Look at an image of Earth taken from space and you’d never guess what’s hidden inside our home planet. Oh, sure, there are a few clues if you know what to look for: volcanoes, continents gliding past each other atop tectonic plates, and deep sea trenches. Those all give clues to activity deep inside our world.
Such clues exist on other planets: volcanoes on Venus and Mars (and on Jupiter’s moon Io, and ice volcanoes on several outer moons of the solar system); some moons have “plates” of ice that slip and slide past each other. Mercury has giant cracks that formed as its surface cooled, but the planet very likely still has a molten core and that can affect the rest of Mercury.
Activity at Earth’s core (or near it) generates our planet’s magnetic field. Geologists point to studies using seismic waves that propagate (travel) through the planet which map out the structure beneath our feet. In general, we stand on the crust of the planet (often referred to as the “lithosphere”). Beneath that lies a mantle, which is mostly hot rock and may be molten in places and heated from below. Inside the shell of the mantle is the core of the planet, which for many years was thought to be completely molten. Then, in the early 20th century, geologists found that the core is actually in two parts: an inner core and an outer core. Motions in the liquid outer core were suspected in the formation of the magnetic field. The inner core was thought to be solid iron.
That picture is changing thanks to new technology that allows scientists to “read” the actions set off by earthquakes very precisely. Illinois geology professor Xioadong Song led a research team that used seismic waves to look at the Earth’s inner core. They found that the inner core has surprisingly complex structure and behaviors.
Seismic waves from earthquakes are used by geologists much like doctors use ultrasound to see inside patients. The team used a technology that gathers data not from the initial shock of an earthquake, but from the waves that resonate in the earthquake’s aftermath. The earthquake is like a hammer striking a bell; much like a listener hears the clear tone that resonates after the bell strike, seismic sensors collect a coherent signal in the earthquake’s coda.
The technology revealed an amazing surprise in Earth’s inner core: it has its own smaller, inner core. So, we have a planet with a core within a core within a core. The structure of the region is pretty complex. The very distinct inner-inner core is about half the diameter of what scientists used to call the inner core. Furthermore, their data show that the iron crystals in the outer layer of the inner core are aligned directionally, north-south. However, in the inner-inner core, the iron crystals point roughly east-west.
As they studied the data, scientists also found that the iron crystals in the inner-inner core behave differently from their counterparts in the outer-inner core. This means that the inner-inner core could be made of a different type of crystal, or a different phase of iron.
Stuck as it is in the center of our planet, you might think the core is an unchanging environment. Yet, planetary scientists think it has evolved radically since Earth first formed. The inner core may be changing in ways that don’t occur in the outer shells. This may have interesting implications for changes in our planet’s magnetic field, for example.
And, if this sort of thing is happening in Earth’s core, it may give astronomers a clue to events occurring in the centers of other planets. Seismic studies of Mars, for example, could tell them just how much activity is still going on at the heart of the Red Planet. Or, what’s happening with Venus (our highly volcanic neighbor planet).