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@abetterjulie Ganymede's surface gravity is 0.146g, which is plenty to keep whatever is on the ground where it is. 1L of liquid water weighs 146g.

@abetterjulie Regardless of gravity, material in space retains its full inertial mass -- that is, it takes the same amount of energy to move it, minus the gravity you're not working against. So something that weighs 1KG here on Earth, and something that weighs 1KG on Ganymede, will require different amounts of energy input to move.

@mos_8502 One more thing---so if I'm inventing a watery moon for a story, I can handwave the math because gravity and inertial mass? It's not a story for like a big publisher or anything, it's just a little game I'm working on.

@abetterjulie Well the thing is, the amount of gravity you experience on the surface of a terrestrial body like a moon is a function of two variables -- the mass of the body, and the distance from the centre of mass to the surface. So if your watery moon happens to have a very dense core, say lots of lead and heavy metals, it could have a much higher surface gravity than you would expect from its size.

@abetterjulie Aha. Retaining an atmosphere is more to do with blocking solar wind than gravity. Earth has its atmosphere because the core is spinning, creating a rotating magnetic field that blocks the solar wind. Mars lacks this, and so the atmosphere is literally blown away.

@mos_8502 A spinning core that creates a magnetic field might just be the coolest thing. I love that.

@abetterjulie Well yes, when a magnetic field rotates, anything conductive will experience an induced electric potential. Detecting that on Earth is hard because we fill the air with so much EM noise, but it's there.

@abetterjulie (And that's how an account about old computers inspired someone to become a physicist)

@mos_8502 my fantasy-inclined brain: What if galaxies are all just giant batteries?

@abetterjulie Well, most spiral galaxies like ours seem to have a supermassive black hole at the centre; this XXXXL black hole has a bunch of matter swirling around it called an accretion disc. As the matter gets closer to the black hole, the friction of the molecules rubbing against each other gets so hot that about half the mass never actually makes it into the black hole, it just gets shot out as X-rays and other such ionizing radiation. That's an awful lot of energy being released.

@mos_8502 so like a giant recycler?

@abetterjulie Sort of, yeah. You could call it that. Certainly everything going into the disc is dead, life itself isn't possible anywhere near that much radiation.

@mos_8502 radiation kind of ruins everything. :(