Building a planet
Ken Tapping, 14th August, 2013
For those of us who had the opportunity to see this year’s Perseid meteor shower, we had the chance to see another small step in the process of planet building. The Perseids and other meteor showers, such as the Leonids, which are due on the night of November 16-17, are just great spectacles to enjoy. World building was much more dramatic in the past. However, on rare occasions it can be pretty dramatic today.
About 4.5 billion years ago, a cloud of swirling cosmic gas and dust collapsed. The swirling caused the cloud to form a rotating disc, shrinking inwards. The material in the centre formed the Sun; the rest of the disc’s dust particles coagulated into lumps; some of those into bigger lumps and so on, leading to the collection of planets we see orbiting the Sun today. However, there are still lots of dust particles and lumps remaining, some hundreds of kilometres in size.
Many of the lumps are mostly ice. Sometimes collisions or other things deflect these into orbits passing close to the Sun, where the increased heating gradually melts and evaporates the ice, leaving streams of grit and dust. When the Earth ploughs through these streams at many kilometres a second, the particles enter our atmosphere and burn up, leaving the glowing trail we call meteors, or shooting stars. Meteor showers such as the Perseids are due to these debris streams.
The other particles, from dust upward in size, orbit the Sun as individuals. We are constantly encountering these, which is why we can see some meteors on any dark night. That is also why there is always a slender chance of encountering something larger, such as the body that came down in Russia not long ago.
We find the remains of our collisions with larger bodies scattered around our world. The most well known example is the Barringer Crater, in Arizona. This crater is about 1.5 kilometres wide and almost 200m deep. It is the result of our being hit, roughly 50,000 years ago by an object about 50m across, weighing roughly 270,000 tonnes, and moving at 20 kilometres a second. The energy released in the impact was equivalent to exploding around 2.5 million tonnes of TNT. However craters are comparatively rare on Earth because our craters get eroded away by the weather and then totally erased by the continuous recycling of our planet’s surface by plate tectonics. To get a better idea of how collisions have contributed to the building of the planets, we need only look at the Moon. That world has no plate tectonics and no weather. The only things changing the surface are impacts or lava flows due to impacts. With the aid of a small telescope we can get an overview of 4.5 billion years of planet building.
In the beginning, there were multiple episodes of major bombardment, which included some large bodies. This led to many of the largest craters we see on the Moon today, some hundreds of kilometres across. With Earth’s larger size and stronger gravity, it is likely that we were bombarded even more dramatically. However, our huge, ancient craters are long gone.
There are still many large objects out there. During the history of life on Earth they have been factors in major extinctions, ranging from 50% to 90% of the species living on our planet. Much more recently, in 1906, something entered the atmosphere and exploded over Siberia, flattening trees for miles. If it had exploded over a city, that city would have been wiped out. Planet building is still going on, and we live on a construction site.
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