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tokens: that the outburst ejecta are comprising small and bright particles. The decrease of the colour temperature and colour shortly after the peak brightness suggests an increase of the mean particle size with time for the particles crossing VIRTIS FOV. The presence of silicatic grains devoid of organic material could be explained by the high grain temperatures causing sublimation of the organics. Quantitatively, the September 13 outburst data are consistent with the silicatic grain hypothesis. However, large colour temperatures (>600 K) are measured at the peak brightness of the September 14 outburst, whereas computed temperatures are 530 K for olivine grains. Several explanations are possible. First, we did not consider grains smaller than 100 nm in radius, whereas we cannot exclude their presence. These grains might reach higher temperatures than 100 nm grains, though this is not expected from model calculations (Lasue et al. 2007 ). In our model, 100 nm grains are compact ( P = 0). Considering instead that these grains are aggregates, their temperature would be higher than the temperature of the equivalent sphere (Xing Hanner 1997 ). Another possibility is that the spectral indices of amorphous olivine differ significantly from those of the refractory material of cometary grains. We performed Mie calculations with inclusions of FeS within an olivine matrix. For a volume fraction of 6 per cent of FeS consistent with the suggestion of Fulle et al. ( 2016a ) for 67P grains, the colour temperature reaches 575 K (Fig. 12 ), which is not far from the extremum values measured during the September 14 outburst. Could fractal agglomerates explain the outburst spectra? For carbon/amorphous olivine mixtures, the colour and colour temperature of their infrared spectrum match the measurement, but this is not the case for the bolometric albedo (Fig. 10 ). Calculations made for units of amorphous olivine provide a bolometric albedo 0.4, i.e. a factor of 2 below the measured values. During 67P outbursts, MIDAS collected a high amount of micrometric grains (Bentley, personal communication), in agreement with an ejecta population dominated by compact grains rather than aggregates. We performed calculations setting the maximum size of the fractal aggregates to a max = 1 μm, and the bolometric albedo is still 0.4. Finally, the main contributors to the scattered and thermal parts of the spectrum could be distinct populations of grains. Cold and bright grains would essentially contribute to scattered light, whereas this is the opposite for warm and dark grains made of strongly absorbing material. These two populations of grains were found in the outburst ejecta of the extraordinary outburst of comet 17P/Holmes in 2007 October ( r h = 2.45 au) (Yang et al. 2009 ). The near-IR spectra of the outburst ejectas showed strong absorption bands at 2 and 3 μm related to water ice (Yang et al. 2009 ). Infrared signatures of ice are not detected in the VIRTIS-H outburst