Europa and Earth water comparison

[Darnok]

How did they calculated this?

http://imgur.com/OnPVJ8A

[Brotoro]

How did they calculate how much water the Earth and Europa have? With Earth, you mostly have to add up the volume of the oceans and other surface water (and smaller contributions from ice caps and underground water and water vapor in the air). With Europa you can tell from its size and density how much of its interior must be rock and how much is water.

Edited March 19, 2015 by Brotoro

[Darnok]

How did they calculate how much water the Earth and Europa have? With Earth, you mostly have to add up the volume of the oceans and other surface water (and smaller contributions from ice caps and underground water and water vapor in the air). With Europa you can tell from its size and density how much of its interior must be rock and how much is water.

Ok, so how do you calculate density of planet covered by ice from such huge distance?

[Vaporo]

Just look at how much Europa affects Jupiter. We can estimate the mass of Jupiter from how quickly its moons orbit it, and we can look at how much Jupiter "orbits" Europa to estimate Europa's mass. Plus, if you have a probe, you can measure Europa's gravitational pull directly.

Edited March 19, 2015 by Vaporo

[AngelLestat]

Iapetus is the natural satellite which it seems has the most % of water according to its volume.

They think measuring its density that it may be 80% water.

[Brotoro]

Ok, so how do you calculate density of planet covered by ice from such huge distance?

The last thing Vaporo said. (We can look at the way Jupiter's moons orbit it to find the mass of Jupiter, but because the masses of the moons are much smaller, they don't have a big effect on Jupiter's motion...so we don't get the moons' masses that way.) You can look at the way the Jovian moons tug at each other, and the way they pull at our spacecraft during flybys of the moons, to get good measurements of the masses of the moons. We also get very accurate measures of the moons' diameters from the space probes, so we can get accurate volumes for them. And from mass and volume, you can get the average density.

Edited March 19, 2015 by Brotoro

[lajoswinkler]

Problem with this animation is that it accounts for free water on Earth's surface and total water in Europa, so it is essentially highly misleading.

Earth contains a lot more water than Europa, but most of it is trapped inside of it, highpressure-stuffed in magma and crystal lattices of lithosphere's minerals. Hydrosphere and atmosphere account for only one part of the total water content.

[Kryten]

Ok, so how do you calculate density of planet covered by ice from such huge distance?

Mass in known from orbit around Jupiter and knowledge of Jupiter's mass, volume is known from apparent diameter. Density is easily calculated given those.

[YNM]

Density, which radius comes from direct observation via angular diameter and radar ranging, mass from indirect observation of the change of spacecraft's orbit.

Other things could come from orbital parameters of Europa itself and it's observable error. If Europa does actually have liquid water below the thick ice, the solid core could be not at the center, and it'd give a slightly different orbir that otherwise expected.