Name the two primary sources of heat in the atmosphere

There are three main sources of heat in the deep earth: (1) heat from when the planet formed and accreted, which has not yet been lost; (2) frictional heating, caused by denser core material sinking to the center of the planet; and (3) heat from the decay of radioactive elements.

It takes a rather long time for heat to move out of the earth. This occurs through both "convective" transport of heat within the earth's liquid outer core and solid mantle and slower "conductive" transport of heat through nonconvecting boundary layers, such as the earth's plates at the surface. As a result, much of the planet's primordial heat, from when the earth first accreted and developed its core, has been retained.

The amount of heat that can arise through simple accretionary processes, bringing small bodies together to form the proto-earth, is large: on the order of 10,000 kelvins (about 18,000 degrees Farhenheit). The crucial issue is how much of that energy was deposited into the growing earth and how much was reradiated into space. Indeed, the currently accepted idea for how the moon was formed involves the impact or accretion of a Mars-size object with or by the proto-earth. When two objects of this size collide, large amounts of heat are generated, of which quite a lot is retained. This single episode could have largely melted the outermost several thousand kilometers of the planet.

Additionally, descent of the dense iron-rich material that makes up the core of the planet to the center would produce heating on the order of 2,000 kelvins (about 3,000 degrees F). The magnitude of the third main source of heat--radioactive heating--is uncertain. The precise abundances of radioactive elements (primarily potassium, uranium and thorium) are poorly known in the deep earth.

In sum, there was no shortage of heat in the early earth, and the planet's inability to cool off quickly results in the continued high temperatures of the Earth's interior. In effect, not only do the earth's plates act as a blanket on the interior, but not even convective heat transport in the solid mantle provides a particularly efficient mechanism for heat loss. The planet does lose some heat through the processes that drive plate tectonics, especially at mid-ocean ridges. For comparison, smaller bodies such as Mars and the Moon show little evidence for recent tectonic activity or volcanism.

We derive our primary estimate of the temperature of the deep earth from the melting behavior of iron at ultrahigh pressures. We know that the earth's core depths from 2,886 kilometers to the center at 6,371 kilometers (1,794 to 3,960 miles), is predominantly iron, with some contaminants. How? The speed of sound through the core (as measured from the velocity at which seismic waves travel across it) and the density of the core are quite similar to those seen in of iron at high pressures and temperatures, as measured in the laboratory. Iron is the only element that closely matches the seismic properties of the earth's core and is also sufficiently abundant present in sufficient abundance in the universe to make up the approximately 35 percent of the mass of the planet present in the core.

The earth's core is divided into two separate regions: the liquid outer core and the solid inner core, with the transition between the two lying at a depth of 5,156 kilometers (3,204 miles). Therefore, If we can measure the melting temperature of iron at the extreme pressure of the boundary between the inner and outer cores, then this lab temperature should reasonably closely approximate the real temperature at this liquid-solid interface. Scientists in mineral physics laboratories use lasers and high-pressure devices called diamond-anvil cells to re-create these hellish pressures and temperatures as closely as possible.

Those experiments provide a stiff challenge, but our estimates for the melting temperature of iron at these conditions range from about 4,500 to 7,500 kelvins (about 7,600 to 13,000 degrees F). As the outer core is fluid and presumably convecting (and with an additional correction for the presence of impurities in the outer core), we can extrapolate this range of temperatures to a temperature at the base of Earth's mantle (the top of the outer core) of roughly 3,500 to 5,500 kelvins (5,800 to 9,400 degrees F) at the base of the earth's mantle.

The bottom line here is simply that a large part of the interior of the planet (the outer core) is composed of somewhat impure molten iron alloy. The melting temperature of iron under deep-earth conditions is high, thus providing prima facie evidence that the deep earth is quite hot.

Gregory Lyzenga is an associate professor of physics at Harvey Mudd College. He provided some additional details on estimating the temperature of the earth's core:

How do we know the temperature? The answer is that we really don't--at least not with great certainty or precision. The center of the earth lies 6,400 kilometers (4,000 miles) beneath our feet, but the deepest that it has ever been possible to drill to make direct measurements of temperature (or other physical quantities) is just about 10 kilometers (six miles).

Ironically, the core of the earth is by far less accessible more inaccessible to direct probing than would be the surface of Pluto. Not only do we not have the technology to "go to the core," but it is not at all clear how it will ever be possible to do so.

As a result, scientists must infer the temperature in the earth's deep interior indirectly. Observing the speed at which of passage of seismic waves pass through the earth allows geophysicists to determine the density and stiffness of rocks at depths inaccessible to direct examination. If it is possible to match up those properties with the properties of known substances at elevated temperatures and pressures, it is possible (in principle) to infer what the environmental conditions must be deep in the earth.

