Some people have said that a meteor
hitting the Earth is like a bug hitting the windshield of a car while the car
is moving. The bug is so small and the windshield is so big that the bug just
splatters. Yes, the bug makes a big mess, but the mess is all on the surface
and doesn't affect the structure of the windshield or the interior of the car.
I believe that, while the bug hitting
the windshield (BHTW) concept is too simplistic to describe what happens when
celestial objects collide with the Earth, the windshield analogy can be
employed to provide a picture of what does happen when objects collide with
There are three major factors that
argue against the simplicity of the BHTW concept. These factors are:
1. The worst earth impacts are
rocks, not bugs.
2. The speed of the impact
objects is orders of magnitude faster (i.e. 24,000 mph vs. 60 mph).
3. The Earth's windshield is actually only
its lithosphere (which is only 4 to 40 miles thick) rather than being the
entire Earth, itself. Furthermore, the Earth is filled
with liquid rock, the equivalent of having the car filled with liquid water,
which would transfer the force if the windshield were deformed inwards or
There are also the mitigating factors
of the air in the atmosphere and the water on the surface of the arth, should
an object happen to collide with Earth in a watery location.
Nevertheless, the BHTW concept can be
modified to provide an easy, everyday understanding of what happens when cosmic
objects collide with earth.
The first change to the BHTW concept is
to change its name to the Rock Hitting The Windshield (RHTW) concept. The RHTW
concept more accurately describes the severe nature of the worst cosmic
impacts. When a rock hits a car windshield, the extent of the damage depends
upon the size of the rock, the angle of impact, the hardness of the rock and
the relative speed of the object, compared to the windshield.
When a small pebble hits a windshield,
often there is no damage at all. Sometimes, depending upon the angle and speed
of the pebble, a small pock mark might result.
The impact of a bigger pebble or a
small rock may cause anything from no damage to a big pock mark with cracks
propagating out into the rest of the windshield. The impacts of larger rocks
and boulders can cause catastrophic damage, ranging destruction of the
windshield to destruction of the car, itself. If the car
had been been filled with water, the transferred energy would have blown out at
least the rear window.
If a small rock were fired from a high
speed rifle at 5000 mph, the effect would be similar to a high speed bullet
The windshield would be pierced and the rock would damage the interior
of the car, as well. Most meteors hit the earth at speeds up to five times
faster than a high speed bullet.
We can apply the RHTW concept to cosmic
impacts. In effect, the Earth has its own windshield, called the lithosphere.
This hard outer shell is 4 to 40 miles thick, depending where on earth it is
measured. However, the thinnest sections of the lithosphere are located at the
bottom of the ocean, where they are protected by miles of ocean water, which
can greatly dissipate the effect of an impact.
Below the lithosphere, the Earth has
various molten layers, which then culminate in the heavy solid core, which is
composed mostly of iron. Regarding impacts, the important characteristic of
these layers is the fact that two of them are not solid, but liquid.
Impacts on Earth's small hard shell
with molten inner layers can have different effects, depending upon the size of
the impact object, the angle of the impact, the hardness of the impact object
and the relative speed of the impact object, mitigated by the effect of the air
in the atmosphere and water at the impact site, if the object hits an area that
contains a substantial depth of water (more than half of the Earth's surface is
located under deep ocean water).
This list of impact factors sounds very
much like the list of factors we examined with the RHTW concept. However, the
size, speed and some other factors can be very different. Nonetheless, viewing
the lithosphere as Earth's windshield can be a useful visualizing device.
DEFINING A SCALE FOR MEASURING
Because of the interplay between the
many factors involved in a cosmic collision, I believe that the best way to
categorize impacts is to rate these impacts based upon their results, rather
than upon how big the impact object was, how fast it was going or what the
angle of impact was (among many other factors).
My Cosmic Impact Rating Scale goes from
one to seven, with one being very minor and seven being the absolute
obliteration of the planet.
1. GRADE 1 IMPACT-Lack of Seismic
A Grade 1 Impact is so minor that the seismic effect of the impact
registers less than 3.0 on the Richter scale.
2. GRADE 2 IMPACTMinor Seismic
A Grade 2 Impact registers between 3.0 and 7.0 on the Richter scale
as a seismic event.
The impact can be felt locally or regionally.
3. GRADE 3 IMPACTMajor Seismic
A Grade 3 Impact registers more that 7.0 on the Richter Scale as a
seismic event, but it does not transfer enough force through the molten layers
of the inner earth to produce a volcanic eruption at the antipode (the spot on
the exact opposite side of the Earth) of the impact site.
4. GRADE 4 IMPACTAntipodal
A Grade 4 Impact hits the Earth with so much force that the
transfer of force through the molten layers of the Earth causes volcanic
eruption at the antipode of the impact, resulting in what is known as a
"hotspot." However, a Grade 4 Impact is not so great as to create continental
5. GRADE 5 IMPACTContinental
A Grade 5 Impact imparts impact forces that are so huge that the
impact not only forms a hotspot at the antipode of the impact site, but it also
causes major continental uplift at and near the antipode. The usual shape of
this continental uplift is "a blob with a tail." The volcanism at the antipode
is extensive and can even result in some ejecta being thrown into space and
(mostly) swept up by the Moon. However, a Grade 5 Impact is not so great as to
create a major moon.
6. GRADE 6 IMPACTMoon Formation
A Grade 6 Impact is so severe that ejecta is thrown into space and in orbit
around the planet. The formation of our Moon is the only sure Grade 6 Impact
that the Earth has ever experienced.
7. GRADE 7 IMPACTObliteration
A Grade 7 Impact erases the planet from existence.