Peer review of: Mineral dust increases the habitability of terrestrial planets but confounds biomarker detection (2020)

Reviewers' comments: 
 
Reviewer #1 (Remarks to the Author): 
 
This paper examines the climatic effect of dust loading into an atmosphere 
in two different scenarios. One with an Earth like world with 
increasing amounts of land to sea while keeping Earth's modern rotation rate. 
In the other case they look at a tidally locked Proxima Centauri b type 
planet with increasing amounts of land at the substellar point and "tiled." 
 
This is a unique approach I've not seen before in the literature and 
has serious implications for observing terrestrial planets with JWST and other 
future instruments. I recommend publication, but I would like the authors to 
make a few changes. I don't need to see the manuscript again if they agree to 
these modest changes. Of course if other referees demand substantial revisions 
then it may be necessary to see it again afterward. 
 
1,) It is never discussed what type of ocean is used. Is it a simple mixed layer depth X 
ocean w/o horizontal heat transport? Is it a fully-coupled bathtub ocean? What is it?! 
Ocean dynamics are important in climate studies, and we should know what it being used. 
 
2.) Line 8: it is not until line 109 that the term "radiatively active mineral 
dust" is mentioned. I think that belongs here in the Abstract. 
 
3.) Lines 23-25 "But most studies so far have focussed on oceanic\aquaplanet" 
scenarios, because of the importance of the hydrological cycle 
and the defintion of habitability requiring stable surface liquid water." 
 
Yet there have been a few large parameter studies that have used non-aquaplanet 
scenarios, in particular: Yang et al. 2014 ApJL 787, L2, and 
Way et al. 2018 ApJS 239, 24. You might mention them and/or others. 
 
4.) Lines 31-32 "More specific treatments of land 
surface features such as topography remain unexplored." 
 
This is a rather strong statement. It has been explored in a few areas, just 
not extensively. Your reference 7 (Del Genio et al.) does have Prox b simulations 
with an aquaplanet set up AND Earth topography (See their Table 4 and in particular 
#18 Day-Land) At least one more I am aware of is Way et al. 2016 GRL 43, 8376 
where they explicitly compare a Venus topography to an Earth topography each 
with an ocean of similar depth (see Simulation A versus C in Table 1). 
Although there is no discussion of dust in either of these works. 
 
5.) I really like your Figure 1! 
 
6.) From Figure 2 it looks to me like you did 4 simulations "TL tiled", 
4 "nTL Tiled" and 5 "TL Continents." It would be nice to have a table 
of all of the simulations with their respective parameters listed. 
If you don't have room in the main text perhaps in the Suppl Mat? 
 
7.) Lines 100-103: Sorry, but I don't quite understand this statement. 
First aren't you extrapolating to get the 70% simulation for "TL Continents?" 
Please say so if that's the case. 
Then you say 20-50% reduction in 2b. But if you mean only at 70% land fraction 
I don't see the same range. Could you be a bit more explicit? The same 
applies for the comment about 20-50 W/m2. 
 
8.) Figure 3 caption: 8.3 km. Can you tell the reader what this is in pressure? 
hPa or mb would be fine. 
 
9.) Figure 4 b & d. Could you please add to the title of each 'plus biomarker gases" 
or something like that to make it easier for the reader to distinguish w/o having 
to read your caption so carefully? 
 
10.) Reference 24: the last author's name is missing a 'c.' Stamenkovic 
 
11.) In your methods section regarding your dust parameterization. Nowhere is it 
explicitly mentioned whether you are using the default Earth parameterization for 
your dust, or if you used your model in interactive mode with with your dust sizes 
to generate a proper parameterization w/o Earth vegetation included. 
Please tell the reader. 
 
12.) Table 2: Could you possibly add a column that lists the partial pressures 
for CO2, O2, O3, CH4 and N2O in ppmv? It's easier to compare with other work w/o 
the reader having to do the work themselves. 
Also nowhere in the text does the reader learn what "MMR" means. 
 
13.) Line 282: Could you please be a bit more explicit about "The surface is 
essentially flat" so that the reader doesn't have to read Lewis et al. and hope 
it's the same? Some mention of roughness length would also seem appropriate 
since it will affect dust lifting. 
 