The problem with this is that the conditions are so extreme at the earth's center that it is very difficult to perform any kind of laboratory experiment that faithfully simulates conditions in the earth's core. Nevertheless, geophysicists are constantly trying these experiments and improving on them, so that their results can be extrapolated to the earth's center, where the pressure is more than three million times atmospheric pressure.

The bottom line of these efforts is that there is a rather wide range of current estimates of the earth's core temperature. The "popular" estimates range from about 4,000 kelvins up to over 7,000 kelvins (about 7,000 to 12,000 degrees F).

If we knew the melting temperature of iron very precisely at high pressure, we could pin down the temperature of the Earth's core more precisely, because it is largely made up of molten iron. But until our experiments at high temperature and pressure become more precise, uncertainty in this fundamental property of our planet will persist.

Earth's internal heat budget is fundamental to the thermal history of the Earth. The flow of heat from Earth's interior to the surface is estimated at 47±2 terawatts (TW)[1] and comes from two main sources in roughly equal amounts: the radiogenic heat produced by the radioactive decay of isotopes in the mantle and crust, and the primordial heat left over from the formation of Earth.[2]

Global map of the flux of heat, in mW/m2, from Earth's interior to the surface.[1] The largest values of heat flux coincide with mid-ocean ridges, and the smallest values of heat flux occur in stable continental interiors.

Earth's internal heat travels along geothermal gradients and powers most geological processes.[3] It drives mantle convection, plate tectonics, mountain building, rock metamorphism, and volcanism.[2] Convective heat transfer within the planet's high-temperature metallic core is also theorized to sustain a geodynamo which generates Earth's magnetic field.[4][5][6]

Despite its geological significance, Earth's interior heat contributes only 0.03% of Earth's total energy budget at the surface, which is dominated by 173,000 TW of incoming solar radiation.[7] This external energy source powers most of the planet's atmospheric, oceanic, and biologic processes. Nevertheless on land and at the ocean floor, the sensible heat absorbed from non-reflected insolation flows inward only by means of thermal conduction, and thus penetrates only several tens of centimeters on the daily cycle and only several tens of meters on the annual cycle. This renders solar radiation minimally relevant for processes internal to Earth's crust.[8]

Global data on heat-flow density are collected and compiled by the International Heat Flow Commission of the International Association of Seismology and Physics of the Earth's Interior.[9]

Based on calculations of Earth's cooling rate, which assumed constant conductivity in the Earth's interior, in 1862 William Thomson, later Lord Kelvin, estimated the age of the Earth at 98 million years,[10] which contrasts with the age of 4.5 billion years obtained in the 20th century by radiometric dating.[11] As pointed out by John Perry in 1895[12] a variable conductivity in the Earth's interior could expand the computed age of the Earth to billions of years, as later confirmed by radiometric dating. Contrary to the usual representation of Thomson's argument, the observed thermal gradient of the Earth's crust would not be explained by the addition of radioactivity as a heat source. More significantly, mantle convection alters how heat is transported within the Earth, invalidating Thomson's assumption of purely conductive cooling.

Cross section of the Earth showing its main divisions and their approximate contributions to Earth's total internal heat flow to the surface, and the dominant heat transport mechanisms within Earth

Estimates of the total heat flow from Earth's interior to surface span a range of 43 to 49 terawatts (TW) (a terawatt is 1012 watts).[13] One recent estimate is 47 TW,[1] equivalent to an average heat flux of 91.6 mW/m2, and is based on more than 38,000 measurements. The respective mean heat flows of continental and oceanic crust are 70.9 and 105.4 mW/m2.[1]

While the total internal Earth heat flow to the surface is well constrained, the relative contribution of the two main sources of Earth's heat, radiogenic and primordial heat, are highly uncertain because their direct measurement is difficult. Chemical and physical models give estimated ranges of 15–41 TW and 12–30 TW for radiogenic heat and primordial heat, respectively.[13]

The structure of Earth is a rigid outer crust that is composed of thicker continental crust and thinner oceanic crust, solid but plastically flowing mantle, a liquid outer core, and a solid inner core. The fluidity of a material is proportional to temperature; thus, the solid mantle can still flow on long time scales, as a function of its temperature[2] and therefore as a function of the flow of Earth's internal heat. The mantle convects in response to heat escaping from Earth's interior, with hotter and more buoyant mantle rising and cooler, and therefore denser, mantle sinking. This convective flow of the mantle drives the movement of Earth's lithospheric plates; thus, an additional reservoir of heat in the lower mantle is critical for the operation of plate tectonics and one possible source is an enrichment of radioactive elements in the lower mantle.[14]

