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Our gold was a crash delivery from space, study finds

Dec. 10, 2010
Courtesy of the University of Maryland
and World Science staff

Gold prices seem oth­er­worldly these days—which may be fit­ting in light of a new study that says al­most all our Earthly stocks of the cov­eted met­al probably crashed here from space long ago.

The study sug­gests gold, plat­i­num, pal­la­di­um and re­lat­ed el­e­ments found in the crusts and man­tles of Earth, the Moon and Mars ar­rived as part of im­pact­ors the size of small plan­ets.

Freshly mined gold. (Cour­tesy Calif. Geo­log­i­cal Sur­vey)


The im­pacts would oc­curred dur­ing the last phase of plan­et forma­t­ion in our so­lar sys­tem, some 4.5 bil­lion years ago, and with­in tens of mil­lions of years of an even big­ger im­pact that pro­duced our Moon, say sci­en­tists. The find­ings are pub­lished in this week’s is­sue of the re­search jour­nal Sci­ence.

Cur­rent un­der­stand­ing of Earth and plan­ets of si­m­i­lar make­up sug­gest our world should have “essen­tially no gold” that we can get at, said Uni­vers­ity of Mar­y­land Ge­ol­o­gy Pro­fes­sor Rich­ard Walk­er, one of the au­thors. That’s be­cause gold and sev­er­al oth­er pre­cious met­als are among a class of el­e­ments that are usu­ally found in com­bina­t­ion with iron, which forms the co­re of Earth.

“Iron-loving el­e­ments are pulled in­to the plan­et co­res as they for­m,” Walk­er ex­plained, so the yel­low met­al should have been too deep for us to reach.

Since this is­n’t the case, Walk­er said, some­thing probably hap­pened to br­ing these el­e­ments to Earth late in its forma­t­ion, af­ter its lay­ers fin­ished sep­a­rat­ing. What sci­en­tists did­n’t know un­til now, he added, was wheth­er this bom­bard­ment oc­curred in big chunks over a rel­a­tively short time or as a “rain” of smaller pieces of ma­te­ri­al over a long­er time.

Walk­er and col­leagues used com­put­er mod­els to as­sess what size ob­jects would best match the needed cri­te­ria. These in­clud­ed pro­vid­ing the right amount of iron-loving met­als to the Earth, Moon and Mars; be­ing large enough to breach the crusts and man­tles of these bod­ies, cre­at­ing lo­cal mol­ten rock ponds from their im­pact en­er­gy and ef­fi­ciently mix­ing in­to the man­tle; and not be­ing so large as to cause the co­res to frag­ment and re-form. The lat­ter would pre­sum­ably cause the new gold to fol­low the old in­to the un­reach­a­ble depths.

The re­search­ers found that they could best meet the cri­te­ria if the im­pactors were few and mas­sive. The larg­est Earth im­pactor should have been about 1,500 to 2,000 miles wide (2,400 to 3,200 km), roughly Plu­to’s size; im­pactors hit­ting the Moon would have been around a tenth as wide.

“These im­pactors are thought to be large enough to pro­duce the ob­served en­rich­ments in highly siderophile [i­ron-loving] el­e­ments, but not so large that their frag­mented co­res joined with the plan­et’s co­re,” said Wil­liam Bot­tke of the South­west Re­search In­sti­tute in San An­to­nio, Tex­as, the lead au­thor of the pa­per.

The team al­so main­tains their pre­dicted pro­jec­tile sizes are con­sist­ent with phys­i­cal ev­i­dence such as the size dis­tri­bu­tions of to­day’s as­ter­oids and of an­cient Mar­tian im­pact scars.


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Gold prices seem otherworldly these days—which may be fitting in light of a new study that said almost all our supplies of the coveted metal probably crashed into Earth from space long ago. The study suggests gold, platinum, palladium and related elements found in the crusts and mantles of Earth, the Moon and Mars arrived as part of impactors the size of small planets during the last phase of planet formation in our solar system, some 4.5 billion years ago. The collisions occurred within tens of millions of years of an even bigger impact that produced our Moon, say the researchers, whose findings are published in this week’s issue of the research journal Science. Current understandings of Earth and planets of similar makeup suggest our world should have “essentially no gold” that we can get at, said University of Maryland Geology Professor Richard Walker, one of the authors. That’s because gold and several other precious metals are among a class of elements that are usually found in combination with iron, which forms the core of Earth. “Iron-loving elements are pulled into the planet cores as they form,” Walker explained, so the yellow metal should have been too deep for us to reach. Since this isn’t the case, Walker said, something probably happened to bring these elements to Earth late in its formation, after its layers finished separating. What scientists didn’t know until now, he added, was whether this bombardment occurred in big chunks over a relatively short time or as a “rain” of smaller pieces of material over a longer time. Walker and colleagues used computer models to assess what size objects would best match the needed criteria. These included providing the right amount of iron-loving metals to the Earth, Moon and Mars; being large enough to breach the crusts and mantles of these bodies, creating local molten rock ponds from their impact energy and efficiently mixing into the mantle; and not being so large as to cause the cores to fragment and re-form. That would cause the new gold to follow the old into the unreachable depths. The researchers found that they could best meet the criteria if the impactors were few and massive. The largest Earth impactor should have been about 1,500 to 2,000 miles wide (2,400 to 3,200 km), roughly Pluto’s size; impactors hitting the Moon would have been around a tenth as wide. “These impactors are thought to be large enough to produce the observed enrichments in highly siderophile [iron-loving] elements, but not so large that their fragmented cores joined with the planet’s core,” said William Bottke of the Southwest Research Institute in San Antonio, Texas, the lead author of the paper. The team also maintains their predicted projectile sizes are consistent with physical evidence such as the size distributions of today’s asteroids and of ancient Martian impact scars.