Carbon, as biologists and chemists can tell you, is really special. We call life on Earth "carbon-based" because that little atom makes all the complicated chemical reactions of life possible. Life as we know it cannot exist without carbon!
Why is carbon so important? First, atoms of carbon have the perfect balance of chemical reactivity and stability. Carbon will easily bond with other elements necessary for life, like oxygen and nitrogen. But it doesn't have such a tight bond with those elements that it requires a huge amount of energy to break those bonds later. This is big because living things are constantly breaking down carbon-based molecules, like glucose, to get energy.
Second, carbon is able to bond to itself and other elements in many shapes, from simple structures to complex rings and chains. This creates many opportunities for interesting chemical reactions!
Lastly, many molecules made with carbon are water-soluble. This means they dissolve in liquid water, making them able move around and interact with other molecules. This is essential for many forms of life. For example, carbon-based proteins, nucleic acids, and sugars in our bodies are dissolved in the watery cytoplasm of our cells.
But is carbon the only member of the periodic table that could support all these processes of life? Since the 1890's, scientists and sci-fi authors have been looking at the element directly below carbon on the periodic table: silicon!
Silicon is an element found in electrical transistors and beach sand that can form bonds in ways that are similar to carbon. Silicon isn't a perfect twin of carbon, though. Silicon is more reactive and less stable when bonding than carbon. It also has the problem of bonding too easily to oxygen. In fact, any silicon in organisms on Earth is already bonded to oxygen!
How different would a planet have to be to support silicon-based life? The cute character "Rocky" in Project Hail Mary accurately shows us some of the differences. For one, an oxygen-rich environment would make silicon-based life basically impossible. Silicon is plentiful in Earth's crust, but it is all locked away, bonded to oxygen as silicon dioxide.
Our planet is also too watery for silicon-based life. Molecules made with silicon can be unstable in water. Silicon life forms would not be able to use water as their solvent to transport and store energy.
What would the planet have for properties? First, it would need to be low in oxygen, and then, it would need a different solvent than water. Scientists who looked into the likelihood of silicon-based life found that a planet or moon with high temperatures and abundant sulfuric acid could possibly support silicon-based life.
Here's a graph showing how water, sulfuric acid, and other possible solvents compare:
Here are some questions I think of when I look at this graph:
💡Water is a great solvent for several reasons. First, it has just the right amount of reactivity. Second, it has just the right amount of solubility. Finally, it is a liquid at a large range of temperatures. The size of the box on the graph shows the range at which the solvent is liquid. What other compounds have a range of liquid temperatures that is equal to or greater than that of water?
💡Three of the solvents on the graph are gases at room temperature; they are only liquids at very cold temperatures. Chemical reactions happen very slowly, if at all, at cold temperatures. Based on their temperature range and location on the reactivity scale, which solvents would likely be too cold to support reactions?
💡Look at the descriptions of reactivity and solubility in each of the four corners of the graph. Based on the scales, if a solvent was liquid at 300oC and had the same reactivity as liquid methane, how would we describe its solubility and reactivity?