Many, many years ago, I used to devour science fiction comics. In one of them, astronauts from Earth were conducting the first-ever exploration of Mars. They were really surprised when a huge alien hauled itself out of the sandy ground and pronounced “My body chemistry is based upon the element silicon”. Even if we can absorb the idea of aliens speaking English, that opening greeting was not exactly what one would have expected.
All the living creatures we know of on our planet, including us, have body chemistries based on carbon. This is why the study of carbon-based molecules is referred to as “organic chemistry”. The reason terrestrial life forms use carbon is that carbon atoms can chain together forming huge molecules, containing other elements, such as hydrogen, oxygen, nitrogen and others. This gives the scope for putting together all the different chemical tools we need: amino acids, proteins and so on, for life as we know it. As far as we know, no other element offers this huge flexibility.
The complicated carbon chemistry driving our forms of life involves water, so we either need to live in places with temperatures higher than the freezing point of water or have ways to keep our bodies above that temperature. By using that highly active gas, oxygen to react with carbon compounds in our food, we can obtain enough energy to keep ourselves warm enough to survive under most of the conditions here on planet Earth. Note that we walk a fine line with oxygen, we need it to live, but too much can damage us. At the same time, our biochemicals break down if they get too hot. This sets the range of temperatures our form of life can survive.
The most common element in the universe is hydrogen, making up 74%. Helium is next, at 24%. Everything else totals up to the other 2%. Of that oxygen contributes 1.04%, Carbon 0.46%. Silicon comes in at 0.07%. Since all the other elements are waste products from energy production in stars, and stars are fuelled by hydrogen, there should be no shortage of new stars for a while.
Space is filled with great clouds of hydrogen and all the other elements. In the depths of the clouds, screened from the ultraviolet emissions from nearby newly born stars, the atoms start to combine, forming a huge list of carbon-based chemicals relevant to our form of life. It is likely that new planets get some of this stuff, so if conditions are suitable, life could get started. What about our silicon alien?
Silicon is very similar to carbon, and although itѻý not the case here on Earth, could there be planets with silicon-based life? Many silicon-based analogues of carbon-based molecules have been made in the laboratory. However, there are problems. Our form of life is based upon the ability of carbon atoms to form long chains, and to function in an oxygen environment. Silicon should be able to form long chains too, but silicon atoms are not as good at this, and would rather hold hands with oxygen atoms than each other. Oxygen could be a poison to silicon-based life forms. In addition, we breathe out carbon dioxide. Our silicon friends would breathe out silicon dioxide - sand. However, silicon molecules are more stable at high temperatures, so if there is silicon-based life it is likely to be on hot planets.
Of course there is another silicon-based option. Those robots exploring the Solar System on our behalf are based on silicon, an essential element in their electronics, rather than using silicon-based chemistry. Maybe the silicon-based life forms we encounter will be more like them.
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Look for the Perseid meteor shower on evenings around the 12th. Saturn rises in the late evening. Jupiter and Venus lie close together in the predawn sky. Venus is brighter and whiter. The Moon will reach Last Quarter on the 15th.