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Cycle of ice turning to gas may feed comets’ tails

Sept. 23, 2005
Courtesy of ESA
and World Science staff

A daily cy­cle in which wa­ter ice turns to gas feeds the “tails” and fuzzy ha­los of com­ets like the one un­der scru­ti­ny by the Eu­ro­pe­an Space Agen­cy’s Ro­set­ta space­craft, mis­sion sci­en­tists say.

Com­ets are mix­tures of dust and ices, which they per­i­od­ic­ally shed as they swing to­wards their clos­est point to the Sun along their stretched-out, el­lip­ti­cal or­bits.

Com­et 67P/Chu­r­yu­mov–Ge­ra­si­men­ko at its clos­est ap­proach to the Sun on Aug. 13 (courtesy ESA)


As sun­light heats the fro­zen nu­cle­us, or co­re, of a com­et, its ice – mainly wa­ter but al­so oth­er “volatile” com­pounds such as car­bon mon­ox­ide and car­bon di­ox­ide – turns di­rectly to gas, sci­en­tists say. 

This gas flows away from the com­et, car­ry­ing dust par­t­i­cles along and build­ing up the char­ac­ter­is­tic bright tail and ha­lo.

Ro­set­ta reached Com­et 67P/Chu­r­yu­mov–Ge­ra­si­men­ko in Au­gust 2014 and has been stu­dy­ing it up close for over a year. On Aug. 13, 2015, the com­et reached the clos­est point to the Sun along its 6.5-year or­bit, and is now re­turn­ing to­wards the out­er So­lar Sys­tem.

Ro­set­ta’s sci­en­tists are in­ves­ti­gat­ing how ac­ti­vity on the com­et and the as­so­ci­at­ed “out­gassing” work.

Us­ing a Ro­set­ta in­stru­ment called the Vis­i­ble, In­fra­Red and Ther­mal Im­ag­ing Spec­trom­e­ter (VIR­TIS), they iden­ti­fied an ar­ea on the com­et’s sur­face where wa­ter ice ap­pears and disap­pears in sync with its spin­ning. Their find­ings are pub­lished Sept. 23 in the re­search jour­nal Na­ture.

“We found a mech­an­ism that re­plen­ishes the sur­face of the com­et with fresh ice at eve­ry rota­t­ion; this keeps the com­et ‘alive,’” said Ma­ria Cristina De Sanc­tis of the In­sti­tute for Space Phys­ics and Plan­e­tol­o­gy of the Na­t­ional In­sti­tute for As­t­ro­phys­ics in Rome, lead au­thor of the stu­dy.

The team stud­ied da­ta gath­ered last Sep­tem­ber, fo­cus­ing on a one-square-kilometer zone on the com­et’s “neck” area. At the time, the com­et was about 500 mil­lion km (300 mil­lion miles) from the Sun and the neck was one of the most ac­tive ar­e­as.

As the com­et spins, tak­ing just over 12 hours to turn around once com­plete­ly, the var­i­ous re­gions re­ceive dif­fer­ent light­ing.

“We saw the tell-tale sig­na­ture of wa­ter ice in the spec­tra [col­or make-up] of the study re­gion but only when cer­tain por­tions were cast in shad­ow,” said de Sanc­tis. “When the Sun was shin­ing on these re­gions, the ice was gone.”

This sug­gests that wa­ter ice on and a few cen­time­ters be­low the sur­face turns to gas when lit up, then flows away, she ex­plained.

Then, as dark­ness re­turns, the sur­face quickly gets cold again. But the deeper ice stays warm for a while, so it can keep turn­ing to gas and find­ing its way out­ward through the po­rous ma­te­ri­al. This gas does­n’t drift away im­me­di­ate­ly; up­on reach­ing the cold sur­face it freezes again, cre­at­ing a lay­er of fresh ice that’s the first to turn to gas af­ter sun­light re­turns. Thus the cy­cle restarts, ac­cord­ing to the sci­en­tists.

“We sus­pected such a wa­ter ice cy­cle might be at play at com­ets,” said Fab­rizio Ca­pac­cioni, prin­ci­pal in­ves­ti­ga­tor with the in­stru­ment at the in­sti­tute, but now “we fi­nally have ob­serva­t­ional proof.”

Down to a few cen­time­ters deep over the ar­ea sur­veyed, wa­ter ice ac­counts for 10-15 per­cent of the ma­te­ri­al and seems well mixed in, the sci­en­tists said.


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A daily cycle in which water ice turns to gas feeds the “tails” and fuzzy halos of comets like the one under scrutiny by the European Space Agency’s Rosetta spacecraft, mission scientists say. Comets are mixtures of dust and ices, which they periodically shed as they swing towards their closest point to the Sun along their elliptical orbits. As sunlight heats the frozen nucleus, or core, of a comet, its ice – mainly water but also other “volatile” compounds such as carbon monoxide and carbon dioxide – turns directly to gas, scientists say. This gas flows away from the comet, carrying dust particles along and building up the characteristic bright tail and halo. Rosetta reached Comet 67P/Churyumov–Gerasimenko in August 2014 and has been studying it up close for over a year. On Aug. 13, 2015, the comet reached the closest point to the Sun along its 6.5-year orbit, and is now returning towards the outer Solar System. Rosetta’s scientists are investigating how activity on the comet and the associated “outgassing” work. Using a Rosetta instrument called the Visible, InfraRed and Thermal Imaging Spectrometer (VIRTIS), they identified an area on the comet’s surface where water ice appears and disappears in sync with its spinning. Their findings are published Sept. 23 in the research journal Nature. “We found a mechanism that replenishes the surface of the comet with fresh ice at every rotation; this keeps the comet ‘alive,’” said Maria Cristina De Sanctis of the Institute for Space Physics and Planetology of the National Institute for Astrophysics in Rome, lead author of the study. The team studied data gathered last September, focusing on a one-square-kilometer zone on the comet’s neck. At the time, the comet was about 500 million km (300 million miles) from the Sun and the neck was one of the most active areas. As the comet spins, taking just over 12 hours to turn around once completely, the various regions receive different lighting. “We saw the tell-tale signature of water ice in the spectra [color make-up] of the study region but only when certain portions were cast in shadow,” said de Sanctis. “When the Sun was shining on these regions, the ice was gone.” This suggests that water ice on and a few centimeters below the surface turns to gas when lit up, then flows away, she explained. Then, as darkness returns, the surface quickly gets cold again. But the deeper ice stays warm for a while, so it can keep turning to gas and finding its way outward through the porous material. This gas doesn’t drift away immediately; upon reaching the cold surface it freezes again, creating a layer of fresh ice that’s the first to turn to gas after sunlight returns. Thus the cycle restarts, according to the scientists. “We suspected such a water ice cycle might be at play at comets,” said Fabrizio Capaccioni, principal investigator with the instrument at the institute, but now “we finally have observational proof.” The same cycle may be at work elsewhere on the comet, he added. Down to a few centimeters deep over the area surveyed, water ice accounts for 10-15% of the material and seems well mixed in, the scientists said.