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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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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. A widely held theory, called panspermia, maintains that living things or their precursors are carried aboard objects such as comets to seed life on new 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. "In our samples, 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 in our study," 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. Wild-2 is thought to have spent most of its time in the Kuiper belt, transiting on unstable orbits within the planetary system before Jupiter's gravity hurled it into the neighborhood of the sun. 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. " 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. "Wherever the cubanite formed, it stayed cool," Berger added. "If this mineral formed on the comet, it has implications for heat sources on comets in general." According to Berger, two ways to generate heat sources on comets are minor collisions with other objects and radioactive 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 demonstrate the increasingly apparent connections between comets and asteroids, Lauretta added. "What we found makes us look at comets in a different way," Lauretta said. "We think they should be viewed as individual entities with their own unique geologic history."