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October 06, 2011

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Finding suggests ocean water could come from comets

Oct. 5, 2011
Courtesy of NASA Jet Propulsion Laboratory
and World Science staff

A characteristic fea­ture of Earth’s ocean wa­ter has al­so been meas­ured in wa­ter from a com­et—sug­gest­ing much of our wa­ter in­deed comes from com­ets, as­tro­no­mers are re­port­ing.

The mea­sure­ments from NASA’s Her­schel Space Ob­serv­a­to­ry in­di­cate that com­et Hart­ley 2, which comes from a dis­tant re­gion of the So­lar Sys­tem known as Kui­per Belt, con­tains wa­ter with the same chem­i­cal sig­na­ture as our oceans. The Kui­per Belt, which sur­rounds the Sun about some 30 to 50 times fur­ther out than Earth’s or­bit, is home to icy, rocky bod­ies in­clud­ing Plu­to, oth­er “dwarf plan­ets” and many com­ets.

Com­et Hart­ley 2. (NA­SA, ESA, and H. Weav­er (Johns Hop­kins U./Ap­plied Phys­ics Lab) )
“Our re­sults with Her­schel sug­gest that com­ets could have played a ma­jor role in bring­ing vast amounts of wa­ter to an early Earth,” said Dar­iusz Lis of the the Cal­i­for­nia In­sti­tute of Tech­nol­o­gy, co-author of a new pa­per in the jour­nal Na­ture, pub­lished on­line Oct. 5. “This find­ing sub­stanti­ally ex­pands the res­er­voir of Earth ocean-like wa­ter in the so­lar sys­tem to now in­clude icy bod­ies orig­i­nat­ing in the Kui­per Belt.” 

Sci­en­tists the­o­rize Earth started out hot and dry, so that wa­ter crit­i­cal for life must have been de­liv­ered mil­lions of years lat­er.

The Her­schel craft peered in­to the com­et’s co­ma, or thin, gas­e­ous at­mos­phere. The co­ma is thought to form as fro­zen ma­te­ri­als in a com­et va­por­ize while on ap­proach to the Sun. This glow­ing en­ve­lope sur­rounds the com­et’s “i­cy dirtball”-like co­re, as­tro­no­mers say, and streams be­hind the ob­ject in a char­ac­ter­is­tic tail.

Her­schel de­tected a sig­na­ture of va­por­ized wa­ter in this co­ma and, to sci­en­tists’ sur­prise, Hart­ley 2 had half as much “heavy wa­ter” as oth­er com­ets an­a­lyzed to date. In heavy wa­ter, one of the two nor­mal hy­dro­gen atoms is re­placed by a hea­vy var­iant, or iso­tope, of hy­dro­gen known as deu­ter­i­um. The re­lat­ion­ship be­tween the amount of heavy and normal wa­ter in Hart­ley 2 was found to be the same as the wa­ter on Earth’s sur­face. 

The amount of heavy wa­ter in a com­et is re­lat­ed to the en­vi­ron­ment where the com­et formed. Based on Hart­ley 2’s path as it swoops by Earth’s neigh­bor­hood eve­ry 6.5 years, as­tro­no­mers be­lieve it comes from the Kui­per Belt. The five com­ets be­sides Hart­ley 2 whose heav­y-wa­ter-to-reg­u­lar-wa­ter ra­tios have been ob­tained all come from an even more dis­tant re­gion in the so­lar sys­tem called the Oort Cloud. This swarm of bod­ies, 10,000 times far­ther afield than the Kui­per Belt, is the well­spring for most doc­u­mented com­ets.

Giv­en the great­er prev­a­lence of heavy wa­ter seen in Oort Cloud com­ets com­pared to Earth’s oceans, as­tro­no­mers had con­clud­ed that com­ets only con­tri­but­ed about one tenth of Earth’s wa­ter. But the new re­sults sug­gest Kui­per Belt com­ets may help fill in the miss­ing link in the story.

How these ob­jects came to pos­sess the tell-tale oceanic wa­ter is puz­zling. As­tro­no­mers had ex­pected Kui­per Belt com­ets to have even more heavy wa­ter than Oort Cloud com­ets, based on where they formed. “Our study in­di­cates that our un­der­stand­ing of the dis­tri­bu­tion of the light­est el­e­ments and their iso­topes, as well as the dy­nam­ics of the early so­lar sys­tem, is in­com­plete,” said co-author Geof­frey Blake, a plan­e­tary sci­entist and chem­ist at Cal­tech. “In the early so­lar sys­tem, com­ets and as­ter­oids must have been mov­ing all over the place, and it ap­pears that some of them crash-landed on our plan­et and made our oceans.”

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A special feature of Earth’s ocean water has also been measured in water from a comet—suggesting that much of our water indeed comes from comets, astronomers are reporting. The measurements from NASA’s Herschel Space Observatory indicate that comet Hartley 2, which comes from a distant region of the Solar System known as Kuiper Belt, contains water with the same chemical signature as our oceans. The Kuiper Belt, which surrounds the Sun about some 30 to 50 times further out than Earth’s orbit, is home to icy, rocky bodies including Pluto, other “dwarf planets” and many comets. “Our results with Herschel suggest that comets could have played a major role in bringing vast amounts of water to an early Earth,” said Dariusz Lis, senior research associate in physics at the California Institute of Technology and co-author of a new paper in the journal Nature, published online Oct. 5. “This finding substantially expands the reservoir of Earth ocean-like water in the solar system to now include icy bodies originating in the Kuiper Belt.” Scientists theorize Earth started out hot and dry, so that water critical for life must have been delivered millions of years later. Herschel peered into the comet’s coma, or thin, gaseous atmosphere. The coma is thought to develop as frozen materials inside a comet vaporize while on approach to the Sun. This glowing envelope surrounds the comet’s “icy dirtball”-like core, astronomers say, and streams behind the object in a characteristic tail. Herschel detected a signature of vaporized water in this coma and, to scientists’ surprise, Hartley 2 had half as much “heavy water” as other comets analyzed to date. In heavy water, one of the two normal hydrogen atoms has been replaced by the heavy hydrogen isotope known as deuterium. The ratio between heavy water and light, or regular, water in Hartley 2 was found to be the same as the water on Earth’s surface. The amount of heavy water in a comet is related to the environment where the comet formed. Based Hartley 2’s path as it swoops by Earth’s neighborhood every 6.5 years, astronomers believed it comes from the Kuiper Belt. The five comets besides Hartley 2 whose heavy-water-to-regular-water ratios have been obtained all come from an even more distant region in the solar system called the Oort Cloud. This swarm of bodies, 10,000 times farther afield than the Kuiper Belt, is the wellspring for most documented comets. Given the greater prevalence of heavy water seen in Oort Cloud comets compared to Earth’s oceans, astronomers had concluded that comets only contributed about one tenth of Earth’s water. But the new results point to Kuiper Belt comets as possibly having performed a previously underappreciated service. How these objects came to possess the tell-tale oceanic water is puzzling. Astronomers had expected Kuiper Belt comets to have even more heavy water than Oort Cloud comets, based on where they formed. “Our study indicates that our understanding of the distribution of the lightest elements and their isotopes, as well as the dynamics of the early solar system, is incomplete,” said co-author Geoffrey Blake, professor of planetary science and chemistry at Caltech. “In the early solar system, comets and asteroids must have been moving all over the place, and it appears that some of them crash-landed on our planet and made our oceans.”