One of the planets I'm thinking of for my book series has all of its landmass in the northernmost half of the northern hemisphere (it is thus mostly a water world) and is completely geologically dead. However, it has an Earth-like atmosphere, is roughly Earth-sized, and has an Earth-like climate (it is much further away from its star than Earth is, but its solar system has a red giant as its sun). Would the land on a planet like this be habitable long-term for humans?
I wouldn't expect long term habitability. A planet's magnetic field provides much of its shielding against stellar and cosmic radiation -- but the magnetic field is created by a "dynamo" action of circulatory motion in the liquid iron core of a planet. Earth's moon once had a magnetic field, but lost it when its core solidified in the billions of years since it formed.
Without a magnetic field, the life of an atmosphere is also shortened; stellar radiation will tend to strip away the upper layers (which will eventually become the entire atmsophere) without the magnetic field to limit its impact zone. Lose the atmosphere, and you lose the oceans (either to evaporation, or to a deep surface freeze like Europa and Callisto).
A moon of a gas giant could harbor life even if it is geologically dead.
Lets consider the two reasons geological activity is usually thought of as necessary, as L.Dutch said.
- A magnetic field is required to protect against the solar wind. In the case of a moon of a gas giant, the gas giant's magnetic field can fulfill that duty for us. We know this is the case because it happens in our own solar system with Saturn and Titan. The Saturnian magnetic field protects Titan's surface from cosmic rays and the solar wind.
- The atmosphere needs to be refreshed, or many gases will end up bound in rocks. On Earth, this happens mostly via volcanism. But if the gas giant has a ring system, gases in the form of ices can be slowly added to the atmosphere of the moon by "stealing" them from the ring system. This could last millions of years, or billions if another moon that IS volcanically active refreshes the rings.
Depends on your definition of habitable, and willingness to use technology for assistance.
The planet can't really have an Earth-like climate without any geological activity, because geological activity is a massive part of what makes Earth's climate what it is. No geological activity basically means no sea-life, period. Without faults, geysers, and volcanoes the ocean would be far colder and the food chain would be non-existent without bottom feeders. This then leads to a huge gap in the food chain for surface creatures as well since there are no fish type creatures to be eaten. Plant life may be able to survive decently, which could sustain in-land animals well, but overall the ecosystem would function very, very differently. Honestly the real question is whether or not such a planet could naturally maintain life at all, which I would consider questionable.
As far as "could we establish a colony there?" Considering we have been working for decades towards colonizing Mars (which is geologically dead), we could definitely build an enclosed, self-sustaining habitat. Your planet even has a few legs up on Mars. Ignoring the ecological concerns, the presence an Earth-like atmosphere means it must also have a magnetosphere (both of which Mars doesn't have) to protect from radiation. Both of those lead into the presence of liquid water implying much better temperatures than Mars.
That all said, I believe geological activity is tied to the presence of a magnetosphere somehow though, so it may be mutually exclusive.
Could this planet naturally sustain life? Doubtful.
Could we establish a colony and inhabit this planet? Yeah, probably.
This is a partial complementar answer.
I have concerns of how that planet could exist in the first place. Oxygen is produced by plants (inclusive marine algae) and cyanobacterias. Earth is 4.6 billions years old, and it needed roughly 2 billions years for starting to have some oxygen in its atmosphere and roughly another 2 billion to have enough oxygen in the atmosphere to become barely breathable for a human. And it needed a stable Sun for getting there long enough. So, your planet needs to be in a comfortable place around its star for a very long time.
However, if the planet is habitable around a red star giant, then it was probably too cold when the star was main sequence. If it was goldilocks when the star was main sequence, then it would be either fried to a crisp or destroyed altogether when the star becomes a red giant. Anyway, you won't be able to get an habitable planet around the red giant star.
Further, red giant stars are very unstable (take a look at Betelgeuse or Eta Carinae). This is really bad news for the habitability of your planet, as it means that any habitability conditions would be very short-lived.
Also, red giant stars have stellar winds much stronger than main sequence stars. This is also bad news, since it means a faster rate of atmospheric erosion, the same atmosphere that isn't shield by a magnetic field due to geologic death.
So, your planet could not even exist in the conditions given to start with. And even if it manages to exist, its habitability conditions would be very short-lived.
Life on earth depends on the planet’s geologic activity, which is one of the main forces that shapes our climate. A planet without any geologic activity cannot have an Earthlike atmosphere.
