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Erth is n engine, tending to obliterte some of the evidence of eventstht re distnt in time, bt memory is retined in its chemistry,its isotopes, the presence of the Moon, perhps lso in geophysiclobservbles sch s the tempertre of the core nd the ntre of themntle immeditely bove the core, nd mybe even in the existenceof plte tectonics nd life. The remrkble growth in the stdy ndnderstnding of Erth hs hppened in prllel with spectclr er

of planetary exploration, relevant astronomical discoveries and com-pttionl nd theoreticl dvnces, ll of which help s to plce Erthnd its interior in perspective tht integrtes the Erth sciences withextrterrestril stdies nd bsic sciences sch s condensed-mtterphysics. However, progress on the biggest chllenges in nderstnd-ing the deep Erth contines to rely minly on looking down rtherthan looking up.

A planetary perspectiveErth is plnet one of mny. There is nothing prticlrly remrk-ble bot or home, except perhps tht it is sitble for life like s rgbly ttology. It hppens to be the lrgest of its type in theSolr System, bt s there re only three others of the terrestril type(Mercry, Vens nd Mrs) this is not prticlrly significnt. Among

planets in general, it is small.In the pst decde, we hve seen n stonishing explosion in or

ctloge of plnets otside the Solr System to bot 250 so fr(see the Extrsolr Plnets Encyclopedi, http://exoplnet.e/ctlog.php). These re mostly plnets tht we sspect re like Jpiter,very different from Erth. Bt s time goes on nd detection meth-ods improve, we cn expect to find bodies tht re Erth-like t lestto the extent of being mde predominntly of rock nd iron, theprimry constitents of or plnet. Some wold clim we might lredybe finding sch bodies1, initilly those tht re more mssive thnEarth.

If plnets were like toms or molecles, or even crystls, we coldspek of their chrcteristics (their DNA, so to spek) in very compctway, just as a handbook might list the properties of a material. Planets

re richer, more complex nd more resistnt to redctionist thinking.Genetics is the science of heredity and variation in biological systems.By nlogy, we cn spek of the genetics of plnet sch s Erth, whilelso cknowledging tht environment hs role in its evoltion ndits current state.

Cosmologists re fmilir with thinking bot time logrithmiclly: lot hppened in very short period of time bck ner the Big Bng.To some extent, it helps to think bot plnet formtion in similrwy (Fig. 1). The events tht defined Erths formtion nd the initilconditions for its sbseqent evoltion re sqeezed into n epoch thtmay have already been over within 100 million years of the formationof the Solr System. In this epoch more hppened inside Erth ndmore energy ws dissipted from within the plnet thn throghot llof sbseqent geologicl time. We hve no direct geologicl record of

this earliest epoch in the form of rocks and must rely instead on othersources of evidence.

Making planetsOr nderstnding of plnet formtion involves for mjor inpts:stronomicl observtions of plces where plnetry systems my cr-rently be forming, the stdy of meteorites tht formed even before theepoch in which the Solr Systems plnets formed, stdy of the plnetsthemselves (Erth mong them), nd theoreticl modelling. None ofthese is very complete or stisfying. The stronomicl observtions

tell s bot disks nd dst nd only indirectly bot possible plnets,the meteorites come from parent bodies that were probably always inorbits beyond Mrs nd re not necessrily representtive of Erthsbuilding blocks, Earth itself is good at concealing its history (throughfrequent surface rejuvenation), and theory is often either too permis-sive (mny djstble prmeters) or flls short of correct descriptionof process. Even so, pictre emerges tht hs ndergone considerbletesting and refinement in recent years.

