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>One of my scale numbers is, everything between Jupiter and the Sun masses less than 2 Earths -- Venus, Mars, Mercury, the Moon, and all the asteroids mass less

by Arizhel 10y ago

>One of my scale numbers is, everything between Jupiter and the Sun masses less than 2 Earths -- Venus, Mars, Mercury, the Moon, and all the asteroids mass less than the Earth. Is that correct? Venus alone is about 90% the size of Earth, and Mars roughly 33%. It really seems like all those bodies put together should be greater than 1 Earth mass (but less than 2 Earth masses).

saalweachter 10y ago
That's size, not mass. Venus is about 81% Earth's mass, and Mars is only a little over 10%. Mercury gets you another 5.5%, the Moon (as above) another 1.2%, and the asteroids are negligible. In the outer solar system, it's basically all the Galilean Moons + Titan. ETA: Hmm, or maybe those are surface gravity numbers you're thinking of?
- Arizhel 10y ago
  Yep, I was roughly equating mass with surface gravity, which I see now is incorrect. I do find it really interesting and a little mind-boggling that the Moon, as huge as it is, with a good 1/6 of Earth's gravity, only has 1.2% of Earth's mass. Same with Mars: a good 1/3 of Earth's gravity, but a mere 10% of its mass. I really thought there'd be more direct correlation between mass and gravity than that. Of course, surface gravity is related to mass with both density and radius, but still, I would have assumed that the densities of these bodies would have been rather similar, as they're all small, rocky worlds.
  - saalweachter 10y ago
    The densities do vary by almost a factor of 2 (~3 g/cc for the Moon, 5.5 g/cc for the Earth), but the larger problem is that surface gravity is a linear function of the radius, and mass is the cube of the radius. Consequently, the mass of an object with a given surface gravity is inversely proportional to the square of the density. If the Moon were the same density as the Earth, and had the same surface gravity as it does now, it'd actually mass less than it does now -- and Uranus, despite have 15x the mass of Earth, has a lower surface gravity, because its density is so low. The rule of thumb for space is "any way your intuition can be wrong, it will be wrong". :-)

by Arizhel 10y ago

saalweachter 10y ago

That's size, not mass. Venus is about 81% Earth's mass, and Mars is only a little over 10%. Mercury gets you another 5.5%, the Moon (as above) another 1.2%, and the asteroids are negligible. In the outer solar system, it's basically all the Galilean Moons + Titan. ETA: Hmm, or maybe those are surface gravity numbers you're thinking of?

Arizhel 10y ago
Yep, I was roughly equating mass with surface gravity, which I see now is incorrect. I do find it really interesting and a little mind-boggling that the Moon, as huge as it is, with a good 1/6 of Earth's gravity, only has 1.2% of Earth's mass. Same with Mars: a good 1/3 of Earth's gravity, but a mere 10% of its mass. I really thought there'd be more direct correlation between mass and gravity than that. Of course, surface gravity is related to mass with both density and radius, but still, I would have assumed that the densities of these bodies would have been rather similar, as they're all small, rocky worlds.
- saalweachter 10y ago
  The densities do vary by almost a factor of 2 (~3 g/cc for the Moon, 5.5 g/cc for the Earth), but the larger problem is that surface gravity is a linear function of the radius, and mass is the cube of the radius. Consequently, the mass of an object with a given surface gravity is inversely proportional to the square of the density. If the Moon were the same density as the Earth, and had the same surface gravity as it does now, it'd actually mass less than it does now -- and Uranus, despite have 15x the mass of Earth, has a lower surface gravity, because its density is so low. The rule of thumb for space is "any way your intuition can be wrong, it will be wrong". :-)