  UltraCarbonaceous Antarctic MicroMeteorites (UCAMMs) represent a small
fraction of interplanetary dust particles reaching the Earth's surface and
contain large amounts of an organic component not found elsewhere. They are
most probably sampling a contribution from the outer regions of the solar
system to the local interplanetary dust particle flux. We characterize UCAMMs
composition focusing on the organic matter, and compare the results to the
insoluble organic matter (IOM) from primitive meteorites, IDPs, and the
Earth.We acquired synchrotron infrared microspectroscopy and micro-Raman
spectra of eight UCAMMs from the Concordia/CSNSM collection, as well as N/C
atomic ratios determined with an electron microprobe. The spectra are dominated
by an organic component with a low aliphatic CH versus aromatic C=C ratio, and
a higher nitrogen fraction and lower oxygen fraction compared to carbonaceous
chondrites and IDPs. The UCAMMs carbonyl absorption band is in agreement with a
ketone or aldehyde functional group. Some of the IR and Raman spectra show a
C$\equiv$N band corresponding to a nitrile. The absorption band profile from
1400 to 1100 cm-1 is compatible with the presence of C-N bondings in the
carbonaceous network, and is spectrally different from that reported in
meteorite IOM. We confirm that the silicate-to-carbon content in UCAMMs is well
below that reported in IDPs and meteorites. Together with the high nitrogen
abundance relative to carbon building the organic matter matrix, the most
likely scenario for the formation of UCAMMs occurs via physicochemical
mechanisms taking place in a cold nitrogen rich environment, like the surface
of icy parent bodies in the outer solar system. The composition of UCAMMs
provides an additional hint of the presence of a heliocentric positive gradient
in the C/Si and N/C abundance ratios in the solar system protoplanetary disc
evolution.
