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Surprises in comet dust

Dust gathered from a comet and brought to Earth tells a tale of a so­lar sys­tem that par­tial­ly turned itself in­side out, re­search­ers say.

Dec. 14, 2006
Special to World Science  
Updated Dec. 15

Dust trail­ing a dis­tant com­et, and gath­ered by a NASA space­craft, has yielded a sur­pris­ing­ly var­ied mix­ture of ma­te­ri­als, as­tro­no­mers say.

Since com­ets are thought to con­tain ma­te­ri­al left over from the ear­ly So­lar Sys­tem, this va­ri­e­ty sug­gests some­thing was mix­ing up the con­tents of the sys­tem in its youth, the re­search­ers add. What caused that, they don’t know.

A re­search­er holds a cu­be of aer­o­gel, a light-as-air foam used to slow down and cap­ture dust par­t­i­cles from the Wild 2 com­et. (© Sci­ence)

“I think of it as the so­lar sys­tem par­tially turn­ing it­self in­side out,” said Don­ald Brown­lee of the Uni­ver­si­ty of Wash­ing­ton, lead au­thor of one of sev­en pa­pers de­s­c­rib­ing the find­ings in the Dec. 15 is­sue of the re­search jour­nal Sci­ence.

One the­or­ized pos­si­bil­i­ty is that the new­born sun blast­ed jets of mat­ter from its poles, said John Brad­ley of the Law­rence Liv­er­more Na­tion­al Lab­o­ra­to­ry in Liv­er­more, Ca­lif. This ma­t­er­i­al might then have rained far out on­to the emerg­ing pla­n­e­tary and co­m­et­ary sys­tem, cir­c­ling the star’s equa­tor.

“It ap­pears to have been a much more dy­nam­ic and per­haps even vi­o­lent” en­vi­ron­ment than ex­pected, added Brad­ley, head of the Li­ver­more team in­volved with the co­m­et dust mis­sion.

Cer­tain char­ac­ter­is­tics dis­tin­guish par­t­i­cles that come from near the Sun from those that in­hab­it dis­tant space, as­tro­no­mers said. The dis­tant par­t­i­cles tend to be glassy, while those close to stars are more crys­tal­line, mean­ing the atoms are ar­ranged in a more or­der­ly way.

This sec­ond type was abun­dant in the an­a­lyzed com­et dust, re­search­ers said. That, they added, sug­gests that as the So­lar Sys­tem formed 4.6 bil­lion years ago, ma­te­ri­al moved from the siz­zling cen­t­ral zone to its icy out­er reaches. 

The mix­ing would have made it­self felt as far as the Kui­per belt, a re­gion of icy bod­ies or­bit­ing the Sun past Nep­tune, and from which the com­et, called Wild 2, comes.

Sci­en­tists be­lieve com­ets con­tain pri­mor­di­al ma­te­ri­al left over from the So­lar Sys­tem’s birth. This is be­cause com­ets cir­cu­late most­ly in dis­tant, cold reaches of the sys­tem, where they stay rel­a­tively un­dis­turbed.

Wild 2 is thought to have in­hab­ited the out­er so­lar sys­tem un­til 1974, when a close en­coun­ter with Ju­pi­ter shifted its or­bit clos­er to Earth. NASA’s Star­dust space­craft left Earth in ear­ly 1999, met the com­et be­yond Mars’ or­bit five years lat­er and re­turned last Jan­u­ary with thou­sands of dust par­t­i­cles.

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Dust trailing a distant comet, and gathered by a NASA spacecraft, has yielded a surprisingly varied mixture of materials, astronomers say. Since comets are thought to contain material left over from the early Solar System, this variety suggests something was mixing up the contents of the system early on, the researchers add. What caused that, they don’t know. “I think of it as the solar system partially turning itself inside out,” said Donald Brownlee of the University of Washington, lead author of one of seven papers describing the findings in the Dec. 15 issue of the research journal Science. Certain characteristics distinguish particles that come from near the Sun from those that inhabit distant space, astronomers said. The distant particles tend to be glassy, while those close to stars are partially crystalline, meaning the atoms are arranged in an orderly fashion. The second type prevailed in the analyzed comet dust, researchers said. That, they added, suggests that as the Solar System formed 4.6 billion years ago, material moved from the sizzling central zone to its icy outer reaches. The mixing would have made itself felt as far as the Kuiper belt, a region of icy bodies orbiting the Sun beyond Neptune, and from which the comet, called Wild 2, came. Scientists believe comets contain primordial material left over from the Solar System’s birth because they circulate mostly in distant, cold reaches of the system, where they stay relatively undisturbed. The comet is thought to have inhabited the outer solar system until 1974, when a close encounter with Jupiter shifted its orbit closer to Earth. NASA’s Stardust spacecraft left Earth in early 1999, met the comet beyond Mars’ orbit five years later and returned last January with thousands of dust particles.