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Probe of comet’s water yields surprises

Dec. 14, 2014
Courtesy of the European Space Agency
and World Science staff

A com­et stud­ied close-up by the Eu­ro­pe­an Space Agen­cy is yield­ing sur­pris­ing se­crets about its wa­ter, sci­en­tists say.

The new da­ta from the agen­cy’s Ro­set­ta space­craft sug­gest most of Earth’s wa­ter came from as­ter­oids, not com­ets, and that com­ets closer to our part of the So­lar Sys­tem have more di­verse ori­gins than pre­vi­ously sus­pected. 

A diagram shows the Kuiper Belt (left) and Oort Cloud (right) in their res­pect­ive pos­i­tions in the So­lar Sys­tem. (Cour­tesy ESA)


Ro­set­ta found the wa­ter va­por from Com­et 67P/Churyu­mov–Ge­rasi­men­ko to be very un­like Earth’s, in mea­sure­ments made in the month fol­low­ing the space­craft’s ar­ri­val at the com­et on Aug. 6.

One of the lead­ing hy­pothe­ses on Earth’s forma­t­ion is that it was so hot when it formed 4.6 bil­lion years ago that any orig­i­nal wa­ter con­tent should have boiled off. But, to­day, two thirds of the sur­face is wa­ter, so where did that come from? 

Scientists think the water came lat­er from im­pact­ing as­ter­oids and com­ets—two dif­fer­ent types of ob­jects that or­bit the Sun. (They have dif­fer­ent make­ups and as­ter­oids lack tails, for in­stance.)

But how much water came from each type of object is de­bat­ed.

Sci­en­tists be­lieve a key to de­ter­min­ing where a par­tic­u­lar body of wa­ter orig­i­nat­ed is the lev­els, with­in it, of a type of hy­dro­gen known as deu­ter­i­um, as com­pared to nor­mal hy­dro­gen. Sim­ula­t­ions show that dur­ing the first few mil­lion years of the So­lar Sys­tem, deu­ter­i­um lev­els should change with dis­tance from the Sun and with time. There­fore deu­ter­i­um lev­els in a par­tic­u­lar body of wa­ter can re­veal some­thing about where and when it orig­i­nat­ed.

Com­ets in par­tic­u­lar are con­sid­ered un­ique tools to study these ori­gins, since they har­bor ma­te­ri­al di­rectly left over from the dust cloud that gave rise to the plan­ets.

But this is­n’t straight­for­ward be­cause many com­et or­bits have got­ten mixed up since long ago. “Long-period” com­ets, a type that is more dis­tant from the sun, are thought to have orig­i­nally formed clos­er in, in the ar­ea of the plan­ets Ura­nus and Nep­tune. And “short-period” com­ets, which now in­hab­it in­ner re­gion­s—like Ro­set­ta’s—were thought to have formed fur­ther out, in the Kuiper Belt be­yond Nep­tune.

Pre­vi­ous mea­sure­ments of oth­er com­ets’ deu­ter­i­um lev­els have var­ied wide­ly, re­search­ers say. Of the 11 com­ets for which there are mea­sure­ments, only the short-period Com­et 103P/Hart­ley 2 was found to match Earth wa­ter in make­up, in ob­serva­t­ions made by the Eu­ro­pe­an Space Agen­cy’s Her­schel mis­sion in 2011. The lev­els now meas­ured by Ro­set­ta are more than three times great­er.

This sug­gests two things, mis­sion sci­en­tists said. 

“This sur­pris­ing find­ing could in­di­cate a di­verse or­i­gin for the Jupiter-family com­ets – per­haps they formed over a wid­er range of dis­tances in the young So­lar Sys­tem than we pre­vi­ously thought,” said Kathrin Al­twegg, lead au­thor of the pa­per re­port­ing the re­sults in the jour­nal Sci­ence this week.

Sec­ond, she said, “our find­ing… adds weight to mod­els that place more em­pha­sis on as­ter­oids [than com­ets] as the main delivery mech­an­ism for Earth’s oceans.” Al­twegg is prin­ci­pal in­ves­ti­ga­tor for an in­stru­ment called Ro­si­na (Or­biter Spec­trom­e­ter for Ion and Neu­tral Anal­y­sis) on the space­craft, which made the mea­sure­ments.

“As Ro­set­ta con­tin­ues to fol­low the com­et on its or­bit around the Sun through­out next year, we’ll be keep­ing a close watch on how it evolves and be­haves,” added Matt Tay­lor, the space agen­cy’s Ro­set­ta proj­ect sci­ent­ist.


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A comet studied close-up by the European Space Agency is yielding surprising secrets about its water, scientists say. The new data from the agency’s Rosetta spacecraft suggest asteroids, not comets, are the source of most of Earth’s water, and that comets of the inner solar system have more diverse origins than previously suspected. Rosetta found the water vapor from Comet 67P/Churyumov–Gerasimenko to be very unlike Earth’s, in measurements made in the month following the spacecraft’s arrival at the comet on Aug. 6. One of the leading hypotheses on Earth’s formation is that it was so hot when it formed 4.6 billion years ago that any original water content should have boiled off. But, today, two thirds of the surface is covered in water, so where did it come from? In this scenario, it came later from impacting asteroids and comets—two different types of objects that orbit the Sun. (Asteroids have different makeups and lack tails.) But how much from each is still debated. Scientists believe a key to determining where a particular body of water originated is the levels, within it, of a type of hydrogen known as deuterium, as compared to normal hydrogen. Simulations show that during the first few million years of the Solar System, deuterium levels should change with distance from the Sun and with time. Therefore deuterium levels in a particular body of water can reveal something about where and when it originated. Comets in particular are considered unique tools to study these origins, since they harbor material directly left over from the dust cloud that gave rise to the planets. But this isn’t straightforward because many comet orbits have gotten mixed up since long ago. “Long-period” comets, a type that is more distant from the sun, are thought to have originally formed closer in, in the area of the planets Uranus and Neptune. And “short-period” comets, which now inhabit inner regions—like Rosetta’s—were thought to have formed further out, in the Kuiper Belt beyond Neptune. Previous measurements of other comets’ deuterium levels have varied widely, researchers say. Of the 11 comets for which there are measurements, only the short-period Comet 103P/Hartley 2 was found to match Earth water in makeup, in observations made by the European Space Agency’s Herschel mission in 2011. The levels now measured by Rosetta are more than three times greater. This suggests two things, mission scientists said. “This surprising finding could indicate a diverse origin for the Jupiter-family comets – perhaps they formed over a wider range of distances in the young Solar System than we previously thought,” said Kathrin Altwegg, lead author of the paper reporting the results in the journal Science this week. Second, she added, “our finding… adds weight to models that place more emphasis on asteroids [than comets] as the main delivery mechanism for Earth’s oceans.” Altwegg is principal investigator for an instrument called Rosina (Orbiter Spectrometer for Ion and Neutral Analysis) on the spacecraft, which made the measurements. “As Rosetta continues to follow the comet on its orbit around the Sun throughout next year, we’ll be keeping a close watch on how it evolves and behaves,” added Matt Taylor, the European Space Agency’s Rosetta project scientist.