The Unique Planet
(or When Jupiter Blinks, Earthlings Should Duck)
by Alex Fink, age 9, from Canada
Surely, a comet or asteroid colliding with Earth must be one of the most
devastating things imaginable. There are a few reasons that comets rarely hit
Earth. None of these seem to be unique, but when the reasons are combined, it
sure is unique. The first is that our planet has an atmosphere. If a comet was
to come close to Earth, there would be so much friction that it would be burned
to bits in the atmosphere before impact with our planet. The remaining part--if
there was any--wouldn't be enough to cause any major problem. (Our moon has no
atmosphere, and just look at the cratered surface it has!)
The moon, by the way, is the second reason. It revolves around the Earth once
a month. It's actually a pretty large moon for the size of Earth. The only
planet that has a larger moon for its size is Pluto! Geometry shows that our
moon shields approximately 0.15% of the area around our planet. In ancient
times, there were more collisions in our solar system, but the moon was closer
to Earth, so shielded our planet more. Therefore, to Earth, the drop in the
number of comets wasn't noticed as much.
Jupiter also has two ways of preventing collisions with Earth. Some comets
are made of rocky materials or ice, while others contain metals such as iron and
nickel. Jupiter pulls in all the types of comets with its gravitational field.
The "king of the planets" has a very large mass--three times greater than all
the other planet masses combined. This makes its gravitational field immensely
strong. In fact, the surface gravity is 2-1/2 times that of Earth! I believe the
next thing Jupiter does to keep Earth free of collisions is to use its magnetic
field. Therefore we have extra iron-nickel comet protection because they would
be more likely attracted to Jupiter's magnetic field. (The actual magnetic field
is 4000 times stronger than that of earth.) However, gravity decreases as a
factor of distance squared, and magnetic force decreases as a factor of distance
cubed. So, as you get further away from the planet, the gravity eventually
overrides the magnetic force.
Now what would have happened if Shoemaker-Levi 9 had not hit Jupiter? There
are lots of other possible ways the situation could have turned out. It could
have stayed out of planet orbits, as other comets do, it could have hit Jupiter
next time around, it could have impacted an inner planet (Earth?), or broken up
and just continued. Fortunately, it was Jupiter. The collision created a
spectacular show, and created something unique to the gas giants: ripples. The
ripples eventually even out. As Jupiter is a gas giant, the gas fills the crater
hole (actually not a hole, but ripples) up as the molecules are not rigidly
locked in place as they would be in a solid planet.
If a comet collides with a solid planet, it will throw material all over, and
it takes way longer to fill up the crater hole. This is of course reduced on
planets with constant geological activity. What would happen in a Earth-comet
collision, given that it avoided Jupiter, went right past the moon, and did not
get destroyed by the atmosphere? As it went through the atmosphere, it would
convert lots of nitrogen and oxygen into NO or NO2. This would make it hard for
life to continue breathing as the new upper air mixed with air as we know it.
Also, those molecules can react to form nitric acid, which could result in acid
rain. Then when it struck the surface, there would be a shock wave. The comet
would burrow partially in the ground. Meanwhile, one shock wave would bounce
through the inside of Earth and another would reflect back at the comet,
exploding and vaporising it.
There would be a flash, similar to one seen in nuclear bomb tests. Also, the
flash front would react with an air disturbance caused by the nitrogen oxides
and a few leftover pieces of the comet, and create an expanding "fire sphere"
destroying anything in its path (which would thankfully not affect things far
away as much as it would at the sphere's centre). A deep crater would remain.
The dust layer in the sky would instantly stop photosynthesis, which would wipe
out all the plants, and take most other living things out too!
Every once in a while, the Earth switches magnetic polarities. (We can tell
the earth does this because of certain shell fossils.) But it is not an
immediate reversal. The magnetic particles gradually shift out of one polarity,
and build up in the opposite. I think that is due to rotating "cells" in the
fluid parts of Earth's mantle and core. Now if the earth switches polarities,
then this same phenomenon should apply to other planets. If Jupiter switches
polarities, there would be large periods when the Earth had no magnetic field
protection from Jupiter. Perhaps the mass extinctions in Earth's history
happened during these periods when Jupiter's magnetic field "blinked," allowing
comets to collide with Earth. Shouldn't it be in mankind's best interest to
predict when (or if) Jupiter switches polarities? Well, it should, but nobody
could make any accurate prediction. Nobody has even made a decent forecast about
We humans should be thankful that Earth is unique compared to other planets
in the universe. After all, it does take lots of time for intelligent life to
evolve, and a collision sets that time back to zero. Hopefully if we evolve long
enough, which I believe we will, we might be able to prevent most collisions!
Comet collisions wouldn't be as likely to happen in the future if humans keep
coming up with new ways to stop comets, but they're not completely preventable.
There might be another Earthlike planet out there, but as far as we know, we are
on a very lucky planet.