Dear Dr. Sneden,
We thank the anonymous referee for helpful comments, which led to an improvement of
the paper. We will first reproduce the referee's question in italics, followed by our
response.
General Comments
The paper discusses a lot of material in very few pages and consequently the description
of the methodology and results is brief, far too brief, in my opinion. It is not possible to
really understand the model or its implications and, essentially, we are asked to accept the
conclusions on faith. I do not know what exospheric temperature is adopted, what the
eddy diffusion profile looks like, what solar flux is adopted, what is assumed for the
aerosol abundance in the atmosphere. These and other assumptions are necessary to
understand the results. I would rather see a full description of the calculations. In some
cases the description is so lacking that I have only the vaguest idea of the problem being
discussed. The most obvious example of this problem is that the 14N/15N results show
up in the conclusion section of the paper but nowhere else. Apparently there was not
space to discuss this aspect of the model at all. Though interesting, the results of the
paper do not require rapid publication and the lack of detail is not justified by a need for
urgent publication.
The paper contains no discussion of how the model has been validated, thus we do not
know if the model provides a good representation of the chemistry occurring in Titan's
atmosphere. There are many high quality observations available to constrain the
composition of the atmosphere, but these are not discussed (I'm thinking especially of
results from CIRS limb profile, for example in the Vinatier et al. papers).
As a skeptical scientist, I would like to know if, for example, the model reproduces the
observed C2H6 altitude profile, before considering whether the D/H ratio in C2H6 is
properly modeled.
In general, the paper seems to be hastily prepared and is not yet of high enough quality
for a professional journal. Moreover, the authors seem unaware of many recently
published papers that are directly relevant to this research. Careful reading of these
recently published papers may, in some cases, cause them to alter their conclusions.
Details are provided in the comments below.
I recommend against publication of this paper, but look forward to a more complete and
useful description of this work in a future publication.
Specific Comments
1. First paragraph, line 8. These references are inappropriate. Rapid escape of H2 from
Titan was first discussed by Hunten in 1973. The importance of this process for chemistry
was probably first discussed in one of the early photochemistry papers (Strobel or Yung).
The most recent paper on H2 escape from Titan is Cui et al. (2008).
2. Page 2, line 7. Again, the H2 escape references are inappropriate.
3. Page 2, line 12. Actually, Strobel (2008) suggests that the rapid CH4 escape is thermal.
4. Page 2. The 1D continuity and diffusion equations are so well known that it is
unnecessary to include them, especially in a brief report. Reference to a standard text
(e.g. Banks and Kocarts, Hunten and Chamberlain) could be given. It would be better to
use the space to describe the model and results. By the way, w is not the vertical
transport velocity but the bulk velocity (averaged over species) of the atmosphere. The
transport velocity is the sum of the bulk velocity and the diffusion velocity.
Reply: The equations have been removed from the revised manuscript.
5. Page 3, line 8. The thermal structure model derived from UVIS data and used in Liang
et al. 2007a, b is now known to be incorrect. A recent paper on the thermal structure is
Mueller-Wodarg et al. (2008).
6. I strongly disagree with the use of a bulk vertical velocity in this 1D model. The
dynamics of Titan's atmosphere, especially the upper atmosphere, is not yet well
constrained and the adopted values could be very far off, even for the northern
hemisphere. More importantly, many of the species modeled in this manuscript are longlived and there will be significant horizontal mixing. A global average is more important
for these species and a vertical velocity makes no sense for a global average - it violate
mass continuity. The long-lived species include CH4 and H2, the focus of the paper, and
so the use of a vertical velocity seems to be an incorrect decision. Too little information is
provided about the model for the reader to form an opinion about how serious this error
is.
7. Section 3, first paragraph. Vertical transport constraints provided by 40Ar
measurements are discussed in Yelle et al. (2008), which strangely is not referenced in
this paragraph. Neither is the eddy diffusion profile used here compared with that derived
in the Yelle et al. (2008) paper.
