Manuscript ApJ80204
Dr. Steve Federman
Editor
The Astrophysical Journal
Dear Dr. Federman
We thank the anonymous referee for helpful comments. All the points are well
taken, and we have made a number of revisions to the paper to clarify the points raised by
the referee. The details are as follows. We will first reproduce the referee’s questions
with italic font in quote, followed by our response.
1. Since the authors here choose to look at only one planet-the best studied one, I
encourage them to go beyond just C,N,O for one particular planet.
Added to the modeling section “The inclusion of Sulfur or Nitrogen species (see eg.
Zahnle et al. 2009) is beyond the scope of this paper and will be explored in later
studies.” Also added to the conclusions “Though we have not included Sulfur and
Nitrogen species in this study as in Zahnle et al. (2009) we have still shown that simple
C, O and H chemistry and their interplay with vertical quenching is consistent with the
detected abundances of CO, CH4, CO2 and H2O ”
2. I found it a little disconcerting that the authors weighed so heavily on the abundance
determination from Swain/Tinetti and collaborators. In particular, those authors
apparent allowance for drastically changing mixing ratios as a function of height would
appear to fly in the face of the models presented here. A safer bet would be to compare to
the allowed mixing ratios of Madhusudhan & Seager (2009), who I believe explore a
much wider range of P-T profiles and constant-with-height chemical mixing ratios, to
match the observed data.
We adjusted the observed values of CO2, CO, H2O and CH4 to include the
Madhusudhan and Seager 2009 results in several places. In most cases their results
bracket the Swain et al. results. Example: “The goal of this investigation is to
understand the chemistry that produces the observed abundances of ~10-5-10-3, ~10-6 -103
, 10-5-10-3, and ~10-6-10-7 for CO, CO2, H2O and CH4, respectively, as derived from the
dayside emission spectrum of HD189733b (Swain et al. 2009a, Madhusudhan & Seager
2009)…..”
3. I am not sure that I would say that "only a small number" of the exoplanets transit.
Dozens!
In the introduction, changed “only a small number” to “many”
4. I think it is perhaps a little misleading to say that equilibrium thermochemstry occurs
at ~1000 bars. In Jupiter's visible atmosphere, for example, one would predict that
essentially all C is in methane. And by in large, that is true. Of course, one can see higher
order hydrocarbons, and one can see a little CO, but really, the vast majority of the C is
still in CH4. So the imperfectness of the equilibrium chemistry approximation is really a
matter of perspective.
We have replaced “Thermoequilibrium chemistry occurs in high temperature and
pressure regimes where chemical timescales are short, typically deep within the
atmospheres of giant planets (~1000 bars)” with “Thermoequilibrium chemistry occurs in
high temperature and pressure regimes where chemical timescales are shorter than
potential disequilibrium mechanisms, typically occurring deep within the planetary
atmosphere (Yung & DeMore 1999, pg 135).”
5. On page 6, the authors mention that they do not include N and S-species, since they
have not yet been detected. I would argue that it is exactly the reason to include these
species, so that people should know what to look for. That does not need to be in this
paper, but I encourage the authors to work in this direction. What interesting chemical
products might we see?
We are aware of this, and will explore S and N chemistry in later studies. In this study
we chose to keep it simple to see if we could explain the observations with just the
detected C, H and O chemistry and the disequilibrium mechanisms. See comment (1)
6. Page 7, the reference should be Smith 1998, not 1997.
Changed “1997” to “1998”
7. Figure 3: I am a little surprized at how important vertical mixing is at yielding a
high mixing ratio of CH4 on both the day and night sides. I generally think of mixing
as allowing a higher CO or CO2 mixing ratio, compared to equilibrium, but I think
that I (and many others) forget about the corresponding effects on CH4. Namely, the
possibility of propping up the CH4 abundance in the upper atmosphere as well, as
shown here. It is MANY magnitudes higher than in equilibrium. I would play that up
quite a bit more.
Thank you for pointing this out. We emphasized this point in the abstract by adding “Our
results suggest that the disequilibrium mechanisms can significantly enhance the
abundances of these species above their thermochemical equilibrium value, so some
caution must be taken when assuming that an atmosphere is in strict thermochemical
equilibrium. For instance, the 10 mbar mixing ratio of CH4 is ~2 orders of magnitude
more than thermochemical equilibrium value.”
