Peer review of: Collisional formation of top-shaped asteroids and implications for the origins of Ryugu and Bennu (2020)

 
Reviewers' comments: 

Reviewer #1 (Remarks to the Author): 

Dear Editor, 

I have read with interest the paper “Formation of top-shaped asteroids by collisional disruption: 
implications for the origin of Ryugu and Bennu” by Michel et al. 

In this paper the authors investigate the question of the origin of the top-shape of the Benu 
and Ryugu asteroids, that are both currently visited by sample-return space mission. 
Constraining their origin is very important because it will allow to better understand their 
evolution and how they acquire their today composition, that will be directly measured in 
laboratory upon come-back to Earth of the different samples, in the next years. As stated in 
the paper, one of the most puzzling characteristics of these two objects is that they show a 
proeminent equatorial ridge, that is heavily cratered, indicating that they acquire their ridges 
very rapidly after their formation. So the question is how is it possible ? and also what is the 
cause of the strong measured difference in their hydration level ? 

In order to solve these mysteries, the author show, using N-body contact-spheres models 
(the PKD grav code) study the gravitational re accumulation of fragments after the 
catastrophic disruption of a parent asteroid. They show that a substantial fraction of the 
remnants may have directly reaccumulated with an oblate shape. That is the main result of 
the paper. Then the rapid action of the YORP effect, may have given to them their top-
shape. Then the author try to estimate the maximum heating temperature during the 
reaccumulation process. They show that depending on the relative original location of the 
reaccumulated material with respect to of the impact point, a wide range of peak 
temperature may be achieved, and thus, which may explain the variety of dehydration level 
observed. They also discuss compaction effect. They conclude that Benu and Rygu may 
both have formed in a catastrophic disruption event, enethough they may have experienced 
different thermal histories. 

I think this is a very interesting and timely paper in the current context of space exploration,  because it gives a new perspective to understand the origin of these top-shaped objects, and it was a major surprise, and it was not expected, that both Ryugu and Benu have the same shape. This paper explains how a same, and very common mechanism, may naturally result in the observed shape. In addition, it tries to do an interesting link between evolutionary processes (disruption) and surface composition. 

I have only minor comments on the paper, mainly clarifications in particular concerning 
compaction and temperature calculation. 

Best Regards, 


Specific comments 

Title vs. abstract : in the title you talk about “top shape”, but in the abstract you talk about “oblate” 
object. Please clarify the link between the two in the abstract. 

Lines 82 to 91 : What is an “individual component” ? a particle ? clarify 

In this paragraph we would like to have more info about the particle size, how it compares to 
the size of the final reaccumulates (or how many particle they contain). What motivates the 
choice of the 18° friction angle. A discussion on the effect of this repose angle would be 
good, as it is not clear how the results depend on this choice. 

Lines 99 to 102: “It should not tae much time” : this is pretty vague. Can you clarify what enters in 
the calculation of the YORP effect timescales? What makes this timescale so short ? 

Line 102 : Authors are talking about “oblate” shape, then “top-shape” . What makes the distinction? 
It gives the feeling that both concepts are the same, but clear distinction is made elsewhere 
in the paper. I think that the authors should clearly define what “oblate “ and “top shape” 
mean in the paper, even though, I guess the frontier between the two may be fuzzy at some 
point 

Figure 2 : Very difficult to read. Labels and numbers are almost unreadable. 

Line 165 : Oblate is defined as c/a>0.75. But if c/a=1 it is spherical? and if c/a>1 it is 
prolate? Can you clarify? And in particular I think this is important when F is computed: 
spherical and prolate objects should not be considered when computing the fraction of 
oblate objects. 

Compaction C ?: I have to admit that I did not understood at all how compaction C is 
calculated. The paper says : “degree of compaction at the center (Ccenter), where the center is 
defined as the fraction of particles within a radius of 5 km from the center of mass of the parent 
body”, but I do not understand it. 