On to the Supplementary Material: 
 
1.) Line 67 add TL: "... between TL land surface configurations..." 
 
2.) Figure S4 certainly makes for depressing, but useful reading. 
Although it was only recently published it would be nice if you mentioned 
it in the context of Figure 5 from Fauchez et al. 2019 ApJ 887, 194, even 
if theirs is for a noise free spectrum, they do mention in their Table 4 
how many transits you need for a given S/N for a specific feature. 
Between your dust and their clouds and hazes, the prospects look bleak 
for detecting anything other than CO2! 
 
 
 
Reviewer #2 (Remarks to the Author): 
 
Overall, I think that the paper is well written, well organized and deals with a topic that has been 
largely overlooked in the literature. I am sure the results of this paper will be of interest to the 
community, as it brings first insights on the effects (and underlying mechanisms) that dust may 
have on the climate of temperate, terrestrial exoplanets. 
 
However, there are two important points I would like the authors to clarify/correct/expand in their 
manuscript before I recommend it for publication. 
 
My first main concern is about the numerical parameterization of the dust cycle. The authors 
mentioned in their manuscript that they used the mineral dust parameterization of Woodward et 
al. 2011 (technical report). The dust emission scheme seems to be tuned and designed with 
empirical coefficients to reproduce the Earth dust cycle, and I am concerned whether it can be 
applied as is on other planets. 
 
It is well known in the planet Mars literature that the dust cycle (and in particular the dust 
injection) is difficult to model (Newman et al. 2002, Kahre et al. 2006, Mulholland et al. 2015, 
Bertrand et al. 2019) with 3-D GCMs because it depends on subtle sub-grid scale 
parameterizations. 
 
Can the authors reproduce roughly the dust cycle of Mars with their model? (any reference of 
previous work done with the UK Met Office GCM? Any additional run?) 
Can the authors explore how sensitive are their results to their dust emission parameterization? 
(e.g. by doing additional runs with various wind threshold friction velocities?) 
Last, the authors assumed that the reservoir of dust is infinite. Is it reasonable especially for 
planets with low land fraction? A discussion on sources versus sinks would be welcome. 
 
My second main concern is about the interpretation and implication of their results regarding the 
habitability of terrestrial exoplanets (see also my more detailed comments hereafter). 
 
Although I am convinced by the results obtained by the authors for 1 bar N2-dominated Earth-like 
planet, I am not sure that they extrapolate well – unlike what is stated by they authors, even in 
the title of this manuscript – to the habitability of planets in general. 
 
The authors performed indeed 3D GCM experiments for tidally locked planets with a 1 bar Earth-
like atmosphere for which they showed that the dust cycle tend to stabilize the climate. This stems 
from the fact that dust reflects efficiently light on the dayside of the planet, thus cooling the 
dayside temperature of irradiated planets which reduces the stratospheric water vapour content 
and delay the moist greenhouse ; but warms the nightside, thus preventing from CO2 surface 
condensation, thus stabilizing the climate of weakly irradiated planets. 
 
(1) On relatively dry (and thus dusty) planets, you would expect the water to migrate on the 
nightside of the planet. See Abe et al. 2011, Menou 2013, Leconte et al. 2013, Yang et al. 2014, 
Turbet et al. 2016, Kodama et al. 2018, 2019. From that perspective, the scenario TL continents is 
relevant. You show for a 1bar N2-dominated Earth-like atmosphere that if you have a dusty 
atmosphere, you expect to decrease the stratospheric water vapour content (Fig 2b, red curve). 
But you also expect to warm the nightside of the planet (Fig 2a, red curve), which should boost 
the hydrological cycle on the nightside of the planet. This means more evaporation, but also a 
much more efficient water flow on the surface. This can be seen in your Figure 3a and 3b where 
adding dust increase the surface temperature of the two nightside gyres which would drastically 
increase water ice flow from nightside to dayside. This should increase in turn the area of wet 
regions, which is the key ingredient for determining the position of the inner edge of the HZ 
according to Abe 2011, Kodama et al. 2018, 2019. 
 
