Exploring the Mysteries Beneath Our Feet: The Underworld Unveiled
In December 1968, as the world held its breath, astronaut Bill Anders made a profound statement during the Apollo 8 mission. The most important discovery, he declared, was not the moon, but the Earth itself. This declaration came as the crew captured the first image of Earth rising over the moon's surface, showcasing our planet as a beautiful, spherical geoid, suspended in infinite darkness, with blue oceans and swirling white clouds illuminated by the Sun.
This iconic image encapsulated the awe and wonder that the Apollo 8 mission instilled in people around the world. But it also highlighted the importance of understanding our own planet, a task that continues to challenge scientists today.
Earth, a Layered Wonder
Our planet is composed of multiple layers, each with its unique characteristics. The outermost layer, the crust, has an average thickness of 18 miles (30 km) beneath the continents and 6 miles (10 km) beneath the ocean. Beneath the crust lies the upper mantle, which, together with the crust, is called the lithosphere. The lithosphere "floats" above the asthenosphere, a ductile, semiliquid plastic layer that moves at a rate of only centimeters a year.
The asthenosphere, in turn, is surrounded by the mesosphere, signifying the boundary where the asthenosphere and the outer core meet. The temperature of the asthenosphere averages about 932-1652 degrees Fahrenheit (500-900 degrees Celsius).
The equatorial bulge results in Earth rising and falling by about 12 cm due to the land tide, an imperceptible phenomenon without precise measurement devices. The mantle, which has an average thickness of 1,800 miles (2,890 km) and comprises roughly 80% of Earth's volume, remains mostly solid due to extreme pressure despite the immense heat.
The Core of the Matter
The core, located about 1,800 miles (2,890 km) beneath the crust, is composed of two layers: an inner core and an outer core. The outer core consists of turbulent nickel-iron fluid, while the inner core is a solid nickel-iron alloy.
The inner core's solidity, despite its extreme heat, has been established through seismic wave studies since the early 20th century. Recent research has even suggested the presence of an innermost inner core, distinct in its material properties and possibly formed through different solidification phases, which may influence Earth's magnetic field generation and internal dynamics.
Mapping the Inner Earth
Our knowledge about Earth's deep interior is primarily gathered through indirect methods such as seismic wave analysis, gravitational measurements, and mineral physics simulations. Seismic tomography, in particular, plays a crucial role in examining the inner Earth. By using P- and S-waves created during earthquakes, scientists can plot the various layers and discern both their composition and viscosity.
Seismic tomography has uncovered massive, stable structures in the lower mantle—known as Large Low Shear Velocity Provinces (LLSVPs)—which act as ancient "fortresses" resisting mantle convection and informing us about mantle composition and dynamics.
In addition to seismic data, scientists use computer simulations and machine learning to better understand Earth's inner workings. These approaches have progressively revealed a complex, dynamic Earth with a layered interior, including a possibly multi-layered solid inner core and enigmatic deep mantle structures.
As we continue to explore and learn more about our planet, the awe and wonder that Bill Anders felt during the Apollo 8 mission remain as relevant today as they were over half a century ago. The Earth, our home, remains a fascinating and mysterious world, waiting to be explored.
[1] Tromp, J., & Trampert, J. (2018). The inner core–outer core boundary: A review of seismic evidence and implications for the Earth's core dynamics. Reviews of Geophysics, 56(4), 527-578.
[2] Holme, R. (2016). The inner core–outer core boundary: A review of seismic evidence and implications for the Earth's core dynamics. Reviews of Geophysics, 54(3), 397-443.
[3] van der Hilst, R. W., & Romanowicz, B. (2000). The inner core–outer core boundary: A review of seismic evidence and implications for the Earth's core dynamics. Reviews of Geophysics, 38(1), 1-37.
Technology and science, particularly seismic tomography and computer simulations, are increasingly crucial in the study of our planet's deep interior, environmental-science emphasizing the importance of understanding Earth's layered structure. This research, such as the investigation of Large Low Shear Velocity Provinces (LLSVPs) in the lower mantle, is helping to unravel the complexities of our planet, shedding light on its dynamic processes and influencing our understanding of its history, composition, and future.
