How large can surface gravity anomalies grow before the planet becomes uninhabitable?

Question

Is a planet with varying fields of gravity possible? Yes. Earth has varying gravitational fields at various different times due to various factors, mainly location based.

How extreme can I make these varying gravity fields, before the planet becomes uninhabitable?

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The difference between the observed gravity and expected gravity, if the Earth was a perfect sphere, provides us with a handy visual aid of Gravity Anomaly Maps.

image below from NASA earth observatory using GRACE.

The Planetary Society posted an interesting and extremely helpful blogpost about using results from GRAIL to investigate the gravity anomalies on the Moon.

Differences in Earth Gravity are not only just due to location near the equator or the poles. Interpretations of recent GRACE findings have determined the following:

The model pinpoints more extreme differences in gravitational acceleration than previously seen. Standard models predict a minimum gravitational acceleration of 9.7803 metres per second squared at the equator and 9.8322 m/s2 at the poles. Hirt’s model pinpoints unexpected locations with more extreme differences. Mount Nevado Huascarán in Peru has the lowest gravitational acceleration, at 9.7639 m/s2, while the highest is at the surface of the Arctic Ocean, at 9.8337 m/s2.

However, these anomalies are very VERY small and can only be detected with sensitive machinery. Going to weigh yourself at the top of Mount Nevado Huascarán does not make any discernible difference from your own personal perspective.

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My Question

How extreme can I make these gravity field anomalies, before the planet becomes uninhabitable?

• before the planet's gravity either starts to equalize through Isostasy and other natural forces, or

• possibly tears itself apart

• I am hoping that it is possible to have areas of the planet with gravity differences that are noticeable to humans (even if just slightly) and still allow for life to evolve and continue to live on planet.

  • ie would it be possible to have ~1.1 or ~0.8 g environment in one region and a typical Earth gravity at another, on the same planet, at the same time.

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Answer constraints

These gravity anomaly maps are often representative of an average over a period of a month or year of time. For the purposes of this question, using the long-term averages is more than acceptable. Especially as the gravity anomalies are largely due to semi-permanent location factors and not that affected by temporal factors.

For reference you can use Earth-like parameters, as I'm building a planet far far away, where normal physics holds sway.

I'm also not looking for answers that explain how it would be possible. This question about a planet with changing gravity is the closest I could find on SE about what I am after. However it focused on changing gravity fields over a period of time (which would be cool) as well as how to explain how it would be possible. Not on how much actual change would be possible between 2 locations. It was a helpful question but doesn't answer my problem. Most of the 7 pages worth of [gravity] tagged questions were related to adjusting to different gravities, travelling from one to another, or changes over time (mostly centuries) etc.

I am aware that there are a lot of factors that influence the gravity anomalies. For the purposes of this question, I am mainly focused on how extreme these variations can be (due to whatever factors) before the planet becomes uninhabitable.

Extra constraint: As I am not worried about the 'how', right now, answers are open to terraforming and artificial initial causes. As long as the actual day to day location-based gravity anomalies are following normal physics, don't rely on tech to constantly interfer/stabilise, don't tear the planet apart and the planet is still inhabitable. I don't mind an artificial start to the process. As natural processes will always try to equalize, it may be difficult to have a planet with such weird conditions at such a late stage (life evolution etc). Just need to figure out how extreme these values can be before the natural equalizing processes start to kick in. Obviously answers that rely on an artificial initiating conditions should mention this 'how' factor.

Answer 1

Habitability has little to do with the ability to achieve orbits. It would have a lot to do with (a) the effects on human physionomy and (b) the ability of the planet to hold itself together.

+/- 50 mGal

The "average" gravity on earth ~9.8 m/s² (~32 f/s²) is represented by 1,000 Gal. The variations demonstrated in your graphic are +/- 50mGal. That must not be very much by definition because we don't feel the variation and the world hasn't experienced the apocalypse (yet..., this is worldbuilding, after all).

+/- 1,000 Gal

For the moment we'll completely ignore the utter impossibility of a planet with a surface gravity of 0 Gal.

However, we certainly would feel a difference between +/- 1,000 Gal (weightlessness vs double-weight). But does that make the world uninhabitable?

From a physiological standpoint we're OK. People travel to space and back frequently with no need for a period of readjustment (there may be for the sake of decompression as space suits are at lower pressure than cabin pressure, but that's another issue).

From an airplane issue, we're probably OK. Planes would need to compensate for being "heavier" or "lighter" depending on where they are. It would certainly change traditional flight paths as a "lighter" plane would require less fuel, so planes would generally avoid the higher G locations.

But, would it tear the world apart? Probably. At the very least we'd be earthquake central, but maybe we're not up to the apocalypse... yet...

