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3-D study of comets reveals “chemical factory” within

Aug. 12, 2014
Courtesy of NASA
and World Science staff

Sci­en­tists have cre­at­ed de­tailed 3-D maps of the at­mo­spheres sur­round­ing com­ets—iden­tifying some chem­i­cals that may have been im­por­tant to the or­i­gin of life, re­search­ers say.

“We achieved truly first-of-a-kind map­ping of im­por­tant mol­e­cules that help us un­der­stand the na­ture of com­ets,” said Mar­tin Cor­di­ner, a re­search­er at the God­dard Cen­ter for As­tro­bi­ol­o­gy at NASA’s God­dard Space Flight Cen­ter in Green­belt, Md., who led the team of in­ves­ti­ga­tors.

E­mis­sion from or­gan­ic mo­le­cules in the at­mos­phere of com­et ISON as ob­served with AL­MA. (Cred­it: B. Sax­ton (NRAO/AUI/NSF); M. Cordiner, NA­SA, et al.)


The 3-D per­spec­tive pro­vides deeper in­sight in­to which ma­te­ri­als are shed from the nu­cle­us, or co­re, of the com­et and which are pro­duced with­in the at­mos­phere, or co­ma, the sci­en­tists said. This helped the team nail down the sources of two key or­gan­ic, or car­bon-con­taining, mol­e­cules, the types of ma­te­ri­als that make up liv­ing things.

The ob­serva­t­ions were con­ducted in 2013 on com­ets Lem­mon and ISON us­ing the At­a­cama Large Mil­lime­ter/sub­mil­lime­ter Ar­ray, or ALMA, a net­work of high-precision in­stru­ments in Chile. 

The ob­serva­t­ions com­bine a de­tailed two-di­men­sion­al im­age of a com­et’s gas­es with a spec­trum, or break­down of col­ors, at each point. From this, re­search­ers can iden­ti­fy the mol­e­cules at eve­ry point and de­ter­mine their speed and di­rec­tion along the line of sight. That al­so re­veals the depth of the co­ma—the third di­men­sion.

The re­search­ers re­ported re­sults for three mo­lec­u­lar types, focus­ing mainly on two whose sources have been hard to dis­cern, ex­cept in com­et Hal­ley. The maps in­di­cat­ed wheth­er each mol­e­cule was flow­ing out­ward evenly in all di­rec­tions or com­ing off in jets or in clumps.

In each com­et, the team found that two mol­e­cules—form­al­de­hyde and HNC (made of one hy­dro­gen, one ni­tro­gen and one car­bon atom­)—were pro­duced in the co­ma. For form­al­de­hyde, this con­firmed what re­search­ers al­ready sus­pected, but the new maps con­tained enough de­tail to re­solve clumps of the ma­te­ri­al mov­ing in­to dif­fer­ent re­gions of the co­ma day-by-day and even hour-by-hour, ac­cord­ing to the sci­en­tists.

For HNC, the maps are said to have set­tled a long-stand­ing ques­tion about the ma­te­ri­al’s source. In­i­tial­ly, it was thought to be pris­tine cosmic ma­te­ri­al com­ing from a com­et’s co­re, but lat­er work sug­gested oth­er pos­si­ble sources. The re­search­ers said the new study proved that it’s pro­duced dur­ing the break­down of large mol­e­cules or or­gan­ic dust in the co­ma.

“Under­standing or­gan­ic dust is im­por­tant,” be­cause some of it “could have been de­liv­ered in­tact to early Earth, there­by fu­el­ing the emer­gence of life” af­ter a com­et im­pact, said Mi­chael Mumma, di­rec­tor of the God­dard Cen­ter and a co-author of the stu­dy.

The ob­serva­t­ions, pub­lished Aug. 11 in the jour­nal As­t­ro­phys­i­cal Jour­nal Let­ters, al­so were con­sid­ered sig­nif­i­cant be­cause rel­a­tively faint com­ets like Lem­mon and ISON con­tain rel­a­tively low lev­els of cru­cial mol­e­cules, mak­ing them hard to stu­dy. The few com­pre­hen­sive stud­ies of this kind so far have been con­ducted on bright, block­bust­er com­ets, such as Hale-Bopp.


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Scientists have created detailed 3-D maps of the atmospheres surrounding comets—identifying some chemicals that may have been important to the origin of life, researchers say. “We achieved truly first-of-a-kind mapping of important molecules that help us understand the nature of comets,” said Martin Cordiner, a researcher at the Goddard Center for Astrobiology at NASA’s Goddard Space Flight Center in Greenbelt, Md., who led the team of investigators. The 3-D perspective provides deeper insight into which materials are shed from the nucleus, or core, of the comet and which are produced within the atmosphere, or coma, the scientists said. This helped the team nail down the sources of two key organic, or carbon-containing, molecules, the types of materials that make up living things. The observations were conducted in 2013 on comets Lemmon and ISON using the Atacama Large Millimeter/submillimeter Array, or ALMA, a network of high-precision instruments in Chile. The observations combine a detailed two-dimensional image of a comet’s gases with a spectrum, or breakdown of colors, at each point. From this, researchers can identify the molecules at every point and determine their speed and direction along the line of sight. That also reveals the depth of the coma—the third dimension. The researchers reported results for three molecular types, focusing mainly on two whose sources have been hard to discern, except in comet Halley. The maps indicated whether each molecule was flowing outward evenly in all directions or coming off in jets or in clumps. In each comet, the team found that two molecules—formaldehyde and HNC (made of one hydrogen, one nitrogen and one carbon atom)—were produced in the coma. For formaldehyde, this confirmed what researchers already suspected, but the new maps contained enough detail to resolve clumps of the material moving into different regions of the coma day-by-day and even hour-by-hour, according to the scientists. For HNC, the maps settled a long-standing question about the material’s source. Initially, it was thought to be pristine interstellar material coming from a comet’s core, but later work suggested other possible sources. The researchers said the new study proved that it’s produced during the breakdown of large molecules or organic dust in the coma. “Understanding organic dust is important,” because some of it “could have been delivered intact to early Earth, thereby fueling the emergence of life” after a comet impact, said Michael Mumma, Director of the Goddard Center and a co-author of the study. The observations, published Aug. 11 in the journal Astrophysical Journal Letters, also were considered significant because relatively faint comets like Lemmon and ISON contain relatively low levels of crucial molecules, making them hard to study. The few comprehensive studies of this kind so far have been conducted on bright, blockbuster comets, such as Hale-Bopp.