Before coming to Trinity I never thought about why the Earth existed, how it was made or what it was made of. Then again, who did? The study of meteorites, the solar system, space and how everything came to be is utterly fascinating. It is certainly the most enjoyable topic I have studied in my four years here, especially when you’re being taught by Ireland’s leading meteoriticist.
Our story begins almost 5 billion years ago when what we know to be the solar system didn’t exist. In its place was a giant cloud of dust and gas consisting predominantly of Hydrogen. Being gravitationally unstable, these particles began collapsing in on one another, and clumped together to form the beginnings of our sun, releasing energy from nuclear fusion when hydrogen forms helium. As this new sun gets bigger and bigger, its gravity increases and it pulls the rest of the cloud around it into an enormous disk with the sun at the centre. This disk is known as a proto-planetary disk and it continues to feed the sun and is also the source material for the planets.
About 4,567 million years ago the matter in this proto-planetary disk is constantly colliding and starts to clump together to form larger and larger chunks until eventually mini-planets or planetesimals, up to several km in diameter, begin to form. The composition of these planetesimals is what is known as ‘primitive chemistry’. Primitive chemistry is the chemistry of the original proto-planetary disk and, as such, the original composition of everything in the solar system. This information is obtained from the study of meteorites that fall to Earth and are believed to be the result of collisions between early planetesimals.
Meteorites can be divided into three main categories: stony meteorites (or chondrites), iron meteorites and achondrites. Chondrites are by far and away the most abundant, accounting for almost 86% of all the meteorites we know about. Their name comes from the fact that they contain tiny spherical grains known as chondrules that were once molten rock. Achondrites do not contain chondrites as they have been melted and their components equilibrated since formation. The study of chondritic meteorites, which are believed to be the building block of our planets, is fascinating and is the chief evidence we have for how our solar system formed. Chondrules in particular are extremely good indicators of what our solar system was like at the time of its formation.
The minerals in chondrules are mainly silicates and would need to have been heated to about 1600 degrees Centigrade. Several theories exist as to how this heat came about and thus how planets were originally created. These include the zapping of small dust and gas particles by gamma radiation from nearby proto-planetary systems. However, a theory originally proposed by Herb Zook in 1980 and more recently resurrected by Trinity College Meteorite expert Ian Sanders, is becoming more and more popular. Dr. Sanders is a lecturer in the Geology Department and does not believe that gamma rays were responsible for chondrules. Instead, the theory is that “when planetesimals grew to about 30 km in radius, the heat produced by the radioactive decay of 26Aluminium in them would have been sufficient for the entire body to exist in a molten state”. When these molten planetesimals collided, they would splash molten droplets into space – chondrules!
Gravity would bring these droplets back to a larger planetesimal and the process would begin again. About 2 million years after the beginning of this process, the 26Aluminium was so depleted that the chondrules could no longer be melted and were frozen as droplet sized spheres housed in these planetesimals. Accurate dating of old chondrules and other early solar system components has confirmed that they are indeed about 2 million years younger than the stony matter found surrounding them.
After this many more collisions took place and planetesimals grew in size to form the planets we see today and meteorites which are mainly the result of planetesimal collisions. The composition of chondrites is very similar to the bulk composition of the Earth and there is little disagreement that these are the early building blocks of our planet and the other rocky planets of the solar system.
Although only about 6 new meteorites each year are recovered for scientific work, they are undoubtedly the greatest storytellers to have come our way and narrate to us a story of astronomical proportions from billions of years before any life on Earth existed.
