Main Belt Comets and other “Interlopers” in the Solar System

Round 1

Reviewer 1 Report

 

The paper presents a good review of exotic objects in the solar system, including active asteroids and some atypical comets. The paper is well written and comprehensive, although I would recommend some updating effort to some of the types of objects mentioned. Specifically, Table 1 in the paper has to be updated, see Table 1 of Jewitt and Hsieh (2022), “The Asteroid-Comet Continuum”, Comets III, edited by K. Meech and M. Combi, University of Arizona Press, in press. From that table, the complete list of suspected Main-Belt comets can be extracted. Also, the latest (to me) MBC analyzed (see Yoonyoung Kim et al. 2022, ApJL, in press), P/2020 O1 (Lemmon-PANSTARRS), could also be added. In addition, an appropriate reference to the paper corresponding to each individual object should be provided in the table and listed in the references.

Concerning the origin of the MBCs, while in most cases long-term integration of their orbits indicate a large stability and therefore a main-belt origin (e.g. Haghighipour, M&PS, 44, 1863, 2009),  a small fraction of them could potentially have an outer solar system origin, particularly in the JFC region (see Hsieh & Haghighipour Icarus, 277, 19, 2016). A recent example of this was tentatively provided by Moreno et al. (2021,  MNRAS, 506, 1733) on P/2021 A5 (PANSTARRS).

Finally,  when describing atypical comets, I would suggest to add to the list the object  249P/LINEAR, described by Fernandez et al. (Icarus, 295, 34, 2017) that could represent a near-Earth counterpart of the main-belt comets.

Author Response

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Reviewer 2 Report

General Comments:

The paper is a very good review presenting the different types of transition objects in the complex asteroid-comets continuum. 

A clear introduction gives the reader the opportunity to understand the context and the background information while the following sections describe in detail the different classes of objects with a specific focus to some special cases.

A subclassification is presented which however I found somehow confusion and believe could be easily improved (see the following)

Also a strong interpretation is presented on the presence of different types of crusts which does not consider a series of results recently published (see for example Ciarniello et al. 2022, Nature Astronomy and others) that should be at least discussed about the inconsistency of the presence of an insulating crust with the water activity mechanism for "classical comets".

Finally a discussion of the implications of the existence of these objects from dynamical point of view is presented and the need for future studies is expressed.

 

Major Points:

 

• The subclassification of MBCs (Table 2)

In the way it is written, this seems a little unclear and confusing to me…

The difference between group Ib and IIa is low vs high albedo?

What do you refer to saying "Objects, similar to a classical comet, with lacking cometary spectral feature... that the spectra is not known? in which sense that is "similar to classical comet", and in which sense the "Atypical comet" is atypical.. in the sense of the albedo only?

This is not completely clear to me

And moreover what characterizes the class Iib, the simple fact that the spectra is of S-type apart from the high-albedo?

If for most MBCs the spectra is not known how can you say it belongs to IIa or IIb?

You might reconsider this classification saying for example that class II is of comets with high albedo of which a subclass is II.S of comets that has S-type spectra, the others, you don't known the spectra so you cannot say they belong to a "different" subclass..

I think it would be good to have a table with various columns, "T_J", "orbital properties", "spectral properties", "albedo" etc..

And I would specify the various properties for each group. This would help identifying immediately the property that distinguish each subclass from the others.

 

• The implications for the different types of crusts:

There are clear observational evidences and recent modeling results that are in conflict with the original idea that comets necessarily have an insulating crust, and these needs to be at least discussed within the paper and in the assessment of the two types of crusts which is instead given as a matter of fact.

 

 

Minor Points:

 

Line 42: (comet nuclei no larger than 60 km) might need a reference

Line 45 (diameter of 100 km) need reference

Line 46 (likely diameter of 150 km) need a reference

Line 55 (comet and asteroid) comets and asteroids

Line 64 (the determination of the axial rotation period) it is not clear how this would help distinguising between asteroids and comets indeed

Line 80-81 (observations do not provide many reliable spectra useful for a taxonomic classification) too genetic. Are you talking about comets in general here? Or MBCs? There are plenty of observations of comets

Line 200-201 (17 active asteroids due to ice sublimation … are reported in the literature)

Are these the ones reported in Tab. 1? Otherwise which are they? and why they are not mentioned in the Table.. in the table there are exactly 17 objects in total, including Gibbs. Clarify

 

From line 226-to line 245. Here a direct comparison is presented between production rates of classical comets and MBCs, however an important parameter is not taken into proper consideration: the heliocentric distance. You cannot compare production rates usually measured for classical comets at 1 au (as an example) to upper limits for MBCs measured at their perihelion distances which are for example 2.2-2.5 AU. Rather production rates measured at similar heliocentric distances (even if not perihelion) would provide much stronger comparison and will show if there is effectively a systematic difference in the general level of activity.

