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Picture an earth-mass, breathable atmosphere planet; tidally locked to a red dwarf; orbiting far enough out that the dayside is not scorching; and with no planet-wide liquid oceans (but probably local seas or lakes, and possibly nightside ice).

I've read information about oceanic, earth-like tidally locked planets, but I haven't seen anything about planets with no oceans. Could portions of this planet's dayside be habitable? What would its climate look like?

My concerns about habitability are:

  1. Without large oceans to circulate heat, day-night temperature differences might be extreme enough to cause hurricane-force winds, frozen night-side air, etc.

  2. Water might get frozen and trapped on the nightside, without oceans to move it back to the dayside. This probably depends on atmospheric patterns, which I don't fully understand despite some research - I have two possible scenarios below:

Habitable dayside scenario: High altitude nightside air would cool, sink, then warm adiabatically, absorbing water. It would then move to the dayside, heat up, rise, then release water and cool.

Desert dayside scenario: High altitude nightside air would cool and sink, but the extreme temperature difference would negate adiabatic warming and cause it to cool further, releasing water. It would then move to the dayside and heat up, absorbing water and carrying it away to the nightside.

Also, is the coriolis force relavent for tidally locked planets? I intuitively don't think so because of the very slow rotation speed, but I've read about coriolis effects on Venus despite it's near-0 rotation.

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  • $\begingroup$ I wonder where the oxygen supply comes from (breathable atmosphere). Oxygen in Earth was produced by plants exposed to sunlight. There has to be sufficient water on the lighted side for plants to produce oxygen faster than it combines with other substances. That implies a lot of water and a lot of life on the dayside. Unless an advanced civilization terraformed the planet, giving it an oxygen rich atmosphere. $\endgroup$ – M. A. Golding Nov 21 '20 at 18:24
  • $\begingroup$ I'm currently envisioning a snowball/eyeball planet with glaciers like described here, which would vastly reduce the surface area available for oxygen to combine with other substances. Then the planet would need a much smaller amount of plant life, and thus much less available water. ...I think $\endgroup$ – Dalas120 Nov 22 '20 at 5:44
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Could portions of this planet's dayside be habitable?

Yes.

What would its climate look like?

Well, that gets complicated... there are multiple possible atmospheric circulation cycles that could apply, depending on details of exactly how fast the planet spins (i.e., how long its year is), how thick the atmosphere is, and so on.

In broad strokes, you likely get wind flowing along the equator from dayside to nightside and then back over the poles from nightside to dayside near the surface, combined with superrotation of the upper atmosphere analogous to Venus. This distorts the temperature bands that would be predicted based strictly on stellar light input, moving the temperate regions away from poles and closer to the equator along the lines of longitude, but also away from the subsolar point and closer to the terminator along lines of latitude.

For more detail, see Simulations of the Atmospheres of Synchronously Rotating Terrestrial Planets Orbiting M Dwarfs: Conditions for Atmospheric Collapse and the Implications for Habitability

My concerns about habitability are:

Without large oceans to circulate heat, day-night temperature differences might be extreme enough to cause hurricane-force winds, frozen night-side air, etc.

It takes surprisingly little atmosphere to transport enough heat to prevent freeze-out.

Water might get frozen and trapped on the nightside, without oceans to move it back to the dayside.

That is indeed an issue. If you are OK with glaciers rather than oceans, however, that's an alternative solution; just cover the entire nightside with an ice sheet (enough water to make an ocean if it were liquid, but it's not), which creeps back over the terminator and melts on the dayside, producing rivers, which then evaporate and snow out on the night side, continuing the cycle.

Also, is the coriolis force relavent for tidally locked planets? I intuitively don't think so because of the very slow rotation speed, but I've read about coriolis effects on Venus despite it's near-0 rotation.

That depends. Yes, it is relevant to the details of airflow, but it does not result in multiple distinct latitudinal circulation bands like we see on Earth.

It can be very relevant to the details of airflow in those scenarios in which the large-scale equatorial-polar surface circulation is modified by the presence of perpetual twin cyclones mirrored across the dayside equator.

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    $\begingroup$ Can you elaborate on what causes the asymmetrical air movement? I would naively expect airflow to be (nearly) radially symmetrical around the substellar-antistellar axis, like giant hadley cells. $\endgroup$ – Dalas120 Nov 20 '20 at 23:32
  • $\begingroup$ @user80711 Not really, no. It isn't something anybody predicted based on simple, intuitive models. It is the result of detailed numerical simulations--the best explanation for "why" is just "because that's how Navier-Stokes predicts fluids should behave". If you read the paper, they note that, at mid-altitudes, there is something similar to the expected rise-on-dayside, fall-on-nightside circulation happening on a great circle over the poles, but it cannot be fully symmetrical because there is some disturbance introduced by the planet's rotation. $\endgroup$ – Logan R. Kearsley Nov 21 '20 at 17:18

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