8. page 4, line 1. The H2 escape flux used in this manuscript is derived in Yelle et al.
(2006) and Cui et al. (2008). The references to Strobel (2008) is inappropriate because
this author interpreted the H2 escape rate but did not derive it from the data. The authors
seem to be unaware of the Cui et al. (2008) paper.
9. page 4, line 24. The basic chemical conversion in Titan's atmosphere is 2CH4 -->
C2H6 + H2. Thus, if the H2 escape flux is 1E10 cm-2s-1 then the CH4 destruction rate
must be 5E9 cm^-2s-1. The authors quote a value of 2.2E9 cm-2s-1. What is going on
here?
10. page 4, 2nd line from the bottom. "most of the photodissociation of CH4 occurs
above ~700 km.." Yung et al. (1984) found that most of the CH4 destruction was due to
the photocatalytic cycle with C2H2 (hv + C2H2 --> C2H + H, C2H + CH4 --> CH3 +
C2H2, followed by CH3 + CH3 --> C2H6). The process peaks near 200 km, well below
700 km. It's odd to see this mistake in a paper with Yung as a co-author. Perhaps
something else was meant?
11. page 5, first partial paragraph. This argument only works for a species in diffusive
equilibrium with zero net flux. It does not apply to H2 or CH4. I cannot tell if the
problem is with the model or the description of the model.
12. page 5 line 8. The fractionation is give as 132 per mil. I think it very unlikely that this
value is estimated with an accuracy of 3 significant figures. An uncertainty for this value
should be provided.
13. Page 5, line 22. The authors mention that their model cannot reproduce the D/H
value in H2 in the upper atmosphere reported in the Mandt et al. (2009) paper. This
deserves more of a discussion than provided here. The D/H reported in the Mandt et al.
paper is not even provided.
Reply: Mandt et al. have withdrawn the paper as their result was suspicious. We also
removed the related discussion too.
14. Page 5, 2nd full paragraph. The authors suggest that the disagreement between the
observed 12C/13C ratio for C2H6 and HCN could be explained by condensation
processes, but provide no real justification for this hypothesis. I think that it is very
unlikely because condensation is the fate of all isotopologues of C2H6 and HCN. The
rate of condensation does not matter. It only changes the altitude of condensation slightly.
15. Page 5, last partial paragraph. The authors assumed model for the origin of CO and
CO2 neglects some recent work. They need to read and discuss the paper by Horst et al.
(2008).
16. Section 4, first paragraph. This is quite odd. The 14N/15N ratio is not discussed at all
in the text then it appears in the first paragraph of the summary. If the authors think that
this is important it must be discussed in the main body of the paper. In fact, what I am
hoping for is to someday see a complete and thorough description of the Liang et al.
hypothesis for the variations of the N isotopes. Liang et al. have published only 1 very
brief report, essentially describing this excellent idea but presented no details for the
model. I have not objection to this, but to now to assume that the matter is settled and to
issue one sentence updates is really not the best way to proceed.
Reply: The description has not moved to the result section.
17. Section 4, 2nd paragraph. Well, I tend to agree, but I think it would be better to say
that the Mandt et al data analysis or results are suspect rather than the data. A serious
problem with the Mandt et al. paper is that they present this very surprising results but
provide very little support of justification for it. I was surprised that it was accepted for
publication.
Reply: Mandt et al. have withdrawn their paper. So our discussion related to this
measurement is removed too.
18. Section 4, last paragraph. See comments above about the O chemistry model.
19. I believe that D/H ratios in C2H2 and C2H6 are available from Cassini/CIRS
measurements but these are not mentioned in the paper. They should be very constraining
for the model.
20. Fig 1. The origin of the data points should be mentioned in the figure caption. The
D/H value from Mandt et al. (2008) should also be shown.
Reply: The references are provided. The cited reference of Mandt et al. is removed.
21. Table 1 lists important reactions for isotope exchange but no rates are given.
Sincerely,
Mao-Chang Liang
Academia Sinica