Comet had watery past, scientists find

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Comet had watery past, scientists find

April 5, 2011
Courtesy of the University of Arizona
and World Science staff

At least one com­et has con­tained liq­uid wa­ter, re­searchers say, shat­ter­ing a long-held be­lief a­mong sci­en­tists that this could nev­er hap­pen.

"Cur­rent think­ing sug­gests that it is im­pos­si­ble to form liq­uid wa­ter in­side of a com­et," sa­id Dan­te Lau­ret­ta of the U­ni­ver­si­ty of Ar­i­zo­na, prin­ci­pal in­ves­ti­ga­tor of a team at the uni­ver­si­ty that is an­a­lyz­ing sam­ples of the com­et Wild-2.

This artist's im­pres­sion shows the ir­reg­u­lar sur­face of com­et Wild-2 and jets spout­ing in­to space. (Cour­tesy NA­SA/JPL-Caltech)

Some scientists subscribe to a the­o­ry known as pan­sper­mia, which holds that liv­ing things or their pre­cur­sors are car­ried a­board ob­jects such as com­ets to seed new life on plan­ets. The pres­ence of liq­uid wa­ter on a com­et might make it eas­i­er for or­gan­isms to sur­vive there, though some mi­crobes and seeds can sur­vive freez­ing al­so.

U­ni­ver­si­ty of Ar­i­zo­na grad­u­ate stu­dent Eve Berger, who led the new stud­y, and col­leagues an­a­lyzed dust grains brought back to Earth from com­et Wild-2 as part of NASA's Star­dust mis­sion. Launched in 1999, the Star­dust space­craft scooped up ti­ny par­ti­cles from the com­et's sur­face in 2004 and brought them back to Earth in a cap­sule that land­ed in U­tah two years lat­er.

"We found min­er­als that formed in the pres­ence of liq­uid wa­ter," Berger sa­id. "At some point in its his­to­ry, the com­et must have har­bo­red pock­ets of wa­ter." The find­ing is to be pub­lished in an up­com­ing on­line e­di­tion of the re­search jour­nal Geo­chim­ica et Cos­mo­chim­ica Ac­ta.

Comets are of­ten called dirt­y snow­balls be­cause they con­sist of most­ly wa­ter ice, pep­pered with rock­y de­bris and fro­zen gas­es. Un­like as­ter­oids, ex­tra­ter­res­trial chunks made up of rock and min­er­als, com­ets sport a tail – jets of gas and va­por that the high-energy par­ti­cle stream com­ing from the sun flushes out of their fro­zen bod­ies.

"When the ice melted on Wild-2, the re­sult­ing warm wa­ter dis­solved min­er­als that were pres­ent at the time and pre­cip­i­tated the i­ron and cop­per sul­fide min­er­als we ob­served," Lau­ret­ta sa­id. "The sul­fide min­er­als formed be­tween 50 and 200 de­grees Cel­si­us [122 and 392 de­grees Fahren­heit], much warm­er than the sub-zero tem­per­a­tures pre­dicted for the in­te­ri­or of a com­et."

Dis­cov­ered in 1978 by Swiss as­tron­o­mer Paul Wild, Wild-2 (pro­nounced "Vilt") had trav­eled the out­er reaches of the so­lar sys­tem for most of its 4.5 bil­lion year his­to­ry, un­til a close en­coun­ter with Jupiter's field of grav­i­ty sent the 3.4 mile-wide com­et on­to a new, high­ly el­lip­ti­cal or­bit bring­ing it clos­er to the sun and the in­ner plan­ets.

Sci­en­tists be­lieve that like man­y oth­er com­ets, Wild-2 o­rig­i­nat­ed in the Kuiper belt, a re­gion ex­tend­ing from be­yond Nep­tune's or­bit in­to deep space, con­tain­ing icy de­bris left over from the for­ma­tion of the so­lar sys­tem. The find­ing of the low-temperature sul­fide min­er­als may be im­por­tant for our un­der­stand­ing of how com­ets formed, which in turn tells us a­bout the or­i­gin of the so­lar sys­tem. In ad­di­tion to pro­vid­ing ev­i­dence of liq­uid wa­ter, re­searchers say, the new­found in­gre­di­ents put an up­per lim­it to the tem­per­a­tures Wild-2 en­coun­tered dur­ing its or­i­gin and his­to­ry.

"The min­er­al we found – cuban­ite – is ver­y rare in sam­ple col­lec­tions from space," Berger sa­id. "It co­mes in two forms – the one we found on­ly ex­ists be­low 210 de­grees Cel­si­us (99 de­grees Fahren­heit). This is ex­cit­ing be­cause it tells us those grains have not seen tem­per­a­tures higher than that." Water is nor­mally li­quid at that temp­er­ature.

Cuban­ite is a cop­per i­ron sul­fide, a com­pound al­so found in o­re de­posits on Earth ex­posed to heat­ed groundwa­ter and in a par­tic­u­lar type of me­te­or­ite.

Ac­cord­ing to Berger, two ways to gen­er­ate heat sources on com­ets are mi­nor col­li­sions with oth­er ob­jects and ra­di­o­ac­tiv de­cay, or dis­in­te­gra­tion, of el­e­ments in the com­et. Heat gen­er­ated at the site of mi­nor im­pacts might gen­er­ate pock­ets of wa­ter in which the sul­fides could form ver­y quick­ly, with­in a­bout a year as op­posed to mil­lions of years. This could hap­pen at an­y point in the com­et's his­to­ry, Berger ex­plained. Ra­di­o­ac­tive de­cay, on the oth­er hand, would point to a ver­y ear­ly for­ma­tion of the min­er­als since the de­cay would oc­cur o­ver time and cause the heat source to flick­er out.

Ac­cord­ing to Lau­ret­ta, the find­ings show that com­ets ex­pe­ri­enced pro­cesses such as heat­ing and chem­i­cal re­ac­tions in liq­uid wa­ter that changed the min­er­als they in­her­it­ed from the time when the so­lar sys­tem was still a protoplan­e­tar­y disk, a swirling mix of hot gas­es and dust. The re­sults also add to evidence of con­nec­tions be­tween com­ets and as­ter­oids, Lau­ret­ta said. "What we found makes us look at com­ets in a dif­fer­ent way... we think they should be viewed as in­di­vid­u­al en­ti­ties with their own u­nique ge­o­log­ic his­to­ry."
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Eli Priyatna 16 Jun, 2011

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Source: http://basistik.blogspot.com/2011/06/comet-had-watery-past-scientists-find.html
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