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Earth’s water is older than the Sun, scientists claim

Sept. 25, 2014
Courtesy 
and World Science staff

Earth’s wa­ter is old­er than the Sun, sci­en­tists have con­clud­ed. The find­ing makes it seem like­ly, they say, that the life-sustaining liq­uid is com­mon in plan­e­tary sys­tems be­yond our own.

Al­though sci­en­tists have no doubt that there was wa­ter be­fore the Sun, it has­n’t been clear wheth­er Earth’s wa­ter, spe­cif­ic­ally, orig­i­nat­ed in that dis­tant past. In­stead, wa­ter could have bro­ken down, then re-formed out of its com­po­nents, as our so­lar sys­tem de­vel­oped.

Wa­ter is now found in some form not just on Earth, but on icy comets and moons, in the shad­owed basins of Mer­cu­ry, and in sam­ples from me­te­orites, the Moon, and Mars. The new work con­cludes that much of this wa­ter likely orig­i­nat­ed as ices float­ing around space be­fore the Sun came to­geth­er.

Ev­i­dence in­di­cates that in its youth, the Sun, like many stars, was sur­rounded by a ring-like “proto­plan­e­tary disk” of gas­e­ous, dusty ma­te­ri­al. That stuff gave rise to the plan­ets. But it has been un­clear, ac­cord­ing to the stu­dy’s au­thors, wheth­er the wa­ter in this disk pre-dated the Sun, or wheth­er chem­i­cal re­ac­tions in the disk it­self re-formed the wa­ter out of its com­po­nents, hy­dro­gen and ox­y­gen. 

Why this is im­por­tant? Conel Al­ex­an­der, a co-author of the new stu­dy, from the Car­ne­gie In­sti­tu­tion for Sci­ence in Wash­ing­ton, D.C., gave one answer. “If wa­ter in the early so­lar sys­tem was pri­marily in­her­it­ed as ice” from the vast voids of space be­tween the stars, he ex­plained, “then it is likely that si­m­i­lar ices… are abun­dant in most or all proto­plan­e­tary disks around form­ing stars.” 

“But if the early so­lar sys­tem’s wa­ter was largely the re­sult of lo­cal chem­i­cal pro­cess­ing dur­ing the Sun’s birth, then it is pos­si­ble that the abun­dance of wa­ter varies con­sid­erably in [new] plan­e­tary sys­tems, which would ob­vi­ously have im­plica­t­ions for the po­ten­tial for the emer­gence of life else­where.”

The re­searchers—led by L. Il­se­dore Cleeves from the Uni­vers­ity of Michi­gan—looked at sam­ples of hy­dro­gen in its reg­u­lar form and in a heav­i­er form, or iso­tope, called deu­ter­i­um. Iso­topes are at­oms of the same el­e­ment that dif­fer in the num­ber of neu­trons, a sub­a­tom­ic par­t­i­cle, per at­om.

Weight dif­ference be­tween iso­topes lead to slight dif­ferences in their be­hav­ior dur­ing chem­i­cal re­ac­tions. As a re­sult, the rel­a­tive amounts of the iso­topes with­in wa­ter can tell sci­en­tists about the con­di­tions un­der which the wa­ter formed. For ex­am­ple, wa­ter-ice in space is deu­ter­i­um-rich be­cause it forms in cold con­di­tions. Wheth­er chem­i­cal pro­cess­ing dur­ing the Sun’s birth could al­so pro­duce such deu­ter­i­um-rich wa­ter was un­clear.

The re­search­ers cre­at­ed com­put­er sim­ula­t­ions of a pro­to­plan­e­tary disk to see wheth­er, by it­self, this chem­i­cal pro­cess­ing could pro­duce deu­ter­i­um-rich wa­ter ice such as is found in me­te­or­ite sam­ples, Earth’s oceans, and comets. They found that it could not, so that our wa­ter probably pre-dates the sun. The find­ings are pub­lished on­line Thurs­day in the jour­nal Sci­ence.

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