Characterization of carbon- and nitrogen-rich particle fragments captured from comet 81P/Wild 2

Abstract

We studied three Stardust fragments with infrared spectroscopy to characterize organic matter; with synchrotron-induced X-ray fluorescence to determine Fe contents and certain elemental ratios to iron; with scanning electron microscopy (SEM) to image sample morphology and to detect semiquantitatively Mg, Al, Si, Ca, and Fe; and with nuclear reaction analysis (NRA) to measure C, N, O, and Si. A fourth fragment was analyzed by SEM only. Fragment C2054,0,35,21 from track 35 (hereafter C21) is extremely rich in C and contains appreciable concentrations of Mg, Al, and Ca, but little Fe. Fragments C2054,0,35,23 (C23), C2044,0,41 (C41), and C2054,0,35,51,0 (C51), from tracks 35, 41, and 35, respectively, consist largely but not exclusively of aerogel. C23 contains Mg and finely dispersed S, but little Al, Ca or Fe. Pooled CI-normalized elemental ratios for C21, C23, and C41 are as follows: Ti/Fe, 5.0; Cr/Fe, 0.84; Mn/Fe, 0.97; Ni/Fe, 2.4; and Zn/Fe, 31. The enrichments in Ti and Zn may be related to the presence of aerogel. Minimum weight percentages of C and N estimated without correcting for the presence of aerogel are 30 and 0.7 for C21; 2.8 and 0.2 for C23; 1.2 and 0.14 for C41. After corrections for the presence of aerogel containing 1.4 wt% C and 0.02 wt% N, the corresponding results are 37 and 0.85 for C21; and 10 and 1 for C23; and ~1 and ~1, for C41 (The results for C41 have large uncertainties). These weight percentages are larger than or comparable to values for carbonaceous meteorites. C/N atomic ratios without/without aerogel corrections are 51/51 for C21, 17/11 for C23, and 10/~1 for C41. Within the uncertainties these values are within the range for carbonaceous meteorites.