Abstract
We present 2-5 $\mu$m spectroscopic observations of the dust coma of 67P/Churyumov-Gerasimenko obtained with the VIRTIS-H instrument onboard Rosetta during two outbursts that occurred on 2015, 13 September 13.6 h UT and 14 September 18.8 h UT at 1.3 AU from the Sun. Scattering and thermal properties measured before the outburst are in the mean of values measured for moderately active comets. The colour temperature excess (or superheat factor) can be attributed to submicrometre-sized particles composed of absorbing material or to porous fractal-like aggregates such as those collected by the Rosetta in situ dust instruments. The power law index of the dust size distribution is in the range 2-3. The sudden increase of infrared emission associated to the outbursts is correlated with a large increase of the colour temperature (from 300 K to up to 630 K) and a change of the dust colour at 2-2.5 $\mu$m from red to blue colours, revealing the presence of very small grains ($\leq$ 100 nm) in the outburst material. In addition, the measured large bolometric albedos ($\sim$ 0.7) indicate bright grains in the ejecta, which could either be silicatic grains, implying the thermal degradation of the carbonaceous material, or icy grains. The 3-$\mu$m absorption band from water ice is not detected in the spectra acquired during the outbursts, whereas signatures of organic compounds near 3.4 $\mu$m are observed in emission. The H$_2$O 2.7-$\mu$m and CO$_2$ 4.3-$\mu$m vibrational bands do not show any enhancement during the outbursts.
Highlights
The Rosetta mission of the European Space Agency accompanied comet 67P/Churyumov–Gerasimenko between 2014 and 2016 as it approached perihelion (2015 August 13) and receded from the Sun
This paper presents the analysis of Visible InfraRed Thermal Imaging Spectrometer (VIRTIS)-H infrared (2–5 μm) spectra acquired during two dusty outbursts that occurred on 2015 September 13 and 14, i.e. about one month after perihelion
We have presented an analysis of the 2–5 μm dust continuum radiation from comet 67P measured at 1.3 au from the Sun with the VIRTIS-H instrument
Summary
The Rosetta mission of the European Space Agency accompanied comet 67P/Churyumov–Gerasimenko between 2014 and 2016 as it approached perihelion (2015 August 13) and receded from the Sun. Grun et al (2016) review observations by nine Rosetta instruments of a dusty outburst which occurred on 2016 February 19, when the comet was at 2.4 au from the Sun. Gaseous outbursts with no dust counterparts were observed (Feldman et al 2016). This paper presents the analysis of VIRTIS-H infrared (2–5 μm) spectra acquired during two dusty outbursts that occurred on 2015 September 13 and 14, i.e. about one month after perihelion. The complementary spectro-imaging observations of these outbursts undertaken with VIRTIS-M in the visible spectral range are presented by Rinaldi et al (in preparation). The VIRTIS-H spectra cover a range where both the dust-scattered sunlight and the dust thermal emission can be studied, to derive constraints on the properties of the dust particles present in the ejecta cloud.
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