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Comment by julienfr112 | Hacker News Reader

Is the gravity really constant ? I mean, big chunk of lava moving with different concentration of iron would make it change, wouldn't it?

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No, as the parent said it isn't. This is why you can create a map of the variations like this one: https://svs.gsfc.nasa.gov/11234

Undersea mountains have gravity signatures very similar to overland mountains, even though the latter are significantly closer to space?

"Closeness to space" doesn't affect the gravitational force due to a particular piece of matter. Now, if water was as dense as rock, then undersea mountains wouldn't make such a difference to the gravitational field.

I'm trying to understand why you think "closeness to space" would be a factor. Maybe you are thinking that the seabed is going to be further from a detector than the terrain would be?

It is true that the further from the Earth the detector is, the smaller the anomaly will be.

Note that even hills are large enough to deflect the gravitational vector at the surface: https://en.wikipedia.org/wiki/Vertical_deflection

https://en.wikipedia.org/wiki/Gravity_anomaly

> big chunk of lava moving with different concentration of iron would make it change, wouldn't it

Why do you think the concentration of iron in magma would not be evenly distributed throughout the mantle?

The magma is not exactly a liquid. Neither is it homogeneous - the roots of the components of continental plates extend down hundreds of kilometers into the mantle. It is also possible to seismologically trace the remains of ancient continental plates which have otherwise all but disappeared from the crust.

Gravity is not magnetism, it does not care if it's iron or anything else.

Um how do you figure?

Gravity cares about distance and mass, and as large parts of the planet move their density distribution shifts affecting both of those factors

Lol this point is true but it’s downvoted.

I love how the IQ of this place keeps spiraling downward.

His point was that lava density could have high-variability.

There might be some confusion in these replies related to the gp's mixture of two terms. g = gravity of the earth which varies and the gravitational constant G which is a Constant and the same across the universe.

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