Earth's outer core

<h2 id="properties">Properties</h2>

The outer core of Earth is liquid, unlike its <a href="/facts/Earth%2527s_inner_core/M2qscVcY">inner core</a>, which is solid.<a class="footnote-ref" id="fnref:5" href="#fn:5">5</a> Evidence for a fluid outer core includes <a href="/facts/Seismology/Lony1LYX">seismology</a> which shows that <a href="/facts/Seismic_wave/1JTeLfac">seismic</a> <a href="/facts/S_wave/CN3pvkJX">shear-waves</a> are not transmitted through the outer core.<a class="footnote-ref" id="fnref:6" href="#fn:6">6</a> Although having a composition similar to Earth's solid inner core, the outer core remains liquid as there is not enough pressure to keep it in a solid state.
Seismic inversions of <a href="/facts/Seismic_wave/1JTeLfac">body waves</a> and <a href="/facts/Seismic_wave/1JTeLfac">normal modes</a> constrain the radius of the outer core to be 3483 km with an uncertainty of 5 km, while that of the inner core is 1220±10 km.<a class="footnote-ref" id="fnref:7" href="#fn:7">7</a>: 94 
Estimates for the <a href="/facts/Temperature/QyGe4gmj">temperature</a> of the outer core are about 3,000–4,500 K (2,700–4,200 °C; 4,900–7,600 °F) in its outer region and 4,000–8,000 K (3,700–7,700 °C; 6,700–14,000 °F) near the inner core.<a class="footnote-ref" id="fnref:8" href="#fn:8">8</a> Modeling has shown that the outer core, because of its high temperature, is a low-<a href="/facts/Viscosity/QLVfrdCz">viscosity</a> fluid that convects <a href="/facts/Turbulence/NpgFUhG5">turbulently</a>.<a class="footnote-ref" id="fnref:9" href="#fn:9">9</a> The <a href="/facts/Dynamo_theory/CVMamd1s">dynamo theory</a> sees <a href="/facts/Eddy_current/KvHNx1v7">eddy currents</a> in the nickel-iron fluid of the outer core as the principal source of <a href="/facts/Earth%2527s_magnetic_field/73G36wmx">Earth's magnetic field</a>. The average <a href="/facts/Magnetic_field/Ta8Ev588">magnetic field</a> strength in Earth's outer core is estimated to be 2.5 <a href="/facts/Tesla_(unit)/K6I5Stn9">millitesla</a>, 50 times stronger than the magnetic field at the surface.<a class="footnote-ref" id="fnref:10" href="#fn:10">10</a><a class="footnote-ref" id="fnref:11" href="#fn:11">11</a>
As Earth's core cools, the liquid at the inner core boundary freezes, causing the solid inner core to grow at the expense of the outer core, at an estimated rate of 1 mm per year. This is approximately 80,000 tonnes of iron per second.<a class="footnote-ref" id="fnref:12" href="#fn:12">12</a>