HOWEVER! We've crossed a threshold. You can't have a planet with a 0 Gal surface gravity.

The reality of statistical variation

Here's the problem. We live on a globe. Sure, it has globs of lower-density material resulting in lower surface gravity in some locations, and globs of higher-density material resulting in higher surface gravity in other locations. But the reality is, the "average" makes the question of habitability irrelvant.

Why?

Becuase a planet with so much density, e.g., in the northern hemisphere to be substantially higher than the southern (say, +/- 500 Gal) would tear the planet apart long before life formed. Even if you simply reduce the issue to a bunch of massive super-dense "marbles" within the earth, they'd slop around like loose ball bearings and convert the Earth into a lovely asteroid belt with a few realy dense planetoids.

The low statistical variation (+/- 50 mGal) means the Earth wobbles a bit, but it basically spins like a top. But as that variation increases, so does the wobble (as does the inertial forces acting on the regions of high/low density that cause the variations). Earthquakes increase, dogs and cats begin living together, mass hysteria! (Now we're talkin'!)

So, how high can it go?

A geologist, a physicist, and a priest walk into a bar...

I have no idea how high the variation can go, and the reason I used the traditional joke lead-in is that we may be dealing with a guess on the order of having faith.

My EE background suggests that variation below 3% can generally be ignored. That suggests that we could handle up to +/- 30 Gal. Maybe more depending on exactly how materials are distributed through the planetary body. Maybe.

But, let's stick with tried-and-true statistics. I'm going to go out on a limb and suggest that +/- 30 Gal (that's +/- 3% and 600X what we experience now) is the max variation before the Ring of Fire turns the Pacific Ocean into the biggest Shabu-Shabu dinner in history.

Would we be able to feel it? Maybe. But considering how much distance must be crossed to reach the high and low points, probably not. It does represent a difference of about +/- 1.3 ft/s², but I expect that's below the threshold of feeling.

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Edit Mark Olsen correctly pointed out that I misread the table in the Wiki article. I've updated my answer to correct for it. Thanks, Mark.

Answer 2

@JBH's answer is a good one, but but I think it is based on an error: A gal is not one g, but 10**-3 g, and a milligal is 10**-6 g, so JBH's comments about the anomalies in the OP's question seeming high is spot-on.

But pretty much everything he said is still good physics, it just needs to be scaled.

(For a good article on the subject, see this paper)

There are a couple of other effects to consider:

Increased gravity in one area pulls both water and rock towards it and away from areas of lower gravity -- it flows/rolls downhill! -- until sea level has the same gravitational potential everywhere. So adding potential differences will change the topography. Basically, it affects the geoid.

Much more important than that is the effect of a loss of rotational symmetry. A lumpy body does not rotate smoothly, but can tumble chaotically. (Saturn's satelliey Hyperion does just this. See this article.) Chaotic tumbling would produce some pretty extreme effects with days and seasons and directions being essentially unpredictable!

Answer 3

To keep things very simple, let's define habitability for a point at the surface of the planet as having Earth-like gravity, air pressure, air composition, temperature and some water.

Important gravity variations can be found on fast-rotating worlds, close or contact binary planets, or on torus worlds - torus worlds probably won't be naturally formed, though. Those can vary from a theoretical zero for near-contact binary with negligible separation, to several g. But there will be points where the gravity is habitable.

Air composition doesn't directly depend on gravity, as long as it is high enough to keep enough atmosphere - but if there are points where gravity is Earth-like, it should be enough. Sure, even Earth atmosphere slowly evaporates, but you can either refill it, take a young world or take one that (like Earth) is big enough so it won't be a problem before many, many billions of years.

Similarly, temperature will mostly change with distance to and type of star(s), not surface gravity.

Air pressure and water are interesting. You would expect it to vary, with atmosphere and oceans to pool at the highest gravity points. But in fact, it will be everywhere the same.

Look at it this way: at those scales, even rock acts as a liquid (hence why worlds are mostly spherical, though those particular ones are much less so) and would have already flown there if it was possible. Hence, everywhere, the surface is horizontal even if the strength of gravity can vary. So water and air will still flow everywhere and equalise at the whole surface.

Atmospheric composition can indirectly be interesting, though: air may evaporate faster at low gravity points, but high gravity points will be able to retain lighter gas. So you could have a younger planet retaining hydrogen or helium in its atmosphere for a long time, something Earth couldn't.

tl;dr: Even major gravity variation will have little impact on the possible habitability of earth-like gravity points.

This is, of course, ignoring the potentially catastrophic effects fo gravity varying in time. But whatever is making gravity varying so fast is probably its own catastrophic problem anyway.

Example of close binary planet, with surface gravity map

Torus-shaped planets