Impact
Events
A Relationship of Size and Velocity
As we have talked about, when a large
object impacts the surface of the Earth, the rock at the site of the impact is
deformed and some of it is ejected into the atmosphere to eventually fall back
to the surface.
Result: A bowl shaped depression with a
raised rim.
But why are some craters larger than
others?
Well, it all depends on certain
factors: Size, velocity, and the angle at which it strikes.
Let's look at these. You'll be
surprised with what science knows....
Meteorite
Flux and Size
Meteorite Flux is a term that means the total mass of extraterrestrial
objects that hit the Earth.
This is a huge number, and according to
some is more than a 100,000 tons each year.
However, in a study led by P. A. Bland
and published in Monthly Notices of the Royal Astronomical Society, this
number is more in the range of 37,000-78,000 tons each year -- still a surprisingly large number.
Much of this material is dust-sized
objects called micrometeorites.
So, each day, tons of micrometeorites
strike the Earth.
But, because of their small size, they
do not usually burn up when entering the Earth's atmosphere; they simply settle
slowly to the surface.
However, this changes as the meteorites
grow in size. Here's some common numbers.
- Meteorites with diameters of about 1 mm
strike the Earth about once every 30 seconds.
Roughly, 1 millimeter = 0.039 inches
Upon
entering the Earth's atmosphere, the friction of passage generates enough heat
to melt or vaporize these small meteorites, resulting in “shooting stars.”
- Meteorites of larger sizes strike the
Earth less frequently. If they have a size larger that 2 or 3 cm, they only
partially melt on passage through the atmosphere, and thus strike the surface
of the earth. (See chart below.)
Roughly,
.8 to 1.2 inches in size
- Objects larger than 1 km are considered
to produce effects that would be catastrophic -- producing global effects.
Roughly,
1 km = 0.62 miles in size
Such
meteorites strike the Earth infrequently -- about once every million years.
Meteorites
10 km (6.21 miles) size hit about once every 100 million years.
So, given this information, here's a
well known chart: It shows object size compared to time between impacts.
Note where the size/time scales are set: once a year a meteorite
1 meter in diameter (3.28 feet) hits the earth.
So, what have we learned:
- Tons of micrometeorites float to the
Earth each year
- Every 30 seconds a millimeter sized
meteorite hits
- And in minutes an inch size hits
- Once a year a three foot meteorite
comes down -- And in between those...
Lots of space rocks are falling each
year. So, when you hear the news of meteorites, you now know more of the real
story.
But there is more to this relationship,
and it deals with speed and the pressure/resistance of air...
Here's
a Quick Review
When
in space, these natural space objects are called meteoroids.
When
it enters the atmosphere, impact pressure causes the body to heat up and emit light, thus forming a fireball, also known as a meteor or shooting star.
Velocity
and Energy Release
As we have seen, we have a range of
meteorite sizes falling to the Earth, and likewise, we have a range of
meteorite speeds involved.
The velocities at which small
meteorites have impacted the Earth range from 4 to 40 km per second (km/sec).
Roughly,
13,123 to 131,233 feet per second
Larger objects are not slowed down much
by the friction of the atmosphere -- thus impacting the Earth with high
velocity.
For example, calculations show that a
meteorite the diameter of 30 meters, weighing about 300,000 tones, traveling at
a velocity of 15 km/sec (33,500 miles per hour) would release energy
equivalent to about 20 million tons of TNT.
Such a meteorite struck at Meteor
Crater, Arizona about 49,000 years ago leaving a crater 1200 meters in diameter
and 200 meters deep.
This is what it looks like up close....
The meteorite that struck the crater is officially called the Canyon Diablo Meteorite and all fragments of the meteorite that are officially labeled bear the Canyon Diablo name.
The name comes from Canyon Diablo, Arizona, which was the closest community to the crater when scientists began investigating the area in the late 1800s.
Published according to the GNU Free Documentation License
The amount of energy released depends on the size and the
velocity of the impacting object.
65 million years ago a meteorite struck
the Yucatan Peninsula, Mexico, created the Chicxulub Crater, 180 km in
diameter, and released energy equivalent to about 100 million megatons of TNT --
thus thought responsible for the extinction of the dinosaurs and numerous other
species.
Here's a view of the Yucatan impact:
For comparison, the amount of energy
needed to create a nuclear winter on Earth as a result of nuclear was is about
8,000 megatons. The energy equivalent of the world's nuclear arsenal is about
60,000 megatons.
So, how do we put this all together to
understand this incoming flux of space material, the speeds it hits, and the
release of energy?
Keeping
It in Perspective
The earth is constantly gaining mass.
And as history has shown, planets gain
mass through collisions with meteorites, asteroids, and comets.
This is how planets actually form: they
start out as lumps of rock or ice and accumulate more and more mass as things
bump into them.
Most of this process happened billions
of years ago: we have overwhelming evidence (the craters on the moon and every
other satellite in the solar system, plus the rocky planets) that this
bombardment was intense for the first few hundred millions years of planet
formation.
It's much less intense now but still goes on. Comet Shoemaker-Levy 9 was a
great example of this process in action.
Here's a video link.
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