FTIR analysis of the organics in IDPs: Comparison with the IR spectra of the diffuse interstellar medium

We have analyzed seven interplanetary dust particles (IDPs) using an infrared microscope with the aim of comparing the 3.4 mu m feature to the same feature observed in the diffuse interstellar medium (DISM) and of searching for additional absorption features due to organic matter. We have developed an in situ extraction method based on the etching of the silicate phases with hydrofluoric acid, which allowed a better detection of the organic fraction. Six out of the seven particles studied present a well resolved 3.4 mu m feature corresponding to aliphatic chains. Some of these particles also present a carbonyl group around 1700 cm(-1) due to ketone and, probably also, carboxylic acid compounds. Here we report the first determination of the column densities of the aliphatic and aromatic fractions in IDPs, as well as other functional groups. With the asymmetric CH3 and CH2 column densities we calculated the CH2/CH3 ratios of the IDPs. The average ratio (CH2/CH3 = 3.7 +/- 0.9) is clearly larger than the one measured for the DISM (2.2), indicating that the aliphatic chains in IDPs are longer (or less ramified) than in the DISM. The 3.4 mu m feature of IDPs is significantly narrower than in the DISM, which suggests that the aliphatic fraction in IDPs is made of relatively simpler and less ramified compounds. We conclude that the carbonaceous compounds found in these IDPs have been formed or evolved in a different environment from the DISM.