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May 20, 2009

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Early cells might have thrived amid asteroid pummeling

May 20, 2009
Courtesy UC Colorado at Boulder
and World Science staff

Earth’s bom­bard­ment nearly four bil­lion years ago by as­ter­oids al­most as large as the U.K. could­n’t have wiped out po­ten­tial early life, and may even have aided it, ac­cord­ing to a new stu­dy.

Sci­en­tists say im­pact ev­i­dence from lu­nar sam­ples, me­te­orites and the pock­marked sur­faces of the in­ner plan­ets paints a pic­ture of a vi­o­lent en­vi­ron­ment in the so­lar sys­tem dur­ing the so-called Hadean Eon 4.5 to 3.8 bil­lion years ago. A cat­a­clys­mic event known as the Late Heavy Bom­bard­ment about 3.9 mil­lion years ago is thought to have been par­tic­u­larly jar­ring.

The bom­bard­ment of Earth by as­ter­oids 3.9 bil­lion years ago may have en­hanced early life, ac­cord­ing to a study. (Cre­dit: NASA/JPL)

Al­though many be­lieve the bom­bard­ment would have ster­i­lized Earth, the study in­di­cates it would have melted only a frac­tion of Earth’s crust, and that mi­crobes could well have sur­vived in sub­sur­face habi­tats, in­su­lat­ed from the de­struc­tion.

“These new re­sults push back the pos­si­ble be­gin­nings of life on Earth to well be­fore the bom­bard­ment per­i­od 3.9 bil­lion years ago,” Oleg Ab­ra­m­ov, a re­search as­so­ci­ate at the Uni­ver­s­ity of Col­o­rad­o at Boul­der. “It opens up the pos­si­bil­ity that life emerged as far back as 4.4 bil­lion years ago, about the time the first oceans are thought to have formed.” 

A pa­per on the sub­ject by Ab­ra­m­ov and ge­olo­g­ist Ste­phen Mo­jz­sis at the uni­ver­s­ity ap­pears in the May 21 is­sue of the re­search jour­nal Na­ture. 

Be­cause phys­i­cal ev­i­dence of Earth’s early bom­bard­ment has been erased by weath­er­ing and plate tec­ton­ics over the eons, the re­search­ers used da­ta from Apol­lo moon rocks, im­pact rec­ords from the moon, Mars and Mer­cu­ry, and pre­vi­ous the­o­ret­i­cal stud­ies to build com­put­er sim­ula­t­ions of the bom­bard­ment. 

Abramov and Mo­jz­sis plugged in as­ter­oid size, fre­quen­cy and dis­tri­bu­tion es­ti­mates in­to their sim­ula­t­ions to chart the dam­age to the Earth dur­ing the Late Heavy Bom­bard­ment, which is thought to have lasted for 20 mil­lion to 200 mil­lion years.

The re­search­ers even cranked up the in­tens­ity of the as­ter­oid bar­rage in their sim­ula­t­ions by 10-fold—enough to va­por­ize the oceans. “E­ven un­der the most ex­treme con­di­tions we im­posed, Earth would not have been com­pletely ster­i­lized,” said Ab­ra­m­ov.

In­stead, deep-sea hot springs known as hy­dro­ther­mal vents may have pro­vid­ed sanc­tu­ar­ies for heat-loving mi­crobes known as “hy­per­ther­mo­phi­lic bac­te­ria” fol­low­ing bom­bard­ments, said Mo­jz­sis. Even if life had not emerged by 3.9 bil­lion years ago, such un­der­ground havens could still have pro­vid­ed a “cru­cible” for life’s or­i­gin on Earth, Mo­jz­sis added. 

The re­search­ers con­clud­ed the un­der­ground mi­crobes liv­ing at tem­per­a­tures rang­ing from 175 to 230 de­grees Fahr­en­heit (79 to 110 de­grees Cel­si­us) would have flour­ished dur­ing the Late Heavy Bom­bard­ment. The mod­els in­di­cate that un­der­ground habi­tats for such mi­crobes in­creased in vol­ume and dura­t­ion as a re­sult of the mas­sive im­pacts. Some ex­treme mi­cro­bi­al spe­cies on Earth to­day—in­clud­ing so-called “un­boil­able bugs” disco­vered in hy­dro­ther­mal vents in Yel­low­stone Na­tional Park—thrive at tem­per­a­tures even some­what hot­ter.

Ge­o­log­ic ev­i­dence sug­gests that life on Earth was pre­s­ent at least 3.83 bil­lion years ago, said Mo­jz­sis. “So it is not un­rea­son­a­ble to sug­gest there was life on Earth be­fore 3.9 bil­lion years ago. We know from the ge­o­chem­i­cal rec­ord that our plan­et was em­i­nently hab­it­a­ble by that time, and this new study sews up a ma­jor prob­lem in or­i­gins of life stud­ies by sweep­ing away the necess­ity for mul­ti­ple or­i­gins of life on Earth.” 