Legend of Figure 3 : It is hard to link the text, explaining the reaccumualtion process, to the 
images. In particular, if a disk is seen, could you clear show the disk in the image witn an 
arrow (for example 
?) or colored particle s? 

Temperature calculation : I do not understand how temperature change is calculated, in 
PKDgrav, that has no equation of state. I guess maybe this calculation comes from the 
impact simulation, but if I remember well, Tillotson EOS does not really gives the 
temperature. Can you clarify this point? 

In particular when computing temperature, I guess this is equilibrium temperature, however 
during the shock, the thermodynamics is not at equilibrium. So what is the meaning of the 
computed temperature? Can you discuss that? 

END 

Reviewer #2 (Remarks to the Author): 

The submitted manuscript investigates the origin of ‘top-shape’ near Earth asteroids coupled to 
difference in hydration levels as found in recently visited NEAs Ryugu (JAXA) and Bennu 
(NASA). This work is very ambitious as it tries to solve two non-trivial problems of current 
asteroid science with one single process: post-impact re-accumulation of a main belt parent 
body. Moreover, the possibility that two such bodies come from one single disruption is 
introduced. This study deserves attention indeed and may pioneer future investigation on 
this topic. 

Nevertheless, this reviewer finds that many parts of the manuscript are somewhat 
speculative, and concerns about the method and conclusions not supported by enough 
compelling arguments, as detailed below. The scenario described by the authors is one of 
the possible explanations to this problem, but they do not provide convincing evidence to 
prefer their solution over alternatives. 

Technically sound data and strong evidence for conclusions are main criteria for publication 
in Nature Communications. Unfortunately, trouble with part of utilized methods and no 
strong evidence for conclusions prevents this reviewer from recommending the publication 
of this research in Nature Communications. 

However, putting forward challenging ideas and theories is a must in science progress and 
it is good to have a chance to make other researchers pay attention to new visions. For this 
reason, I am sure that at least part of the submitted research may find suitable publication 
in other specialized journals. 

Instead, this reviewer finds that the study concerning material heating during shattering 
phase is very interesting. Were the ansatz dropped of having twin generation of the two 
rubble-piles, the study looks indeed very promising to help and understand different levels 
of hydration in observed NEAs. I encourage the authors to follow that innovative path 
which is potentially capable to shed light to interpretation of new observations. 


Major worries about the submitted manuscript are pointed out here. 

1) Concerns on twin origin of Ryugu and Bennu. 

A number of lucky circumstances need to combine: 

a) Two bodies, with similar shape and different size, are produced in the main belt after the 
catastrophic collision of a single common parent body, based on SPH+N-body numerical 
simulations. 

b) Both bodies end up as NEAs after reaching suitable MMR through Yarkovsky drift and 
surviving potential collisional disruption in the main belt. 

c) Both bodies happen to have orbits and Delta(V) such that two space agencies target 
them for selected space missions. 

Did the authors try to estimate the probability of such a chain of events to occur? (a) and 
(b) could be softened in the case that tens or hundreds of similar rubble-piles are created 
in the proposed single collision, however the authors do not mention how many similar 
bodies are typically created in their numerical simulations. Though (c) may still be a 
challenge to work around. 

- The authors state (‘Results’): 

“In addition, Bennu and Ryugu have been linked to asteroid families in the inner main belt that 
have dynamical ages of 1 Ga [12]. If the ridges on those bodies formed at approximately the 
same time, 
they would be subject to a billion years of impacts from main belt projectiles.” 

On one hand, NEAs dynamical time in the main belt before they reach strong resonaces 
through Yarkovsky drift is of the order of tens to ~100 Ma, as shown by CRE analysis in late ’90. 
Only iron meteorites show CRE ages close to 1 Ga. On the other hand, were Ryugu and 
Bennu created 1 Ga ago, they would both hardly survive on their way to a resonance, given 
that collisional lifetimes in the main belt are less than 500 and 350 Ma, respectively for their 
sizes (estimating collision lifetimes from Farinella and Vokrouhlicky, 1999). 

