Surprises in comet dust
Dec 15,2006 00:00 by World Science

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

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.