View Full Version : Composition of planetary interior

2008-Sep-16, 03:58 PM
Simple question, but still puzzles me:
How do we know the composition of planetary interiors in our solar system, particularly Earth?
We haven't drilled past the crust, and magma we can analyze is only from the upper mantle, so how can we confidently say that the core consists of so-and-so much iron and nickel; couldn't it be iron and lead instead?
And how do we calculate the size of the core?

2008-Sep-16, 04:10 PM
Simple question...
The simple answer is through the clues of pressure, density, magnetic variations, world-wide vibrations, and probably a few other things I'm missing.

2008-Sep-16, 04:16 PM
The size of the core is known from seismic measurements. Basically, some seismic waves bounce on the mantle/core interface (and on the outer core/inner core one) and the travel times allow us to reconstruct the size and shape of the boundary.

Exchanging the nickel of a typical model of the inner Earth for lead would noticeably increase the planet's mass - any realistic composition has to add up to the right mass, which such a one wouldn't. Also, it would be hard to explain just where all the extra lead came from, since meteoroidal and stellar abundances give us a baseline for how much of each element to expect to have gone into the Earth's makeup. (Allowance has to be made, of course, for the fact the planet's accretion was a violent affair that lost us a lot of volatiles and that some stuff, notably hydrogen, mostly didn't accrete in the first place.) However, the exact composition isn't as well known as popular books may seem to suggest - we're dealing with estimates, and debate is ongoing on the exact composition of the core.

2008-Sep-16, 10:13 PM
Modeling, seismic data (if available), density, moment of inertia, elemental comparisons (e.g., why are meteorites rich in iron and nickel while those elements are relatively rare at Earth's surface? Differentiation.), and a combination of math and educated guesswork. It's probably easier for gas giants, with their simple compositions than for terrestrial planets; knowledge of our own planet's interior remains imperfect after decades of geophysical work.

2008-Sep-17, 01:28 PM
Just to add, to what Romanus and AndreasJ said, seismic measurements give us a lot of information. For example, the speed of these waves is very dependent upon density of the material, and that is going to be very sensitive to composition. There has also been work done in the laboratory, with high pressure/high temperature equipment, to create model materials and check their properties, to see if they match the measured ones from the Earth.

I just happened to see this article (http://www.rdmag.com/ShowPR.aspx?PUBCODE=014&ACCT=1400000100&ISSUE=0809&RELTYPE=MS&PRODCODE=00000000&PRODLETT=CA&CommonCount=0) about exactly that kind of research.

The planetary building blocks magnesium, silicon, oxygen and iron are the most abundant minerals in the lowermost mantle. A team of scientists led by Jung-Fu Lin at The Univ. of Texas at Austin’s Jackson School of Geosciences synthesized materials from these building blocks in a diamond anvil cell, a device containing two interlocking diamond pieces that squeeze the sample like a vice. They subjected the sample to more than 1.3 million times standard atmospheric pressure. Shining a laser through the transparent diamonds, they then heated the sample to almost 3,000C (5,400F) for several days.

2008-Sep-18, 07:44 PM
The simple answer is through the clues of pressure, density, magnetic variations, world-wide vibrations, and probably a few other things I'm missing.

We stuck in our thumb, and pulled out a...

No, that's not it...

Yeah - what you said. Earthquakes help a good bit, too, as they're like sonograms of our planet.