Getting heat to cold places, especially during winter, ranks as a great human concern and source of comfort. And though we may not ever think about Earth’s giant interior furnace, humans exist due to the benefits of its fine-tuned flow.
Recent research shows that Earth is likely to be the universe’s heat flow champion for rocky planets of its size and age. In spite of Earth’s small size and age (4.6 billion years), the flow of heat from the interior to the surface is 42,000,000,000,000 watts.1 Approximately 70,000 measurements show that, on average, every square meter of Earth’s continental surface is warmed by 92 milliwatts as a result of heat flow from Earth’s interior. Meanwhile every square meter of ocean surface is warmed by 67 milliwatts.2 (Heat flows more readily through continental silicates than it does through oceanic basalts.)
At Earth’s surface, incident radiation from the Sun dominates heat flow from Earth’s interior. The Sun warms every square meter of Earth’s surface on average by 340.2 watts.3 That is, internal heat flow accounts for only 0.024 percent of Earth’s surface warming. However, incident radiation from the Sun does not penetrate very deeply. Any deeper than several tens of meters into Earth’s crust and internal heat flow dominates solar heat.
Sources of Interior Heat Flow
There are two major sources of heat flow from Earth’s interior. Heat produced through the radioactive decay of long-half-life radioisotopes in Earth’s interior (radiogenic heat) accounts for about 58 percent of the present total heat flow.4 Primordial heat—that is, heat lost as Earth continues to cool from the heat generated by the accretion processes that formed it—accounts for the remaining internal heat flow.
Earth has a remarkably high level of primordial heat because of its unique formation process. Like all rocky planets, it formed through the accretion of planetesimals, dust, and gas. The gravitational collapse that occurred through this accretion generated a lot of internal heat. However, Earth’s accretion history did not end there.
Earth experienced three other major accretion events. The most significant of these was the merger event that led to the formation of the Moon a little less than 100 million years after Earth’s initial formation. Earth and Theia (a planet 15–45 percent of Earth’s original mass) merged, increasing Earth’s mass, producing the Moon, and substantially augmenting Earth’s heat of accretion.5 Shortly after the Moon-forming event Earth received a “late veneer”—a bombardment by large asteroids and comets.6 Then, about 3.9 billion years ago, Earth received the Late Heavy Bombardment of large asteroids and comets.7 Consequently, Earth’s primordial heat from accretion was boosted far beyond what is typical for other rocky planets of its size.
Earth possesses an even more remarkable level of radiogenic heat. Compared to the average abundance levels in rocky exoplanets, Earth possesses 90 times as much potassium, 340 times as much uranium, and 610 times as much thorium.8 One reason why Earth is so super-endowed with these elements is due to the timing of the solar system’s formation and the unique manner in which the solar system formed. Thus, all the solar system planets are enriched with these three elements. For a number of reasons, Earth is super-endowed with these elements far beyond the levels in other solar system planets. (I go into further detail in my books Why the Universe Is the Way It Is9 and Improbable Planet.10)
It is not only the amounts of potassium, uranium, and thorium that matter for potential life on a planet but also where within a planet these elements reside. On Mars, measurements show that the weight of the polar ice cap has not depressed (pushed down) the underlying lithosphere (crust). The observed upper limit on this depression shows that the present-day mantle heat flow must be less than 7 milliwatts per square meter.11 This implies that most of Mars’ potassium, uranium, and thorium resides in its crust and that its mantle is depleted of these elements. Such a lack explains why the planet’s magnetic field, tectonic activity, and volcanism shut down about 4 billion years ago. Meanwhile these features have been sustained on Earth.
Interior Heat Flow Pathways
In 1862, Britain’s famous physicist Sir William Thomson, Baron Kelvin, calculated that Earth could not be older than 98 million years.12 His calculation was based on estimates of Earth’s internal heat flow at the time. It assumed all of Earth’s internal heat flow came from the residual heat from a single accretion event (the condensation of Earth from planetesimals) and that heat flow from Earth’s interior occurred strictly as a result of conductive cooling. As already noted, his first assumption was incorrect. His second assumption was also incorrect.
