Conditions for Life

The elements that are most often present in biological matter are: carbon (C), hydrogen (H), nitrogen (N), oxygen (O), phosphorous (P), and sulfur(S), or CHNOPS. These are rather common elements in the Universe. In order of decreasing abundance, in the Universe we have the following elements: H, He, O, C, N, Ne, Si, Mg, Fe, S, etc. Other elements important to life are: Na (sodium), Ca (calcium), K (potassium). In contrast, the crust of the Earth is composed principally of Si, O, Al, Fe, etc. Thus, it seems clear that biogenic (=related to life) elements are quite common in the Universe. However, the presence of the right elements is not enough. Indeed the complex functions performed by cells in biological matter are due to complex molecules, such as proteins, enzymes (=promoters of chemical reactions), DNA and RNA. What are the conditions necessary to make such molecules?

In interstellar space, simple molecules, such as water, ammonia, methane, carbon dioxide, etc., are easily produced. These molecules can also readily form on a planet. One would hope that by preparing the right soup of simple molecules, and with the injection of some energy to get reactions started, biologically relevant molecules could be produced. The Miller and Urey's experiment did just that, and some amino-acids, which are building blocks of more complex molecules present in cells of biological matter, formed. However, the road to make biologically functional matter, such as a working (living) cell, is much longer, and as of today nobody has found the recipe to do that, although the main steps to get there are broadly sketched.

The key element in living matter is carbon. Its abundance and versatility (it can form bonds with one, two, three, and even four other molecules) are well known. We expect life in other worlds be based on carbon chemistry; nonetheless, the possibility of using other elements, such as silicon, has been explored. However, silicon, although abundant, doesn't allow the richness of bonds and configurations that carbon does.

Another important molecule for life is water; the presence of a liquid substance is very important for life: chemicals can be moved easily by diffusion, and a liquid can act as a solvent for nutrients and waste products. Water is in liquid form over a wide range of temperature (from 0 C to 100 C); it can also participate in chemical reactions. Other molecules exist in the liquid form, but their temperature range is narrower and the boiling temperature lower than water's. Since chemical reactions proceed at increasingly higher rates at higher temperature, life based on liquids such as methane or ammonia would have a much lower chance of changing and evolving, assuming that it can be started at all.