In summary, dust here would decrease the habitability of terrestrial planets. 
 
(2) As the CO2 partial pressure increases (e.g. up to several bars) in a terrestrial planet, the heat 
redistribution between day and nightside also increases (see for example Turbet et al. 2018 Fig. 
11, Fauchez et al. 2019 Fig. 10). This should partly move a planet toward a 'nTL'-like configuration 
where the effect of dust on the nightside should be significantly lowered (see your Figure 2a, green 
curve). 
 
For thick CO2-dominated planets representative of planetary atmospheres used to estimate the 
position of the outer edge of the Habitable Zone (Kopparapu et al. 2013, Kadoya & Tajika 2019 
and many other references), your Figure 2a seems to predict the opposite effect of your claim. In 
summary, dust here would again decrease the habitability of terrestrial planets. 
 
To end on that second main comment, new additional runs (e.g. taking self-consistently into 
account the land fraction as a function of the temperature of the nightside ; e.g. with thick 
multibars CO2-dominated atmospheres) would be needed to support the claim that dust do really 
increase the habitability of terrestrial planets as stated in the title, in the abstract and throughout 
the text. 
 
Last but not least, I have appended below a detailed list of remarks which mainly consists of minor 
remarks and additional clarifications on my two main concerns. 
 
 
----------------------------------------------------- 
 
 
-l18 - « the first potentially-habitable terrestrial exoplanets1 » 
 
The reference points to Wordsworth et al. (2011) which studied GJ581d. However, the planet was 
shown later not to exist. It was likely an artefact of stellar activity. See Robertson et al. 2014 
(https://ui.adsabs.harvard.edu/abs/2014Sci...345..440R/abstract). It may be more relevant to cite 
the first non-disputed potentially habitable terrestrial-size planet : Kepler-62f. Ref : Borucki et al. 
2013 (https://ui.adsabs.harvard.edu/abs/2013Sci...340..587B/abstract) 
 
-l21 – ref 2 and 3 
 
It may be more relevant to cite : 
 
Dole 1964 (https://ui.adsabs.harvard.edu/abs/1964hpfm.book.....D/citations) 
Kasting et al. 1993 (https://ui.adsabs.harvard.edu/abs/1993Icar..101..108K/abstract) 
Barnes 2017 (https://ui.adsabs.harvard.edu/abs/2017CeMDA.129..509B/abstract) 
 
-l23-25 - « But most studies so far have focussed on oceanic “aquaplanet” scenarios, because of 
the importance of the hydrological cycle and the definition of habitability requiring stable surface 
liquid water. » 
 
This is also because water-rich planets are one of the likely outcome of planetary formation 
models. See for instance Tian & Ida 2015 
(https://ui.adsabs.harvard.edu/abs/2015NatGe...8..177T/abstract). 
 
-l26-28 - « For a planet’s climate to be stable enough for a sufficiently long period of time to allow 
the development of complex organisms (e.g. around 3 billion years for Earth 9 ), the presence of 
significant land cover is likely required »The surface is essentially fla 
 
This is extremely speculative! I suggest to rephrase « is likely required » by « may be required ». 
 
- l28 - « responsible for the stabilisation of CO2 levels » 
 
Please add « responsible on Earth for the long-term stabilisation of CO2 levels » 
 
- l30 - « to simulate the effects of the presence of land 11,12,13 » 
 
You may also cite the previously mentioned Turbet et al. 2016 (land planet) and Del Genio (ocean 
with continents) et al. 2019. 
 
- l40 - « the presence of dust in the atmosphere of super-Earth planets has been reported 17 » 
 
The reference 17 looks weird to me. The flat spectrum reported in Kreidberg et al. 2014 could be 
as well due to the presence of non-mineral clouds or photochemical hazes. Please give additional 

evidence/reference proving this must be dust. 
 
- l85-86 - « The effect of dust is to sustain a CO2 greenhouse effect at a lower stellar irradiance 
than when dust is absent » 
 
Do you mean « to sustain a greenhouse effect »? Is this a typo? 
 
