CU-led study cap­tures proof of a land­slide on a comet

The Denver Post - - NEWS - By Sarah Ka­plan

It was 3 a.m., and as­tronomer Mau­r­izio Pa­jola had been up for hours look­ing through im­ages taken by the Rosetta space­craft of its dumpy, duck-shaped comet. Pa­jola had just started a new job study­ing Mars’ moon Pho­bos at NASA’s Ames Re­search Cen­ter. The only time he could con­tinue this work on the Rosetta mission was the mid­dle of the night.

His eyes were be­gin­ning to glaze over when he spot­ted some­thing un­usual: A patch of some­thing bright and white shined out from the comet’s dim sur­face. This photo was from De­cem­ber 2015. Pa­jola started flip­ping back though the cat­a­logue of im­ages taken by Rosetta’s high power OSIRIS in­stru­ment un­til he ar­rived at July 4. There, streak­ing across a cliff called Aswan in the comet’s north­ern hemi­sphere, was a gash more than 200 feet long and wide enough for a per­son to fall through.

The next im­age, taken by one of Rosetta’s less pow­er­ful cam­eras on July 10, showed a plume of gas and dust burst­ing from the comet. Five days later, OSIRIS got an­other good look at the site in ques­tion. The cliff face had col­lapsed, re­veal­ing This hand­out pic­ture re­leased by the Euro­pean Space Agency shows an OSIRIS NAC im­age of the Aswan cliff taken in De­cem­ber 2015. The white ar­row shows the bright Aswan cliff with the wa­ter ice ex­posed. Euro­pean Space Agency the ra­di­ant ma­te­rial be­neath.

In a pa­per pub­lished Tues­day in the jour­nal Na­ture Astron­omy, Pa­jola and his col­leagues re­port that the event he spot­ted was a land­slide — the first cap­tured on a comet. The col­lapse of the dark or­ganic ma­te­rial coat­ing the cliff face re­vealed that pris­tine wa­ter ice lies be­neath the comet’s sur­face.

The land­slide was the most dra­matic of sev­eral ge­o­logic phe­nom­ena that Rosetta sci­en­tists have wit­nessed on Comet 67P/ Churyu­mov-Gerasi­menko, a lump of ice and rock about the size of Mount Fuji.

In a sec­ond pa­per pub­lished in the jour­nal Science, the as­tronomers de­scribe how the comets’ sur­face is con­stantly chang­ing as a re­sult of its ro­ta­tion and the glare of the sun.

“These im­ages are show­ing that comets are some of the most ge­o­log­i­cally ac­tive things in the so­lar sys­tem,” Pa­jola said. “We see frac­tures in­creas­ing, dust cov­er­ing ar­eas that were not dusted be­fore, boul­ders rolling, cliffs col­laps­ing.”

The land­slide, which took place some­time around July 10, 2015, would not have looked like a land­slide on Earth. Comet 67P is so small it hardly has any grav­ity, so in­stead of tum­bling down like an avalanche, much of the ma­te­rial that broke off from the frac­tured cliff face pro­duced an “out­burst.” Some 22,000 cu­bic me­ters of ma­te­rial, enough to fill nine Olympic swim­ming pools, puff­ing up above the sur­face to form a cloud of dust and gas. This sug­gests an un­am­bigu­ous link be­tween out­bursts (which give comets their char­ac­ter­is­tic co­mas) and de­struc­tive events like land­slides, the sci­en­tists say.

Aswan’s col­lapse also of­fered a cru­cial peek into the comet’s in­te­rior.

Based on the ex­posed patch’s high albedo — a mea­sure­ment of how much light it re­flects — the sci­en­tists say that it must be com­posed of wa­ter ice.

In the Science pa­per, for which Pa­jola is a coau­thor, re­searchers de­scribe how ge­o­logic events like the cliff col­lapse are driven by the heat of the sun. In sum­mer 2015, Comet 67P was ap­proach­ing its per­i­he­lion — its clos­est ap­proach to the sun.

“This is the time where you get max­i­mum ac­tiv­ity, the time where you get max­i­mum amount of change,” said Ramy ElMaarry, a plan­e­tary sci­en­tist at the Univer­sity of Colorado at Boul­der who was the lead au­thor on the Science study.

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