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Range of asteroid types could have seeded life, study finds

Jan. 19, 2011
Courtesy of NASA
and World Science staff

A wid­er va­ri­e­ty of as­ter­oids than pre­vi­ously thought were ca­pa­ble of seed­ing Earth with the kind of ami­no ac­ids that are key in­gre­di­ents of life, ac­cord­ing to new NASA re­search.

Ami­no ac­ids are com­po­nents of pro­teins, mo­le­cules used by life to make struc­tures like hair and nails, and to speed up or reg­u­late chem­i­cal re­ac­tions. Ami­no ac­ids come in two va­ri­eties that are mir­ror im­ages of each oth­er, like your hands. Life on Earth uses the left-hand­ed kind ex­clu­sive­ly. Since life based on right-hand­ed ami­no ac­ids would pre­sumably work fi­ne, sci­en­tists have been try­ing to find out why Earth-based life fa­vored left-hand­ed ami­no ac­ids.

In March 2009, re­search­ers at NASA’s God­dard Space Flight Cen­ter in Green­belt, Md., re­ported the dis­cov­ery of an ex­cess of the left-hand­ed form of the ami­no ac­id iso­va­line in sam­ples of me­te­orites that came from carbon-rich as­ter­oids. This sug­gests, they said, that per­haps left hand­ed life got its start in space, where con­di­tions on these roam­ing rocks fa­vored the crea­t­ion of left-hand­ed ami­no ac­ids. As­ter­oid im­pacts could have sup­plied this ma­te­ri­al, en­riched in left-hand­ed molecules, to Earth. The bi­as to­ward left-hand­edness would have been per­pet­u­at­e as this ma­te­ri­al was in­cor­po­rat­ed in­to emerg­ing life.

In the new re­search, the team re­ports find­ing ex­cess left-hand­ed iso­va­line, or L-iso­va­line, in a much wid­er va­ri­e­ty of carbon-rich me­te­orites. “This tells us our in­i­tial dis­cov­ery was­n’t a fluke; that there really was some­thing go­ing on in the as­ter­oids where these me­te­orites came from that fa­vors the crea­t­ion of left-hand­ed ami­no ac­ids,” said NASA God­dard’s Dan­iel Glavin, lead au­thor of a pa­per on the re­search pub­lished on­line in Me­te­or­it­ic and Plan­e­tary Sci­ence Jan. 17.

The re­search in­di­cates that “some pro­cess in­volv­ing liq­uid wa­ter fa­vors the crea­t­ion of left-hand­ed ami­no ac­ids,” Glavin said. “We can tell how much these as­ter­oids were al­tered by liq­uid wa­ter by an­a­lyz­ing the min­er­als their me­te­orites con­tain. The more these as­ter­oids were al­tered, the great­er the ex­cess L-iso­va­line we found.” 

“In the me­te­orites with the larg­est left-hand­ed ex­cess, we find about 1,000 times less iso­va­line than in me­te­orites with a small or non-detectable left-hand­ed ex­cess. This tells us that to get the ex­cess, you need to use up or de­stroy the ami­no ac­id, so the pro­cess is a dou­ble edged sword,” said Glavin.

What­ev­er it may be, the wa­ter-altera­t­ion pro­cess only am­pli­fies a small ex­ist­ing left-hand­ed ex­cess, it does not cre­ate the bi­as, ac­cord­ing to Glavin. Some­thing in the pre-so­lar neb­u­la—a vast cloud of gas and dust from which our so­lar sys­tem, and probably many oth­ers, were born—cre­ated a small in­i­tial bi­as to­ward L-iso­va­line and pre­sumably oth­er left-hand­ed ami­no ac­ids.

One pos­si­bil­ity, he went on, is radia­t­ion. Space is filled with ob­jects such as mas­sive stars, neu­tron stars, and black holes that pro­duce many kinds of radia­t­ion. It’s pos­si­ble that the radia­t­ion en­coun­tered by our so­lar sys­tem in its youth made left hand­ed ami­no ac­ids slightly more likely to be cre­ated, or right-hand­ed ami­no ac­ids a bit more likely to be de­stroyed, ac­cord­ing to Glavin.

It could al­so be that oth­er young so­lar sys­tems en­coun­tered dif­fer­ent radia­t­ion that fa­vored right-hand­ed ami­no ac­ids, he not­ed: if life emerged in one of these sys­tems, the bi­as to­ward right-hand­ed ami­no ac­ids might be built in just as it may have been for left-hand­ed ami­no ac­ids he­re.

* * *

A wider variety of asteroids than previously thought were capable of seeding Earth with the kind of amino acids that are key ingredients of life, according to new NASA research. Amino acids are used to build proteins, molecules used by life to make structures like hair and nails, and to speed up or regulate chemical reactions. Amino acids come in two varieties that are mirror images of each other, like your hands. Life on Earth uses the left-handed kind exclusively. Since life based on right-handed amino acids would presumably work fine, scientists have been trying to find out why Earth-based life favored left-handed amino acids. In March 2009, researchers at NASA’s Goddard Space Flight Center in Greenbelt, Md., reported the discovery of an excess of the left-handed form of the amino acid isovaline in samples of meteorites that came from carbon-rich asteroids. This suggests, they said, that perhaps left handed life got its start in space, where conditions in asteroids favored the creation of left-handed amino acids. Meteorite impacts could have supplied this material, enriched in left-handed molecules, to Earth. The bias toward left-handedness would have been perpetuated as this material was incorporated into emerging life. In the new research, the team reports finding excess left-handed isovaline, or L-isovaline, in a much wider variety of carbon-rich meteorites. “This tells us our initial discovery wasn’t a fluke; that there really was something going on in the asteroids where these meteorites came from that favors the creation of left-handed amino acids,” said NASA Goddard’s Daniel Glavin, lead author of a paper on the research published online in Meteoritics and Planetary Science Jan. 17. The research indicates that “some process involving liquid water favors the creation of left-handed amino acids,” Glavin said. “We can tell how much these asteroids were altered by liquid water by analyzing the minerals their meteorites contain. The more these asteroids were altered, the greater the excess L-isovaline we found.” “In the meteorites with the largest left-handed excess, we find about 1,000 times less isovaline than in meteorites with a small or non-detectable left-handed excess. This tells us that to get the excess, you need to use up or destroy the amino acid, so the process is a double edged sword,” said Glavin. Whatever it may be, the water-alteration process only amplifies a small existing left-handed excess, it does not create the bias, according to Glavin. Something in the pre-solar nebula—a vast cloud of gas and dust from which our solar system, and probably many others, were born—created a small initial bias toward L-isovaline and presumably other left-handed amino acids. One possibility, he went on, is radiation. Space is filled with objects such as massive stars, neutron stars, and black holes that produce many kinds of radiation. It’s possible that the radiation encountered by our solar system in its youth made left handed amino acids slightly more likely to be created, or right-handed amino acids a bit more likely to be destroyed, according to Glavin. It could also be that other young solar systems encountered different radiation that favored right-handed amino acids, he noted: if life emerged in one of these systems, the bias toward right-handed amino acids might be built in just as it may have been for left-handed amino acids here.