Does the composition of a planet’s core make a difference for whether it can sustain advanced life? As research continues to show, the size, density, structure, and composition of a planet’s core carry huge implications for life.
For several decades, geologists have used seismometers to determine the structure and composition of Earth’s core. On November 26, 2018, the Interior Exploration Using Seismic Investigations, Geodesy, and Heat Transport (InSight) robotic lander touched down on the Martian surface (see figure 1). For four years, InSight’s seismometer, Seismic Experiment for Interior Structure (SEIS), measured seismic activity on Mars with the goal of developing an accurate 3-dimensional model of Mars’s interior (see figure 2).
Figure 1: Artist’s Rendition of InSight on Mars InSight’s seismometer is to the lower left. Credit: NASA
Figure 2: One of the Last Images Taken by InSight The image, taken on December 11, 2022, shows the SEIS instrument tethered to InSight and the Martian horizon. Credit: NASA
In late 2022, a Martian dust storm rendered InSight’s solar panels inoperable, leading to the lander’s decommissioning. Just before this happened, the SEIS instrument detected seismic waves from a distant marsquake and from a meteoritic impact on the other side of Mars. These detections were the first time that SEIS had successfully measured deep-traveling seismic waves from Mars’s far side traveling through its core (see figure 3).
Figure 3: Seismic Waves Detected by InSight’s SEIS Traveling through Mars’s Core Credit: NASA/JPL/University of Bristol/University of Maryland
Properties of Mars’s Core A team of 34 planetary astronomers, led by University of Bristol’s Jessica Irving, published their analysis of the Martian seismic waves in a recent issue of Proceedings of the National Academy of Sciences USA.1 Their analysis allowed them “to construct the first seismically constrained models for the elastic properties of Mars’ core.”2 Determining the elastic properties enabled the researchers to determine the size, density, and composition of the Martian core.
Irving’s team discovered that Mars’s core is different from Earth’s core in several respects. The seismic waves traveling through Mars’s core revealed that the Martian core has a radius of 1,780–1,810 kilometers (1,106–1,125 miles), or 53% of Mars’s radius, and a density of 6.2–6.3 grams/cubic centimeter. For comparison, Earth’s core radius is 55% of the planet’s radius and its density ranges from 9.5 to 14.5 grams/cubic centimeter. The low density of Mars’s core implies that 20–22% of its core by weight is composed of sulfur, oxygen, carbon, and hydrogen, with sulfur making up the bulk of these light elements. Light elements make up just 5% of Earth’s core and include silicon, sulfur, oxygen, carbon, and hydrogen where none of these light elements predominate. Mars’s core is mostly, if not entirely, liquid. Earth has an inner solid core and an outer liquid core. Earth’s inner solid core, has a radius of 1,221 kilometers (760 miles). The outer liquid core extends out to 3,480 kilometers (2,162 miles) from Earth’s center.
Why Such a Different Core? Irving’s team identified two explanations for why Mars’s core is different from Earth’s. The first is that the two bodies had different accretion histories and hence formed from different materials and had different initial core temperatures. The second is that the two cores are subject to different physical conditions—for example, different pressures, temperatures, and oxygen fugacity (partial pressure of oxygen).
Both explanations are in play. Mars formed through the accretion of planetesimals; so did the proto-Earth. However, a few tens of millions of years later, the proto-Earth collided with Theia, another rocky planet about twice the mass of Mars. Earth is nine times more massive than Mars and its density is 40% greater. Therefore, Earth’s core pressures are much greater than Mars’s.
Planetary astronomers eagerly seek more detailed data on Mars’ core, especially more accurate determinations of the Martian core’s elemental composition. Irving’s team showed that such a detailed model of the Martian core will be forthcoming once a multilocation network of seismometers is set up on the Martian surface. Meanwhile, the team intends to continue their analysis of InSight seismic data to further refine their present model of Mars’s interior structure.
Why No Martian Magnetosphere? The different size, density, structure, and composition of Mars’s core compared to Earth’s explains why Mars has lacked a magnetosphere (see figure 3) for the past 4 billion years. For a planet to possess a magnetosphere sufficiently strong, stable, and enduring enough to protect surface life from deadly stellar and cosmic radiation, it must have a large liquid iron core where that liquid iron is circulated to establish a dynamo.
Figure 4: Diagram of Earth’s Magnetosphere Credit: NASA
The total volume of liquid iron in Earth’s core is seven times greater than the total volume of liquid iron in Mars’s core. Since the Moon-forming event 4.47 billion years ago, Earth has had strong circulation mechanisms operating within its liquid core.3 For Mars, these circulation mechanisms shut down 4.0 billion years ago, perhaps even earlier.
Earth’s Remarkable Core I dedicate three chapters of my book Designed to the Core to describing the many design features of the Moon’s and Earth’s interiors that together explain how Earth—for the past 4.5 billion years—has possessed a magnetosphere sufficiently powerful and enduring to protect life from deadly solar and cosmic radiation. Briefly, both Earth and the Moon started off with a large, hot liquid iron core thanks to the collision between the proto-Earth and Theia. The early rapid cooling of their cores circulated the liquid iron. When this cooling rate began to subside, the tidal forces that Earth and the Moon exerted on one another sustained the circulation of liquid iron in their cores.
As the Moon spiraled away from Earth, tidal circulation of Earth’s liquid iron began to subside. Just before the Moon’s increasing distance threatened to shut down circulation of Earth’s liquid iron, a solid iron core began to form at Earth’s center. The combination of a solid inner core and a liquid outer core sped up the circulation of liquid iron. What drove the new circulation was (1) the huge temperature difference between the top of the solid core and the top of the liquid core, and (2) the squeezing out of light elements from the inner core into the outer core and the rise of these light elements toward the top of the liquid core.
As Earth’s interior continues to cool, the solid core’s diameter is increasing in size by about a millimeter per year. Right now, the relative sizes of Earth’s inner and outer cores are optimal for advanced life. The degree of multiple fine-tuned features required to explain Earth’s magnetosphere is so extraordinary that Earth may well be the only rocky planet that has sustained a strong magnetic field and life-protecting magnetosphere for 4.5 billion years. As Psalm 24:1 declares, “The earth is the Lord’s, and everything in it.”
Endnotes
Jessica C. E. Irving et al., “First Observations of Core-Transiting Seismic Phases on Mars,” Proceedings of the National Academy of Sciences USA 120, no. 18 (April 24, 2023): id. e2217090120, doi:10.1073/pnas.2217090120.
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