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The ESA mission Comet Interceptor will target an Oort Cloud or interstellar comet during its first approach to the heliocentric distance r h ≈ 1 au, and has been designed to handle the dust loss rate of 1P/Halley as an upper limit of activity level. In the framework of the adopted model of pebble-made nuclei, any extrapolation at r h ≈ 1 au of the dust loss rate observed beyond Jupiter is impossible, because the dust loss rate at r h > 3.8 au depends on the nucleus surface area, A p , composed of water-poor pebbles, whereas at r h < 3.8 au the dust loss rate depends on the nucleus surface area, A r , composed of water-rich pebbles. Pebble-made nuclei are characterized by all possible values A p / A r ⩾ 1 2 . The water-driven activity model of pebble-made nuclei has been applied to comets 1P/Halley and C/1995O1 Hale-Bopp. We find that all comets with a nucleus radius smaller than 3.2 - 0.9 + 2.3 km will be characterized by a dust loss rate lower than 1P/Halley at r h ≈ 1 au, i.e. will be safe for Comet Interceptor, whichever their ratio A p / A r ⩾ 1 2 will be. We also find that the lower limit of the average refractory-to-water–ice mass ratio of all comets is 3.4. [ABSTRACT FROM AUTHOR]

Recent data of molecular clouds and protoplanetary disks constrain the composition and structure of the disk and planetesimals. Laboratory experiments suggest that dust accretion in disks stops at pebble sizes. Sublimation and recondensation of water ice at the disk water-snow line suggest that pebbles split into water-rich and water-poor ones. The same conclusion has been recently reached by models of cometary activity consistent with the structure of porous Interplanetary Dust Particles (IDPs) and of porous dust collected by the Stardust and Rosetta missions. The observation of crystalline water ice in protoplanetary disks by the Herschel satellite, the erosion of comets, and the seasonal evolution of the nucleus color require that the two pebble families have a water-ice mass fraction close to 33% and 2%, respectively. Here, we show that the diversity of comets is thus due to random mixtures with different area fractions A p and A r of water-poor and water-rich pebbles, predicting most of the data observed in comets: why the deuterium-to-hydrogen ratio in cometary water correlates to the ratio A p / A r , which pebbles dominate the activity of Dynamically New Comets (DNCs), what is the origin of cometary outbursts, why comets cannot be collisional products, and why the brightness evolution of DNCs during their first approach to the Sun is actually unpredictable. [ABSTRACT FROM AUTHOR]

The ESA mission Comet Interceptor will target an Oort or interstellar comet during its first approach to the Sun. Meanwhile, the Vera Rubin LSST Survey will observe hundreds of active comets per month beyond 4 au from the Sun, where water vapour pressure is expected to be too low to eject dust. We discuss observations of dust tails at heliocentric distances larger than 4 au in order to retrieve the physical parameters driving cometary activity beyond Jupiter by means of a probabilistic tail model, which is consistent with the activity model defining the gas coma parameters due to the sublimation of carbon monoxide, molecular oxygen, methane, ethane, and carbon dioxide since the activity onset at 85 au from the Sun. We find that: (i) All the observed dust tails are consistent with the adopted activity model; (ii) The tail fits depend on three free parameters only, all correlated to the nucleus size; (iii) Tail fits are always improved by anisotropic dust ejection, suggesting activity of Oort nuclei dominated by seasons; (iv) Inbound seasons suggest cometary activity before the ejection of protocomets into the Oort cloud, as predicted by the activity model; (v) Oort nuclei larger than 1 km may be characterized by a fallout up to ≈100 m thick deposited during ≈60 yr inbound; (vi) On the other side, Oort nuclei smaller than 1 km may appear more pristine than Jupiter Family Comets when observed at 1 au from the Sun. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

We develop an activity model based on ice sublimation and gas diffusion inside cm-sized pebbles making-up a cometary nucleus. Our model explains cometary activity assuming no free parameters and fixing the nucleus surface temperature T s, its gradient below the nucleus surface at thermal equilibrium, the pressure inside the porous pebbles, and the gas flux from them. We find that (i) the nucleus erosion rate and water vapour flux are independent of the nucleus refractory-to-ice ratio, which affects the dehydration rate only; (ii) water-driven dust ejection occurs in thermal quasi-equilibrium at T s > 205 K; (iii) the smallest and largest ejected dust sizes depend on the nucleus surface temperature and its gradient at depths of few cm; and (iv) the water-driven nucleus erosion rate is independent of the water vapour flux. Regarding comet 67P/Churyumov–Gerasimenko, we find that (i) during the northern and southern polar summers, the nucleus active areas are ≈5 km2; (ii) >95 per cent of the southern pristine nucleus has a refractory-to-water-ice mass ratio >5; and (iii) the different temperature dependences of the dehydration and erosion rates explain the seasonal cycle: at perihelion, dm-sized chunks ejected by the sublimation of CO2 ices are rapidly enveloped by an insulating crust, preserving most water ice up to their fallout on the northern dust deposits; the inbound water-driven activity at low temperatures triggers a complete erosion of the fallout if its water-ice mass fraction is >0.1 per cent. [ABSTRACT FROM AUTHOR]

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 10 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. [ABSTRACT FROM AUTHOR]