Comet 67P outbursts and quiescent coma at 1.3 au from the Sun: dust properties from Rosetta/VIRTIS-H observations

Abstract

We present 2-5 $\\mu$m spectroscopic observations of the dust coma of\n67P/Churyumov-Gerasimenko obtained with the VIRTIS-H instrument onboard Rosetta\nduring two outbursts that occurred on 2015, 13 September 13.6 h UT and 14\nSeptember 18.8 h UT at 1.3 AU from the Sun. Scattering and thermal properties\nmeasured before the outburst are in the mean of values measured for moderately\nactive comets. The colour temperature excess (or superheat factor) can be\nattributed to submicrometre-sized particles composed of absorbing material or\nto porous fractal-like aggregates such as those collected by the Rosetta in\nsitu dust instruments. The power law index of the dust size distribution is in\nthe range 2-3. The sudden increase of infrared emission associated to the\noutbursts is correlated with a large increase of the colour temperature (from\n300 K to up to 630 K) and a change of the dust colour at 2-2.5 $\\mu$m from red\nto blue colours, revealing the presence of very small grains ($\\leq$ 100 nm) in\nthe outburst material. In addition, the measured large bolometric albedos\n($\\sim$ 0.7) indicate bright grains in the ejecta, which could either be\nsilicatic grains, implying the thermal degradation of the carbonaceous\nmaterial, or icy grains. The 3-$\\mu$m absorption band from water ice is not\ndetected in the spectra acquired during the outbursts, whereas signatures of\norganic compounds near 3.4 $\\mu$m are observed in emission. The H$_2$O\n2.7-$\\mu$m and CO$_2$ 4.3-$\\mu$m vibrational bands do not show any enhancement\nduring the outbursts.\n