"Long before it's in the papers"
January 27, 2015


Giant asteroids battered early Earth, NASA scientists say

Aug. 1, 2014
Courtesy of NASA
and World Science staff

New re­search in­di­cates that more than four bil­lion years ago gi­ant as­ter­oid im­pacts heavily “re­pro­cessed” – or melted, mixed, and bur­ied – Earth’s sur­face. 

The study fo­cus­es on changes in the up­per­most lay­ers of the early Earth dur­ing the ge­o­log­ic eon called the “Hadean,” about 4 to 4.5 bil­lion years ago.

Ar­tis­tic con­cep­tion of the ear­ly Earth, show­ing a sur­face pum­meled by large im­pacts, re­sult­ing in ex­tru­sion of deep seated mag­ma on­to the sur­face. At the same time, dis­tal por­tions of the sur­face could have re­tained liq­uid wa­ter. (Cred­it: Si­mone Marchi/SwRI )

Re­search­ers from in­sti­tu­tions in­clud­ing NASA’s So­lar Sys­tem Ex­plora­t­ion Re­search Vir­tu­al In­sti­tute at the agen­cy’s Ames Re­search Cen­ter in Mof­fett Field, Ca­lif. pub­lished their find­ings in a pa­per in the July 31 is­sue of the jour­nal Na­ture.

“A large as­ter­oid im­pact could have bur­ied a sub­stant­ial amount of Earth’s crust with im­pact-generated melt,” said Y­vonne Pen­dle­ton, di­rec­tor of the in­sti­tute. “This new mod­el helps ex­plain how re­peat­ed as­ter­oid im­pacts may have bur­ied Earth’s ear­li­est and old­est rocks.”

Plan­et forma­t­ion mod­els in­di­cate Earth went through a se­quence of ma­jor growth phases: in­i­tially ac­cre­tion of plan­etes­i­mals – plan­e­tary em­bryos – over many tens of mil­lions of years, then a gi­ant im­pact that led to the forma­t­ion of our moon. Fol­low­ing this the mod­els point to a “late bom­bard­ment” when gi­ant as­ter­oids sev­er­al tens to hun­dreds of miles in size per­i­od­ic­ally hit an­cient Earth, dwarf­ing the one that much lat­er killed the di­no­saurs, an ob­ject es­ti­mat­ed to be six miles wide.

Re­search­ers es­ti­mate this bom­bard­ment con­tri­but­ed less than one per­cent of Earth’s pre­s­ent-day ma­te­ri­al, but still had a pro­found ef­fect. Pri­or to four bil­lion years ago Earth would have been “resur­faced” re­peat­edly by im­pact-caused melt­ing. Fur­ther­more, large col­li­sions may have re­peat­edly boiled away ex­ist­ing oceans in­to steamy at­mo­spheres. De­spite this, the find­ings are said to be com­pat­ible with the claim of liq­uid wa­ter on Earth’s sur­face as early as about 4.3 bil­lion years ago based on ge­o­chem­i­cal da­ta.

The new re­search re­veals that as­ter­oidal col­li­sions not only se­verely al­tered the ge­ol­o­gy of the Hadean eon Earth, but likely al­so played a ma­jor role in the sub­se­quent ev­o­lu­tion of life.

“Pri­or to ap­prox­i­mately four bil­lion years ago, no large re­gion of Earth’s sur­face could have sur­vived un­touched by im­pacts and their ef­fects,” said Si­mone Marchi, sen­ior re­searcher at the South­west Re­search In­sti­tute in Boul­der, Col­o­rad­o, and the pa­per’s lead au­thor. “The new pic­ture of the Hadean Earth emerg­ing from this work has im­por­tant im­plica­t­ions for its hab­it­abil­ity.”

The re­search­ers found that on av­er­age, Hadean Earth could have been hit by one to four im­pactors that were more than 600 miles wide and ca­pa­ble of glob­al steril­iz­a­tion, and by three to sev­en im­pact­ors more than 300 miles wide and ca­pa­ble of glob­al ocean va­por­iz­a­tion.

“The lag be­tween ma­jor col­li­sions was long enough to al­low in­ter­vals of more clem­ent con­di­tions, at least on a lo­cal scale,” said Marchi. “Any life emerg­ing dur­ing the Hadean eon likely needed to be re­sist­ant to high tem­per­a­tures, and could have sur­vived such a vi­o­lent pe­ri­od... by thriv­ing in niches deep un­der­ground or in the ocean’s crust.”

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New research indicates that more than four billion years ago giant asteroid impacts heavily “reprocessed” – or melted, mixed, and buried – Earth’s surface. The study focuses on changes in the uppermost layers of the early Earth during the geologic eon called the “Hadean,” about 4 to 4.5 billion years ago. Researchers from institutions including NASA’s Solar System Exploration Research Virtual Institute at the agency’s Ames Research Center in Moffett Field, Calif. published their findings in a paper in the July 31 issue of the journal Nature. “A large asteroid impact could have buried a substantial amount of Earth’s crust with impact-generated melt,” said Yvonne Pendleton, director of the institute. “This new model helps explain how repeated asteroid impacts may have buried Earth’s earliest and oldest rocks.” Planet formation models indicate Earth went through a sequence of major growth phases: initially accretion of planetesimals – planetary embryos – over many tens of millions of years, then a giant impact that led to the formation of our moon. Following this the models point to a “late bombardment” when giant asteroids several tens to hundreds of miles in size periodically hit ancient Earth, dwarfing the one that much later killed the dinosaurs, an object estimated to be six miles wide. Researchers estimate this bombardment contributed less than one percent of Earth’s present-day material, but still had a profound effect. Prior to four billion years ago Earth would have been “resurfaced” repeatedly by impact-generated melting. Furthermore, large collisions as late as about four billion years ago may have repeatedly boiled away existing oceans into steamy atmospheres. Despite the heavy bombardment, the findings are said to be compatible with the claim of liquid water on Earth’s surface as early as about 4.3 billion years ago based on geochemical data. The new research reveals that asteroidal collisions not only severely altered the geology of the Hadean eon Earth, but likely also played a major role in the subsequent evolution of life. “Prior to approximately four billion years ago, no large region of Earth’s surface could have survived untouched by impacts and their effects,” said Simone Marchi, senior researcher at the Southwest Research Institute in Boulder, Colorado, and the paper’s lead author. “The new picture of the Hadean Earth emerging from this work has important implications for its habitability.” The researchers found that on average, Hadean Earth could have been hit by one to four impactors that were more than 600 miles wide and capable of global sterilization, and by three to seven impactors more than 300 miles wide and capable of global ocean vaporization. “The lag between major collisions was long enough to allow intervals of more clement conditions, at least on a local scale,” said Marchi. “Any life emerging during the Hadean eon likely needed to be resistant to high temperatures, and could have survived such a violent period in Earth’s history by thriving in niches deep underground or in the ocean’s crust.”