Convection plays a far more important role in heat transport than does conduction in both Earth’s mantle and liquid core (see figure 1).13 For Earth’s crust the dominant heat transport mode is volcanic advection. The dominant heat transport modes in the liquid outer core and mantle are convection. The dominant heat transport mode in the solid inner core is conduction. As a transport mechanism, convection is an important reason why Earth’s internal heat flow has been sustained for several billion years.
Benefits of Earth’s Interior Heat Flow
The present temperature of the mantle just under Earth’s oceanic crust is 1,410°C.15 The current high temperature of the upper mantle means that the upper mantle material has a low viscosity (flows more easily). A familiar analogy would be the difference between a cold stick of butter and a stick of butter that has been melted in a saucepan.
It is thanks to the mantle’s low viscosity that tectonic plates in Earth’s crust are able to move relative to another—a feature of Earth for the past 3.8 billion years. This tectonic activity transformed Earth from a waterworld, where only water existed on its surface, to a planet possessing both surface oceans and surface continents. The combination of surface oceans and continents and enduring, strong tectonic activity established the biogeochemical cycles that allowed Earth’s surface temperature to be sustained at an optimal level for life in spite of the ongoing brightening of the Sun.16 The same combination has been recycling many of Earth’s life-essential nutrients. Without Earth’s enduring, strong interior heat flow, at best, only microbial life could have existed on Earth and for only several million years. In that event, microbial life never would have been able to physically and chemically transform Earth’s surface environment so that plants, animals, and humans could exist.
The current cooling rate of Earth’s mantle is 70–130°C per billion years.17 This cooling is slow enough to pose no short-term threat to any current life-forms. The cooling does imply, however, that Earth’s mantle will become more viscous. Eventually, it will be so viscous that plate tectonic activity will shut down. When that happens, advanced life and, eventually, all life will go extinct.
It is thanks to Earth’s unique strong, enduring interior heat flow that it has both a solid inner core and a liquid outer core. Earth’s liquid core is almost entirely comprised of the ferrous elements iron, cobalt, and nickel. These easily magnetized elements driven by convection currents in the outer core explain why Earth has sustained a powerful magnetic field throughout at least the past 3.7 billion years.18
This powerful, enduring magnetic field has shielded Earth’s surface life from deadly high-energy particles flowing in from the Sun and equally deadly high-energy cosmic rays. Without the shield, solar radiation would have sputtered both Earth’s atmosphere and Earth’s surface water into interplanetary space.
If it were not for Earth’s astoundingly powerful and enduring internal heat flow from the furnace in its core, there would be no civilization. In fact, there would be no humans on Earth. Nor would there be animals, trees, large plants, oceans, or atmosphere. It is thanks to the amazingly fine-tuned designs of Earth’s formation and of its interior structure and heat flow that billions of humans can thrive on Earth, enjoy high-technology civilization, gain an enormous amount of knowledge and understanding and use them to fulfill the purpose and gain the destiny for which our Creator made us.
Francis Lucazeau, “Analysis and Mapping of an Updated Terrestrial Heat Flow Data Set,” Geochemistry, Geophysics, Geosystems 20, no. 8 (August 2019): 4000–24, doi:10.1029/2019GC008389; J. H. Davies and D. R. Davies, “Earth’s Surface Heat Flux,” Solid Earth 1, no. 1 (February 22, 2010): 5–24, doi:10.5194/se-1-5-2010.
Lucazeau, “Analysis and Mapping,” 4000.
