The right elements & organics. Where do we find them?
Liquid water seems unique and indispensable for living cells to form and develop. There is a long list of good reasons for this, which we will come back to later. But equally essential, of course, is the matter that makes up life. Is this kind of matter available everywhere in the Solar System?
The elements
Logically, living matter is built with the chemical elements that are abundantly available everywhere. Our Solar and Milky Way systems are not unique in this respect. Like other places in the universe, it mainly contains hydrogen and helium. Take a look at the top ten most abundant elements in the Milky Way:
All these elements have not always been there in the universe. Briefly, it went like this:
At the time of the big bang, matter did not yet exist in the form of atoms. The first atoms to form were Hydrogen, and to a lesser extent Helium. These elements could then start to concentrate in stars. In the cores of those stars, much more Helium was formed at first, and heavier elements in later stages. Thus, the first 26 elements from Mendeliev’s table were formed everywhere in the young universe. At the end of their existence, stars explode, scattering all those elements around in the environment. All the elements thus formed are lighter than iron up to and including iron itself (Fe, element 26).
Some stars became very heavy, and exploded in very violent Supernovas. Even heavier elements such as heavier metals and even heavier radioactive elements were formed in these kinds of explosions.
On Earth, life is largely composed of the six elements often referred to in 1 word CHNOPS. So: carbon (C), hydrogen (H), nitrogen (N), oxygen (O), phosphorus (P), and sulphur (S).
Helium is of no interest to life because it is a very inert element. That is, it does not bind with other elements to form molecules.
Phosphorus only comes in at position No 17 in this list! Our Sun – hence our Solar System – has a relatively high content of phosphorus. But even then, Phosphorus is by far the least abundant of the CHNOPS series. So it is not surprising that plant growth, for example, is often limited by the limited phosphorus available in the soil. Fertilisers therefore usually contain phosphates (but also nitrogenous molecules). We will see later that phosphates and free oxygen gas were very likely the limiting factors on Earth for complex multicellular life (= animals and plants) to arise and thrive.
Molecules
If we start looking on a smaller scale, namely within our Solar System, we see that those individual elements occur just about everywhere. However, if we look within the Solar System at the molecules they form, we do see a very uneven distribution across space.
The main cause has to do with the distance to our star, the Sun. For this, we need to go back to the origins of our Solar System.
A giant cloud of gas and dust was disrupted, probably by an explosion from a nearby supernova. This caused that whole cloud to start moving and rotating, causing molecules to collide and start clumping. That clumping starts because of the attraction between molecules because of the (partial) charges they carry, and once they acquire minimal mass, gravity takes the leading role. Then gravity between those larger clumps began to take on a larger role, and the cloud began to collapse towards the centre. More than 99% of the matter present concentrated centrally, forming a protostar. This is a star not yet heavy enough to cause spontaneous nuclear fusion in its core.
When the protostar of our Solar System became heavy enough, the Sun began to shine for the first time. This happens when the matter at the centre is pushed together so hard (by gravity) that nuclear mergers spontaneously occur.
Remaining matter that had not been swallowed by the star consisted mainly of small molecules such as:
Gases:
- Hydrogen gasH2
- Helium He
Volatile matter, gas or solid (ice):
- WaterH2O
- Ammonium NH3
- Methane CH4
- Carbon dioxideCO2
- Carbon monoxide CO
Solid matter:
- Metals and metal oxides
- Silicon compounds (rock minerals)
Note that a large proportion of these small molecules (volatile matter) are in gas form when closer to the star, and in solid form (frozen) when further from the star. That imaginary boundary between ‘closer’ and ‘further’ is called the ice line. The ice line is actually somewhat different for each molecule.
Meanwhile, the Sun’s radiation increased, and the solar wind was born. This is a star’s constant radiation of very fast particles (protons, electrons, ions) in all directions. The solar wind started blowing away the remaining gas cloud all around the protoplanets. Between the Sun and the ice line, extra matter was blown away because that’s where most of the volatile matter was in gas form. So you got a differentiation into roughly two major zones:
- A denser zone (between the Sun and the ice line) where mainly minerals and metals remained, with a much lower density of matter.
- A further zone (beyond the ice line) with higher density and very much frozen debris (comets, planetoids and frozen dust)
In the dense zone, the rocky planets formed, and beyond that, the large gas and ice giants. The debris of frozen volatile matter did subsequently drift to all directions partly due to disturbance by large planets. As a result, the inner part of the Solar System, among others, experienced a meteor shower during the first hundreds of millions of years. As a result, the rocky planets were additionally enriched with this so-called volatile matter, including water, long after their formation. The share or importance of this enrichment for the inner planets is very much debated.
The many ice chunks from those early days of the Solar System can still be found in several places today in the form of comets, asteroids and space dust. Examination of these ‘primordial chunks’ shows that in addition to the dominant ‘basic molecules’, they also naturally contain a variety of organic substances, such as simple sugars, fatty acids, amino acids, alkanes and alkenes, alcohols, etc., and sometimes even nucleotides (building blocks of DNA/RNA). These, too, ended up on the rocky planets afterwards with the meteor rains.
We can conclude that because of this genesis, the elements and building blocks of life are distributed throughout the Solar System. In principle, therefore, there is no region where the building blocks of life are not available.
