We've got a good head start at our original table here, but is there anything else we need or have missed that impact on human habitability of worlds? It's helpful to compare and contrast places in the solar system? (The original table comes from https://bit.ly/3pH1i3T)
$\begingroup$ What's the goal of your table, and what do you mean by habitability? Are you talking about a self-sustaining colony or supported exploration? As a high level view this is pretty good, it's not nearly detailed enough for decision making for either. Without more details it will be hard to answer your question. $\endgroup$– GdDNov 19, 2020 at 8:41
$\begingroup$ I think the local resources categories are rated too low for Mars or too high for Venus. Specifically on Venus, extracting matter from the surface is difficult because of the atmosphere while on Mars you can just dig. Also, Venus has no water ice which is an extremely valuable resource that Mars has $\endgroup$– DragongeekNov 19, 2020 at 11:27
$\begingroup$ Small solar system bodies isn't applicable to Titan. It might need its own column. $\endgroup$– WaterMoleculeNov 19, 2020 at 15:33
If by "human habitability" you mean "a world on which an unprotected human can survive for at least some time", then the requirements are pretty stringent. Off the top of my head:
- There must be oxygen in the atmosphere, otherwise humans will quickly asphyxiate. At least 6% O2 concentration is needed to live, and at least 19.5% is needed to be able to properly perform physical activity (these numbers assume atmospheric pressure equal to Earth's). But the O2 concentration shouldn't be too high, as this is unhealthy and increases risk of fire.
- There can't be any toxic gases in the atmosphere. This includes toxic levels of gases that exist in Earth's atmosphere, like CO2.
- The temperature should be within human tolerances. What these are is variable, but a world that's consistently below the freezing point of water or above the boiling point wouldn't be one I'd like to live in.
- The gravity shouldn't be too high, lest humans have difficulty standing up (or living, at extremely high gravity). 3-4 g seems to be the maximum limit for the average person. On the other hand, gravity shouldn't be too low either: while low gravity isn't harmful to humans in the short term, low-gravity worlds would have trouble holding on to an atmosphere.
- Radiation levels shouldn't be too high. Fortunately, a thick atmosphere can protect the surface of a world from radiation, as already happens on Earth. An atmosphere on its own can get blown away from the solar wind, so you'll also want a strong magnetosphere to protect the atmosphere from this (but Venus and its thick atmosphere show that this isn't a strict requirement).
For longer-term habitability, you'd need water and soil that allows the growth of Earth life, so that humans can grow their own food. You'd also want a non-flammable gas in the atmosphere (e.g. nitrogen) to dilute the oxygen and so reduce fire risk.
On the other hand, if you allow for technological assistance, then the requirements are less stringent. For example, allowing the humans to have breathing gear means that they no longer need O2 in the atmosphere or care about toxic gases. A reasonably thick atmosphere would still be required, to protect against radiation, prevent formation of abrasive regolith and because humans don't do well at low atmospheric pressures.
1$\begingroup$ +1 People live in places where the mean average temperature is well below the freezing point of water and, amazingly, have lived in some of these places for thousands of years without the benefit of modern technology ( en.wikipedia.org/wiki/Iqaluit ). Places where the mean temperature is above the boiling point of water would be a much different story. $\endgroup$ Nov 19, 2020 at 15:41