- l86-87 : «  implying that dust moves the outer edge of the habitable zone away from a parent 
star » 
 
I dont agree with that. As the CO2 partial pressure increases, the heat redistribution between day 
and nightside also increases. This can be observed in Turbet et al. 2018 
(https://ui.adsabs.harvard.edu/abs/2018A%26A...612A..86T/abstract) Figure 11, Fauchez et al. 
2019 (https://ui.adsabs.harvard.edu/abs/2019ApJ...887..194F/abstract) Figure 10, and many 
other references. 
 
So I guess this should partly move the planet toward a 'nTL'-like configuration where the effect of 
dust on the nightside should be significantly lowered (see your Figure 2a, green curve). 
 
This could be tested by running your GCM with an atmosphere of e.g. 10bar dominated by CO2. 
 
Also by looking at the Figure 17 of Forget et al. 2013 
(https://ui.adsabs.harvard.edu/abs/2013Icar..222...81F/abstract) that did similar simulations for 
the case of early Mars, it seems the effect of dust is relatively low. Could you comment on that? 
 
- Figure 2 
 
I dont understand how you calculed your « approximate change in stellar radiation ». Where does 
this come from? 
 
- l102-103 
 
Could you connect your results with the observational results on Mars obtained by Heavens et al. 
2018 (https://ui.adsabs.harvard.edu/abs/2018NatAs...2..126H/abstract)? 
 
- l104-106 : « In contrast to the effect on the outer edge, both our TL and nTL simulations result 
in a reduction in stratospheric water-vapour when including dust, demonstrating that the inward 
movement of the inner edge of the habitable zone is a ubiquitous feature of atmospheric dust » 
 
Are you sure this mechanism hold when you are much closer (in insolation) to the inner edge of 
the HZ. On relatively dry (and thus dusty) planets, you would expect the water to migrate on the 
nightside of the planet. See Abe et al. 2011, Turbet et al. 2016, Kodama et al. 2018 
(https://ui.adsabs.harvard.edu/abs/2018JGRE..123..559K/abstract), which makes your scenario 
TL continents more relevant. If you have a dusty atmosphere, you expect to decrease the 
stratospheric water vapour content (Fig 2b, red curve). But you also expect to warm the nightside 
of the planet (Fig 2a, red curve), which should boost the hydrological cycle on the nightside of the 
planet : more evaporation ; but also more efficient water flow on the surface (this can be seen in 
your Figure 3a and 3b where adding dust increase the surface temperature of the two nightside 
gyres which would drastically increase water ice flow from nightside to dayside), which should 
increase the area of wet lands, which could push back the inner edge of the HZ according to 
Kodama et al. 2018. 
 
I understand your numerical results, but again I dont agree with your extrapolations on the 
position of the inner limit of the HZ. 
 - l-109-117 « In summary radiatively active atmospheric dust increases the size of the habitable 
zone for tidally-locked planets, both by moving the inner edge inwards and outer edge outwards » 
… 
 
I dont agree with this statement, because I think (cf my previous comments) that your results 
cannot simply be extrapolated to the conditions for which the HZ limits have been calculated. 
 
- Figure 4 : Could you please specify the instrument used in these synthetic spectra? Is it 
NIRSPEC/Prism? 
 
- l-266 – Methods 
 
Did you consider lifted dust particles as a possible source of CCN? 
 
Did you consider (or plan to consider, for a future study) the effect of sea salt particles that would 
be relevant even in the aquaplanet case? 
 
Eventually, could you say a few words about the dust optical properties? What database did you 
use? And did you use the same database in the GCM and the PandExo simulations? 
 
- l-282 : « The surface is essentially flat » 
 
What do you mean by « essentially flat »? 
 
 
- SUPPLEMENTARY MATERIALS 
 
-l28-32 : « On a tidally-locked planet, the increase in temperature due to long-wave emission from 
the dust is stronger than the decrease in direct heat-transport from the day-side, therefore the 
night-side warms. On a non-tidally-locked planet, the decrease in heat-transport from equatorial 
regions is stronger than the increase due to long-wave emission from dust, therefore the polar 
regions cool. » 
 
Again, you demonstrate that for an Earth-like atmosphere. But is this still true for a thick CO2-
dominated atmosphere? 
 
