Sound­ings Shapeshift­ing wings

Air & Space Smithsonian - - Front Page -

IT TAKES ONLY about three mil­lion years. That’s the ges­ta­tion pe­riod of a so­lar sys­tem: three mil­lion years for a star to form from an im­mense cloud of gas that spi­rals in­ward, leav­ing be­hind the mol­e­cules of sil­i­cates and met­als that will co­a­lesce into plan­ets. That may sound like a long time, but build­ing the foun­da­tion of a so­lar sys­tem is a big job; sci­en­tists have been try­ing to un­der­stand why it doesn’t take longer than three mil­lion years. Last Novem­ber astronomers an­nounced that they have found ev­i­dence to sup­port one ex­pla­na­tion of how this process was helped along by an­a­lyz­ing a me­te­orite that is older than the plan­ets them­selves.

Roger Fu, a grad­u­ate stu­dent at the Mas­sachusetts In­sti­tute of Tech­nol­ogy who refers to him­self as a pa­le­o­mag­netist, got a read­ing on the strength of the mag­netic field in the disk of gas that be­came our so­lar sys­tem. He did it by mea­sur­ing the strength of the mag­netic field in mil­lime­ter-size con­stituents of the Se­markona me­te­orite, a one-and-a-half-pound rock named for the place in In­dia where it was found and re­garded as one of the most prim­i­tive me­te­orites ever dis­cov­ered. “Peo­ple have been model­ing mag­netic fields in the early so­lar sys­tem since the 1950s,” says Fu. “There are all sorts of dif­fer­ent mod­els of how you ex­plain the rate of this gas ac­cre­tion in the early so­lar sys­tem. Our re­sults are con­sis­tent with the lat­est mag­netic-field-based mod­els for this trans­port.”

The re­sult doesn’t say pre­cisely what mag­netic ef­fects cause the ac­cel­er­a­tion of gas to­ward the cen­tral star, but it does es­tab­lish that mag­netic fields were most likely the dom­i­nant mode of trans­port. “There are mod­els based on non­mag­netic ef­fects,” says Fu. “For ex­am­ple, the baro­clinic

in­sta­bil­ity is some­thing peo­ple talk about.” That in­sta­bil­ity is a con­cept in fluid dy­nam­ics based on the idea that tem­per­a­ture vari­a­tions cause tur­bu­lence.

Ari­zona State Univer­sity as­tronomer Steve Desch, a spe­cial­ist in the very early stages of so­lar sys­tem for­ma­tion and co-au­thor of the pa­per Fu pub­lished in the jour­nal Sci­ence, called Fu’s mea­sure­ments “as­tound­ing and un­prece­dented.”

The mea­sure­ments are as­tound­ing for a num­ber of rea­sons: first, be­cause they are made on such a small scale. Us­ing a very sen­si­tive mag­ne­tome­ter that re­lies on su­per­con­duc­tiv­ity, Fu and fel­low re­searchers mea­sured the mag­netic fields of the me­te­orite’s chon­drules, molten droplets in the pri­mor­dial gas cloud that hard­ened into stony rub­ble and, through cy­cles of heat­ing and cool­ing, gath­ered into me­te­orites. Then Fu went even smaller.

Within the chon­drules are min­er­als called olivines. “Olivines, like the sands in Hawaii,” says Desch. “Some sands are black, some green be­cause they con­tain olivine crys­tals. In some of these olivines are mi­cron-size grains of iron-nickel metal, and those are the things that are mag­netic.”

The grains get mag­ne­tized when they’re ex­posed to a mag­netic field, and if they cool”— be­low a cer­tain, still very high tem­per­a­ture—“they re­main mag­ne­tized.” The grains in the Se­markona me­te­orite have kept this mag­ne­ti­za­tion for bil­lions of years.

“That’s a mag­net that lasts,” says Desch. “And they’re each like lit­tle bar mag­nets.” In other words, the di­rec­tion of the po­lar­iza­tion can be de­ter­mined: which end is pos­i­tive, which is neg­a­tive.

“The re­ally cool thing,” Desch con­tin­ues, “is that in each chon­drule, the mag­netic fields recorded by these iron-nickel in­clu­sions have a uni­form di­rec­tion, but one chon­drule’s field points in a dif­fer­ent di­rec­tion from another. They all go in dif­fer­ent di­rec­tions. That tells you they were mag­ne­tized be­fore they grouped into this [me­te­orite].” And the mag­netic field that mag­ne­tized them, there­fore, ex­isted when they were float­ing as droplets in the gas that formed the so­lar sys­tem.

“We can now say some­thing about how chon­drules formed, and we can say some­thing about the mag­netic field of the proto-so­lar disk, which tells us how it evolved,” says Desch. “Pretty fun­da­men­tal re­sult.”

Based on their mea­sure­ments and cal­cu­la­tions, Fu and the re­searchers de­ter­mined that the in­fant so­lar sys­tem had a mag­netic field roughly equal to what a hiker’s com­pass de­tects to­day, so, ac­cord­ing to Desch, the an­cient mag­netic field that ex­isted be­fore Earth be­came a planet is com­pa­ra­ble to the mag­netic field on Earth to­day.

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