Could a planet with no satellites and no tectonic plates be habitable? [closed]

Question

If there was an earth-like planet in the habitable zone, without tectonic plates or a satellite what would its characteristics be like (structure, geology etc.)?

Answer 1

You would have severely reduced crust eccentricity and no tides. You would still have volcanoes, but no earthquakes. Water would spread out more, meaning that you would need to have a lot less water for there to be liveable land.

Yes, but it wouldn't stay that way

Consider the following simplified diagram (I made it, feel free to share with credit). The vertical axis represents global mantle heat flow, though there are other factors like the presence of water that influence where the red and blue lines are. The horizontal axis represents the mantel's potential temperature.

Tectonic plates can exist between the red and the blue line, there are essentially two forms without tectonic plates:

Magma ocean

A magma ocean consists of a molten rock down several hundred kilometers. In this situation, a solid lid may form but constantly founder and disintegrate, where cooled material sinks deep into the planet's core.

If the potential temperature becomes cool enough a freeze up can occur when the forming lid freezes into the underlying mantle (c).

The presence of water in the crust may aid subduction, but the primary determining factor is the heat flow.

Early planets are often hot, but it is not inconceivable that a newly formed planet with a magma ocean can sustain a life, however, sooner or later a freezeup (c) will occur (unless potential energy stays too high), leading to g or d.

Stagnant lid

Venus doesn't just lack water, it has a very high temperature, and as such lack the heat flow required for plates to form (the lid temperature is too close to the surface temperature). Were Venus to cool down dramatically, it would be a habitable planet with a solid lid (i.e. no tectonic plates).

However, (depending on the plate thickness, in several hundred thousand, to several millions of years), the lid would melt from the increasing heat flow, leading to the formation of plates (along the thick yellow line), which might lead to massive earthquakes and catastrophic volcanic activity.

Under normal cyclical conditions, melt (e): The formation of plates would then once again lower the heat flow, either leading to stable plates (g) (or a trench lock (j) if the heat flow is less than heat production from radioactivity and there is enough water in and on the crust, leading to the oceanic base to freeze to the mantle).

If however the heat flow increases too much during melting, a magma ocean (b) may once again form.

If the mantle heat flow dips too low due to planetary (not atmospheric !) cooling, a Ridge lock may occur, in which the plates form into a lid.

If you go with catastrophic heat loss, expect a cataclysmic lid melting event.

I recommend you read (1) if you want to know more about the technicalities of plates.

Atmosphere

Then there is the problem of retaining an atmosphere. It is the magnetic field caused by the molten core which protects a planet's atmosphere from being stripped by solar wind. This would be a problem with a solid-lid, Venus for instance has a very weak magnetosphere.

However, in recent models, posted in (2) a planet with shallow oceans and an earth-like atmosphere can be habitable, and retain its atmosphere in a close solar orbit, with moderate temperature through dynamical heat transport.

In these models, the temperature range achieved varied greatly with respect to the speed of axial rotation, where a more moderate temperature range was received with a very slow rotation.

The dark (facing away from the sun) side of the planet radiates away a lot of the heat, but surprisingly in the optimal model, the temperature range is surprisingly limited:

Where the verical axis is surface area, for Venus with roughly 60% of the surface covered with water, where interestingly it wouldn't be the poles that freeze, but the water masses on the dark side.

1. Evolution of the mode of convection within terrestrial planets Norman H. Sleep Journal of Geophysical Research, VOL. 105, NO. E7, Pages 17,563-17,578, 25 July , 2000
2. Was Venus the First Habitable World of our Solar System? M.J. Way et al. Geophysical research letters, VOL. 43, NO. 16, Pages 8376–8383, 28 August, 2016

Answer 2

It could be habitable.

A planet without a satellite that was otherwise like Earth would be pretty much identical except that it would lack tides and the phases of the moon would not be available as a measure of time and it would be darker at night (like a new moon all the time).

In the absence of tectonic plates, there would be craters but not mountains and the planet overall would be flatter with less elevation differences (which would also disfavor deep oceans and would cause any rivers to be shorter or flow much more slowly; water would tend to be in ice or in ponds or crater lakes caused by impacts from space). It would also lack volcanos and geysers (which were critical to the evolution of life in the first place but for which substitutes in the formation of the first life could probably be found, for example, in solar heated ponds). It might also have a colder interior because tectonic plates are associated with a liquid magma core caused in substantial part by nuclear fission of elements within the planet. And, it would probably have less background radiation. But, none of these things are terribly essential to habitability. For example, people in Kansas manage tolerably well without mountains or oceans or volcanos or geysers.

UPDATE: A comment helpfully notes the example of Venus which is tectonically active (it has volcanos, for example) but not tectonic plates, a distinction that I had not focused on previously. The differences between a world with tectonic activity that lacks tectonic plates and one with tectonic plates is even smaller. The way that this plays out on Venus illustrates how small of an impact a lack of tectonic plates would have on its habitability. This also illustrates the fact the some of the major features of Earth's topography, like the Pacific Ocean are a consequence of the event that formed the Moon, so the lack of a satellite would also possibly mean it was more likely to be flatter.

Venus has two "continents," Ishtar Terra in the north (about the size of Australia) and Aphrodite Terra (about the size of South America) near the equator. . . . It was thought that these "continents" are tectonic features similar to what we see with the Himalayas, compression of the crust that creates mountains. Ishtar also has the tallest mountain on Venus, Maxwell Mons, 11 km above the mean surface (similar to Earth's sea level) of Venus. For comparison, the top of Mt. Everest is 8.8 km above sea level and the top of Mauna Loa is 9.2 km above the ocean floor. Most of Venus is relatively flat, compared to the Earth, with more than 50% of it within 500 meters of the mean surface. Venus has more craters than we see on Earth (more erosion on Earth), but no craters smaller than about 3 km due to the effects of the atmosphere (slows them down or breaks them up before they hit the surface).

If it was a collision that caused Venus to rotate clockwise (opposite of the Earth), this probably happened long ago at the same time that the planets were still forming and that a Mars-sized object hit the Earth and formed the Moon. It is likely, based on what we see now, that what happened on Venus 500 million years ago was due to a turning over of the surface. Without the water to allow for plate tectonics, it is harder for Venus to get rid of its internal heat. So, the mantle gets hotter and eventual melts the crust, forming a whole new surface. This process probably takes about 100 million years and is probably when most of the volcanoes on Venus formed—very little erosion to weather them away.

Venus does have tectonic activity: faults, folds, volcanoes, mountains, and rift valleys. However, it does not have global tectonics as there is on Earth—plate tectonics. This is thought to be due to the fact that Venus is hot and dry. To have true plate tectonics, you need to have subduction zones so that one plate can ride over the other. This happens on Earth, but not on Venus.

Tectonics is a very general word for the processes that go on to “building” a surface: volcanoes and earthquakes, for example. These existed/exist on Venus. On Earth, we have these and much of this is due to plate tectonics which is the movement of the plates which gives us the processes discussed above.

Answer 3

This has been noted in other answers here, before.

Without techtonics, the mountains will erode and low areas will fill in, and without any mechanism to throw up new mountains etc. you will be left with a flat planet.

In fact, the contenents are made from lighter rock which float higher in the mantle, and these were (and are still being) made as a refining process due to techtonic activity.

So, no continents.

A rather flat world covered with a uniform ocean.

Habitable? By what? You didn't specify. There is life deep inside the crust miles down living slowly off chemical activity. That would still work. But with no shallows and no land, don’t expect anything more complex than worms.