Energy Stays Behind: Two Engines and the Atmospheric Greenhouse of the Hadean

Sun Apr 05 2026 · en

Keywords: Hadean energy retention, solar radiation, Earth’s internal heat, greenhouse effect, Faint Young Sun paradox, carbonate-silicate thermostat, Earth habitability, radiogenic heat, tidal heating

In most parts of the Earth, day and night differ in temperature — but only by a modest amount, typically a few to a couple of dozen degrees. If you stood on the lunar surface, however, with the same solar distance and the same radiation intensity, the sunlit side reaches 120 °C while the shadowed side plunges to −170 °C. Heat arrives quickly and leaves just as fast, because the Moon has no atmosphere — nothing to make energy linger.

Once an atmosphere formed around the early Earth, the energy delivered by solar radiation began to stay at the surface for much longer. Heat rising from the Earth’s interior was likewise intercepted, unable to radiate freely into space. Two engines fed energy into the system, and the atmosphere acted as a gate — for the first time allowing energy to remain at the surface long enough to sustain liquid water, and establishing the physical preconditions for energy to later be locked into chemical bonds.

Solar flare photographed from the International Space Station over the Atlantic Ocean — Photographed shortly after orbital sunrise from the International Space Station at roughly 263 miles (423 km) above the Atlantic. Sunlight refracts across the cloud-covered ocean surface; the energy carried by that light every second exceeds humanity’s total annual energy consumption by a factor of ten thousand. Image credit: NASA / ISS

The First Engine: Light from Space

The Sun is Earth’s primary energy source. Today, at Earth’s orbital distance, the Sun delivers a continuous flux of approximately 1361 W/m² — a value known as the solar constant [Feulner, 2012] . Not all of this radiation is absorbed, however: Earth’s albedo causes roughly 30% of incoming solar radiation to be reflected directly back into space.

After correcting for albedo, the effective solar energy reaching Earth’s surface averages approximately 240 W/m².

In the Hadean, the Sun was roughly 25–30% dimmer than today, with a luminosity of only about $0.70 \sim 0.75 \, L_\odot$ [Feulner, 2012] . This means the total solar radiation reaching the early Earth was considerably lower — and this is the heart of the Faint Young Sun Paradox: at such a flux, liquid water should have been impossible, yet Earth clearly maintained warm oceans.

In addition to visible and infrared radiation, the young Sun blasted the Hadean Earth with intense ultraviolet (UV) radiation. No ozone layer existed yet and there was virtually no free oxygen in the atmosphere, so high-energy UV could reach the surface and the sea directly, injecting large amounts of radiative energy into the upper ocean.

The Second Engine: Heat from Earth’s Interior

Earth’s second energy engine lies deep underfoot. Today the Earth still releases around 47 terawatts (TW) of heat to the surface every second [Davies et al., 2010] — roughly three times the total energy consumption of human civilisation. In the Hadean, this figure was 3 to 4 times higher, approximately 150–200 TW.

Earth’s internal heat is the sum of three contributions:

① Primordial Heat: Around 4.6 billion years ago, Earth accumulated from countless planetesimals, and every collision converted kinetic energy into heat. The final giant impact — the Theia collision that formed the Moon — injected enormous energy into the proto-Earth, melting its outer layers and generating a magma ocean thousands of kilometres deep [Elkins-Tanton, 2012] . That primordial heat has been stored in the interior ever since, leaking out slowly over billions of years [Chambers, 2004] .

② Radiogenic Heat: Earth’s interior is rich in radioactive isotopes of uranium (²³⁸U, ²³⁵U), thorium (²³²Th), and potassium (⁴⁰K). Their decay releases heat continuously, and this is the dominant source of Earth’s internal heat today. In the Hadean, these isotopes were far more abundant — their radioactive decay had barely begun — so the rate of radiogenic heat production was correspondingly higher [Korenaga, 2008] .

The decay chain of uranium-238 illustrates this:

^{238}_{92}\text{U} \;\longrightarrow\; ^{206}_{82}\text{Pb} \;+\; 8\,^4_2\text{He} \;+\; 6\,\bar{\nu}_e \;+\; \text{energy}

This chain releases a total of approximately 51.7 MeV and has a half-life of roughly 4.47 billion years — comparable to Earth’s age — meaning the concentration of uranium-238 inside the Hadean Earth was roughly twice what it is today.

③ Tidal Heating: In the immediate aftermath of its formation, the Moon orbited at only about 20–25 Earth radii (today it sits at roughly 60) [Dickey et al., 1994] . The intense tidal friction generated by such proximity produced additional heat inside the early Earth and rapidly slowed Earth’s rotation.

