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Lorcan Faol
2004-Mar-10, 11:53 PM
I don't understand how the density of other planets or moons in the solar system is found. I was just reading about some characteristics of planets, such as their density, size, orbits, simple stuff like that. But I cannot find information on how their density is actually determined. Surely it can't just be based on the gravitational pull of that planet or moon, or a small sample of the rock from that planet, which is what this book seems to be suggesting. And with the Jovian planets, I can't even begin to guess how their density is found. My Astronomy text book does not explain, at all, how density is found, and then it goes into some other information concerning the density which I am supposed to know, but still never explaining how it is actually determined. Now I am not expected to know how the density of these planets is found for my class, I am just curious for my own personal knowledge.

Mars
2004-Mar-11, 12:07 AM
It is determined through it's gravitational pull and the volume of the planet. Mass has been determined to have a gravitational pull of X.

I don't have the numbers, but that is it in a nutshell.

Russ
2004-Mar-11, 12:09 AM
I don't understand how the density of other planets or moons in the solar system is found. I was just reading about some characteristics of planets, such as their density, size, orbits, simple stuff like that. But I cannot find information on how their density is actually determined. Surely it can't just be based on the gravitational pull of that planet or moon, or a small sample of the rock from that planet, which is what this book seems to be suggesting. And with the Jovian planets, I can't even begin to guess how their density is found. My Astronomy text book does not explain, at all, how density is found, and then it goes into some other information concerning the density which I am supposed to know, but still never explaining how it is actually determined. Now I am not expected to know how the density of these planets is found for my class, I am just curious for my own personal knowledge.

I'm suffering from a brain lockup but I'll take a stab at this. Density is mass per unit volume. For example gr/cm^3 or grams per cubic centemeter. By analyzing a planet or moon's orbit you can figure out the minimum mass for the the object. Assuming the object is close enough, you can determine its angular size and distance which gives you it's size from which you can determine its volume. Divide mass by volume and you density, well....actually average density.

Those of you not suffering from a combination of a cold and spring allergies can correct my mistakes on this. :)

daver
2004-Mar-11, 12:13 AM
The mass can be found by watching the planets moons go around
(yes, it is computed just by the gravitational pull of the planet). For bodies without satellites, they essentially guess, based on the likely makeup of those bodies.

Manchurian Taikonaut
2004-Mar-11, 12:29 AM
Some of the moons of Jupiter were understood for some time. A lot of people knew a rough idea about the mass and density of these moons.
From our observationas throughout history we know orbital periods of Io, Europa, and Ganymede are nearly in a perfect 1:2:4 ratio. In 1920 this knowledge paved the way for the first estimate of the satellites' masses within an accuracy of 20%, they could also divide its estimated Mass by its aproximate volume.
Finally in the 70s NASAs Voyager spacecraft flew past the Jovian system, took high-resolution pictures of the moons, and conducted experiments that provided the first accurate measurements of the moon's dimensions and mass. These in turn were used to calculate the mean density of the planet. Neptune the planet was vistited by Voyagers as it passed out tword the edge of our solar systems area. Neptune has a denisty of about 1.5-1.8 g/cm3 or a mean density 1.64 g/cm3 .

2004-Mar-11, 12:35 AM
Density is mass/volume, or in other words, the measurement of how compressed and tightly packed matter is. The mass is found by observing the gravitational effects of a planet on its moon, or if it doesn't have any moons, on the sun or other planets. The volume is much easier to measure of course, just find the angular diameter and use 4/3(pi)r^3. This measurement is accurate only with perfect spheres, and they do not exist in nature, so it will be slightly off. Divide mass by volume to find density. For Earth, the mass is 6(10)^24 kg and the volume is (i think) 1.01(10)^21 m^2 so the density is 5,500 kg/m^2. Surprisingly Earth is the most dense planet in the solar system, with Mercury close behind. Of course, the gas giants and the sun are much less dense. Hope this helps. :D

JohnOwens
2004-Mar-11, 12:46 AM
...For Earth, the mass is 6(10)^24 kg and the volume is (i think) 1.01(10)^21 m^2 so the density is 5,500 kg/m^2....

Almost exactly in accordance with my figures, except... m^3 rather than m^2, of course.

Lorcan Faol
2004-Mar-11, 01:22 AM
Thank you very much... This actually all makes perfect sense the way you guys explain it. I just wasn't using my brain very well to put 2 and 2 together, earlier, to see that this actually makes sense... #-o

AGN Fuel
2004-Mar-11, 01:29 AM
Density is mass/volume, or in other words, the measurement of how compressed and tightly packed matter is. The mass is found by observing the gravitational effects of a planet on its moon, or if it doesn't have any moons, on the sun or other planets. The volume is much easier to measure of course, just find the angular diameter and use 4/3(pi)r^3. This measurement is accurate only with perfect spheres, and they do not exist in nature, so it will be slightly off. Divide mass by volume to find density. For Earth, the mass is 6(10)^24 kg and the volume is (i think) 1.01(10)^21 m^2 so the density is 5,500 kg/m^2. Surprisingly Earth is the most dense planet in the solar system, with Mercury close behind. Of course, the gas giants and the sun are much less dense. Hope this helps. :D

The fun bit gets to be when you use various methods to determine the interior structure of a body. Examples of this include the seismic experiments performed by the Apollo astronauts, where they set up seismographs and whacked the moon with spent LM ascent stages, S-IVB's, explosive charges, etc. They could then determine the internal nature of the moon from the seismic wave patterns (what's that??? It was hollow??! :roll: )