Most sci­en­tists be­lieve a rogue plan­et as large as Mars smacked Earth with a glanc­ing b­low 4.5 bil­lion years ago, va­por­iz­ing it­self and part of Earth. The col­li­sion would have cre­at­ed an im­mense va­por cloud from which moon­lets, and lat­er our moon, co­a­lesced, Mo­jz­sis said. “That event, which pre­ced­ed the Late Heavy Bom­bard­ment by at least 500 mil­lion years, would have ef­fec­tively hit Earth’s re-set but­ton,” he said. 

“But our re­sults strongly sug­gest that no events since the moon forma­t­ion were ca­pa­ble of de­stroy­ing Earth’s crust and wip­ing out any bi­o­sphere that was pre­s­ent,” Mo­jz­sis said. “In­stead of chop­ping down the tree of life, our view is that the bom­bard­ment pruned it.” 

The re­sults al­so sup­port the po­ten­tial for mi­cro­bi­al life on oth­er plan­ets like Mars and per­haps even rocky, Earth-like plan­ets in oth­er so­lar sys­tems that may have been re­sur­faced by im­pacts, said Ab­ra­m­ov.

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Earth’s bombardment nearly four billion years ago by asteroids almost as large as the U.K. couldn’t have wiped out potential early life, and may even have aided it, according to a new study. Scientists say impact evidence from lunar samples, meteorites and the pockmarked surfaces of the inner planets paints a picture of a violent environment in the solar system during the so-called Hadean Eon 4.5 to 3.8 billion years ago. A cataclysmic event known as the Late Heavy Bombardment about 3.9 million years ago is thought to have been particularly jarring. Although many believe the bombardment would have sterilized Earth, the study indicates it would have melted only a fraction of Earth’s crust, and that microbes could well have survived in subsurface habitats, insulated from the destruction. “These new results push back the possible beginnings of life on Earth to well before the bombardment period 3.9 billion years ago,” Oleg Abramov, a research associate at the University of Colorado at Boulder. “It opens up the possibility that life emerged as far back as 4.4 billion years ago, about the time the first oceans are thought to have formed.” A paper on the subject by Abramov and geologist Stephen Mojzsis at the university appears in the May 21 issue of the research journal Nature. Because physical evidence of Earth’s early bombardment has been erased by weathering and plate tectonics over the eons, the researchers used data from Apollo moon rocks, impact records from the moon, Mars and Mercury, and previous theoretical studies to build computer simulations of the bombardment. Abramov and Mojzsis plugged in asteroid size, frequency and distribution estimates into their simulations to chart the damage to the Earth during the Late Heavy Bombardment, which is thought to have lasted for 20 million to 200 million years. The researchers even cranked up the intensity of the asteroid barrage in their simulations by 10-fold—enough to vaporize the oceans. “Even under the most extreme conditions we imposed, Earth would not have been completely sterilized,” said Abramov. Instead, deep-sea hot springs known as hydrothermal vents may have provided sanctuaries for heat-loving microbes known as “hyperthermophilic bacteria” following bombardments, said Mojzsis. Even if life had not emerged by 3.9 billion years ago, such underground havens could still have provided a “crucible” for life’s origin on Earth, Mojzsis added. The researchers concluded the underground microbes living at temperatures ranging from 175 to 230 degrees Fahrenheit (79 to 110 degrees Celsius) would have flourished during the Late Heavy Bombardment. The models indicate that underground habitats for such microbes increased in volume and duration as a result of the massive impacts. Some extreme microbial species on Earth today—including so-called “unboilable bugs” discovered in hydrothermal vents in Yellowstone National Park—thrive at temperatures even somewhat hotter. Geologic evidence suggests that life on Earth was present at least 3.83 billion years ago, said Mojzsis. “So it is not unreasonable to suggest there was life on Earth before 3.9 billion years ago. We know from the geochemical record that our planet was eminently habitable by that time, and this new study sews up a major problem in origins of life studies by sweeping away the necessity for multiple origins of life on Earth.” Most scientists believe a rogue planet as large as Mars smacked Earth with a glancing blow 4.5 billion years ago, vaporizing itself and part of Earth. The collision would have created an immense vapor cloud from which moonlets, and later our moon, coalesced, Mojzsis said. “That event, which preceded the Late Heavy Bombardment by at least 500 million years, would have effectively hit Earth’s re-set button,” he said. “But our results strongly suggest that no events since the moon formation were capable of destroying Earth’s crust and wiping out any biosphere that was present,” Mojzsis said. “Instead of chopping down the tree of life, our view is that the bombardment pruned it.” The results also support the potential for microbial life on other planets like Mars and perhaps even rocky, Earth-like planets in other solar systems that may have been resurfaced by impacts, said Abramov.