Peer review of: Near-infrared observations of active asteroid (3200) Phaethon reveal no evidence for hydration (2020)

Reviewers' comments: 

 

Reviewer #1 (Remarks to the Author): 

This work presents, as far as I am aware of, the first collection of infrared (3-microns) spectra of 
Phaethon to detect the 2.7-2.9 microns absorption feature associated to the presence of hydrated 
silicates. This by itself deserves publication. The presented spectral data are used to shed some light 
into the origin of Phaethon, discussing two different scenarios: a 'dry' and a 'wet' origin. Although 
the lack of detection of such hydration feature is not as constraining as expected, and still the two 
scenarios are possible, I recommend this work for publication after some revision. I would like to see 
again the corrected manuscript before final acceptance. 

General comments. 

Methodology  

- Line 81: I suggest to include here the value for Phaethon's rotational period. 

- Lines 113-116: what is the reason for using the value of p_v = 0.122 for Phaethon's albedo, and not 
other measurements like the one reported by Kareta et al. (2018) (p_v=0.08) or the more recent 
measurement by Masiero et al. (2018), p_v=0.16? The same applies to the G value used by the 
authors: there are several publications in the literature providing lower values (even negative ones) 
for G in the case of Phaethon. Although I do not think this should be explicitly written in the main 
text, I would like to know what was the criterion of the authors to select such values of albedo and G 
slope for their modeling, and to actually see how changing these parameters changes their results. 

- Line 119: I think the reader might wonder about the meaning of this K/V ratio. Please add a couple 
of sentences to explain why you need this value in your model. 

Results 

This section is composed of one single paragraph. I suggest to integrate it into the Discussion 
section, changing it to something like "Results and Discussion". 

- Line 124: the authors refer here to thermally-corrected spectra (blue) and thermally-uncorrected 
spectra (black) in Figure 2, while the plot only shows the thermally-corrected ones. Please correct 
this. 


- How are the uncertainties associated to the depth values computed? Is this uncertainty related to 
the dispersion in the data points? This needs to be properly described. 


- Figure 2 is a nice way of presenting all your data in a single plot, but it does not help visualize your 
results. I suggest the authors to either change it (maybe you can show half your sample in one plot 
and half in another, so you can zoom in a bit) or keep it and select one or two spectra (maybe the 
ones with highest band depths) and present them in a separate plot. In addition, the caption of this 
figures says that the spectra are thermally corrected and normalized....to what wavelength? 2.45 
microns? Please add this to the caption. 

Discussion 

- Lines 167: regarding the connection between Phaethon and Pallas, it should be added here the 
similarity between the albedo of Phaethon and those of both Pallas itself and the members of the 
Pallas family. In this respect, it is interesting the work from Ali-Lagoa et al. (2013), where they show 
that the albedo of the B-types in the Pallas collisional family (p_v = 0.14) is higher than the average 
albedo of B-types (p_v=0.07). This comment directly connects to what has been stated by the 
authors in line 210, where they mention the mismatch between the albedo values of Phaethon and 
Pallas. Although Kareta et al. (2018) obtained a value of p_v=0.08 for Phaethon, a very recent 
publication by Masiero et al. (2018), using WISE data, provides a value of p_v = 0.16. This should at 
least be mentioned and considered by the authors in their discussion. 


- Figure 3. Please indicate in the caption which spectrum of Phaethon is shown in the plot (Set #?) 

- Lines 184-194. This paragraph is a bit confusing. The authors comment on the possibility of 
Phaethon having an anhydrous origin. Then, in line 189 they suddenly introduce Bennu: 

"(...) However, Bennu is likely also from the New Polana family...". It seems to me that some 
preceding info/sentence is missing. Later they state (line 191) that Phaethon is from the New Polana 
family. Is this a working hypothesis or is based on a published work? If so, the corresponding 
reference should be provided. 

Minor corrections: 

 

- Table 1. Rotational phase has no units. 

- Line 138: "The red dotted line represents a continuum..." 

- Line 167: "... de Leon et al. (2010)..." 

- Line 172: "The surface temperature of Phaethon is..." 

- Line 184: "Unlike Phaethon, Pallas..." 



Reviewer #2 (Remarks to the Author): 

Review of "Surface of Active Asteroid (3200) Phaethon" by Takir et al 

General Comments: 

This paper presents a solid observation, making the case that the surface of Phaethon shows no 3 
micron hydration band. Unfortunately, the observation is not matched by a comparably good 
analysis. After a lot of words amounting to about half the paper (lines 142 - 229) the final sentence 
states the conclusion succinctly: 

“It is unclear whether Phaethon was originally hydrated and has since lost volatiles from its surface 
and interior via dehydration or it was formed from anhydrous material.” 

Specific Comments: 

Page 2: The perihelion activity cannot be due to buried ice because it would need to be within a 
thermal skin depth of the surface in order to respond at perihelion, as observed, but the 
temperature at this depth is much too high for ice to survive. Deep ice could exist but would not 
respond to perihelion heating because of the long conduction time. 

The panels of Figure 1 all look very similar - what is the reader meant to see by comparing the 
panels? The figure is not effective. 

