"Long before it's in the papers"
September 30, 2015


Comet watched by probe is actually two in one, scientists say 

Sept. 30, 2015
Courtesy of ESA
and World Science staff

Two com­ets crashed long ago to cre­ate the dumb­bell-shaped Com­et 67P/Chur­yu­mov-Ge­ra­si­men­ko, say sci­en­tists with a Eu­ro­pe­an Space Agen­cy mis­sion stu­dy­ing the com­et.

The reason for the odd form has been a mys­tery since the agen­cy’s space­craft Ro­set­ta, vis­it­ing the com­et, re­vealed the shape in July 2014. 

Rosetta's comet (© ESA/Rosetta/Navcam – CC BY-SA IGO 3.0)

Re­search­ers at­trib­ut­ed the shape ei­ther to a long-a­go merg­er, or to ero­sion of a sin­gle com­et to cre­ate a “neck.” 

Now, sci­en­tists say it’s clear that sep­a­rate com­ets col­lid­ed, some­what slowly, to form 67P. The re­search­ers ana­lyzed close-up im­ages to study lay­ers of ma­te­ri­al seen all over the comet’s nu­cle­us, or co­re.

The im­ages show “an out­er en­ve­lope of ma­te­ri­al or­gan­ized in dis­tinct lay­ers… we think these ex­tend for sev­er­al hun­dred me­ters,” or yards, deep, said Mat­teo Mas­sironi, lead au­thor from the Un­ivers­ity of Pa­do­va, It­a­ly, and an as­so­ci­ate sci­ent­ist with Ro­set­ta’s OSI­RIS cam­era sys­tem.

The lay­ers are “a bit like an on­ion”—or rather, “two sep­a­rate on­ions of dif­fer­ing size” that grew sep­a­rately be­fore squash­ing to­geth­er, he said. The find­ings are re­ported in the re­search jour­nal Na­ture and were pre­sented Sept. 27 at the Eu­ro­pe­an Plan­e­tary Sci­ence Con­gress in Nantes, France.

Mas­sironi and col­leagues used im­ages snapped be­tween Aug. 6 of last year and March 17 of this year to iden­ti­fy over 100 ter­races, or step-like land­forms, on the sur­face of the com­et, and par­al­lel lay­ers of ma­te­ri­al in ex­posed cliff walls and pits. They then used a mod­el to work out thei di­rec­tions and depth of these lay­ers.

It soon be­came clear, they said, that they were co­her­ently ar­ranged all around the com­et’s lobes, or dis­tinct “halves” of the dumb­bell, and in some places about 650 me­ters deep. 

“This was the first clue that the two lobes are in­de­pen­dent,” Mas­sironi said. An­oth­er: “the lay­ers are in­clined in op­po­site di­rec­tions close to the com­et’s neck,” where­as their di­rec­tion would be con­sist­ent if it had formed as a sin­gle ob­ject.

“We al­so looked at the rela­t­ion­ship be­tween the lo­cal gra­vity and the ori­enta­t­ions of the in­di­vid­ual fea­tures all around the re­con­struct­ed com­et sur­face,” he added. 

On Earth, lay­ers of rock are hor­i­zon­tal be­cause they gen­er­ally form at right an­gles to the force of gra­vity, which is down­ward. On an oddly shaped ob­ject like 67P, lay­ers would al­so be ex­pected to form at right an­gles to gra­vity, but the gravita­t­ional field is more com­pli­cat­ed. So the team used mod­els to com­pute the strength and di­rec­tion of gra­vity at each lay­er.

In one case, they mod­eled the com­et as a sin­gle body, as it is now. In the oth­er, they mod­eled it as if it were two sep­a­rate com­ets, each with its own gravita­t­ional field cen­tered more or less at the mid­dle. The lay­ers, the re­search­ers said, made more sense in the sec­ond pic­ture.

But “it must have been a low-speed col­li­sion” to avoid dis­rupt­ing the lay­ers more than they have been, Mas­sironi said. Moreo­ver, “the strik­ing struc­tur­al si­m­i­lar­i­ties” be­tween the two parts sug­gest they “formed through a si­m­i­lar ac­cre­tion pro­cess,” added co-au­thor Bjorn Davids­son of Upp­sa­la Un­ivers­ity, Swe­den. “Lay­er­ing has al­so been ob­served on the sur­face of oth­er com­ets dur­ing pre­vi­ous fly­by mis­sions.”

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Two comets crashed long ago to create Comet 67P/Churyumov-Gerasimenko, with its “rubber duck” shape, say scientists with a European Space Agency mission studying the comet. The origin of the comet’s form, also often described as a dumbbell, has been a mystery since the agency’s spacecraft Rosetta, visiting the comet, revealed the shape in July 2014. Researchers attributed the shape either to a long-ago merger, or to erosion of a single comet to create a “neck.” Now, scientists using high-resolution images to study layers of material seen all over the nucleus, or core, say it’s clear that separate comets collided somewhat slowly to form 67P. The images show “both lobes have an outer envelope of material organized in distinct layers… we think these extend for several hundred meters,” or yards, deep, said Matteo Massironi, lead author from the University of Padova, Italy, and an associate scientist with Rosetta’s OSIRIS camera system. The layers are “a bit like an onion, except in this case we are considering two separate onions of differing size” that grew separately before squashing together. The findings are reported in the research journal Nature and were presented today at the European Planetary Science Congress in Nantes, France. Massironi and colleagues used images snapped between Aug. 6 of last year and March 17 of this year to identify over 100 terraces, or step-like landforms, on the surface of the comet, and parallel layers of material in exposed cliff walls and pits. They then used a model to work out thei directions and depth of these layers. It soon became clear, they said, that they were coherently arranged all around the comet’s lobes, or distinct parts of the dumbbell, and in some places about 650 meters deep. “This was the first clue that the two lobes are independent,” Massironi said. Another clue: “the layers are inclined in opposite directions close to the comet’s neck,” whereas their direction would be consistent if it had formed as a single object. “We also looked at the relationship between the local gravity and the orientations of the individual features all around the reconstructed comet surface,” he added. On Earth, layers of rock are horizontal because they generally form at right angles to the force of gravity, which is downward. On an oddly shaped object like 67P, layers would also be expected to form at right angles to gravity, but the gravitational field is more complicated. So the team used models to compute the strength and direction of gravity at each layer. In one case, they modeled the comet as a single body, as it is now. In the other, they modeled it as if it were two separate comets, each with its own gravitational field centered more or less at the middle. The layers, the researchers said, made more sense in the second picture. But “it must have been a low-speed collision” to avoid disrupting the layers more than they have been, Massironi said. Moreover, “the striking structural similarities” between the two parts suggest they “formed through a similar accretion process,” added co-author Bjorn Davidsson of Uppsala University, Sweden. “Layering has also been observed on the surface of other comets during previous flyby missions.”