Andrew C. Kren, Peter Pilewskie, and Odele Coddington, “Where Does Earth’s Atmosphere Get Its Energy?”, Journal of Space Weather and Space Climate 7 (March 20, 2017): id. A10, doi:10.1051/swsc/2017007; Fei Feng and Kaicun Wang, “Determining Factors of Monthly to Decadal Variability in Surface Solar Radiation in China: Evidences from Current Reanalyses,” Journal of Geophysical Research: Atmospheres 124, no. 16 (August 2019): 9161–82, doi:10.1029/2018JD030214.
The KamLAND Collaboration, “Partial Radiogenic Heat Model for Earth Revealed by Geoneutrino Measurements,” Nature Geoscience 4 (September 2011): 647–51, doi:10.1038/ngeo1205; Lucazeau, “Analysis and Mapping”; Davies and Davies, “Earth’s Surface Heat Flux.”
Hugh Ross, Improbable Planet(Grand Rapids, MI: Baker, 2016), 48–60; Hugh Ross, “Yet More Reasons to Thank God for the Moon,” Today’s New Reason to Believe (blog), November 22, 2016, https://reasons.org/explore/blogs/todays-new-reason-to-believe/read/todays-new-reason-to-believe/2016/11/22/yet-more-reasons-to-thank-god-for-the-moon.
Ross, Improbable Planet, 65–72, 97–105; Hugh Ross, “Late Heavy Bombardment Intensity and the Origin of Life,” Today’s New Reason to Believe (blog), June 29, 2009, https://reasons.org/explore/blogs/todays-new-reason-to-believe/read/tnrtb/2009/06/29/late-heavy-bombardment-intensity-and-the-origin-of-life.
The heat flow percentages are from two papers: S. T. Dye, “Geoneutrinos and the Radioactive Power of the Earth,” Reviews of Geophysics 50, no. 3 (September 2012): id. RG3007, doi:10.1029/2012RG000400; Ricardo Arevalo Jr., William F. McDonough, and Mario Luong, “The K/U Ratio of the Silicate Earth: Insights into Mantle Composition, Structure, and Thermal Evolution,” Earth and Planetary Science Letters 278, issues 3–4 (February 25, 2009): 361–69, doi:10.1016/j.epsl.2008.12.023.
The heat flow rates are taken from Ricardo Arevalo Jr., et al., “The K/U Ratio of the Silicate Earth,” 361–69.
Lujendra Ojha et al., “Depletion of Heat Producing Elements in the Martian Mantle,” Geophysical Research Letters 46 (November 28, 2019): 12756–63, doi:10.1029/2019GL085234.
William Thomson, “4. On the Secular Cooling of the Earth,” Proceedings of the Royal Society of Edinburgh 4 (1862): 610–11, doi:10.1017/S0370164600035124. A pdf of Thomson’s paper is available here.
Emily Sarafian et al., “Experimental Constraints on the Damp Peridotite Solidus and Oceanic Mantle Potential Temperature,” Science 355, issue 6328 (March 3, 2017): 942–45, doi:10.1126/science.aaj2165.
Hugh Ross, “Carbon Cycle Requirements for Advanced Life, Part 1,” Today’s New Reason to Believe (blog), November 18, 2019, https://reasons.org/explore/blogs/todays-new-reason-to-believe/read/todays-new-reason-to-believe/2019/11/18/carbon-cycle-requirements-for-advanced-life-part-1; Hugh Ross, “Carbon Cycle Requirements for Advanced Life, Part 2,” Today’s New Reason to Believe (blog), November 25, 2019, https://reasons.org/explore/blogs/todays-new-reason-to-believe/read/todays-new-reason-to-believe/2019/11/25/carbon-cycle-requirements-for-advanced-life-part-2.
Ricardo Arevalo Jr., et al., “The K/U Ratio of the Silicate Earth,” 361–69.
Alexandra Witze, “Greenland Rocks Suggest Earth’s Magnetic Field Is Older Than We Thought,” Nature 576 (December 10, 2019): 347, doi:10.1038/d41586-019-03807-7.
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.