| 词条 | Inner core super-rotation |
| 释义 |
Inner core super-rotation is the theory that the inner core of Earth is rotating more quickly than its outer core. The theory is based on discrepancies in the time that p-waves take to travel through the inner and outer core. The theory was initially greeted with skepticism but more recent studies suggest there may be a semi-solid layer at the boundary between the inner and outer cores. TheoryEarth rotates on its axis once every 24 hours but because Earth's solid inner core is decoupled by the liquid outer core from its mantle and crust, it could be rotating at a different rate. This possibility was first proposed in 1966; the proposal is based on anomalies of travel times of p-waves traveling through the inner and outer cores.[1] Earth's inner core is primarily composed of iron compounds and despite temperatures of over {{convert|5000|K|lk=on}} is solid because of pressure in excess of 300 GPa. The inner core is thought to rotate 0.2 to 3° per year more quickly than the rest of the Earth, a phenomenon known as super-rotation.[1][2] There remains, however, a lack of consensus among geologists about its cause.[3] Supporting evidenceSong and Richards' observation of inner core super-rotation was published in Nature in 1996; they observed discrepancies in travel times of p-waves through the inner and outer cores.[5][1] Song and Richards analyzed records of seismic waves that originated in the South Sandwich Islands earthquakes and travelled through the inner core to Fairbanks, Alaska.[7] The earthquake and recorder are nearly antipodal and sub-polar, and a thirty-year data record with almost forty events was available.[4] The seismic waves took almost identical paths; some (referred to as PKPBC), however, only traveled through the outer core and the rest (known as PKPDF) also traveled through the inner core. A comparison of the travel times from the earthquakes over a thirty-year period showed an up-to-0.3-second difference between the earliest and latest earthquakes. There were also large discrepancies in travel times of three earthquakes located in the South Sandwich Islands in 1991 that were recorded by seismometers in Alaska.[7] By 1996, the anisotropy of the inner core had been well established due to differences in travel times through the inner core. Waves traveling north-south were moving around 3% faster than those traveling across the equator. The exact structure and axis of anisotropy, however, was not known. The axis was thought to be close to 10° from the axis of the Earth and the western side of the core was thought to be more anisotropic.[7] Based on the data, the relative rotation was inferred to be 0.3 to 0.5° per year, meaning the inner core will rotate once with respect to Earth's surface about every thousand years.[1] Recent findings have further constrained the rotation of the inner core but suggest a more complex rotation. Supercomputer-generated 3D numerical models found a westward drift of the outermost inner core, despite the eastward super-rotation of the bulk inner core, with changes in direction that correlated with changes in Earth's magnetic field based on electromagnetic coupling with the outer core and physically through a torque caused by the growth of the inner core.[5] The inner rotation could be caused by Lorentz forces caused by the cycling of outer mantle fluid. Because the cycling of outer mantle fluid is responsible for Earth's inner dynamo, the rotation of the inner core could by linked to the cycling and may be partially responsible for the geodynamo.[7] Initial skepticism and responseDespite the presence of a growing body of supportive work, there was skepticism about the 1996 findings until around 2005.[1] A study by some French scientists claimed uncertainties about the hypocenters of the earthquakes caused errors in the measurement of travel times.[6] In 2003, an earthquake occurring in the South Sandwich Islands allowed accurate doublet data to confirm the discrepancy in travel time, confirming the initial 1996 findings and largely settling the debate. Zhang and Song published their findings in 2005 in Science and the relative rate of motion was adjusted to 0.41° per year based on the new data from the doublet.[7][8] Skepticism also arose because for the inner core to rotate, it must have a viscosity low enough to deform in response to topography along the inner-core boundary, and to gravitational torques produced by large mass variations in the Earth's mantle. The rotation and presence of large solids would result in significant changes in shape over time, meaning the outer core would need to be viscous or solid enough to adjust or rotate regardless of the solids. The predicted viscosity of the inner core based on these constraints is below 3 × 1016 Pa·s.[9] Models after Song and Richards' theory was published (within a year) limited the possible viscosity of the inner core to less than 1016 Pa·s or greater than 1020 Pa·s.[10] Models of the viscosity of the inner core based on gravitational inner torque caused by the Moon, Sun, and planets constrain the viscosity of the core between 2–7 × 1014 Pa·s, which fits within the bounds necessary for inner core rotation to be possible.[11] More recent studies have also suggested a semi-solid (or "mushy") layer at the inner-outer core boundary to better adjust for rotation with the formation of solids. The semi-solid layer would be dendritic, also providing insight into the anisotropy of the inner core and the solidification of the outer core. The dendritic structure of the layer would have contributed to the difference in formation of the western and eastern inner core, which are thought to be melting and solidifying respectively.[12][13] The existence of the mushy zone within the inner core depends on the way the core formed. The zone most likely exists if the inner core cooled slowly and solidified from the center outwards.[14] See also
