[jamest (OP)]

Heather sent us this query:

'What would be the difference to our Earth if the collision which resulted in the formation of the Moon had not happened?'

[paul cotter]

I don't really know where to start with this question and I am only attempting an answer as no one else has responded. Everything imaginable would be different and I doubt we would be here. The earth would probably be a different size with a different rotation period. The amount of water on the surface could well be different(more/less) and of course tides would be much diminished. The distance from the sun might be different with consequent thermal changes. That is all I can think of offhand but I am sure there would be more.

[Halc]

Paul hits almost all of the relevant points.
Yes, without the mass gained from the hit, Earth would be perhaps 8% less massive, and it would arguably not have been Earth at all, Earth being the planet we know.

The hit gave the planet quite the spin rate (perhaps a 10 hour day?), but it also gave us significant tidal drag.  Without that, I suspect not-Earth today would have a day longer than 24 hours, but that's a guess.  The tilt would have been different, some other randomish value.

The planet would orbit closer or further from the sun (depending on the direction of the hit, which is totally unknown).  That means either too hot or too cold for liquid oceans, so there'd be very little noticeable tides without liquid water.

As for life, well just about any ancient change would probably affect that.  Maybe Theia brought life to Earth, but it probably started/arrived later than that in my opinion.

Without the hit, where would Theia have gone instead?  Was it from outside our solar system, or was it something that would just be elsewhere today, some extra planet finding a stable home in some other orbit somewhere?

Theia brought minerals to us.  Our chemical composition would be different without whatever it brought to Earth.  For instance it probably had a lot more titanium than did the pre-hit Earth.

[evan_au]

In terms of geology, Theia splashed a lot of Earth's crustal material (light Silica/Aluminum minerals) into orbit, forming the Moon. Without this, Earth would have had a much thicker crust, covering the entire planet.
- With a planet-wide crust, there would be no floating continents, no continental drift, and no subducting of carbon-containing compounds
- This appears to be the situation on both Mars and Venus. CO2 builds up in the atmosphere; in the case of Venus, it led to a runaway greenhouse gas effect.

The impact may also have impacted Earth's insides, too - some of Theia's heavy core may have punched into the mantle, producing an asymmetry we see in the mantle today. It may even have affected Earth's magnetic field (speculation!).

Finally, some speculate that if Theia originated in the outer solar system, it may have brought a lot of the water that is now in Earth's oceans.

But it was a long time ago; the Earth has been resurfaced many times by continental drift, so it is hard to reconstruct the details.

https://en.wikipedia.org/wiki/Theia_(planet)

[alancalverd]

Without this, Earth would have had a much thicker crust, covering the entire planet.

Pedantically, of course the crust does cover the entire planet but just has a lot of water floating on top. As I see it, relatively rapid radiation cooling of a solid crust with a liquid interior is bound to lead to fractures and hence tectonic drift and magma upwellings, with the "continents" merely being random chunks that happen to be above the water. The fascinating question is where the water came from!