 

Line 265 (since they can be spectrally similar to classical comets) apart from lacking emission lines. The spectra resembles more that of a cometary nucleus

 

Line 293: (schema) scheme

 

Line 433: (Ziolkowski [41]) a bit confusing the use of references, sometime the author names are used, other times they are not, please uniform the references style

 

Line 579 (this) these

 

Line 677-678 (although cometary nuclei and asteroid are two types of objects that are completely different in origin and composition) This phrase is quite contradicting some evidences presented in the paper itself which show that this might not be true in general

 

 

End: Other important future developments would be definitely performed thanks to JWST observations of spectral features of faint objects and by the LSST Rubin Telescope devoted to sky survey and expected to discover and potentially study in colors a large number of Solar System objects

 

Author Response

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Reviewer 3 Report

The review by Orofino is a clean and nice review on anomalous small bodies in the solar system.

Nevertheless, I have a few relevant comments that, in my opinion, call for some modifications before that the paper is published.

I would not qualify the objects addressed in this review as “extraneous”. This word suggests that they have been captured from outside of the solar system. I’m sure that the author is not suggesting this. But other authors have made a big fuss about extrasolar objects captured in the solar system. So the ambiguity should be avoided.

It is said multiple times that the members of the Taurid complex originated from the break-up of a larger progenitor. This statement, however is never supported by a strong reference or a strong argument. Instead, this review ignores the paper by Valsecchi et al., 1995 in Icarus, 118, 169, which argued the exact opposite. The large dispersion in inclinations of the Taurid complex is a counter-argument to the common break-up hypothesis, because the inclination is roughly preserved on long timescales. Valsecchi and co-authors found that there are two dynamical channels, one from the Jupiter family of comets, the other from the main belt, that can put objects in the Taurid complex region in q,Q space and they showed that the apparent cluster in perihelion longitude is just the outcome of an observational bias. In the spirit of reviewing the subject, this paper should not be ignored. Granted, it is an old paper. If there are strong new results that invalid the conclusions by Valsecchi et al., they should be reported as well, but I don’t know of any and I did not find a strong reference in the current review.

The paper insist that MBCs are anomalous because none or very few belong to Group 1a (Table 2), unlike what would be expected. However, I think that group 1a is very restrictive. For instance, the spectral type B is not considered, whereas several MBCs (those associated to the Themis family) are B-type. This type is also very primitive. For instance Ryugu, an asteroid of CI meteorite composition, is type Cb. Themis, which is of Type B, has been reported to have ice at its surface (Campins et al., 2010 in Nature). Also, in group 1a the albedo is required to be <0.07. It is true that this low albedo is typical of comets, but many primitive asteroids have albedo <0.15. How many MBCs would fall in Group 1a if the B-type were considered as well as albedos up to 0.15? I think the majority would. Maybe the simplest explanation for MBCs is that they are asteroids which preserved some ice in their sub-surface,  unlike the carbonaceous meteorite parent bodies where the ice melted giving origin to the wide-spread water alteration.

The genetic relationship of many MBCs with asteroid Themis and its family should also be discussed. I’m surprised that Themis does not enter in Table 1 together with Ceres.

I would not say that evidence for the degassing activity of Ceres is compelling. It is true that observations of degassing have been claimed with Herschel observations, but the Dawn mission did not report any degassing, and the mission had an extended lifetime precisely to cover the passage of Ceres through perihelion. Many suspect now that the Herschel observations are not correct.

The section of Ryugu (2.2) should be updated. It is clear now from isotopic analyses that Ryugu is an asteroid of CI meteorite composition. It is identical to CI meteorites in any isotope ratio that has been investigated.

The paper quotes Snodgrass et al. to claim that it is unlikely that C-type asteroids come from beyond Jupiter. But, being a fair review, it should also quote Kruijer et al., 2017 in PNAS, saying the exact opposite. The isotopic argument of Kruijer et al is compelling and today I would say that it is widely accepted in the meteoritic community that all parent bodies of carbonaceous chondrites (i.e. C-type asteroids) formed beyond Jupiter, possibly even Saturn.

A list of the 30 active Taurid complex members is missing. It should accompany Table 1. It would be nice to show which of them are S-type. The activity of so many S-types is indeed surprising. I discovered their existence only at the end of the paper (line 700).   I only knew Oljato.

There is no upper limit to the size of comets. The fact that we have seen only moderately large objects in historic times is just a size distribution effect. Big objects are rare. But Jupiter family comets come from the scattered disk, where big objects (e.g. Eris, as big as Pluto) are found. These objects have a non-zero probability to become comets one day. Also, Centaurs are intermediate stage objects, on the path for dynamical transfer from the scattered disk to the Jupiter family group. Some are really big, e.g. Chiron.

A few minor comments. At line 20 in the abstract it is unclear whether “This” refers only to the last category of objects (inclined asteroids) or to all those mentioned in the previous phrase. I would say the latter, but in this case the plural would be more appropriate.

At line 125 “it” should be deleted.  

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