Current models of planetary formation26 have had some success inexplining observtions nd hve the following fetres. Almost 4.6billion yers go, n interstellr clod of gs nd dst collpsed nderthe ction of grvity. Anglr momentm grnteed tht the collpsewold be into disk rond the forming str (the Sn) rther thnmerely into the Sn lone. This disk hd rdis of perhps 50 stro-

nomicl nits (au), where 1 au is the distnce between Erth nd theSn. Almost ll the mss of this disk ws otwrds of the eventl orbitof Earth. The particular mix of elements was nothing unusual, havingbeen set by ncleosynthesis for the hevy elements nd the otcome ofthe Big Bang for the lightest elements. Conversion of the gravitationalenergy of infll into het ssred tht tempertres wold be high inthe inner prt of the disk, sfficient to vporize mch of the infllingdst. Sbseqent cooling llowed the formtion of dst embedded inthe primrily hydrogen gs. Throgh gentle collisions, these prticlesaggregated into larger particles up to a centimetre or more in size.

Meteoritic evidence strongly indictes the formtion of lrgerplnetesimls tht were kilometres or more in size, on timescleof less thn million yers. This process is poorly nderstood:plnetesimls my hve risen throgh gentle collision nd sticking of

smller grins or they my hve risen throgh grvittionl instbili-ties in the disk. Sch processes re presmed to hve occrred throgh-ot the Solr System. The timing of formtion of these bodies is wellestblished t rond 4,567 million yers go, nd their collpse fromthe interstellar medium can only have occurred a million years or lessbefore this becse of evidence for the presence of short-lived rdioc-tive elements. This precisely determined dte therefore well defines theorigin of the Solar System.

Almost billion bodies 10 km in dimeter wold be needed to mken Erth. However, it is not thoght likely tht plnetesimls werethe ctl bilding blocks of Erth. A dense swrm of sch bodies innerly circlr low-inclintion orbits is grvittionlly nstble on short timescle. In less thn million yers, mch lrger bodies (pln-etry embryos) re formed tht re Moon- to Mrs-sized. These rise

becse of grvittionl focsing of impcts between bodies with lowreltive velocity. Otwrd from the steroid belt, these embryos my

A planetary perspective on the deep EarthDavid J. Stevenson

Earths composition, evolution and structure are in part a legacy of provenance (where it happened to form)

and chance (the stochastics of that formation).

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exceed Erth in mss, bt in the terrestril zone they re still well shortof Earths final mass. This means we must build Earth from a modestnmber (100 or fewer) of these embryos, bt dding in sprinklingof plnetesimls. The ggregtion of embryos to even bigger bodiestkes fr longer thn their formtion, extending from tens of mil-lions of yers to s mch s 100 million yers, becse it reqires theexcittion of eccentric orbits so tht the embryos hve n opportnityto collide35.

Erth nd its compnion terrestril plnets re tiny prt of theSolr System nd it shold come s no srprise tht the presence of thegint plnets, especilly Jpiter, the most mssive nd closest of these toErth, wold hve role in Erths formtion. Jpiter mst hve formed

while the hydrogen-dominated gas of the solar nebula was mostly stillpresent6, nd stronomicl observtions sggest tht the gs my hvebeen present in sfficient bndnce for bot 5 million yers t most.We cold perhps imgine tht the formtion of Erth postdted theformtion of Jpiter, nd some models re of this kind. Relisticlly, fll nderstnding of Erths formtion probbly reqires fll nder-stnding of Jpiters formtion. Jpiter is enriched in hevy elementsreltive to the Sn, nd some prt of tht enrichment is likely to bepresent s core. It is likely, lthogh not certin, tht this core wsformed first, with the gs then plced on top. Bt whichever story iscorrect, the formation of Jupiter involves much more than the physicsinvolved in bilding bodies sch s Erth becse we mst nderstndgs ccretion s well s the ccretion of solids. At present, this nder-stnding is incomplete. Models of Erth ccretion re in mny wys

mch more detiled thn models of gint-plnet formtion, bt theyare contingent on understanding Jupiter.

