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Comment by dev1n | Hacker News Reader

Some possible reasons; Mars has a c02 atmosphere, water ice, it has a ~24 hour day as opposed to the moons 28 day-day, Earth and Mars have similar sizes.

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Besides, it wouldn't exclude the possibility of an extinction level event that disables the earth-moon system. Moon seems more suited for the role of an outpost, re-fuelling station and the final flight to Mars/elsewhere.

Dumb question: it can't be coincidence that our days are almost identical in length. Why are they?

It is a coincidence. The rotational speeds of planets formed through a lot of chaotic events in the early solar system. E.g. Venus spins in the "wrong" (retrograde) direction, completely violating conservation of angular momentum, probably because of a planetary scale collision.

We can try to explain solar system formation through mathematical models and simulation (Phobos and Deimos would be clues) but we can't go back in time and find the real explanation. No law of physics mandates that Mars' rotational period should be similar in magnitude to Earth's.

I don't know why you got downvoted but I couldn't find anything helpful regarding your question. I found some references to the fact that mars and earth have ~24hr days but no explanation as to why they are so similar. Mars also rotates (on its axis) the opposite direction of Earth which I found interesting. So many questions about our little local solar neighborhood, so few answers!

I'm pretty sure everything about the particular properties of our solar system is, strictly speaking, a coincidental result of the initial conditions of the big bang. But to speculatively address the intent of your question, I'd guess it has to do their relatively similar size, solidity and distance from the sun.

I'm not sure the premise is correct. Why can't it be coincidence?

I suppose it could, but of all possible values, it seems exceedingly strange that they'd fall on essentially the same number. It seems much more likely that it has basic physics behind it, but I'm not sure what it is.

random chance is as good as physics. the earth was hit by a mars-sized impactor allegedly, so if not for the moon, we would have a completely different day length. (assuming there would be some 'us'.)

Why does the Mars 24.5 hr day versus the Moons 28 day-day matter? What are the consequences of this big difference between Earth day and the moon/Mars day?

For constant power on Mars you need 2 kg of batteries for every 1 kg of solar panels. On the moon you need 50+ kg.

14 consecutive days of darkness in the moon creates issues with solar energy, heating, etc.

24.5 hours matches our circadian cycle. On the Moon extra power would be necessary for artificial lighting.

I don't think that lighting is going to be the difference between a viable and unviable non-terran colony.

I think you underestimate the importance and cost of the artificial lighting :P

On the moon it means you need enough solar power in batteries to power your gear, and to power appropriate spectrum lighting for plant growth.

If you can rely on solar power more reliably, then you can construct (significantly more complex than those required on earth) green houses which still benefit from natural light.

In all fairness, my parent poster was talking about human circadian rhythms, not greenhouses.

I actually agree that there are numerous challenges associated with very long days and nights. Artificial lighting per se is not among them. Solar power is a reasonable one.

The temperature swings on the moon are a major engineering challenge. 500f difference between noon and midnight.

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