References1. ^1 2 3 4 {{cite journal |last1=Mohazzabi |first1=Pirooz |last2=Skalbeck |first2=John D. |title=Superrotation of Earth's Inner Core, Extraterrestrial Impacts, and the Effective Viscosity of Outer Core |journal=International Journal of Geophysics |date=2015 |volume=2015 |pages=1–8 |doi=10.1155/2015/763716 }} 2. ^{{cite book |doi=10.1007/978-1-4020-4423-6_149 |chapter=Inner Core Composition |chapter-url=https://www.ucl.ac.uk/EarthSci/people/lidunka/papers/55.pdf |title=Encyclopedia of Geomagnetism and Paleomagnetism |pages=420–422 |year=2007 |last1=Vočadlo |first1=Lidunka |isbn=978-1-4020-3992-8 }} 3. ^{{cite journal |last1=Waszek |first1=Lauren |last2=Irving |first2=Jessica |last3=Deuss |first3=Arwen |title=Reconciling the hemispherical structure of Earth's inner core with its super-rotation |journal=Nature Geoscience |date=20 February 2011 |volume=4 |issue=4 |pages=264–267 |doi=10.1038/ngeo1083 |bibcode=2011NatGe...4..264W }} 4. ^1 {{cite journal |last1=Song |first1=Xiaodong |last2=Richards |first2=Paul G. |title=Seismological evidence for differential rotation of the Earth's inner core |journal=Nature |date=July 1996 |volume=382 |issue=6588 |pages=221–224 |doi=10.1038/382221a0 |bibcode=1996Natur.382..221S }} 5. ^{{cite journal |last1=Livermore |first1=P. W. |last2=Hollerbach |first2=R. |last3=Jackson |first3=A. |title=Electromagnetically driven westward drift and inner-core superrotation in Earth's core |journal=Proceedings of the National Academy of Sciences |date=16 September 2013 |volume=110 |issue=40 |pages=15914–15918 |doi=10.1073/pnas.1307825110 |pmid=24043841 |pmc=3791745 |bibcode=2013PNAS..11015914L }} 6. ^{{cite journal |last1=Poupinet |first1=Georges |last2=Souriau |first2=Annie |last3=Coutant |first3=Olivier |title=The existence of an inner core super-rotation questioned by teleseismic doublets |journal=Physics of the Earth and Planetary Interiors |date=February 2000 |volume=118 |issue=1–2 |pages=77–88 |doi=10.1016/S0031-9201(99)00129-6 |bibcode=2000PEPI..118...77P }} 7. ^{{cite journal |last1=Xu |first1=Xiaoxia |last2=Song |first2=Xiaodong |title=Evidence for inner core super-rotation from time-dependent differential PKP traveltimes observed at Beijing Seismic Network |journal=Geophysical Journal International |date=March 2003 |volume=152 |issue=3 |pages=509–514 |doi=10.1046/j.1365-246X.2003.01852.x |bibcode=2003GeoJI.152..509X }} 8. ^{{cite journal |last1=Zhang |first1=J. |last2=Song |first2=X |last3=Li |first3=Y |last4=Richards |first4=PG |last5=Sun |first5=X |last6=Waldhauser |first6=F |title=Inner Core Differential Motion Confirmed by Earthquake Waveform Doublets |journal=Science |date=26 August 2005 |volume=309 |issue=5739 |pages=1357–1360 |doi=10.1126/science.1113193 |pmid=16123296 |bibcode=2005Sci...309.1357Z }} 9. ^1 2 3 4 {{cite journal |last1=Creager |first1=K. C. |title=Inner Core Rotation Rate from Small-Scale Heterogeneity and Time-Varying Travel Times |journal=Science |date=14 November 1997 |volume=278 |issue=5341 |pages=1284–1288 |doi=10.1126/science.278.5341.1284 |bibcode=1997Sci...278.1284C }} 10. ^{{cite journal |last1=Buffett |first1=Bruce A. |title=Geodynamic estimates of the viscosity of the Earth's inner core |journal=Nature |date=August 1997 |volume=388 |issue=6642 |pages=571–573 |doi=10.1038/41534 }} 11. ^{{cite journal |last1=Koot |first1=Laurence |last2=Dumberry |first2=Mathieu |title=Viscosity of the Earth's inner core: Constraints from nutation observations |journal=Earth and Planetary Science Letters |date=August 2011 |volume=308 |issue=3–4 |pages=343–349 |doi=10.1016/j.epsl.2011.06.004 |bibcode=2011E&PSL.308..343K }} 12. ^{{cite journal |last1=Huguet |first1=Ludovic |last2=Alboussière |first2=Thierry |last3=Bergman |first3=Michael I. |last4=Deguen |first4=Renaud |last5=Labrosse |first5=Stéphane |last6=Lesœur |first6=Germain |title=Structure of a mushy layer under hypergravity with implications for Earth's inner core |journal=Geophysical Journal International |date=1 February 2016 |volume=204 |issue=3 |pages=1729–1755 |doi=10.1093/gji/ggv554 |bibcode=2016GeoJI.204.1729H }} 13. ^{{cite journal |bibcode=2013AGUFMDI34A..01B |title=The structure of melting mushy zones, with implications for Earth's inner core (Invited) |journal=Agu Fall Meeting Abstracts |volume=2013 |pages=DI34A–01 |last1=Bergman |first1=M. I. |last2=Huguet |first2=L. |last3=Alboussiere |first3=T. |year=2013 }} 14. ^{{cite book |doi=10.1016/S0074-6142(02)80263-7 |chapter=56 the Earth's core |title=International Handbook of Earthquake and Engineering Seismology |volume=81 |pages=925–933 |series=International Geophysics |year=2002 |last1=Song |first1=Xiaodong |isbn=978-0-12-440652-0 }} Further reading{{Refbegin}}
4 : Rotation|Structure of the Earth|Geodynamics|1996 in science |
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