We also added to the results section under CH4 and heavier hydrocarbons “The
abundance of CH4 produced via quenching is also many orders of magnitude larger than
the expected thermochemical equilibrium values (see Figures 2 and 3) with very little
thermochemically derived CH4 (<10-9) present above 10-4 bars. “
8. Related to Figure 3: However, this also leads me to wonder what may be causing the
dramatic differences in the CH4 mixing ratios computed here, compared to those
presented in Zahnle et al. (2009b, arXiv:0911.0728). This needs to be discussed in the
paper. I realize that Zahnle et al. looked at atmospheres as a function of isothermal T and
Kzz, but I am struck that Zahnle et al. find a dramatic fall-off in the CH4 mixing ratio at
P < 1 mbar and 0.1 mbar for Kzz of 10^9 and 10^11, respectively. However, Figure 3
here shows a high mixing ratio of CH4 (at Kzz=10^10) all the way out to 10^-8 bars. The
corresponding level at which the C2H2 abundance is maximized in this paper is likewise
shifted up to much lower pressures, compared to Zahnle et al. This needs to be addressed
and discussed.
Zahnle et al. used the solar spectrum (See figure 3 in Zahnle et al. (2009b,
arXiv:0911.0728) as seen from 0.032 AU. We used a K2V spectrum as seen from 0.032
AU. The UV flux below 3000Å is ~2 orders of magnitude less for a K2V star. This will
result in lower photolysis rates than in Zahnle et al, and thus allowing more methane
higher up. We added a sentence in the modeling section that says “We use the UV stellar
spectrum from HD22049 which is also a K2V star (Segura et al. 2003). This K2V star
has ~2 orders of magnitude less flux below ~3000 Å than the solar spectrum used in
Zahnle et al (2009a, 2009b) giving significantly different results.
9. Additionally, Zahnle et al. (2009b) show very little sensitivity to the bottom boundary:
1 and 100 bars were used, but the chemistry at, say, millibar pressures was essentially
unaffected by the choice of the boundary. I get the feeling that the opposite would be true
here, where the mixing ratios at 3 bars are basically carried very high into the
atmosphere before photochemistry takes over. Actually, based on this work, the authors
could make the case that non-equilibrium thermochemistry is crucially important for this
planet, but that photochemistry is not particularly important, for P> 1 microbar.
I believe in the conclusions that we mention quenching is more dominant than
photochemistry for H2O and CO --“Vertical quenching determines the lower boundary
values and thus the mixing ratios of CO and H2O, which are not significantly affected
photochemically. CO2 can be photochemically produced above its quench level value by
the reaction described in equation (13), and CH4 can be readily photochemically
destroyed.” We added a sentence that says “However as a whole, the vertical quenching
primarily dictates the abundances of these species in the observable portion of the
atmosphere.”
10. I generally found the paper a little light in references to work of others. I have
pointed out some points of comparison with Zahnle et al (2009b). Additionally, Lodders
& Fegley (2002) and Zahnle et al. (2009a) have previously commented on the change in
CO and CO2 mixing ratios as a function of metallicity.
Added references to “CO2 is more readily affected by metallicity than the other species
because it has two oxygen’s as opposed to CO’s one oxygen (see eg, Zahnle et al. 2009,
Fegley & Lodders 2002 )”
11. The high temperature opacity databases are pretty well understood now, save for
CH4 and CO2. I do not think that it is proper to give the impression that the whole field
of radiative transfer modeling in hot Jupiter atmospheres is in a state of flux because the
opacities are uncertain. That just is not true. The opacity databases are generally very
good, and spectra of brown dwarfs of Teff the same as those of hot Jupiters can be
modeled generally quite well (e.g., Cushing et al. 2008, Stephens et al. 2009). Differences
between groups in estimating mixing ratios of atoms and molecules are dominated by
choices made (allow changes to the mixing ratios with height, choice of initial T-P
profile), and not by differences in opacity databases.
Removed “Line lists used in radiative transfer models are also not well known and are
constantly changing at these high temperatures which can play a significant role in
dictating the retrieved abundances from the observations (Tinetti 2010 private
communication).” as it is not crucial to our argument.
Misc.
We also modified equation 8 using the enthalpies and entropies at 1000 K and NOT 298
K. This does not change equation 8 significantly and therefore does not effect the
location of the quench level.