Earth heat transport occurs by conduction, mantle convection, hydrothermal convection, and volcanic advection.[15] Earth's internal heat flow to the surface is thought to be 80% due to mantle convection, with the remaining heat mostly originating in the Earth's crust,[16] with about 1% due to volcanic activity, earthquakes, and mountain building.[2] Thus, about 99% of Earth's internal heat loss at the surface is by conduction through the crust, and mantle convection is the dominant control on heat transport from deep within the Earth. Most of the heat flow from the thicker continental crust is attributed to internal radiogenic sources; in contrast the thinner oceanic crust has only 2% internal radiogenic heat.[2] The remaining heat flow at the surface would be due to basal heating of the crust from mantle convection. Heat fluxes are negatively correlated with rock age,[1] with the highest heat fluxes from the youngest rock at mid-ocean ridge spreading centers (zones of mantle upwelling), as observed in the global map of Earth heat flow.[1]

The evolution of Earth's radiogenic heat flow over time

The radioactive decay of elements in the Earth's mantle and crust results in production of daughter isotopes and release of geoneutrinos and heat energy, or radiogenic heat. About 50% of the Earth's internal heat originates from radioactive decay.[17] Four radioactive isotopes are responsible for the majority of radiogenic heat because of their enrichment relative to other radioactive isotopes: uranium-238 (238U), uranium-235 (235U), thorium-232 (232Th), and potassium-40 (40K).[18] Due to a lack of rock samples from below 200 km depth, it is difficult to determine precisely the radiogenic heat throughout the whole mantle,[18] although some estimates are available.[19]

For the Earth's core, geochemical studies indicate that it is unlikely to be a significant source of radiogenic heat due to an expected low concentration of radioactive elements partitioning into iron.[20] Radiogenic heat production in the mantle is linked to the structure of mantle convection, a topic of much debate, and it is thought that the mantle may either have a layered structure with a higher concentration of radioactive heat-producing elements in the lower mantle, or small reservoirs enriched in radioactive elements dispersed throughout the whole mantle.[21]

An estimate of the present-day major heat-producing isotopes[2]

Isotope	Heat release W/kg isotope	Half-life years	Mean mantle concentration kg isotope/kg mantle	Heat release W/kg mantle
232Th	26.4×10−6	14.0×109	124×10−9	3.27×10−12
238U	94.6×10−6	4.47×109	30.8×10−9	2.91×10−12
40K	29.2×10−6	1.25×109	36.9×10−9	1.08×10−12
235U	569×10−6	0.704×109	0.22×10−9	0.125×10−12

Geoneutrino detectors can detect the decay of 238U and 232Th and thus allow estimation of their contribution to the present radiogenic heat budget, while 235U and 40K are not thus detectable. Regardless, 40K is estimated to contribute 4 TW of heating.[22] However, due to the short half-lives the decay of 235U and 40K contributed a large fraction of radiogenic heat flux to the early Earth, which was also much hotter than at present.[14] Initial results from measuring the geoneutrino products of radioactive decay from within the Earth, a proxy for radiogenic heat, yielded a new estimate of half of the total Earth internal heat source being radiogenic,[22] and this is consistent with previous estimates.[21]

Primordial heat

Primordial heat is the heat lost by the Earth as it continues to cool from its original formation, and this is in contrast to its still actively-produced radiogenic heat. The Earth core's heat flow—heat leaving the core and flowing into the overlying mantle—is thought to be due to primordial heat, and is estimated at 5–15 TW.[23] Estimates of mantle primordial heat loss range between 7 and 15 TW, which is calculated as the remainder of heat after removal of core heat flow and bulk-Earth radiogenic heat production from the observed surface heat flow.[13]

The early formation of the Earth's dense core could have caused superheating and rapid heat loss, and the heat loss rate would slow once the mantle solidified.[23] Heat flow from the core is necessary for maintaining the convecting outer core and the geodynamo and Earth's magnetic field; therefore primordial heat from the core enabled Earth's atmosphere and thus helped retain Earth's liquid water.[21]

Earth's tectonic evolution over time from a molten state at 4.5 Ga,[11] to a single-plate lithosphere,[24] to modern plate tectonics sometime between 3.2 Ga[25] and 1.0 Ga[26]

Controversy over the exact nature of mantle convection makes the linked evolution of Earth's heat budget and the dynamics and structure of the mantle difficult to unravel.[21] There is evidence that the processes of plate tectonics were not active in the Earth before 3.2 billion years ago, and that early Earth's internal heat loss could have been dominated by advection via heat-pipe volcanism.[24] Terrestrial bodies with lower heat flows, such as the Moon and Mars, conduct their internal heat through a single lithospheric plate, and higher heat flows, such as on Jupiter's moon Io, result in advective heat transport via enhanced volcanism, while the active plate tectonics of Earth occur with an intermediate heat flow and a convecting mantle.[24]

Geothermal energy
Geothermal gradient
Planetary differentiation
Thermal history of the Earth
Anthropogenic heat

Media related to Earth's internal heat budget at Wikimedia Commons

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