• 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.”

Check out more from Reasons to Believe @Reasons.org


  1. 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.
  2. Irving et al., “First Observations,” p. 1.
  3. Hugh Ross, Designed to the Core (Covina, CA: RTB Press, 2022), 177–206.

About The Author

Dr. Hugh Ross

Reasons to Believe emerged from my passion to research, develop, and proclaim the most powerful new reasons to believe in Christ as Creator, Lord, and Savior and to use those new reasons to reach people for Christ. I also am eager to equip Christians to engage, rather than withdraw from or attack, educated non-Christians. One of the approaches I’ve developed, with the help of my RTB colleagues, is a biblical creation model that is testable, falsifiable, and predictive. I enjoy constructively integrating all 66 books of the Bible with all the science disciplines as a way to discover and apply deeper truths. 1 Peter 3:15–16 sets my ministry goal, "Always be prepared to give an answer to everyone who asks you to give the reason for the hope that you have. But do this with gentleness and respect, keeping a clear conscience." Hugh Ross launched his career at age seven when he went to the library to find out why stars are hot. Physics and astronomy captured his curiosity and never let go. At age seventeen he became the youngest person ever to serve as director of observations for Vancouver's Royal Astronomical Society. With the help of a provincial scholarship and a National Research Council (NRC) of Canada fellowship, he completed his undergraduate degree in physics (University of British Columbia) and graduate degrees in astronomy (University of Toronto). The NRC also sent him to the United States for postdoctoral studies. At Caltech he researched quasi-stellar objects, or "quasars," some of the most distant and ancient objects in the universe. Not all of Hugh's discoveries involved astrophysics. Prompted by curiosity, he studied the world’s religions and "holy books" and found only one book that proved scientifically and historically accurate: the Bible. Hugh started at religious "ground zero" and through scientific and historical reality-testing became convinced that the Bible is truly the Word of God! When he went on to describe for others his journey to faith in Jesus Christ, he was surprised to discover how many people believed or disbelieved without checking the evidence. Hugh's unshakable confidence that God's revelations in Scripture and nature do not, will not, and cannot contradict became his unique message. Wholeheartedly encouraged by family and friends, communicating that message as broadly and clearly as possible became his mission. Thus, in 1986, he founded science-faith think tank Reasons to Believe (RTB). He and his colleagues at RTB keep tabs on the frontiers of research to share with scientists and nonscientists alike the thrilling news of what's being discovered and how it connects with biblical theology. In this realm, he has written many books, including: The Fingerprint of God, The Creator and the Cosmos, Beyond the Cosmos, A Matter of Days, Creation as Science, Why the Universe Is the Way It Is, and More Than a Theory. Between writing books and articles, recording podcasts, and taking interviews, Hugh travels the world challenging students and faculty, churches and professional groups, to consider what they believe and why. He presents a persuasive case for Christianity without applying pressure. Because he treats people's questions and comments with respect, he is in great demand as a speaker and as a talk-radio and television guest. Having grown up amid the splendor of Canada's mountains, wildlife, and waterways, Hugh loves the outdoors. Hiking, trail running, and photography are among his favorite recreational pursuits - in addition to stargazing. Hugh lives in Southern California with his wife, Kathy, and two sons.

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