While sitting in my back yard last night, I saw a shooting star, and my bride suggested it might be a good subject for a column. As always, I agree with my wife, so let’s have at it.
This subject goes along pretty nicely with the recent columns about the moon, and even has a considerable amount to do with the elements and that we are all made of the good Dr. Sagan’s “star stuff”.
A meteor entering the atmosphere, as seen from the International Space Station. Image credit: NASA
A “shooting star” is what we see when little (and sometimes not so little) bits of cosmic debris enter our atmosphere and burn up due to the friction they encounter as they pass through the air at the very high speeds at which they travel. The proper name for a shooting star is “meteor”. If the meteor is large enough that some of it survives its fiery passage through the atmosphere and hits the ground, it is referred to as a “meteorite”. If you’ve ever been to see the gigantic “Meteor Crater” in the desert outside Winslow, Arizona, you have seen the result of the impact of a fairly large (house-sized) meteorite made primarily of iron and nickel with the ground. It should be called “Meteorite Crater”, but that’s probably not going to happen. About 50,000 years ago, this meteorite hit the ground going about 8 miles per second, leaving a crater about 4000 feet in diameter. Today, it is about 560 feet deep and the rim of the crater rises about 150 feet above the surrounding ground level. It was originally about 100 feet deeper with the rim about 50 or 75 feet higher, but erosion by wind and rain have worn down the rim and caused some sediment to accumulate in the bowl of the crater. The impact was so powerful that most of the meteorite was vaporized when it hit, and some of the soil and rock on the ground were fused into a grainy, sandstone-like rock. I have a little piece of it.
The Arizona crater is still there to see (and you should see it, if you ever get out to Arizona, along with the Grand Canyon, the Petrified Forest and the Painted Desert) because it’s in a fairly dry location, where wind and water haven’t had much time to erode it away. The Earth has been pummeled with thousands of similar impacts since its formation four-and-a-half billion years ago, but the erosion has erased most of the craters. Without erosion, the surface of the Earth would be almost as pockmarked and cratered as that of the Moon.
We still have run-ins with large meteors. In 2013, an asteroid that was in an orbit around the Sun that brought it too close to the Earth entered the atmosphere and exploded over Chelyabinsk, Russia. As it heated up, it grew brighter than the sun and when it exploded, the force generated (thirty times more than the atomic bomb dropped on Hiroshima) shattered windows for miles about, causing hundreds of injuries.
In 1908, another similar event occurred over the Tunguska River area in Siberia, although this one was of much greater power. No remnants of the meteor have been found, so it is unknown what is was made of. It is thought that, like the Chelyabinsk meteor, the Tunguska object exploded above the ground when it became superheated by the friction of its passage through the atmosphere. The explosion was so powerful that it flattened a forest for about 800 square miles, knocking down about 80 million trees. The circular pattern of destruction is still visible today. The explosion is estimated to have been equivalent to about 15 megatons of TNT (about 1000 times greater than the Hiroshima bomb). The shock wave generated by the explosion shook the Earth with a force equivalent to an earthquake registering 5.0 on the Richter Scale. The sound was heard hundreds of miles away, and the dust thrown up into the atmosphere made for interesting sunsets and other atmospheric effects all over the world for weeks afterwards.
Another impact event you might have heard of is the one that supposedly caused an extinction event 65 million years ago. An asteroid, estimated to have been about 6 miles in diameter, impacted the Earth in the area of what is now the Yucatan Peninsula and the Gulf of Mexico. The energy of this impact was so great, equivalent to over one billion tons of TNT, that it threw so much debris into the atmosphere that sunlight was blocked over much of the Earth, causing drastic changes in the climate that led to the extinction of approximately 75% of all the plant and animal life on Earth, including the dinosaurs.
So, over the course of the history of the Earth, impacts by other bodies moving through the area were not exactly rare. In fact, the very existence of Earth is a result of a whole lot of material slamming together to form the early planet, through a process called accretion. Picture taking a bunch of small lumps of clay and throwing them together, one by one, so they stick together, forming a bigger and bigger lump. This is the prevailing view of how the Earth originally formed. An impact between this early Earth and another really large planet-sized body, knocking off a big chunk of the Earth, is what likely formed the Moon.
So, one interesting aspect to all of these asteroids and comets slamming into each other to form the Earth is that all of the minerals and most of the water on the planet originated somewhere else and ended up here when the comet or meteor they were part of impacted the Earth. All the metallic elements on the Periodic Chart from Iron on the densest end to Lithium on the lightest, were formed in the hearts of stars, through a process called nuclear fusion. In fusion, the nuclei of two atoms combine under great heat and pressure to form one larger nucleus. If you’ve been reading these articles for a while, you know that the identity of an element depends on the number of protons it has in its nucleus. If, for instance, two Hydrogen nuclei (one proton each) fuse together to form a nucleus with two protons, the result is no longer hydrogen–it’s helium, because any atom with two protons is helium. The fusion of hydrogen into helium releases vast amounts of energy, and this hydrogen fusion is the process that most stars are in for most of their lives. The light and heat given off by our Sun is the result of the conversion of hydrogen to helium. As the star starts to run out of hydrogen, it expands into a red giant star and the helium nuclei start to fuse to form heavier elements, like lithium, oxygen and, eventually, iron. Iron, with 26 protons in its nucleus, is the heaviest element that can be formed this way. All of the elements heavier than iron on the Periodic Chart, like copper, gold, and uranium were formed when very massive stars become unstable and explode in what is called a supernova. So much energy is released in a supernova that iron and other elements that were present in the star fuse to form the heavier elements. All this material is blasted through the galaxy by the force of the supernova. Some of the debris ends up smashing into the Earth, and that is where all our metals came from. It’s why certain elements are found in more abundance in certain areas than in others—veins of gold found in a mountain in California originated when a gold-rich meteorite slammed into the area millions or billions of years ago. We have lots of iron on Earth because lots of iron is produced in the dying stars, so most meteors have large amounts of iron in them.
Similarly, most of our water formed on asteroids and other bodies in the early solar system and was brought here during collisions as the Earth was forming and during its early life. The fact that so much of it accumulated is because Earth is, again, right in the “Goldilocks Zone”. We have just the right gravity, just the right atmosphere and just the right temperature for water that ends up here to remain, rather than boiling away or evaporating out into space.
So, the next time you see a shooting star and make your wish, take a little time to think about where that meteor started its life and think about the billions of years it has been moving through the galaxy, only to hit our atmosphere and burn up, just at the moment you, during your incredibly short little life, happened to be looking up. If that doesn’t make you feel lucky, and grateful to be part of such a wonderous experience, I don’t know what would.