Thanx Mark. If you wouldn't mind keeping an ear open to that channel and posting when part 5 turns up, I'd appreciate that.

I've saved the series as a vignette for my honours-level course "Comparative Anatomy of the Terrestrial Planets". Whilst I don't agree with all that's said, it's a good summary of existing ideas.

What I think thay have missed out on is the importance of plate tectonics, which itself depends on the physical size of the planet. Mars is too small, Earth is just right. Explaining the differences between Earth and Venus is quite interesting and there are multiple hypotheses there with very different outcomes:-

In essence they boil down to end members of:

a) Earth just happens to have plate tectonics and Venus doesn't, therefore PT is an uncertain event

b) Venus has no PT because of its atmospheric and volatile history (i.e. loss of water leads to change in tectonic style from continuous PT to episodic resurfacing as in Venus)

Planets larger than Earth run into problems and at about 2.5 Me the increased gravity (which makes mountains smaller) and the increase in ocean depth (which comes from the volume to surface area ratio), leads to a planet with global oceans and essentially no land.

I'm not sure at what size PT becomes neccesary, but it's somewhere between Mars and Earth and may be affected by the Moon-forming impact. (only Earth has a big Moon, only Earth has PT, ergo PT is caused somehow by the Moon-forming impact, or tidal evolution of the E-M system) - a poor argument, but one to keep in the back of one's mind.

- BTW, a retraction. Having learned MUCH more about the Moon-forming impact, I can no longer sustain my original arguments about "loss of crust" for the origin of PT. Strike One...

The reason why PT is so important, is that it:

1) Maintains a duality of land and ocean, allowing evolution to progress from marine to terrestrial environments, and hence allow the key discoveries that lead to technology - Fire, Metalworking, Electricity, Computers, Radiotelescopes, Rockets.

2) Acts as a chemical reprocessing system, returning atmospheric constituents such as CO2 and key biological nutrients such as S, N, and especially P, from seabed sequestration and deep burial.

3) Allows the complex chemical recycling and concentration that leads to ore deposits that facilitate the lattter stages of (1).

There are other reasons, but they are less vital. In short, my hypothesis is "No PT, No Technology"