Comparison of the Oxidation State of Fe in Comet 81P/Wild 2 and Chondritic-Porous Interplanetary Dust Particles

TL;DR

This study compares the iron oxidation states in comet Wild 2 and interplanetary dust particles, revealing significant differences that suggest different formation environments and challenging the idea that CP-IDPs originate from Wild 2-like comets.

Contribution

It provides the first large-scale, direct comparison of Fe oxidation states in Wild 2 samples and CP-IDPs, using synchrotron X-ray microprobe analysis.

Findings

CP-IDPs are more oxidized than Wild 2 grains.

Wild 2 formed at a lower oxygen fugacity than CP-IDPs.

Comets similar to Wild 2 may not be the source of CP-IDPs.

Abstract

The fragile structure of chondritic-porous interplanetary dust particles (CP- IDPs) and their minimal parent-body alteration have led researchers to believe these particles originate in comets rather than asteroids where aqueous and thermal alteration have occurred. The solar elemental abundances and atmospheric entry speed of CP-IDPs also suggest a cometary origin. With the return of the Stardust samples from Jupiter-family comet 81P/Wild 2, this hypothesis can be tested. We have measured the Fe oxidation state of 15 CP-IDPs and 194 Stardust fragments using a synchrotron-based x-ray microprobe. We analyzed ~300 nanograms of Wild 2 material - three orders of magnitude more material than other analyses comparing Wild 2 and CP-IDPs. The Fe oxidation state of these two samples of material are >2{\sigma} different: the CP-IDPs are more oxidized than the Wild 2 grains. We conclude that comet Wild 2 contains material that formed at a lower oxygen fugacity than the parent body, or parent bodies, of CP-IDPs. If all Jupiter-family comets are similar, they do not appear to be consistent with the origin of CP-IDPs. However, comets that formed from a different mix of nebular material and are more oxidized than Wild 2 could be the source of CP-IDPs.