Planetary embryologyWe hve evidence bot some of the plnetesimls becse they re thepresmed sorce of most meteorites, bt the mch lrger embryos hvenot left direct evidence of their existence. Nonetheless, it is likely thttheir properties re importnt for nderstnding Erth. They formedso qickly tht they probbly prtly melted, owing to the presence ofthe short-lived rdioctive isotope 26Al. They my even hve been bigenogh to ndergo melting by the conversion of grvittionl energyof formtion into het. Prtil melting cn be expected to cse theseparation of a liquid iron alloy from the partly molten silicate mantle,and these embryos may even have had atmospheres. In short, they are

plnets with iron cores, short-lived bt possessing properties derivedfrom plnetry processes rther thn the properties of the precrsorplnetesimls. These differences from plnetesimls cn rise in nmber of wys: ingssing (the incorportion of solr nebl gs, sholdthe surface of the embryo be molten), the role of pressure (the mineralphses within the embryo nd its crst cn be different from those in low-pressre plnetesiml becse of self-grvity), nd the loss ofmteril by escpe (either becse of high tempertres or throghcollisions). Close enconters, tidl disrption nd the cretion of debrisdring collisions re processes tht re not crrently well incorportedinto models of planet formation.

The embryos responsible for forming Erth were not indeed coldnot hve been bilt from plnetesimls tht formed t 1 au, becsethe colescence of the embryos necessrily reqires their scttering

rond the inner prt of the Solr System

35

. It is therefore incorrect tothink of Erths provennce nd composition s being precisely defined,

nd different from, sy, those of Vens. On the other hnd, some dif-ferences re expected prely by chnce nd, importntly, it is thoghtnlikely tht ny of the Erth-forming embryos formed ot t loc-tions where wter ice cold condense. Indeed, Erth is reltively dry,t lest for the wter inventory tht we cn mesre (the ocens ndpper mntle), nd or wter my hve risen throgh wter-ber-ing plnetesimls coming from greter distnces rther thn throghwter incorported in the primry embryos. This remins somewhtcontroversil, nd one of the gols of Erth science is to get betterunderstanding of Earths complete water budget.

Giant impacts and lunar formation

The likely dominnce of the embryos s bilding-blocks for Erth impliesthe predominnce of gint impcts. We shold not think of Erthsformtion s the stedy ccmltion of mss bt rther s series ofinfreqent, highly trmtic events seprted by periods of cooling ndheling. The lrgest, nd possibly the lst, of these events is thoght tohve been responsible for the formtion of the Moon7,8 (Fig. 2). Recentisotopic evidence9 now dtes this event t s mch s 100 million yersfter the origin of the Solr System. Mny fetres of the event woldlso pply to erlier non-lnr-forming events, except tht those woldhve been less extreme. The impct origin of the Moon ws once con-troversil ide, bt it hs grdlly been ccepted for two resons: thelack of a realistic alternative, and growing evidence for its compatibilitywith the dt isotopic dt in prticlr. Prticlrly importntly, itis thought to set the stage for Earths subsequent evolution.

The lnr-forming collision plsibly involved the obliqe impctof Mrs-mss plnetry embryo (10% of Erths mss) with the ~90%complete Erth. The impct velocity wold probbly hve been domi-nted by the infll into the mtl grvity field, nd most of this energywold hve been converted into het. Unlike energy, nglr momen-tm is mch more nerly conserved throghot geologicl time, ndthis kind of impct explins well the crrent nglr momentm ofthe ErthMoon system. The men tempertre rise of Erth reslt-ing from this collision can be estimated as T 0.1GM/RCp 4,000 K,whereG is the grvittionl constnt, Mnd R re Erths mss ndrdis, respectively, nd Cp is the specific het of rock. Previos impctswold hve heted Erth p to hot, nerly isentropic stte ( stte inwhich entropy is nerly niform with depth) close to, or prtly in excessof, melting. Convective cooling below the freezing point is inefficient,

so the stte immeditely before impct is hot, except perhps right tthe surface.We expect tht the impct heting wold hve been neven becse

