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 Earth yet!

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.

THE END