Many people, myself included, have wondered if the human race should be worried about the sun exploding at some point in our future (it seems to be well established that we shouldn't be worried.) On a similar train of thought, I am curious as to what would happen to Earth if one of the fundamental forces, gravity, were to suddenly become weaker for some unexplained reason. I know that we generally consider mass in the present to have a gravitational pull that is constant - but what if, for reasons beyond our current understanding, the pull of gravity spontaneously became slightly weaker?

Specifically, what would happen to Earth's atmosphere if gravity were to suddenly become 9/10 as strong as it is currently? Would humans be able to survive?

I have found many answers about what would happen if it was absent, weaker from the beginning of the universe, etc, but nothing that answers this version of the question specifically.

EDIT: narrowed question

  • $\begingroup$ I think this is a meaningless question. There is no evidence that the gravitational constant has ever changed, and no theoretical evidence to suggest that it will ever change. $\endgroup$ Oct 7 '16 at 16:50
  • $\begingroup$ @JohnRennie I think whether or not $G$ will ever change is beside the point. OP simply wants to know how the world around us would change if $G$ were to decrease. Of course, the question is probably too broad as written, but that's a different consideration. $\endgroup$
    – DanielSank
    Oct 7 '16 at 16:54
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    $\begingroup$ Is something like 'would earth be inhabitable by humans' narrow enough? Other option is 'what would happen to earth's atmosphere...' $\endgroup$
    – SheerSt
    Oct 7 '16 at 16:59
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    $\begingroup$ Here's the deal we should make regarding these questions, some of which are interesting, in my opinion. If a plausible reason is given for the, (usually catastrophic) event, then that's an answerable question. If not, I feel they should be migrated to worldbuildingSE $\endgroup$
    – user108787
    Oct 7 '16 at 17:38
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    $\begingroup$ On the other hand, asking about what kind of an atmosphere a planet nominally like Earth but with 9/10's the gravity is answerable, and fairly easily. Where I live, normal 'atmospheric' pressure is more than 10% lower than at sea level. Somehow I survive, as does the rest of the biome. I leave questions on long-term sustainability of the atmosphere with respect to light element loss and solar wind to somebody else... $\endgroup$
    – Jon Custer
    Oct 7 '16 at 18:54

The athmosphere of the Earth continously escapes even today, but it does it slowly. What is needed for a molecule to escape:

  1. To have enough kinetical energy to escape the gravity well of the Earth (around $11.2 \frac{km}{s})$, in the higher athmosphere or around the Equator it is little bit smaller).

  2. It has to be enough high to not collide with another air molecule.

The process is significantly fastened by the Solar Wind.

The currently estimated characteristic time of the escape of the athmosphere is estimated to the order of some billions of years. (*)

The same estimation says some tens of millions of years for the Moon. Thus, if the Moon would have an Earth-like athmosphere, it wouldn't survive in a geological timescale, but it would be quite enough for us. (*)

These long numbers aren't surprising, because the speed of sound ($\approx$ the mean speed of the molecules) is some hundreds $\frac{m}{s}$, while the escape speed is ten-hundred times higher as this.

Thus, if the gravity would be suddenly weakened to its 90%, it wouldn't have any visible effect to the atmosphere. Maybe we would lose it 10billions of years instead 12.

But, other effects could be important. For example, the balance of the Earth crust would be significantly altered, it would probably cause earthquakes. Much smaller changes in the crust cause major problems.

(*): here professional source says, the escape of the atmosphere on the Earth is essentially negligible.


Even though it's a hypothetical question I think the specific answer would be that the atmosphere would expand slightly depending on how much gravity was lost.

  • $\begingroup$ Using the baryometric height formula, it would expand with around 10%. Its pressure would decrease also with 10% (not due to the expansion, but due to the decrease of its total weight). Probably none of them would result major changes. Maybe weather would change a little bit. $\endgroup$
    – peterh
    Oct 7 '16 at 19:53

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