Imagine the water as a shell on a ball. If the ball gets bigger, and the volume of the shell is constant, what happens to the thickness?
Mountains don’t just grow higher but the entire crust lifts up and expands. Essentially the Earth gains in surface area while the volume of water distributed upon it remains the same, effectively shallowing the oceans.
Wrap the planet in a low level warp field, of course!
Stranger
Earth’s (and Mars’) atmosphere is already much thinner than its mass could support. For comparison, Venus is a bit lighter than the Earth and much hotter, but it’s still able to hold onto an atmosphere a hundred times thicker than Earth’s. Last I heard, the best explanation for this is that we lost a whole bunch of atmosphere in some cataclysmic event, probably the same event that formed the Moon.
I don’t think that would be a significant effect. Physical strength among individuals differs by much more than 10% even now, so it’s not as if humanity would be divided into a superhumanly strong race born before the change in gravity versus weaklings born afterwards. Also, the bodies of those born before the change would quickly adapt to the new gravity, and their muscles would weaken. Astronauts on microgravity missions need to exercise to prevent the muscle decay which would set in even after a short time.
G, the gravitational constant affects the attraction between all objects, F = GMm/r^2. If you reduce the force between Earth and me by 10%, then you reduce the attraction between the Earth and Sun, leading to orbital instability as previously discussed.
It’s unclear if it’s even meaningful to talk about “changing the value of G” at all, and even if we could, we know of no mechanism by which it could be done in a small locality. Changing the mass of the Earth, though, is something that it’s easy to discuss. In that case, we just have to decide things like how abruptly we’re changing it, and maybe what distribution for the removed mass.
Given the incredible difference in masses between the sun and the earth, I don’t see this changing the earth’s orbit one whit. Now, the moon…
Kinetic energy is not conserved. Energy is. chancing the Earth’s mass also changes the potential energy in its orbit. If the mass of the Earth drops so does its potential energy. Momentum is also conserved, and that changes as well if the mass changes.
If rebounding is a buoyancy-related phenomenon, it might slow down if gravity is turned down.
Again, to reiterate a point that was previously made, the Earth’s mass is basically irrelevant to its orbital parameters. The Sun contains 99.86% of all the mass in the solar system. If you removed 10% of the Earth’s mass, or in some other magical way caused it to lose 10% of its gravitational force, the orbital change would be effectively non-existent, and difficult or impossible to measure.
Consider, for instance, the L3 Lagrange point in the Earth-Sun system, which on a straight line drawn from the Earth through the center of the Sun to the other side, is the point on exactly the opposite side of the Sun from the Earth and (almost) the same distance away. A small spacecraft at L3 would orbit the Sun with the same period as the Earth, even though its mass is many orders of magnitude less than the Earth’s mass, because both are insignificant compared to the mass of the Sun.
For completeness, I’ll note that L3 is very slightly closer to the Sun than the Earth is, but only because of the effects of the Earth’s gravity. It’s also unstable because it’s a saddle point – stable in two directions, but unstable (repulsive) in the other two. But if Earth didn’t exist, any small spacecraft could orbit the Sun at essentially (for all practical purposes) the same distance from the Sun as the Earth and with the same period.
Yes I understand all that. If the Earth were 10% less massive than it is and were in the same orbit as it is, then the orbit would remain basically unaffected. But what we seem to be talking about is that somehow magically the Earth loses 10% of its mass or G is reduced by 10%. In those cases, the orbit would change due to conservation of energy, and momentum. And I’m pretty sure the Earth would move into a higher orbit with a perihelion at its current point.
Well, that depends on the process by which the mass is decreased, and in particular on where the energy comes from to haul away the extra mass. But it’s a lot easier to come up with scenarios that don’t change the Earth’s orbit than ones that do.
You’d have to find some place to put all that material. Much more practical to increase the radius of the planet by c.1250 miles.
The o.p. didn’t actually specify the process why which “gravity on Earth was lessened by 10%”. Instantaneously reducing the mass of the planet or arbitrarily changing the gravitational constant without a corresponding change in kinetic energy would both be aphysical mechanisms that would violate conservation of momentum and would result in a severe discontinuity in the stress-energy tensor. Ignoring that physical impossibility, just somehow magically removing a distributed mass from the core and mantle of the Earth would also remove the specific orbital energy (and the gravitational potential energy associated with that mass, of course) without any real change in the orbit of the Earth about the Sun. It would, however, have an impact upon the Sun-Moon, where the barycenter will still be within the radius of the Earth but will move within about 1100 km.
Stranger
OK, I agree with that, but we’re talking about different scenarios. Basically there are three; to put it simply,
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The OP postulates a 10% reduction in Earth’s gravity but says nothing about loss of mass. In that seemingly impossible theoretical situation, the Earth’s orbit wouldn’t change in any measurable way.
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Somewhat more realistic but pretty fantastical is some imaginary process that removes 10% of the Earth’s mass in some benign, non-disruptive fashion. Again, that wouldn’t change the Earth’s orbit.
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The physically implausible scenario where the Earth loses 10% of its mass as in (2), but for some reason retains all its original kinetic energy. Unless fundamental laws of physics also change, this is exactly equivalent to accelerating the lighter Earth to the same momentum as before, and thus to a higher velocity. Barring some other peculiarities about exactly how this happened, it would have the effect you suggest. The Earth would gradually rise to a new aphelion on the opposite side of its orbit, and return to perihelion at the same point in its orbit where all this happened. But there’s no inherent reason that such an orbit should be “unstable”.
Yes it would, although the change would not be very large. Currently, the Earth’s orbit is growing larger by 15 cm/year. Most of that is due to tidal drag, but some (I think about 1 or 2 cm/year) is due to the Sun’s mass loss. Now the Sun loses a very small fraction of an Earth mass per year, yet the change in the Earth’s orbit is still measureable. So the Earth losing 10% of its mass should have a measureable effect on its orbit.
Keeping the same energy is not the same thing as keeping the same momentum. It’s impossible to do both at once, if you’re changing the mass.
That’s a valid point. I should have said “no significant orbital change”, since we’ve long been able to measure tiny changes like the moon moving away from us a couple of centimeters per year.
Note that though the Sun loses an estimated 1.5 x 1017 kg of mass per year due to emission of energy, it also loses an estimated 4 x 1016 kg on an annualized average due to coronal mass ejections. It’s no surprise that this is having a tiny effect on planetary orbits. (This compares to the mass of the Earth at 6 x 1024 kg.)
This thread makes me mindful (ha) of the studies focused on damage to the brain and visual cortices due to prolonged space flight.
Although we are only talking about a 10% reduction, could such a shift cause premature vision and brain damage? What about the mechanisms of the human heart and circulatory system?
Just because the gravity has shifted from 1.0 to 0.9 ( instead of, say, 1.0 to 1.5 ) doesn’t mean there would be no repercussions.
A few posts here have talked about humans moving around in 0.9 gravity. What about the impact on their bodies just existing in 0.9 ?
Although we obviously have no data from people living for any duration in a 0.9 g field, space physiologists have done a lot of bed rest and other simulated fractional gravity studies to evaluate the impact of living in fractional gravity. While these are generally concerned with accelerations of 0.5 g or less, it seems unlikely that being in a 0.9 g field would cause serious detrimental effects for adults, and while it might have some impact upon infant and childhood development there is no real reason to think that it would be severely detrimental, i.e. a lack of bone densification, spinal alignment, et cetera. The general consensus by space physiologists is that 0.5 g or greater is probably the minimum threshold to avoid the most severe syndromes from living in less than an Earth gravity field, but again, that is extrapolation, not direct experience.
Stranger