Dr Jon Wade of the OU is an example of the mature student who got hooked on Earth Sciences – and has that typical enthusiasm of someone who has come late onto his subject.
Some intriguing stuff for starters: the nebula history of planet formation goes back to Immanuel Kant, 1755, but it was Pierre-Simon Laplace who supposed that the nebula had to be rotating round the baby Sun, developing concentric rings from which the planets formed. It appears he got this idea from a rather unusual character, Emanuel Swedenborg, who got the idea from a dream…(look the rest up.)
So we think we know about how gravity caused all nebula bits to coalesce then wham, you have your planetesimals then planets. But how the little bits of stuff start to aggregate is still not well understood. Once the bits get bigger then you start to get radioactive decay of Aluminium 26, then the lumps get bigger and differentiation starts, so you get a denser iron rich core and silicates on the surface. This happens when the lumps get to about 100km across, he says. Look up Pallasites – odd and lovely meteoritic lumps that have come from these planetesimals where you get iron and silicates (usually olivine) mixed up in them.
There is a lot of rust in this talk: the cores of Mercury, Earth and Mars are similar but iron in the mantle is hardly seen in Mercury, more in the Earth’s and even more in Mars’ (weight/atomic percentages being roughly 1%, 8% and 18% respectively). This means that the planets nearer to the Sun lost their oxygen more easily.
So, is core formation the key to planetary habitability? (His words) How long did the water last? How significant is this rustiness? And why is Mars so lopsided? Mars’ northern boundary is lower than the southern, which has lots of craters and a high level of minerals that have reacted with water. The northern bit is an old ocean basin, the result of a massive impact. It has a long dead volcano 22km in height. Why no evidence of tectonic activity? The implication is that there was a lot of water on Mars between about 4 billion years ago and 3.5 billion years ago, which means a lot of sedimentary activity took place on Mars in the first 500 million years of its life. Its magnetic field was weak, as were its tectonics, and there may have been as much as 3km depth of water at one point. The presence of the iron (more of his lively speculations) means the water was taken up in the rocks and sediments. Easy to understand why Mars is the colour it is – yes, it really is the rust – and why the lack of water prevented life from evolving.
Luckily the constant upheavals in the Earth’s mantle mean that new stuff keeps coming to the surface, so it never went the way of Mars. Copper, nickel, iron, manganese, all stayed stuck deep in Mars’ core. Iron is very important for life on Earth. However, we did have a boring billion years (his words) around 2.5 billion years ago, where there was major oxidation (too much iron again) and a large upsurge in methane in the atmosphere, which killed off any attempt at incipient life formation. Look at the surface basalts. He says Earth’s core is gradually growing, but that it’s crystallisation that’s keeping the heat going, rather than radioactivity. He says radioactive elements don’t like staying in the core.