Peer review of: Nucleobase synthesis in interstellar ices (2019)


RR2
Reviewers' comments:
Reviewer #1 (Remarks to the Author):
The authors have directly addressed most of my concerns about this manuscript and the addition of the 15N work has made their results significantly more convincing. The paper is quite interesting and should be of value to the community. I only have one remaining quibble.
The language suggesting the photolysis temperature was “a key factor” in the detection of nucleobases is still too strong. The authors concede that they made no attempt to directly study the effects of conducting the photolysis at different temperatures and are making a hypothesis based on the comparison of their results to a previous study. The language should be adjusted to reflect that their assertion that temperature may play an important role is merely a hypothesis.
If the authors correct this, I have no further need to review the manuscript.

Reviewer #2 (Remarks to the Author):
The changes made by the authors have adequately addressed most of the points outlined, and all of the work that has been done to improve the manuscript is appreciated. My only remaining concern is the following:
Original comment: “In particular, did all samples undergo purification with the cation-exchange material? It is possible that the low yields of sugars (glyceraldehyde, glycoaldehyde dimer, etc.) are simply due to their low retention in the purification step...”
[Author Reply] “We have validated our experimental procedure for all target molecules in the present study. In particular, the standard reagents of the target molecules were treated in the same analytical procedure including the cation-exchange column chromatography (Fig. S34 with the unpublished data)”
Reviewer response: Thank you for the clarification. In the caption to Figure S34, please note which concentrations were used.
If the standards in all figures containing co-injected samples were subjected to the same analytical procedure, including purification step, please add a sentence to this effect in the Methods section.
If this was not done for all standards in the figures, then the recoveries of all relevant species at low concentrations under this procedure should be tested and added to Fig. 34. It is a good sign that several molecules showed better than 95% recovery, but one cannot assume that every other molecule would have similarly high recovery. Particularly, analytes of very low (ppb or ppm-level) concentrations can exhibit significantly lower recovery efficiencies, and if this was not accounted for already by subjecting the standards and their calibration solutions to identical treatment, the concentrations (and therefore some of the conclusions) may be skewed.

Reviewer #3 (Remarks to the Author):
Dear editor,
The manuscript has improved significantly after the corrections. I think this version is not confusing to the reader because the cases where a detection was not clear are regarded as upper limits. The quality of the work and the final results are certainly worth publication in this journal. It is of high relevance for a broad scientific community. I congratulate the authors for this work.
I just have a small clarification that the authors can take as optional: the UV field in the diffuse interstellar medium is indeed 10^8 photons cm-2 s-1, but the more relevant value is 10^4 photons cm-2 s-1, i.e. the secondary UV field in dense clouds where ice mantles are present, this value varies in circumstellar regions of YSOs. I have no personal interest in citing Shen et al. since I am not one of the authors, if you prefer you can cite the earlier reference od Cecchi-Pestellini & Aiello (1992) or leave the value of 10^5 photons cm-2 s-1 without any mention to a given astrophysical environment.