The Refractory-to-Ice Mass Ratio in Comets

Fulle, Marco; Blum, J.; Green, S. F.; Gundlach, B.; Herique, A.; Moreno, F.; Mottola, S.; Rotundi, A. and Snodgrass, C. (2019). The Refractory-to-Ice Mass Ratio in Comets. Monthly Notices of the Royal Astronomical Society, 482(3) pp. 3326–3340.



We review the complex relationship between the dust-to-gas mass ratio usually estimated in the material lost by comets, and the Refractory-to-Ice mass ratio inside the nucleus, which constrains the origin of comets. Such a relationship is dominated by the mass transfer from the perihelion erosion to fallout over most of the nucleus surface. This makes the Refractory-to-Ice mass ratio inside the nucleus up to ten times larger than the dust-to-gas mass ratio in the lost material, because the lost material is missing most of the refractories which were inside the pristine nucleus before the erosion. We review the Refractory-to-Ice mass ratios available for the comet nuclei visited by space missions, and for the Kuiper Belt Objects with well defined bulk density, finding the 1-σ lower limit of 3. Therefore, comets and KBOs may have less water than CI-chondrites, as predicted by models of comet formation by the gravitational collapse of cm-sized pebbles driven by streaming instabilities in the protoplanetary disc.

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