Table 2 and Figure 2, compared: 

The text states "Phaethon has no observable 3 μm band with >2σ detection at 2.90 μm". This 
statement is supported by Table 2, except for Set 4, which is >3sigma significant. But comparison 
with Figure 2 seems to contradict the Table. Specifically, >3sigma detection in Set 4 is not evident at 
all in Figure 2, while spectra from Sets 9 and 10 show clear dips at 2.9 microns relative to nearby 
continuum and yet these Sets are given 1 sigma errors in Table 2. I don't see the connection 
between the uncertainties in Table 2 and the spectra in Figure 2. Could that be explained a little in 
the text? 

 

Figure 2 caption refers to a “gray band”. What gray band? 

 

Page 5 Pheathon is mis-spelled 

 

Page 6 “the surface of this asteroid is not hydrated”. Here is a good place to state to what depth the 
dehydration extends. It's a few tens of microns, isn't it? 

 

Page 7: The fact that the surface still looks as uniform as it does suggests that the current mass loss 
is being outpaced or matched by the processes that serve to change and homogenize the surface, 
and thus that they act relatively quickly. 

 

The natural process is ballistic resurfacing, as on comets. 

 

Page 8 line 184: Some words are missing from the start of the sentence, or is it just not capitalized? 

 

Page 8 Ohtsuk - no such person 

 

Page 8 “the Earth’s crossing asteroid”. What? 

 

Page 9: "Phaethon may experienced" - may have 


Reviewer #3 (Remarks to the Author): 

 

The paper deals with new interesting spectroscopic data of Phaethon. Phaethon has been studied 
during its several passages close to the Earth, in particular during the 2017 campaign, and especially 
in the visible region. Only very few information is available on its NIR spectrum. So these data 
deserve to be published. 

My main comment is that the authors claim their results as definitive, that is, Phaethon is an 
anhydrous abject, as clearly reported in the title. Instead I think that their results refer to a portion 
of the surface of Phaethon and all the paper should be written keeping this in mind. 

After that, I think the paper should be published. It is important also as support of the Destiny 
mission. 

Please find below my comments and some small corrections: 

Page 2 lines 63-64: “B-types are carbonaceous and primitive asteroids and many of them are located 
in the high-inclination Pallas”, please include a reference. 

Page 4 lines 103-104: “we subtracted spectra of Phaethon and the standard star SAO 39985 at beam 
position A from spectra at beam B.” The authors should give some more details of the reduction 
procedure. 

Page 5 line 124: “shows the thermally-corrected LXD spectra (Sets 1 to 10) of Phaethon (blue)” I do 
not see the blue… 

Page 5 line 125: 2.90…micron is missing 


Page 5 line 126-127: “and slope of 0.00 are shown in Table 2” between 0.00 and are shown there 
should be “and they are shown” 

Page 5 line 127: Pheathon is Phaethon 

 

Page 6 line 143-144: “This study revealed that Phaethon is featureless at the 3-µm band, suggesting 
that the surface of this asteroid is not hydrated” I partially agree with this conclusion as the authors 
didn’t study the complete surface of Phaethon. It is probable that the surface of Phaethon is not 
hydrated owing to the strong heating, but this specific investigation is not enough to make a final 
conclusion on the whole surface. It could be better to say that it is not hydrated on the investigated 
portion of the surface. 

 

Page 6 line 146-151: As before, even if it could be true that the whole surface of Phaethon is quite 
unifrom, it cannot be excluded that some variatons could be present, see for example Lazzarin et al. 
2019 PSS. In this paper the authors find also an absorption band around 0.43 micron, typical of 
hydrated minerals. 

 

Page 6 line 152-153: include this sentence “while a small fraction of its total mass of ~10^14 kg (Li 
and Jewitt 2013 )” between commas. 

 

Page 7 line 155: “The fact that the surface still looks as uniform as it does” ….the same as above, the 
authors have observed only a frction of the surface of Phaethon and they generalize the unifomity 
found to the whole surface. I do not agree with this statement. 

 

Page 8 line 184: “unlike Phaethon” unlike should have U capital letter. 

 

Page 8 lines 188-190: “…mechanism like thermal degradation of the surface materials or solar wind 
sweeping (Jewitt 189 and Li, 2010). However, Bennu is likely also from the New Polana family and 
was found to be hydrated…” I do not like how the phrase However Bennu…starts. There not 
connection with the previous one, it should be rewritten better. 

 

Page 8 lines 195-199: “A volatile-rich past for Phaethon would by necessity imply a period of intense 
devolatilization upon its injection into its current orbit, which would provide an easy explanation for 
the creation of the Geminids. Jewitt and Hsieh (2006) suggested that the Earth’s crossing asteroid 
2005 UD is dynamically similar to Phaethon and that both are thought to be fragments generated by 
the breakup of a primitive precursor object. The scenario of Yu et al…..” 

The authors should explain better the introduction of the similarity between Phaethon and 2005 UD 
in this context. They explain the consequence of the possible similarity with Polana and Pallas, but 
they do not explain that one with 2005 UD. 

 

Last, I think that the title should be less categorical, for example it could be written as a question: Is 
the surface of Phaethon totally dehydrated? Or something similar..