the vrios prts of Erth wold be shocked to differing extents, btthe immedite post-gint-impct stte wold relx to very hot con-figuration, in which all or most of the rock and iron is in molten formand some silicate (perhaps even tens of per cent) is in vapour form. Inmost simltions of this kind of impct, disk forms, derived mostlyfrom the impcting body. For the expected rditing srfce re ndrditing tempertre (~2,000 K), the cooling time to remove bot hlfof the impct energy is rond 1,000 yers, perhps somewht shorterfor the disk. This is very short period reltive to the time betweenmjor collisions, bt very importnt one. Dring this short period,the Moon forms, most of the core of the projectile merges with the core

of the proto-Erth, some of the pre-existing Erths tmosphere my beblown off, nd significnt prt of the deep, initilly molten, mntle

Collapse of an

interstellar cloud to

form the solar nebula

Planetesimals form

Moon-to-Mars-sized

embryos form rapidly

after this

Jupiter exists

Solar nebula eliminated

Substantial proto-Earth

(more than 50% of final

mass) might already exist

Last giant impact occurs,

plausibly lunar forming

Earths surface rapidly

cools after this impact

Life originates

Rock record (cratons)

develops

Present1 Myr 10 Myr 100 Myr 1,000 Myr

Figure 1 | A logarithmic view of the time of planetary formation. The leftend corresponds to the initiation of a collapse to form the solar nebula, and

is close to 4,567 million years ago. Much happened in the 1 to 100 million

years (Myr) immediately after this, and the logarithmic scale correctlyemphasizes the importance of this 100-Myr period, despite the shortness of

this period compared with Earths age.

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of the Erth will freeze withot hving the opportnity to differentite(because the crystals are advected vigorously by the turbulent convec-tive motions that accompany the cooling).

The Moon probbly did not form immeditely fter the gint impct,even thogh orbitl times for mteril plced bot Erth re less thn dy. Insted, it seems to be necessry to wit for hndreds to tho-sands of years, the timescale of disk cooling, as it is thought likely thatthe Moon did form completely molten. For resons not flly nder-stood, the need to cool the disk is of greter importnce thn the shorter

timescles of dynmicl evoltion. Perhps lnr formtion shold notbe thoght of s disconnected from the provennce nd evoltion of thedeep Erth. The reson is tht, fter the gint impct, some exchngeof mteril my hve tken plce between Erth nd the disk, ided bythe vigoros convection of both the liqid nd vpor prts of ech ndthe presence of common silicte tmosphere. This pictre of rpidexchnge mkes the disk more Erth-like, rther thn like the projec-tile tht ws responsible for its formtion. The pictre ws originllymotivted by desire to nderstnd the remrkble similrity of Erthnd Moon oxygen isotopes8 bt lso finds spport in tngsten9 ndpossibly silicon10 isotopic evidence. However, we do not yet hve fllyintegrted model of lnr formtion tht is dynmiclly stisfctory swell as chemically acceptable.

Core formationThe core-formtion events (one event per gint impct) re prticlrlyimportnt becse core formtion is the biggest differentition processof Erth: it involves one-third of Erths mss nd lrge energy relese,becse the iron is bot twice s dense s the silictes. To sbstntilextent, it lso defines the composition of Erths mntle. In the imme-dite ftermth of gint impct, we expect sbstntil prt of thecore of the projectile to be emlsified with the molten mntle of thepre-impct proto-Erth. The core nd mntle mterils re thoght tobe immiscible (like water and oil) despite the very high temperatures,perhaps as high as 10,000 K for some of the material. If the material ismixed down to smll scle (perhps even to the point where there recentimetre-sized droplets of iron immersed in the liqid silicte) thenthe iron nd silicte cn chemiclly nd thermlly eqilibrte t high

tempertre nd pressre (Fig. 3). The composition of the core ndthe iron content of the mntle were presmbly set dring these eqili-brtion episodes. The silicon nd hydrogen contents of the mntle mylso be ffected by this eqilibrtion, s both re solble in iron t highpressre nd tempertre. These elements re prticlrly significnt:silicon content ffects the minerlogy of Erths mntle, nd the fteof hydrogen my hve mch to sy bot the totl wter inventory ofErth t this erly epoch nd the flow of mntle rocks. However, mchof Earths water may have been delivered later.