<h2 id="light-elements-of-earths-outer-core">Light elements of Earth's outer core</h2>
<h3>Composition</h3>
Earth's outer core cannot be entirely constituted of iron or iron-nickel <a href="/facts/Alloy/I7w7WQWR">alloy</a> because their densities are higher than geophysical measurements of the <a href="/facts/Density/92p6hh9I">density</a> of Earth's outer core.<a class="footnote-ref" id="fnref:13" href="#fn:13">13</a><a class="footnote-ref" id="fnref:14" href="#fn:14">14</a><a class="footnote-ref" id="fnref:15" href="#fn:15">15</a><a class="footnote-ref" id="fnref:16" href="#fn:16">16</a> In fact, Earth's outer core is approximately 5 to 10 percent lower density than <a href="/facts/Iron/QK1JYy8E">iron</a> at Earth's core <a href="/facts/Temperature/QyGe4gmj">temperatures</a> and <a href="/facts/Pressure/eyc37GRG">pressures</a>.<a class="footnote-ref" id="fnref:17" href="#fn:17">17</a><a class="footnote-ref" id="fnref:18" href="#fn:18">18</a><a class="footnote-ref" id="fnref:19" href="#fn:19">19</a> Hence it has been proposed that light <a href="/facts/Chemical_element/0jducgWD">elements</a> with low <a href="/facts/Atomic_number/bgAr581S">atomic numbers</a> compose part of Earth's outer core, as the only feasible way to lower its density.<a class="footnote-ref" id="fnref:20" href="#fn:20">20</a><a class="footnote-ref" id="fnref:21" href="#fn:21">21</a><a class="footnote-ref" id="fnref:22" href="#fn:22">22</a> Although Earth's outer core is inaccessible to direct sampling,<a class="footnote-ref" id="fnref:23" href="#fn:23">23</a><a class="footnote-ref" id="fnref:24" href="#fn:24">24</a><a class="footnote-ref" id="fnref:25" href="#fn:25">25</a> the composition of light <a href="/facts/Chemical_element/0jducgWD">elements</a> can be meaningfully constrained by high-<a href="/facts/Pressure/eyc37GRG">pressure</a> experiments, calculations based on <a href="/facts/Seismic/Lony1LYX">seismic</a> measurements, models of <a href="/facts/Accretion_(astrophysics)/atcWRsDP">Earth's accretion</a>, and <a href="/facts/Chondrite/mbzbGm5A">carbonaceous chondrite meteorite</a> comparisons with <a href="/facts/Primitive_mantle/lc2Tt4qY">bulk silicate Earth (BSE)</a>.<a class="footnote-ref" id="fnref:26" href="#fn:26">26</a><a class="footnote-ref" id="fnref:27" href="#fn:27">27</a><a class="footnote-ref" id="fnref:28" href="#fn:28">28</a><a class="footnote-ref" id="fnref:29" href="#fn:29">29</a><a class="footnote-ref" id="fnref:30" href="#fn:30">30</a><a class="footnote-ref" id="fnref:31" href="#fn:31">31</a> Recent estimates are that Earth's outer core is composed of <a href="/facts/Iron/QK1JYy8E">iron</a> along with 0 to 0.26 percent <a href="/facts/Hydrogen/BoYHjl0J">hydrogen</a>, 0.2 percent <a href="/facts/Carbon/ukrgQexo">carbon</a>, 0.8 to 5.3 percent <a href="/facts/Oxygen/acyn6o8r">oxygen</a>, 0 to 4.0 percent <a href="/facts/Silicon/cgWx0hdr">silicon</a>, 1.7 percent <a href="/facts/Sulfur/IK0amTfM">sulfur</a>, and 5 percent <a href="/facts/Nickel/KwMCHYRP">nickel</a> by weight, and the <a href="/facts/Temperature/QyGe4gmj">temperature</a> of the <a href="/facts/Core-mantle_boundary/EWHuuEdT">core-mantle boundary</a> and the inner core boundary ranges from 4,137 to 4,300 <a href="/facts/Kelvin/E3trzC4C">K</a> and from 5,400 to 6,300 <a href="/facts/Kelvin/E3trzC4C">K</a> respectively.<a class="footnote-ref" id="fnref:32" href="#fn:32">32</a>

<h4>Constraints</h4>
<h5>Accretion</h5>

The variety of light elements present in Earth's outer core is constrained in part by <a href="/facts/Accretion_(astrophysics)/atcWRsDP">Earth's accretion</a>.<a class="footnote-ref" id="fnref:33" href="#fn:33">33</a> Namely, the light elements contained must have been abundant during Earth's formation, must be able to partition into <a href="/facts/Liquid/t9GkLWzt">liquid</a> iron at low <a href="/facts/Pressure/eyc37GRG">pressures</a>, and must not volatilize and escape during Earth's accretionary process.<a class="footnote-ref" id="fnref:34" href="#fn:34">34</a><a class="footnote-ref" id="fnref:35" href="#fn:35">35</a>

<h5>CI chondrites</h5>
<a href="/facts/CI_chondrite/KyADcSh9">CI chondritic meteorites</a> are believed to contain the same planet-forming elements in the same <a href="/facts/Ratio/mFmhQt7T">proportions</a> as in the early <a href="/facts/Solar_System/QIcLSazQ">Solar System</a>,<a class="footnote-ref" id="fnref:36" href="#fn:36">36</a> so differences between CI meteorites and <a href="/facts/Primitive_mantle/lc2Tt4qY">BSE</a> can provide insights into the light element composition of Earth's outer core.<a class="footnote-ref" id="fnref:37" href="#fn:37">37</a><a class="footnote-ref" id="fnref:38" href="#fn:38">38</a> For instance, the depletion of <a href="/facts/Silicon/cgWx0hdr">silicon</a> in Earth's <a href="/facts/Primitive_mantle/lc2Tt4qY">primitive mantle</a> compared to CI meteorites may indicate that silicon was absorbed into Earth's core; however, a wide range of silicon concentrations in Earth's outer and <a href="/facts/Earth%2527s_inner_core/M2qscVcY">inner core</a> is still possible.<a class="footnote-ref" id="fnref:39" href="#fn:39">39</a><a class="footnote-ref" id="fnref:40" href="#fn:40">40</a><a class="footnote-ref" id="fnref:41" href="#fn:41">41</a>

<h3>Implications for Earth's accretion and core formation history</h3>
Tighter constraints on the concentrations of light elements in Earth's outer core would provide a better understanding of <a href="/facts/Accretion_(astrophysics)/atcWRsDP">Earth's accretion</a> and <a href="/facts/Internal_structure_of_Earth/678dYXU7">core formation</a> history.<a class="footnote-ref" id="fnref:42" href="#fn:42">42</a><a class="footnote-ref" id="fnref:43" href="#fn:43">43</a><a class="footnote-ref" id="fnref:44" href="#fn:44">44</a>

