Review Reports

Round 1

Reviewer 1 Report

In the conclusions it will be nice to add a short paragraph on the advantages/disadvantages of the pebbles model in explaining how the comets formed with respect to the other current ideas.

Author Response

We added a paragraph at the end of Sect. 6, which reads "Advantages of the pebble-collapse model of comet formation are that it is entirely based on a large body of empirical evidence from the laboratory as well as on established numerical simulations of hydrodynamic processes. The pebble-collapse model explains a number of observations of comets, particularly of comet 67P, such as the low bulk density, the ultra-low mechanical strengths, the low thermal conductivity, the presence of ultra-low-density dust particles and the internal homogeneity. However, there are observations of comet 67P that the model cannot explain. Among these are the observed layering, the presence and variety of macroscopic ($\gtrsim 1 \, \mathrm{m}$) geologic features, and the measured increase of permittivity in a shallow ($\lesssim 25 \, \mathrm{m}$) sub-surface region, for which alternative formation models may have explanations, unless they are evolutionary processes. These models are discussed in Weissman et al. 2022.".

Reviewer 2 Report

This is a well written and assertive overview of issues related to the formation and nature of comets.  However, this is a subject where we possess few constraining observations but lots of ideas from computer modelers.  The paper is overly focused on the author's favorite, and currently popular, strand of models about the streaming instability and "pebbles".  Unfortunately, we have essentially no direct evidence for the existence of pebbles. To their credit, the authors do note this fact, but this mention is buried on page 14 and easy to miss.   

One suggestion is for the authors to list, in a separate section, specific predictions (i.e. of things that have not already been measured)  that are expected if the pebble idea is correct.   That way, a few decades from now, we might be able to decide how much of this subject matter is meaningful.

Still, I do accept that this is an interesting and coherent description of the author's opinions, and would be happy to see it published without change.  (I also nominate the authors for some sort of prize, for the maximum number of times the word "pebbles" has ever appeared in a single manuscript!!).

Author Response

To emphasize that the direct evidence for the existence of pebbles is weak, we modified the last paragraph of Sect. 1 into "It must be emphasised that we only provide one plausible, but not the only possible scenario of the formation of planetesimals and evolution into comets. This model heavily relies on the existence of pebbles, for which only indirect evidence exists [3]. Alternative planetesimal-formation models exist and are discussed in detail by Weissman et al. [4].".

In Section 6, we added the following paragraph to specify our predictions for future missions and observations:
"Future missions to and astronomical observations of comets may be able to test the following most crucial predictions of the pebble-cloud-collapse model of planetesimals and subsequent evolutionary processes into comets: 
[i] A strong positive correlation between the surface temperature at sunrise and the insolation at local noon for a subsurface made of pebbles, in contrast to no such dependency for a makeup without large void spaces [149], measurable by thermal IR mapping. Such measurements would deliver, from remote observations only, invaluable information about the presence and size of pebbles in a shallow sub-surface layer. 
[ii] Proof of the absence or presence of internal volatile differentiation of comets, as predicted by Malamud et al. [150] for comets consisting of pebbles, measurable by long-wavelength radar. Such measurements would deliver information about the formation time and/or abundance of radiogenic nuclei and about the thermal conductivity in the deep interior of the comet nucleus and, thus, would confirm or disprove the gravitational-collapse theory. 
[iii] Search for traces of the collisional evolution of cometary nuclei, measurable by radar through the amplitude and length scale of internal inhomogeneities. Due to the obvious distinction of the internal makeup of bodies of type A1, B1, A2, B2 and A/B3 (see Fig. 1), deep-penetrating radar measurements covering the entire body would provide information about the internal distribution of water ice, the refractory component and the void spaces inside the body. 
[iv] Determination of the physical properties and orbital parameters of extinct comets. Due to a memory effect [see 134] or differentiation [see 150], the depth to which dust activity is possible strongly depends on the original size of the comet-precursor body so that larger comets should be getting extinct on a different timescale than smaller ones [see 228]. 
[v] Determine whether positive relief features [106] are local remnants of impacts, measurable by high-frequency radar through local permittivity enhancement. If moderate-velocity impacts on the surface of a planetesimal lead to local compaction of the material and, thus, to a loss of the pebble structure, further dust activity is impossible, due to a considerable enhancement of the tensile strength. Such measurements would deliver information about the collision history of the planetesimals and the abundance of small-scale (metre- to decimetre-sized) objects in the region in which the planetesimals resided throughout most of their lifetime before becoming comets."

Reviewer 3 Report

This paper investigates the formation of comets, including from dust to pebbles, collapse of pebble clouds into planetesimals,  evolution of planetesimals towards comets, and discussed which planetesimal can become a comet. I enjoy reading this paper. However, the influences of  the comet's gravitational field and its structure to the formation of comets is also important.

Such as the the euqilibria of 1P/Halley, 9P/Tempel 1, and 103P/Hartley 2,  the rotation speeds of Halley and Tempel 1 are lower than other minor celestial bodies, thys the distance between the mass center and the equilibria are large.

 

see  <Analysis of the potential field and equilibrium points of irregular-shaped minor celestial bodies>

 

In addition, the comparison of different forces on the dust grain also affect the  formation of comets.

see  <Hamiltonian Formulation and Perturbations for Dust Motion Around Cometary Nuclei>

 

The author should consider the irregular shapes of the comets to the equilibria, the comparison of different forces.

Author Response

We added a sentence at the end of first paragraph of Sect. 5.1.4: "It should be mentioned that the binarity of low-density objects has also important consequences for the escape of dust particles when dust activity is present \citep[see, e.g.,][]{Wang.2014,Jiang.2017."

Round 2

Reviewer 3 Report

The paper can be accept in its present form.