- On the utilized methods: 

The authors state: 

“We performed a series of simulations of disruptions of 100-km-diameter asteroids with 
microporosity, as expected for parent bodies of dark (geometric albedo < 0.1) asteroid 
families [17],…” 

Reference [17] suggests microporosity might be present in parent bodies of CC meteorites 
and they measure it for such meteorites to be up to some 20%. However, Reference [19] 
used material properties of porous targets that provided the best match to impact 
experiments on pumice targets, that typically have porosity larger than 65% (70% in the 
material chosen by [19]). Crush-up curves of CC meteorites are not necessarily represented 
by that of pumice, thus flawing the method used. 
Current SPH simulations may be good to represent damage on very porous pumice-
like target. However, pumice does not presumably match any of the known asteroid 
materials: even CC meteorites have porosities way lower than pumice. 

To date, no comparison of high-speed laboratory shattering experiments and SPH results 
is known by this referee to have been published for materials other than pumice (Ref. [19]). 
Typically, SPH simulations of asteroid shattering on non-porous targets show extremely 
high damage (sometimes down to resolution size) with the formation of a somewhat small 
largest fragment and a swarm of equal small size fragments. That pattern does not match 
what laboratory impact experiments show for low micro-porosity materials. Therefore, it is 
questionable to what extent SPH results can be currently trusted for shattering of non-high-
porosity asteroids. 

2) Concerns on top-shape formation. 

Some of the statements in the ‘Introduction’ section look speculative, not supported by strong 
evidence and instead seem to drive the whole paper. 

“However, the equatorial ridges of both Ryugu and Bennu appear to be their oldest surface 
features, predating the formation of large equatorial craters [5,6,1]. These geologic 
characteristics suggest that Ryugu and Bennu formed directly as top-shapes, or achieved 
such a shape early after their 
formation.” 

Both Ryugu and Bennu show their largest crater-like structures right on their equatorial 
ridge. It is at least statistically suspicious that both bodies underwent their largest impacts ---
after formation--- in the ‘equatorial’ region. Other NEAs (e.g. 2008 EV5) show similar features 
according to shape models derived from radar observation. It cannot be ruled out that such 
features correspond to locations of large boulders landed at the end of the re-accumulation 
process (this is at least at the same speculative level as authors’ ansatz). Once in the inner planet 
region, the YORP effect may have spun up such bodies to the point that the ridge formed and 
boulders on equatorial region were lost or lofted and re-landed off-equator, due to centrifugal 
force being larger than gravitational attraction at that location. In fact, no large boulders are 
currently located on the ridge of these bodies, but many are scattered at larger latitudes. 
Current spin rate for Ryugu is way below threshold for 
instability. Bennu’s spin period (4.276 hr) is instead not that far from typical periods for 
instability corresponding to low density (C/B-type) asteroids. Both rotation states may have 
been reached from higher spin rates after tidal interaction with some former satellite. 

The occurrence of oblate shapes in formed rubble-piles does not imply necessarily the 
formation of a ridge on its own. No compelling evidence for the formation of a ridge while in 
the main belt is offered by simulations, instead this is left to YORP evolution based on 
former studies on ridge (and satellite) formation for NEAs. The argument seems weak. 

3) Other concerns. 

The authors state that: 

“These observations suggest that the two bodies have different levels of hydration, assuming that 
the mineralogy of their surfaces is representative of their bulk compositions.” 

This looks like a strong assumption indeed, not supported by any evidence. Different 
mineralogical features on the surface may well correspond to impacts of bodies with 
different composition and hydration levels. A few, or even just one sub-catastrophic 
collision by a strongly hydrated body may provide enough material to garden the target 
suitably. 

- Missing data in ‘Results’? 

According to sentence: 
"… using the Soft-Sphere Discrete Element Method (SSDEM) and assuming specific sets of 
values of friction parameters to compute the contact forces between the particles 
reaccumulating to form aggregates (Methods)." 

Values for such friction parameters should be provided. The available version of the 
manuscript does not include any value of friction parameters in Methods to be checked.