<h4>Consequences for Earth's accretion</h4>
Models of Earth's accretion could be better tested if we had better constraints on light element <a href="/facts/Concentration/wqBmz5od">concentrations</a> in Earth's outer core.<a class="footnote-ref" id="fnref:45" href="#fn:45">45</a><a class="footnote-ref" id="fnref:46" href="#fn:46">46</a> For example, accretionary models based on core-mantle element partitioning tend to support proto-Earths constructed from reduced, condensed, and volatile-free material,<a class="footnote-ref" id="fnref:47" href="#fn:47">47</a><a class="footnote-ref" id="fnref:48" href="#fn:48">48</a><a class="footnote-ref" id="fnref:49" href="#fn:49">49</a> despite the possibility that <a href="/facts/Redox/Pyd5RVcj">oxidized</a> material from the outer <a href="/facts/Solar_System/QIcLSazQ">Solar System</a> was accreted towards the conclusion of <a href="/facts/Accretion_(astrophysics)/atcWRsDP">Earth's accretion</a>.<a class="footnote-ref" id="fnref:50" href="#fn:50">50</a><a class="footnote-ref" id="fnref:51" href="#fn:51">51</a> If we could better constrain the concentrations of <a href="/facts/Hydrogen/BoYHjl0J">hydrogen</a>, <a href="/facts/Oxygen/acyn6o8r">oxygen</a>, and <a href="/facts/Silicon/cgWx0hdr">silicon</a> in Earth's outer core, models of Earth's accretion that match these concentrations would presumably better constrain Earth’s formation.<a class="footnote-ref" id="fnref:52" href="#fn:52">52</a>

<h4>Consequences for Earth's core formation</h4>

The depletion of <a href="/facts/Goldschmidt_classification/8p9CWrpe">siderophile elements</a> in <a href="/facts/Earth%2527s_mantle/l9DjRLcc">Earth's mantle</a> compared to chondritic meteorites is attributed to metal-silicate reactions during formation of Earth's core.<a class="footnote-ref" id="fnref:53" href="#fn:53">53</a> These reactions are dependent on <a href="/facts/Oxygen/acyn6o8r">oxygen</a>, <a href="/facts/Silicon/cgWx0hdr">silicon</a>, and <a href="/facts/Sulfur/IK0amTfM">sulfur</a>,<a class="footnote-ref" id="fnref:54" href="#fn:54">54</a><a class="footnote-ref" id="fnref:55" href="#fn:55">55</a><a class="footnote-ref" id="fnref:56" href="#fn:56">56</a> so better constraints on <a href="/facts/Concentration/wqBmz5od">concentrations</a> of these elements in Earth's outer core will help elucidate the conditions of formation of <a href="/facts/Earth%2527s_core/678dYXU7">Earth's core</a>.<a class="footnote-ref" id="fnref:57" href="#fn:57">57</a><a class="footnote-ref" id="fnref:58" href="#fn:58">58</a><a class="footnote-ref" id="fnref:59" href="#fn:59">59</a><a class="footnote-ref" id="fnref:60" href="#fn:60">60</a><a class="footnote-ref" id="fnref:61" href="#fn:61">61</a>
In another example, the possible presence of <a href="/facts/Hydrogen/BoYHjl0J">hydrogen</a> in Earth's outer core suggests that the <a href="/facts/Accretion_(astrophysics)/atcWRsDP">accretion</a> of Earth’s <a href="/facts/Water/0lkXnJPK">water</a><a class="footnote-ref" id="fnref:62" href="#fn:62">62</a><a class="footnote-ref" id="fnref:63" href="#fn:63">63</a><a class="footnote-ref" id="fnref:64" href="#fn:64">64</a> was not limited to the final stages of <a href="/facts/Accretion_(astrophysics)/atcWRsDP">Earth's accretion</a><a class="footnote-ref" id="fnref:65" href="#fn:65">65</a> and that <a href="/facts/Water/0lkXnJPK">water</a> may have been absorbed into core-forming metals through a hydrous <a href="/facts/Magma_ocean/PGcKDMRg">magma ocean</a>.<a class="footnote-ref" id="fnref:66" href="#fn:66">66</a><a class="footnote-ref" id="fnref:67" href="#fn:67">67</a>

