Water and deuterium-to-hydrogen ratio in comets

Abstract

ABSTRACT Recent models find that nuclei of comets are a mixture of water-rich and water-poor cm-sized pebbles. We aim to fit the correlation of the deuterium-to-hydrogen ratio, D/H, with the nucleus active area fraction. Pebble parameters, constrained by the Rosetta mission data, depend on where pebbles accreted in the protoplanetary disc. The diversity of comets depends on the fraction of water-rich versus water-poor pebbles in each nucleus. Our model explains why the D/H values correlate to the nucleus active area fraction, and also implies that: (i) the seasonal colour cycle of the nucleus is opposite to the dust coma one; (ii) the perihelion dust size distribution is steeper than the average; (iii) water–ice content is anticorrelated with the amount of supervolatiles and with the D/H ratio. (iv) the fallout is composed of supervolatiles-depleted water-poor pebbles, whose D/H ratio was measured by the Rosetta mission; (v) the D/H average in nuclei may be different than that measured in comae and cannot be retrieved by local sample-return missions; (vi) mixing of pebbles in the outer protoplanetary disc excludes any correlation of the water content, of the D/H ratio and of the nucleus size with the disc location where comets were born.