A hot start for some comets?

Some comets may be made up of material ejected from the sun or other stars during the early days of their formation, in contrast to the icy outer solar system origin previously envisioned, scientists announced on March 13. Particles of the comet Wild 2, returned to Earth last January by NASA's Stardust spacecraft, contain the iron-magnesium silicate mineral olivine-in particular, a magnesium-rich form known as forsterite (shown in a sample from Stardust). Olivine is common in the solar system, but it is believed to be formed at high temperatures near stars. The samples also contain other minerals rich in calcium, aluminum, and titanium that were likely formed at high temperatures. More than 150 researchers worldwide are now studying the tiny comet grains that were captured in aerogel collectors aboard Stardust. Scientists hope to determine how much of the comet material came from outside the solar system and how much came from our own solar nebula, said Donald Brownlee, principal investigator for the $200 million Stardust mission.