It is likely tht some of the projectile iron is not mixed down to thesmllest scles bt insted finds its wy to the core jst hors fterthe impct (Fig. 3b). This iron will not eqilibrte, either thermllyor chemiclly, nd it ths crries memory of previos core-formingevents t erlier times in smller bodies (the embryos discssed erlier).

The emerging picture is a complex one in which we should not expectthe core or mntle of Erth to hve simple chemicl reltionship thtinvolves the lst eqilibrtion t prticlr pressre nd tempertre,bt rther to hve been formed nder rnge of thermodynmic condi-tions involving a number of significant events at different times2,11.

Erths tmosphere t the time of gint impct might hve beenmostly stem nd crbon dioxide (CO2) probbly both were impor-tnt. It is possible, bt not certin, tht lrge prt of the tmosphere wsblown wy immeditely fter the gint impct. Wter vpor is, however,mch more solble thn CO2 in mgm, so tht even if the tmospherewere ejected into spce, otgssing from the nderlying mgm ocenwold replenish mch of it. An importnt fetre of wter vpor is thtit hs strong greenhose effect, nd tht my hve llowed the reten-tion of n nderlying mgm ocen, even for the long periods between

gint impcts. However, this type of tmosphere cn rin ot if there isinsfficient energy spplied to its bse (snlight lone is insfficient) nd,

as a consequence, any steam atmosphere may collapse on a geologicallyshort timescle, leding to n Erth srfce tht is ctlly cool (ble tohave liquid water) even while the interior is very hot.

Mantle differentiationThe mntle of the post-gint impct Erth will cool very fst t first12,limited only by the black-body radiation that can escape from the top ofthe trnsient (initilly silicte vpor) tmosphere. The therml strctreof the mntle is expected to be close to isentropic becse tht is the stte

of netrl boyncy nd therefore the stte preferred by convection, pro-vided tht viscosity is low. The ntre of the freezing within this convect-ing stte is of gret importnce nd is thermodynmiclly determined.Mny mterils hve the property tht if they re sqeezed isentropiclly,they ndergo freezing even s they get hotter. Eqivlently, they melt ifthey re decompressed isentropiclly from frozen bt hot, high-pressrestte. The former correctly describes the freezing of Erths solid innercore (the hottest plce in Erth, yet frozen) wheres the ltter correctlydescribes the melting responsible for the genertion of bsltic mgm,the dominnt volcnism on Erth nd most volminosly expressed tthe low mntle pressres immeditely beneth mid-ocen ridges. Recentwork13,14 sggests tht this pictre my not pply for the deeper prt ofEarths mantle, so that freezing may begin at mid-depths.

Even so, there will eventually come a point (perhaps as soon as a few

thosnd yers) fter gint impct when the bottom prt of the mntle

a

b

c

Lunar-forming giant impact

Core

Core

Magma disk

Silicate vapour

atmosphere

Radiative cooling

Blobs of iron settlingto core

Partly

solidified mantle

Rest of disk falls

back on Earth

Newly formed

Moon, mostly or

partly molten

Figure 2 | The effect on Earth of the giant impact that formed the Moon.

a, A giant planetary embryo collides with the nearly complete Earth. b, Amagma disk is in orbit about Earth, while blobs of iron from the planetaryembryo settle down through the mantle to join the existing core. c, Theoutermost part of the magma disk coalesces to form the Moon as the resultof radioactive cooling, while the rest falls back to Earth. Inside Earth, the

mantle nearest the core has partly solidified, and the mantle might acquirea layered structure.

