Composition of the Earth Questions and Answers

Answer: A

The earliest-formed condensate among these is corundum.

As the hot solar nebula cooled, minerals condensed in order of decreasing condensation temperature. Calculations and calcium-aluminium-rich inclusions in meteorites show that refractory oxides of aluminium and calcium came first: corundum (Al\(_{2}\)O\(_{3}\)) at about 1700 K, followed by hibonite and perovskite, then melilite and spinel.

Forsterite and enstatite, the magnesium silicates, condense later at about 1400 K, and anorthite also forms below the refractory oxides, so none of them precedes corundum.

Answer: C

The strongest evidence is that both have similar oxygen isotopic composition.

Different bodies of the solar system, such as Mars and the parent bodies of meteorites, have distinct ratios of the three oxygen isotopes, like a fingerprint. Lunar rocks lie on the same mass-fractionation line as terrestrial rocks, which shows that the Moon formed from material of the same reservoir as the Earth, as the giant-impact hypothesis requires.

Proximity says nothing about origin, since a captured body could be close too. Basaltic crusts are found on many rocky bodies, and the bulk SiO\(_{2}\) and MgO of the silicate Moon and Earth are only broadly alike, so these are much weaker evidence.

Answer: B

Lunar surface samples are remarkably depleted in volatile elements.

Apollo samples contain much less of volatile elements such as sodium, potassium, lead, zinc and water than terrestrial rocks, and are relatively enriched in refractory elements such as calcium, aluminium, titanium and uranium. This pattern is explained by the high temperatures of the giant impact and the lunar magma ocean, which drove off the volatiles.

So non-volatile and refractory elements, including refractory trace metals, are the ones that are relatively enriched, not depleted.

Answer: B

The magma-ocean concept was developed to explain the Moon.

Apollo 11 samples included fragments of anorthosite. To explain a crust made of light plagioclase-rich rock, Wood and Smith proposed in 1970 that the outer few hundred kilometres of the young Moon were molten: plagioclase floated to form the anorthositic highlands, while olivine and pyroxene sank to form the mantle. The europium anomalies of lunar basalts and the KREEP component fit this model.

Magma oceans have since been suggested for the early Earth and Mars too, but the concept was first developed for lunar petrology and geochemistry. Jupiter is a gas giant without a rocky crust of this kind.

Answer: A

The unaccreted gas and dust were blown away during the T-Tauri phase.

T Tauri stars are young, pre-main-sequence stars with strong stellar winds and intense radiation. When the young Sun passed through this phase, its wind is thought to have swept the remaining gas and fine dust out of the inner solar system, ending the growth of the terrestrial planets and leaving them poor in volatiles.

The nebular phase is the earlier stage when that gas and dust were present. 'Corona phase' and 'S-Tauri phase' are not stages of stellar or planetary evolution.

Answer: D

The Widmanstatten structure is associated with siderites, the iron meteorites.

When an etched surface of an iron meteorite is examined, it shows a criss-cross pattern of intergrown plates of kamacite and taenite (two iron-nickel alloys). The pattern forms by extremely slow cooling, of a few degrees per million years, inside the cores of asteroids, and it is the classic proof that a specimen is an iron meteorite.

Aerolites are stony meteorites and siderolites are stony-irons, which lack the full pattern, and tektites are natural glasses produced by impacts, not meteorites at all.

Answer: D

In the Big Bang model the temperature of the universe fell from about \(10^{13}\) K to 3 K.

About a microsecond after the Big Bang the universe was at roughly \(10^{13}\) K, hot enough for quarks to combine into protons and neutrons. As it expanded it cooled: to about \(10^{9}\) K during the first few minutes, when helium nuclei formed, and to about 3000 K some 380,000 years later, when atoms formed and the cosmic background radiation was released. That radiation has now cooled to about 2.7 K, the '3 K' of the option.

A negative temperature on the kelvin scale (−100 K) is impossible, and the other ranges end at \(10^{5}\) K or \(10^{3}\) K, far above today's temperature of the universe.

Answer: D

The key evidence comes from the Doppler effect, the red shift of distant galaxies.

Light from almost all distant galaxies is shifted towards longer (redder) wavelengths, showing that they are moving away from us. In 1929 Edwin Hubble found that the recession speed is proportional to distance (Hubble's law), which is exactly what a uniformly expanding universe produces.

Supernova explosions were later used as standard candles to show that the expansion is accelerating, but the primary evidence for expansion is the red shift. Black holes and galactic collapse say nothing about the expansion of the universe.

Answer: A

Carbonaceous chondrites are primitive material that has survived since the formation of the solar system.

Carbonaceous chondrites, especially the CI type, have never been melted or differentiated. Their composition, apart from the most volatile elements, matches that of the Sun's photosphere, and they contain the oldest dated solids of the solar system (calcium-aluminium-rich inclusions, about 4.567 billion years old), together with water, organic compounds and presolar grains.

Iron meteorites and achondrites come from bodies that melted and differentiated, and olivine peridotite is a mantle rock, so none of them preserves primitive solar-system material. 'Iron chondrite' is not a meteorite class.

Answer: C

These meteorites are the achondrites.

Achondrites are stony meteorites without chondrules. They are igneous rocks, basalts, cumulates and breccias, formed by melting and differentiation on their parent bodies and later knocked off by impacts. The HED group (howardites, eucrites and diogenites) is linked to the asteroid Vesta, and other achondrites come from the Moon and Mars.

Enstatite is a pyroxene mineral (it also names the enstatite chondrites). Carbonaceous chondrites are unmelted, primitive meteorites, not igneous ones. Ignimbrites are terrestrial volcanic rocks deposited by pyroclastic flows.