-l39-42 : « As discussed in Lewis et al. 1 , land positioned on the day-side of a tidally-locked 
planet is the most interesting and useful case to consider, because any land located on the night-
side is likely to be locked away under permanent ice-caps and so be unable to influence the 
planetary climate or carbon-silicate cycle » 
 
I dont agree with the fact that any land located on the nightside is unable to «  influence the 
planetary climate ». Such land can for example become a cold trap on which water or other 
condensible can accumulate, thus affecting the climate at the global scale. 
 
See Menou 2013 (https://ui.adsabs.harvard.edu/abs/2013ApJ...774...51M/abstract) 
Leconte et al. 2013 (https://ui.adsabs.harvard.edu/abs/2013A%26A...554A..69L/abstract) 
or Yang et al. 2014 (https://ui.adsabs.harvard.edu/abs/2014ApJ...796L..22Y/abstract) 
 
- l55-56 : « stratospheric water-vapour contents are quite high (similar to aquaplanet experiments 
2 ) » 
 
From your Figure S2, it seems that it reaches ~ 0.5ppm maximum, which is more than one order 
of magnitude lower than present-day Earth stratosphere which is considered to be pretty dry. Why 
do you consider your stratospheric water-vapour contents « quite high »?? 
 
- l58-59 : « The dust reduces the stratospheric water-vapour content down to amounts 
comparable to the 100% land simulations » 
 
What does it mean? Where does the water come from in your 100% land simulations? Shouldn't 
be the water content equal to 0?! 
 
- l72-74 : « Figure S4 presents the same results shown in Figure 4 but with the star moved further 
away, such that its apparent magnitude is similar to TRAPPIST-1 rather than Proxima Centauri. 
These simulated observations could therefore represent those of TRAPPIST-1e. » 
 
Did you also adapt the size and spectrum of the star to TRAPPIST-1? Did you take the planetary 
parameters of TRAPPIST-1e? (in both the climate calculations and PandExo?) 
 
How do the SNR of the CO2 and other features compare to what have been calculated in the 
literature, e.g. in Lustig-Yaeger et al. 2019 
(https://ui.adsabs.harvard.edu/abs/2019AJ....158...27L/abstract) and Fauchez et al. 2019? 
 
 
 
 
Reviewer #3 (Remarks to the Author): 
 
By incorporating airborne mineral dusts in general circulation models, the authors show that the 
extent of habitable zone can be substantially widened and the grey opacity of dusts tend to mute 
transmission features of biomarker molecules. Speaking as someone from the exoplanet 
community, the type of “dusts” that is much more detrimental to the detection of biomarker is the 
high-altitude clouds/haze primarily produced photochemically produced by UV flux. This is entirely 
different from the mineral dusts elevated from the planet’s surface that the authors focused on in 
this work. The famous “detached” haze layer (400km above the surface) on Titan is believed to be 
hazes produced photochemically rather than mineral dusts lofted from the surface. It is this haze 
layer that gives rise to many interesting features of titan such as the fuzzy appearance, orange 
coloration and strong forward-scattering. More importantly, in the near future biomarker molecules 
in exoplanets will be studied by transmission spectroscopy which probes the upper atmosphere. It 
is again the high-altitude hazes/clouds that most severely mute transmission features of 
biomarkers. The creation, growth and sedimentation of hazes and their effects on transmission 
spectroscopy have been extensively studied both in labs and in simulations 
(https://arxiv.org/abs/1712.02808,https://arxiv.org/abs/1805.10488,https://arxiv.org/abs/1902.
10151) 
 
The authors showed interesting results on the extent of habitable zone when mineral dusts is 
considered. I would be curious to see how high-altitude photochemical haze in GCM further affects 
the habitability. I understand that this would be an arduous undertaking. However, in its current 
form, I am afraid that the manuscript is not original enough or comprehensive enough that it 
warrants the publication in a high profile journal like NatureComms. I suggest that the authors 
submit the manuscript to Astrophysical Journal. Astrobiology or Icarus.