These three heat flows combined, rising from the interior to the surface via volcanic eruptions, hydrothermal circulation, and mantle convection [Davies et al., 2010] .

Earth’s Energy Budget: An Elegant Balance

With two engines continuously adding energy, how did Earth maintain thermal equilibrium rather than growing steadily hotter or cooler?

The answer is radiative balance. Earth absorbs energy from the Sun while simultaneously shedding heat to space as infrared radiation. In long-term equilibrium, input equals output, and Earth maintains a stable effective radiating temperature. This temperature is governed by:

T_{\text{eff}} = \left[\frac{S_0\,(1 - \alpha)}{4\,\sigma}\right]^{1/4}

where $S_0$ is the solar constant (today approximately 1361 W/m²), $\alpha$ is Earth’s albedo (approximately 0.30), and $\sigma$ is the Stefan–Boltzmann constant ( $5.67 \times 10^{-8}$ W/m²/K⁴).

Plugging in these values gives approximately −18 °C.

That is the surface temperature Earth would have in the absence of an atmosphere — well below the freezing point of water. Earth’s actual global mean surface temperature today is around +15 °C. The difference of roughly 33 °C is the additional warming contributed by the atmospheric greenhouse effect.

In the Hadean, with a dimmer Sun ( $S_0$ at roughly 70–75% of today’s value), the same equation gives an effective temperature of around −30 °C. To sustain liquid water, the greenhouse effect at the time would have needed to contribute more than 40–60 °C of additional warming — far more intense than the greenhouse effect operating today.

The Atmosphere: Earth’s Insulation System

The layer of thick atmosphere of the Hadean is what resolved this energy balance problem.

Greenhouse gases in the atmosphere — chiefly CO₂ and H₂O — are largely transparent to the short-wave solar radiation arriving from above, letting sunlight pass through to the surface. But they strongly absorb the long-wave infrared radiation emitted upward from the warm surface, preventing it from escaping directly to space. The absorbed infrared re-radiates in all directions; the downward component warms the surface again, producing the so-called greenhouse effect [Zahnle et al., 2010] .

The CO₂ molecule has a strong infrared absorption band at around 15 micrometres, which falls squarely near the peak of the surface emission spectrum. Water vapour absorbs infrared radiation across a much broader range of wavelengths and is the single most powerful greenhouse gas in today’s atmosphere.

CO₂ concentrations in the Hadean atmosphere were extremely high — estimated at hundreds to thousands of times the present level (~420 ppm) [Zahnle et al., 2010] [Holland, 2002] . Such concentrations created a thick thermal blanket, locking heat near the surface that would otherwise have been lost to space, and elevating global temperatures into the range where liquid water could persist.

This mechanism also has self-regulating properties. When Earth cools, carbonate dissolution slows, atmospheric CO₂ accumulates, the greenhouse effect intensifies, and Earth warms again. This is the carbonate–silicate thermostat [Walker et al., 1981] . This feedback kept Earth’s surface temperature broadly habitable over billions of years — while Venus and Mars took very different paths.

Schematic of the greenhouse effect in the early Earth's atmosphere — **Figure**: The early Earth’s atmosphere during the Hadean. Thick CO₂ and water vapour produce an intense greenhouse effect, compensating for the radiation deficit caused by the faint young Sun and maintaining global mean temperatures in the range where liquid water can exist.

Energy Truly Stays Behind for the First Time

The intricate interplay between the two engines — solar radiation maintaining the climate, internal heat driving geological activity, and the atmosphere regulating the temperature balance — together created a stable liquid-water planet. This is not simply a question of temperature: the longer energy lingers at the surface, the more opportunities it has to react with matter.

This physical framework is the starting point for the next chapter. In the following article, we ask: when energy no longer merely flows across the surface but begins to interact with molecules for longer periods, what happens — and how did it first become locked into chemical bonds?

Comparing the Two Engines

Property	Solar Radiation	Earth’s Internal Heat
Source	Hydrogen fusion (solar core)	Primordial heat + radiogenic decay + tidal friction
Hadean power (estimated)	~9,400 TW (net absorbed, 70% solar luminosity)	~150–200 TW
Primary surface expression	Heating atmosphere and ocean, driving water cycle and photochemistry	Volcanic eruptions, hydrothermal circulation, plate tectonics
Timescale	Instantaneous (light speed, 8-minute delay)	Slow release over billions of years

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