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is mostly frozen. A very important question then arises: does the inter-stitil melt of this two-phse medim move p or move down nderthe ction of grvity? It is very nlikely to be immobile. It is likely tht itgoes down (most probably because it is richer in iron than the coexist-ing solid), but in either case the mantle will differentiate internally into lyered strctre (Fig. 4). This does not necessrily men tht Erthdeveloped primordil lyering tht hs been preserved throghotgeologicl time nd is perhps present still s prt of the complex strc-tre observed t the bse of the mntle by seismologists nd given bythem the nromntic nme of D (see pge 269). An erly differentitionevent for the silicte portion of Erth is fvored by some geochemists15,lthogh, interestingly, it my hve been erlier nd it my hve involved

the formation of a primordial crust. It could perhaps be the cumulativeconseqence of gint impct events, rre exmple of n Erth memorythat even pre-dates the last giant impact.

The verge Erth srfce environment dring ccretion mynot hve been very hot, even thogh there were ndobtedly shortperiods of time dring which it ws so hot tht rocks were vporized.These trmtic events reset the clock for sbseqent evoltion ndemphasize the importance of the last such global event. Soon after thelst globl trmtic event, it my even be possible to hve hd rocksthat survived throughout subsequent Earth history. Certainly, zircons tiny, very resistnt prts of rocks hve been dted bck to ~4.4 bil-lion yers16, nd it is not nresonble to expect zircon discoveries thtdte bck to within few hndred million yers of the lnr-formingimpct. Zircons re not the sme s hnd specimens nd rocks tht cn

be stdied in context (n intct strctre, sch s srviving crton),but the gap is closing between the geological record as usually definednd the events tht cn only be dted throgh gross isotopic signls forEarth as a whole.

Core memoryThe composition of Erths core is different from pre ironnickel ndthis is presmbly becse of the modest solbility of other elements,especilly oxygen, silicon nd slphr. The simplest view of Erthscore is that it is a hot fluid cooled from above. Significantly, Earths corehs sperhet: it is hotter thn the tempertre it wold hve been ifliqid iron lloy coexisting with pper to mid-mntle silictes hd snkisentropiclly to the core. We cn estimte this sperhet by knowingthe tempertre t which iron lloys freeze t the known pressre of

Earth near its centre and by the seismological determination of the sizeof its inner core. This sperhet is crrently bot 1,000 K or so, nd

may initially have been in excess of 2,000 K. Unlike the mantle, the corecnnot lose energy directly to the srfce or to spce nd it is thereforelikely tht prt of this sperhet is memory of the primordil Erth ndmy be telling s something bot the specific processes responsible forcore formtion. Loss of primordil het, together with the ltent hetrelesed s the inner core freezes, is potentilly sfficient to mintinconvection in the oter core over geologicl time, lthogh even this is insome dobt given crrently fvored vles of the therml condctivityof the core. In ddition, boyncy cn be provided by the exclsion ofpart of the light elements from the inner core or perhaps from materialexsolving from the outer core and attaching itself to the mantle.

Erths mgnetic field is generted by dynmo: vigoros convection in

the liqid, electriclly condcting, oter core mplifies the existing mg-netic field and thereby balances the tendency of the electrical current andssocited fields to ndergo decy. It is possible tht these energy sorceswere insfficient to generte Erths mgnetic field even for the periodwhen we know it mst hve existed17. A modest mont of rdiogenic het,most plsibly from the decy of40K, is sggested soltion to the short-fall. The experimental support for this is equivocal, but given the possiblyhigh tempertres for prt of the core-forming mterils, it my be moredifficult to keep things out of the core (that is, to avoid the core becomingtoo low in density) than to get them in! The amount of potassium neededwold be modest nd so it might not be pprent s mrked depletion ofpotssim in Erths mntle reltive to elements of similr voltility. Thebility of Erth to generte mgnetic field my lso be linked to the pres-ence of n efficient mechnism for eliminting het throgh the plnets

surface. Plate tectonics is a particularly efficient mechanism.

