Солнечная система и ее тайны

Планеты Созвездия НЛО

What Life Needs and Creates

We have already discussed the basic attributes of life, and found that many of them relate to requirements or needs from the environment. What does life need from the typical environment of a planet? Let us answer this question from the perspective of a life form.

What Life Needs

Life needs the elements necessary to synthesize its molecular building blocks (carbon, oxygen, nitrogen, phosphorus, etc). In addition, life needs a solvent that is easily accessible in its environment. In the case of Earth life, that solvent is water. And life needs access to energy - solar light, chemical, or other - in order to continue to synthesize its building blocks and extract necessary elements from its environment.

While we talk about planetary environments suitable for life, we may also want to distinguish between the environments that allow for life to emerge, and environments that allow life to survive over geologically long periods of time. Scientists still do not know enough about life’s origins in order to determine whether life can always survive where it can emerge (and vice-versa), or whether the two environments need to be considered separately. The expectation is that, at minimum, a planetary environment should be conducive to the abiotic synthesis of the basic building blocks of life. We do not know what the minimal set of molecular building blocks required for life is, or even what the original set was on Earth. However, this fundamental uncertainty is allayed by the fact that we expect much of the organic chemistry involved in life to be similar. We are going to need the same elements, the same solvents, and a source of energy, just as we would on Earth.

The conditions discussed above are readily available on the surfaces of rocky planets, as long as they have some liquid water on their surface. This requires that they reside in the habitable zone and have an atmosphere with surface pressure above water’s triple point.

What life creates

Once life has emerged and sustained itself long enough such that it has diversified into life forms adapted to very different local environments on the planet, we can assume that the planet has a biosphere stable on geological timescales. That allows the biosphere to become a significant factor in the geology and chemistry of the planet. For example, new minerals may form and accumulate on the surface or at the bottom of bodies of water. Gases emitted by life may accumulate in the atmosphere or in liquid water reservoirs. Similarly, certain minerals and gases may be significantly depleted by the biosphere. The history of life on Earth is full of examples and has led to a planet fully transformed by life from its original pre-biotic state.

In the sections that follow, we will use the lessons we have learned about life in order to aid our search for alien life beyond Earth.

Солнечная система и ее тайны