into California. Meanwhile the continental plate has drifted over the site. That allowed it to spread basalt
through Southern Idaho and to form the present volcano in the Yellowstone National Park.
There is now an interesting thing to be seen in Oregon where great river valleys have cut through the plain of the old
flood basaltes. Where formerly there were valleys in the upper plain, the flood basaltes were deeper, so that now
those deeper basaltes protrude over the upper rims of the newer valleys like fingers over the edge.
And they remind one of Handel's Messiah: "Every valley shall be exalted, and the rought laces plain."
John Dobson, February 25, 2006, Hollywood, California