Plate tectonics and lifeWe nderstnd why Erths mntle convects: there is no lterntive mech-nism for eliminting het. However, we do not nderstnd why Erthhas plate tectonics. It is sometimes described as merely a property of theprticlr form tht mntle convection tkes on or plnet, bt this begsthe qestion. Plte tectonics is neither mndtory nor common (there isno cler evidence of its existence on ny other plnet so fr). Nonetheless,mny think its presence is deterministic: given the specific prmetersof present-dy Erth, it is the behvior expected, in the sme sense thta physicist setting up a convection experiment on a layer of fluid heatedfrom below need not be concerned bot whether his chosen flid wsonce a vapour or a solid. Even in this point of view, the presence of plate

tectonics is history-dependent. For exmple, the mont nd distribtionof wter my be importnt, s it is well estblished tht wter in rocks

Core

Iron

dropletsMetalpond

SilicateMetal

diapir

Core

Completely

molten

mantle

Unequilibrated

iron blobs

a b

Silicate

liquidus

Figure 3 | Two contrasting

views of what might have

happened during core

formation. a, There is amagma ocean boundedbelow by a mostlysolid lower region: thedispersed iron aggregatesbefore descending tothe core. b, Some of the

iron from the core of theprojectile responsiblefor a giant impact isimperfectly mixed anddescends to the coreon a short timescale asdistorted blobs hundredsof kilometres in diameter,without equilibrationwith the mantle.

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hs mjor effect on their melting properties nd response to stress.Erths wter bdget is likely to be dependent on its history. The srfce

environment is profondly inflenced by the presence or bsence of plte-tectonic cycle, nd tht environment is, in trn, inflencing theexistence of life nd is then ffected by the presence of life. Everythingffects everything else: the development of life on Erth is not likely to bedisconnected from the composition of Earths core.

Where do we go from here?The remrkble dvnces over recent yers nd decdes hve beennotble for their strongly interdisciplinry chrcter, nd some of thisdvnce hs come bot throgh thinking of Erth s plnet nd relt-ing it to the environment in which it formed. Even so, the biggest chl-lenge seems to reqire looking inside the plnet: we need to nderstndbetter the phse reltionships between Erths constitents, the wy inwhich mntle convection works nd how to integrte this with plte

tectonics, the connection between the deep Erth nd or ocen ndtmosphere, nd the genertion of Erths mgnetic field. The origin

and development of life are also clearly questions for Earth science andwill resist compelling nswers ntil we hve better chrcterized thethermodynmic, chemicl nd flid dynmicl environments. The deepErth is deeply significnt nd lso deeply informtive for Erths srfceand all of Earth science. David J. Stevenson is in the Division of Geological and Planetary Science,

California Institute of Technology, Pasadena, California 91125, USA.

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) in a 3-planet system. Astron. Astrophys.469,L43L47 (2007).

2. Halliday, A. N. & Wood, B. J. in Treatise on Geophysics Vol. 9 (ed. Schubert, G.) 1350(Elsevier, Amsterdam, 2007).

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Author Information Reprints and permissions information is available at

npg.nature.com/reprints. Correspondence should be addressed to the author(djs@gps.caltech.edu).

Dense suspension, vigorously

convecting

Might be well mixed

Much higher viscosity, melt

percolative regime

Meltsolid differentiation?

High-density material

might accumulate at the base

Iron-rich melt might descend

Core

Figure 4 | Mantle cooling and differentiation during the later stages of a

magma ocean. As the magma of the mantle cools, a stage is eventuallyreached at which dense iron-rich interstitial liquid (red) percolates throughthe solid matrix (blue) to accumulate just above the core.

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