<h3>Implications for Earth's magnetic field</h3>

<a href="/facts/Earth%2527s_magnetic_field/73G36wmx">Earth's magnetic field</a> is driven by <a href="/facts/Convection_(heat_transfer)/3r5MHqdg">thermal convection</a> and also by chemical convection, the exclusion of light elements from the inner core, which float upward within the fluid outer core while <a href="/facts/Density/92p6hh9I">denser</a> elements sink.<a class="footnote-ref" id="fnref:68" href="#fn:68">68</a><a class="footnote-ref" id="fnref:69" href="#fn:69">69</a> This chemical convection releases <a href="/facts/Gravitational_energy/SJpNw6G5">gravitational energy</a> that is then available to power the <a href="/facts/Dynamo_theory/CVMamd1s">geodynamo</a> that produces Earth's magnetic field.<a class="footnote-ref" id="fnref:70" href="#fn:70">70</a> <a href="/facts/Carnot_cycle/cjN9V9q8">Carnot efficiencies</a> with large uncertainties suggest that compositional and thermal convection contribute about 80 percent and 20 percent respectively to the power of Earth's geodynamo.<a class="footnote-ref" id="fnref:71" href="#fn:71">71</a> Traditionally it was thought that prior to the formation of <a href="/facts/Earth%2527s_inner_core/M2qscVcY">Earth's inner core</a>, Earth's geodynamo was mainly driven by thermal convection.<a class="footnote-ref" id="fnref:72" href="#fn:72">72</a> However, recent claims that the <a href="/facts/Thermal_conductivity/qWnSG3nQ">thermal conductivity</a> of <a href="/facts/Iron/QK1JYy8E">iron</a> at core <a href="/facts/Temperature/QyGe4gmj">temperatures</a> and pressures is much higher than previously thought imply that core cooling was largely by conduction not convection, limiting the ability of thermal convection to drive the geodynamo.<a class="footnote-ref" id="fnref:73" href="#fn:73">73</a><a class="footnote-ref" id="fnref:74" href="#fn:74">74</a> This conundrum is known as the new "core paradox."<a class="footnote-ref" id="fnref:75" href="#fn:75">75</a><a class="footnote-ref" id="fnref:76" href="#fn:76">76</a> An alternative process that could have sustained Earth's geodynamo requires Earth's core to have initially been hot enough to dissolve <a href="/facts/Oxygen/acyn6o8r">oxygen</a>, <a href="/facts/Magnesium/XyIhd2FG">magnesium</a>, <a href="/facts/Silicon/cgWx0hdr">silicon</a>, and other light elements.<a class="footnote-ref" id="fnref:77" href="#fn:77">77</a> As the Earth's core began to cool, it would become <a href="/facts/Supersaturation/YaTN0D1L">supersaturated</a> in these light elements that would then <a href="/facts/Precipitation_(chemistry)/s3BgsLLY">precipitate</a> into the <a href="/facts/Lower_mantle_(Earth)/KQQBHWp8">lower mantle</a> forming <a href="/facts/Oxide/DXtoVe1C">oxides</a> leading to a different variant of chemical convection.<a class="footnote-ref" id="fnref:78" href="#fn:78">78</a><a class="footnote-ref" id="fnref:79" href="#fn:79">79</a>
The magnetic field generated by core flow is essential to protect life from interplanetary radiation and prevent the atmosphere from dissipating in the <a href="/facts/Solar_wind/BvuB3dT8">solar wind</a>. The rate of cooling by conduction and convection is uncertain,<a class="footnote-ref" id="fnref:80" href="#fn:80">80</a> but one estimate is that the core would not be expected to freeze up for approximately 91 billion years, which is well after the Sun is expected to expand, sterilize the surface of the planet, and then burn out.<a class="footnote-ref" id="fnref:81" href="#fn:81">81</a>[better source needed]

<h2 id="see-also">See also</h2>
<ul><li><a href="/facts/Hollow_Earth/jNdSySBK">Hollow Earth</a></li>
<li><a href="/facts/Geological_history_of_Earth/Jx7ClmF4">Geological history of Earth</a></li>
<li><a href="/facts/Large_low-shear-velocity_provinces/xfcluSMY">Large low-shear-velocity provinces</a></li>
<li><a href="/facts/Lehmann_discontinuity/4SvssIMk">Lehmann discontinuity</a></li>
<li><a href="/facts/Rain-out_model/aKVTQNv5">Rain-out model</a></li>
<li><a href="/facts/Seismic_tomography/xlRG9zac">Seismic tomography</a> – technique for imaging the subsurface of Earth using seismic waves</li>
<li><a href="/facts/Travel_to_the_Earth%2527s_center/njeFGe3z">Travel to the Earth's center</a></li>
<li><a href="/facts/Solid_earth/9cTx04Rb">Solid earth</a></li></ul>

<h2 id="external-links">External links</h2>

The Wikibook Historical Geology has a page on the topic of: Structure of the Earth

<h2 id="references">References</h2>

<ol>
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</ol>

Earth's outer core open-in-new

Earth's outer core