As others have noted, the dynamo-induced magnetic field is crucial for protecting our atmosphere and oceans. But there’s also atmospheric carbon dioxide, the blanket that keeps our planet warm enough for life. And plate tectonics and volcanism play a crucial role in the planetary carbon cycle by regulating levels of atmospheric CO2.
Basically, life (plants, sea creatures, etc.) takes carbon dioxide from the atmosphere, reducing the greenhouse effect that keeps our planet warm. Carbon dioxide also dissolves in rainwater (creating “acid rain”).
Volcanoes pump that carbon back into the atmosphere, and with no plate tectonics, there are basically no volcanoes. In other words, geologic processes are like our planetary thermostat, and they have a huge effect on both the temperature of the planet and its atmospheric composition.
Without the greenhouse effect, the earth would be far too cold for much of its surface life to exist. It would turn into a giant snowball. (And yes, global warming is a serious problem, but with no CO2 whatsoever, our balmy planet would be way too cold).
... if your "long term" is short enough, and your planet didn't form naturally.
If you can magic a planet like you describe into existence, the problems caused by no geologic activity wont show up except on geologic time-scales. But such a planet could never form.
The biggest deal is the lack of magnetic field. This is usually considered the primary benefit of a geologically active planet. Contrary to what you often hear, a magnetic field is not, on its own, a good radiation shield. Atmospheres are good radiation shields, and you need a magnetic field to protect an atmosphere.
Without a magnetic field, the atmosphere would be hit by charged particles that stream from the sun. The atmosphere would be slowly stripped away. Very slowly. Over millions of years. It would not be a huge consideration on human timescales, but planets last much longer than that.
If your planet somehow has an Earth-like atmosphere right now, it will keep it for a little while. But, how does it have an atmosphere in the first place? How did it form when solar wind was constantly stripping it away? How did life evolve to fill the atmosphere with an Earth-like concentration of oxygen? Did geologic activity stop suddenly? It won't happen naturally. Did someone terraform it?
A geologically caused magnetic field protects Earth from cosmic radiation, but it is not the only solution to the problem. For example Venus' thick atmosphere offers a similar level of protection without the use of a geologically caused magnetic field. Venus's atmosphere interacts with solar winds to create an ionosphere.
Solar winds are stripping Venus of its atmosphere. Especially lighter gasses found in the upper atmosphere. However in the 4 billion years the process has been taking place Venus hasn't lost its atmosphere yet. So long term as in kingdoms or Empires should be doable.
Habitable by whom or what?
Geological activity on Earth creates a magnetic field, which reduces the rate at which the solar wind blows away our atmosphere (most especially, our water as vapour in that atmosphere, since it's the lightest atmospheric gas). ISTR without our magnetic field it would take around 200M years to end up as dry as Mars.
So what would substitute for a magnetic field to maintain an Earthlife-compatible environment for at least, say, 2000M years? (A wild guess at the minimum span of time for complex life forms, which was around 3500M years here on Earth).
Stronger gravity. A bigger planet. Two Gees does not merely double 200My to 400My. Thermally activated gas molecules have to escape a deeper gravitational well, and the effect of quite a small increase in gravity is substantial. Unfortunately, it also makes the planet a lot less friendly to space transportation. It may also block the only known path to a technological civilisation. We needed hands to develop technology. We developed hands by virtue of our ancestors being tree-dwellers. High gravity may make trees impossible, or it may make the consequences of falling too severe for large animals to ever evolve in that niche. There may be other evolutionary paths.
More water. Ten times as much to last ten times as long. This has the drawback that there would probably be no dry land anywhere, and life that evolved there would either be marine or hadean (extreme pressure dark-dwelling life on an ocean floor under maybe 40km of water). Life could almost certainly evolve in these environments. Maybe intelligent life. Almost certainly not technological civilisation. However, it's not impossible that terrestrial intelligent life arises just as the last of the water is being lost. They'd work out that they were on a dying planet with maybe as little as 20My left before the environment goes to hell. Long enough, surely!
TL;DR. Life in such an environment -- yes. A technological civilisation -- maybe. A space-going civilisation -- probably only if geological and evolutionary time-scales coincide happily (scenario 2).
From what we know a geologically planet is necessary to support life:
- it replenishes the atmosphere/hydrosphere of gases and elements like water, sulfur, phosphorus, coming from the mantle/core. All of those are needed from life
- it ensures the presence of a magnetic field, shielding the planet from cosmic radiation