Internal structure and densities of the solid worlds I - chemical evidence


Clouds of interstellar gas and dust from Wikipedia

Origin of planetary material:

Solar systems form from clouds of interstellar gas and dust. Where does this material come from?


The Milky Way - all sky composite from Astronomy Picture of the Day.

The result of all this is that huge molecular clouds of interstellar gas and dust are spread throughout the galaxy. Sometimes, that material can be compressed by the stellar winds of neighboring stars to the point that it contracts under its own gravity, leading to....


The Rosette Nebula detail from Astronomy Picture of the Day.

Formation of Solar Systems:

Nebulae. Occasionally, conditions are such that clouds of gas and dust (leftovers of the big bang, material ejected from stars during their lifetimes, and the shrapnel of ancient supernovae) become sufficiently compressed by the stellar winds of adjacent stars or the shock waves of supernovae that they form relatively dense clouds of material called nebulae (sing. nebula) like the Rosette Nebula (detail at right.) Particularly dense regions of nebulae can continue to contract as gravity sucks material to their centers, where it piles up forming a protoplanetary disk. Material concentrated in the center of the disk becomes a protostar while material circling it in the disk may accrete into planets. The Sun and Solar System formed in this way. The protostar becomes hot for at least two reasons.

Stages in origin of Sun:

We now turn to what was going on in the disk of gas (mostly hydrogen and helium) with traces of dust surrounding the protosun when our solar system formed.



Reconstruction of protosolar nebula University of Arizona - Astronomy 170B1.

Origin of planets:

A modern analog to this stage in the Solar System's development may be provided by the star Fomalhaut in whose accretion disk the second extrasolar planet ever directly to be imaged orbits.

Paradox of Earth mass and density:

The fact that the Solar System formed from homogeneous material leads to a paradox. We know:

This yields a bulk density of 5.51 103 kg m-3. But this presents a difficulty: Any Earth rocks that we can put our hands on are considerably less dense.

We have access to three general classes:


Granite - Joshua Tree National Park___________________Basalt - Sullivan Bay, Santiago, Galápagos Islands___________________Mantle xenolith in basalt from Wikipedia

The average density of surface rocks is nothing like the bulk density of planet Earth. One way or another, the low density of surface rocks must be offset by denser material somewhere. This paradox allegedly led Sir Edmund Halley to propose in 1692 that large parts of the interior of Earth were actually hollow. Today, serious possible explanations include:

In fact, there are reasons to think that all of these are true.

Lines of evidence:


Chondrite Wikipedia.

Meteorites:

Meteorites come in several compositional classes:

The last three types seem to represent debris from the collisions of differentiated planetary embryos (like Vesta). The chondrites, however, represent material left over from the formation of the Solar System. One subset, CI carbonaceous chondrites are considered particularly good representations of the composition of the Solar Nebula, as their composition (gasses and volatile substances notwithstanding) compares very well with that of the sun's atmosphere, as revealed in absorption spectra.

Earth composition vs. chondrites:

Chondrite normalization: CI carbonaceous chondrites are considered sufficiently reliable reflections of the Solar System's overall composition that it is common to refer to the concentrations of trace substances in rock samples by comparison to their concentrations in chondrites. These are typically displayed in spider diagrams or "spidergrams" (right.) As fig 2.3 0f the text indicates, Earth's crust and upper mantle are significantly enriched in some elements and depleted in others, especially iron, which is known to be common in the sun and in chondrites. The presence of an iron reservoir deep in the Earth might explain Earth's density paradox.

Laboratory simulations of the deep Earth:

What about self-compression or chemical alteration of mantle silicates? We can actually study the behavior of silicate minerals under conditions of temperature and pressure that they might encounter in the very deep Earth using equipment such as:


Such tests experimentally verify that: Such studies demonstrate that olivine can be transformed into material approaching 5 103 kg m-3. That can't account for Earth's density paradox (Earth = 5.52 103 kg m-3). Something denser like iron (7.8 103 kg m-3) must be down there as well.



Key concepts and vocabulary: