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How far does a moon need to be from the roche limit for sapient life to evolve?

This is something I've been wondering about for ages. Life on exomoons seems to be a really unexplored subject in science, despite having quite a presence in fiction. I only know that at a distance of about 10 parent radii, the odds of a runaway greenhouse effect due to proximity to the host are reduced significantly.

I know this is probably very unorthodox, but out of desparation, I looked to Kurzgesagt's video on what would happen if Luna crashed into Earth. Using an equally unorthodox method (messing around in Universe Sandbox), I got an upper tidal-force limit of either 1.54E+21 or 2.53E+21 Newtons exerted upon the moon before the oceans would start regularly rising and receding by 'hundreds of kilometres'. Whilst not a barrier to life on its own, I imagine this would significantly limit the odds of sapient life evolving (which is what I'm particularly interested in). Obviously, though, this video is about Earth.

Originally, I asked a few extra things; I've since edited this to be more simple.

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  • $\begingroup$ Not seeing the wordbuilding element here. This is a better fit for astronomy.stackexchange.com $\endgroup$ Commented Jul 20, 2023 at 19:42
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    $\begingroup$ I believe the question could be rephrased as: How far away from Roche limit must a moon be to avoid the worst of the close to Roche limit affects? Plus some other questions that should be asked in a different question. $\endgroup$ Commented Jul 20, 2023 at 20:04
  • $\begingroup$ @GaultDrakkor Simplified somewhat per your comment. Thank-you! $\endgroup$
    – Kazon
    Commented Jul 21, 2023 at 23:03

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There's a lot more that could be explored regarding habitability of exomoons:

  • Tidal Locking - Exomoons that are tidally locked to their host planet could have very uneven heating similar to how one side of the Moon always faces Earth. This could create temperature extremes and concentrated frozen/arid regions. Complex life may only thrive in the terminator zone between extremes. Slowing a moon's rotational period could help distribute heating.

  • Orbital Eccentricity - A circular orbit is ideal for stable climate and tides. A highly elliptical orbit could cause tidal forces and heating to fluctuate dramatically between orbital periapse and apoapse. Any life would have to adapt to these cyclic changes.

  • Geomagnetic Field - A strong magnetic field generated by a rotating iron core helps shield a moon's atmosphere and surface from cosmic and stellar radiation and solar wind erosion. Not all moons may have dynamo magnetic fields.

  • Plate Tectonics - Geological activity driven by interior convection and plate movement aids geochemical cycles, atmosphere regulation, and habitat renewal. Smaller moons may lack active plate tectonics.

  • Orbital Resonance - Moons often fall into orbital resonances with each other or their planet. This could enhance tidal heating or make orbits more chaotic over time. Resonances may need to be avoided for long-term stability.

  • Neighboring Moons - Large nearby moons can perturb each other's orbits over time through gravitational interactions. Multiple moons need adequate spacing for stable orbits.

  • Comet/Asteroid Impacts - Impacts could cause mass extinctions if too frequent. Larger moons have stronger gravity to attract more frequent catastrophic impacts. But impacts also deliver organics and volatiles beneficial for life.

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A frame challenge.

This is something I've been wondering about for ages. Life on exomoons seems to be a really unexplored subject in science, despite having quite a presence in fiction.

There actually are a number of scientific articles discussing the possibility of life on exomoons orbiting exoplanets.

Here is a link to a list of articles by astrophysicist Rene Heller.

https://arxiv.org/a/heller_r_1.html

Of these, numbers 35, 37, 50, 53, 54, 55, 58, and 59 have titles mentioning the theoretical habitability of exomoons.

Large exomoons are likely to be tidally locked to their planets, which will keep them from becoming tidally locked to their stars even if they orbit very close to dim low mass stars. So with one side eternally facing the planet and the other side eternally facing away from the planet, and the moon orbiting the planet several times during each orbit of the planet around the star, each side of the moon will alternately face the star and face away from the star.

And with almost all potentially habitable exomoons tidally locked to their planets, they won't have large tides due to the changing direction to the planet. Any tidal bulges in hypothetical bodies of liquid will remain permanently located over certain locations instead of moving around the moons.

No matter how strong the tidal forces on a tidally locked moon are, the tidal bulges will not move around the moon and a specific location will not experience rising and falling tides. Your fear of giant tides is unrealistic.

However, since all orbits of moons must be at least slightly elliptical, the moons will move at least slightly closer and father from their planets. That will cause tidal heating of exomoons. Some degree of tidal heating can help a moon which is otherwise too told to be warm enough for life.

Too much tidal heating can cause excessive volcanism on a moon, making it a volcanic hell like Io, the innermost Galilean moon of Jupiter. Io is intact, and so must be far beyond the Roche limit for Jupiter. In fact Jupiter's innermost moon, Metis, which is beyond Jupiter's Roch limit, orbits only 0.303times as far from Jupiter as Io does.

And a lesser amount of tidal heating can cause a runaway greenhouse effect on a moon, like that suffered from other causes by the planet Venus, which is clearly outside the Sun's Roche limit.

Any hypothetical exomoon large enough to have sufficient ex scape velocity to retain a dense atmosphere for geological eras of time is likely to to have a high density due to being made of relatively dense matter and to its internal matter being compressed by the weight of matter above it.

Titan, the largest moon of Saturn, has a density of 1.8798 grams per cubic centimeter. The two largest moons of Jupiter, Ganymede and Calisto, have densities of 1.942 and 1.8344 grams per cubic centimeter.

The giant planets in our solar system have densities ranging from Saturn, 0.687 grams per cubic centimeter, to Neptune, 1.638 grams per cubic centimeter.

And the four rocky terrestrial planets in our solar system, which are much more likely to retain dense atmospheres for geological eras of time, have densities ranging from 3.9 grams per cubic centimeter for Mars to 5.514 grams per cubic centimeter for Earth.

Thus any moon of a giant planet large and massive enough to be habitable is likely to be denser than the giant planet which it orbits.

And the calculation for the Roche limit of a particular planet and particular moon includes in part on the ratio of their densities. And since potentially habitable exomoons of giant planets would normally be much denser than their planets, I suspect that the Roche limits would be quite close to the the planet in the cases of large and potentially habitable exomoons.

So I suspect that in most cases of giant planets and their moons which are large enough to potentially be habitable, the Roche limits for the large and relatively dense moons are likely to be much closer to the planet than the "habitable edge", the minimum distance from the planet where the moon would have excessive tidal heating and experience a runaway greenhouse effect.

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