The goal of all our research into the mantle is to learn its story—and since it is by far the largest part of the Earth, the mantle's story is the planet's story. The crust we live on, with its continents and ocean basins, the seas and the atmosphere, the living things that arose on it 4 billion years ago, and the civilization that depends on it—all of that is just the thin skin of the mantle.
Earth formed as the Sun itself was being born in a great disk of hot gas that came together when a nearby star exploded in a supernova. (Here's a longer version of the story.) Earth was created hot, from the energy of many smaller bodies falling into it and adding to its mass. As the Sun ignited, it blew away almost all of the light gases hydrogen and helium from around Earth (these gases became part of Jupiter and Saturn). The same story applies to Mercury, Venus, and Mars, the other terrestrial (rocky) planets.
A last big chunk—a body the size of Mars—crashed into Earth, splashing so much material into nearby space that the Moon formed from the wreckage. If any of the Earth had solidified by then, it was certainly returned to melt. By this time the mantle had formed, an ocean of molten rock surrounding an iron core. Slowly it sorted itself out and crystallized into rock.
Crystallizing a melt is like refining it—as the melt cools and the first solid mineral grains form, they settle out, leaving the liquid with a different composition. The cooling melt then favors a different set of minerals, and the remaining liquid changes further. Eventually the last bit becomes highly enriched in a certain set of elements that didn't get picked for the team, so to speak. In the Earth's early magma ocean, these incompatible elements wound up in the crust, where they slowly built the continents. And the mantle settled down for 4 billion years of slow stirring, driven by heat from beneath and cooling from above.
Earth is a special planet in many ways, and its mantle seems to be special too, when you compare it to the other rocky planets. Mercury, from the evidence, has a rather thin and quiet mantle—thin because the early Sun blew away a lot of its substance, quiet because the planet is too small to have retained much heat. Mercury seems to have calmed down inside very early.
Mars is another small planet, and while its enormous volcanoes must have deep sources, beyond those it has few signs of having an active mantle like Earth's. It appears to have its own version of high continents and low basins (like empty oceans), but these are frozen in place where they formed. It's the surface of Mars, with its river-washed gorges and large-scale weather systems, that resembles Earth so interestingly. Down below, the mantle of Mars seems to be pretty dead.
Venus is only slightly smaller than Earth, and it appears to have a live mantle, including its own version of hotspots. It looks as though around a half-billion years ago, the whole surface of Venus was wiped clean. Researchers are still digesting the huge trove of data from the Magellan mission to Venus, and they haven't settled on a favorite story yet. But my favorite one is that Venus has a thick, stiff crust that every once in a while collapses of its own weight, since solid rock is denser than molten rock. At that point, hot stuff below bursts out and covers the whole planet in a fresh layer of rock. This happens only rarely because Venus's very thick atmosphere of carbon dioxide keeps the surface hot, and it takes a long time to build up for a turnover. (Here's more about Venus geology.)
Earth is very different. Water was there from the beginning, and life started there as soon as it could. Both prevented carbon dioxide from building up, keeping the surface cool. A cool surface means that the density-driven cycle happens much faster than on Venus. Thus early on, Earth's mantle evolved the system of recycling the crust that we call plate tectonics. The oceanic, underwater part of the crust, which makes up most of the Earth's surface, is constantly being recycled—forming fresh at the volcanic mid-ocean ridges, slipping out of sight at the subduction-zone trenches. As it sinks, the crust carries with it a great deal of water and carbon.
This process keeps the entire mantle moist, youthful and active. Lab experiments show that water, in particular, makes mantle rock melt more easily and makes molten rock more fluid. Just a little bit of water in the Earth's mantle recipe makes all the difference. It keeps the volcanoes popping, the mid-ocean ridges bubbling, and the crustal plates slipping. And that in turn is what keeps the mountains high and the valleys low,
puts gold in the hills and oil in the ground, and in every other way maintains our landscape and lifestyle. Here's to the mantle!
