Peer review of: The root of anomalously specular reflections from solid surfaces on Saturn’s moon Titan (2020)

Reviewers' comments: 

Reviewer #1 (Remarks to the Author): 

The manuscript discusses the discrepancies between the specular reflections detected by the 
Arecibo Observatory/Green Bank Telescope and the instruments onboard the Cassini spacecraft. The 
detection of specular reflections are an important tool in order to identify recently present liquids on 
Titan's surface. 

In this work, however, the authors present new evidence that the Arecibo data rather point to 
paleaolakes and paleoseas than recently existing liquids. Of particular interest are the areas of Tui 
and Hotei Regio. Several former studies have proposed that these surface features are cryo-volcanic 
flows or possibly present dry lakes/seas.Thus this work provides important evidence to reveal the 
nature of these key features on Titan. It also expands the usage of specular reflections i.e. the NRCS 
not only for the detection of recent liquids on a planetary surface but also areas, which have been 
covered by liquids in the past. Thus this work significantly furthers or knowledge of Titan's methane 
hydrologic cycle. This work will be certainly be of high interest to the scientific community, in 
particular with the increasing interest in the study of exoplanets. Thus, I gladly recommend the 
publication of this work. I only have a few small points, which the authors might consider. 

1. The paper jumps from the abstract to the results. I rather would use the second paragraph of the 
abstract as a short general introduction into the topic and motivation of the presented work. 

2. I recommend to remove the VIMS data from Figure 1. They are diffiult to recognize and 
considerably decrease the contrast of the radar images. It is enough to show the VIMS 5-µm bright 
spots in Figure 3. Instead I would consider to show the subsets VIMS 5µm data of Tui and Hotei 
Regio similar to Figs. 3b and d. This would be particularly interesting for Tui regio, where no stereo 
topography is available. 

Reviewer #2 

This  paper  is  interesting  and  proposes  an  interpretation  of  Arecibo  and  Green  Bank  radar  specular 
observations  of  some  low  latitude  regions.  The  authors  propose  that  the  mentioned  observations  are 
correlated  to  possible  paleolakes/paleoseas  discussed  by  MacKenzie  et  al.,  on  the  basis  of  infrared 
spectro-imaging.  However,  some  aspect  (detailed  below)  have  to  be  clarified.  For  instance,  the 
“anomalous  reflections”  have  to  be  identified  quickly  in  the  text  …  don't  forget  that  Nature 
Communication  has  a  pretty  broad  audience.  Similarly,  it  is  not,  for  me,  always  clear  what  we 
observed: distribution of incident angle? … ? 

Beginning of the text: the authors should be very clear concerning the "anomalous reflections". 
Are they the two around NRCS ~ 3.5-4.0 acquired in 2003 and 2005? 

l. 55-99 and Fig. 1: How did the authors deal with the georeferencing with Cassini and ensure 
consistency with AO/GBT observations? Did they make estimations of uncertainties related to 
this technical aspect? 
My feeling is that AO/GBT geometry could be much less constrained than Cassini ones. 

l. 88-91: "The NRCS of Titan's surface depends sensitively on incidence angle (e.g., ref. 2,8,9 ) 
and the AO/GBT specular reflections are reflections from the surface at nadir (0°) incidence. 
The incidence angles of the RADAR images of the AO/GBT locations, however, range from 10-77°" 

> So, are 0° incidence observations included in AO/GBT set? If the letter range between 10 and 77° 

this is not the case. Please, be more clear. 

Fig 1 caption: “Figure 1: A. Map of Saturn’s moon Titan. The monochrome swaths are Cassini 
RADAR 7 images, which are overlain on a Cassini VIMS 34 mosaic. The purple and red dots are 
AO/GBT subradar locations; red dots are locations where the maximum-NRCS was ≥4 standard 
deviations above the noise. Dot radii are linearly proportional to NRCS. Colored tracks are Cassini 
RADAR altimetry observations where color indicates NRCS. There is a high concentration of large, 
red dots at ≈70-135° W, 15-30° S and many Cassini altimetry observations in this region also have a 
high NRCS. B. Boxed area enlarged.” 

> in Fig 1, many Cassini radar swaths indicates a NRCS not really correlated to AO/GBT NRCS (or 
even frankly anticorrelated), for instance : 

How can the authors explain this aspect? 

> In both cases (AO/GBT and Cassini): specular nadir reflections? 

l. 118-121: “The southeastern part of this area is Hotei Regio, Tui Regio is the southwestern part, and 
the  area  in-between  is  southern  Xanadu.  Hotei  and  Tui  Regiones  have  very  similar  properties  (see 
below).  Figure  1  shows  that  Hotei  Regio  has  a  high  NRCS  in  both  AO/GBT  and  Cassini  altimetry 
observations.” 

> to improve the readability please indicate Hotei Regio, Tui Regio and Xanadu on the map of Fig. 1, 
for instance using contours (with clear labels like “Hotei”, “Tui” and “Xanadu”). 

l. 139-140: “Nine AO/GBT locations were imaged by RADAR before and after the AO/GBT 
observation; no surface changes are detected.” 

>  Not  logical,  please  reformulate  your  sentence  …  “...  before  and  after  the  AO/GBT  specular  (?) 
observation...” 

l. 143-147: 
"The  low  frequency  of  rain  events  on  Titan,  long  reversion  timescale,  low  frequency  of  clouds  over 
Hotei  and  Tui  Regiones,  and  RADAR  image  constraints  all  suggest  that  transient  liquids  are  not 
responsible  for  the  AO/GBT  specular  observations.  An  explanation  that  does  not  invoke  temporal 
changes  is  presented  later  in  this  paper.  Transient  liquids  are  therefore  an  unlikely  and  unnecessary 
hypothesis." 

>  A  few  references  are  needed  here.  The  point  is  important  and  need  to  be  supported  by  some 
additional  theoretical/observational  arguments.  Even  if  they  are  far  to  be  perfect  (particularly 
concerning  the  representation  of  the  methane  cycle)  don't  we  have  any  clue  in  term  of  rainfalls 
frequency  from  GCM?  On  the  VIMS/ISS  side:  are  there  new  observations  analysis  showing  some 
indication of rainfalls frequency? Rodriguez et al.? Corlies et al.? I would be more easily convinced by 
an (even crude) estimation  of the  probability  of observing a transient liquid  feature. No way to  be a 
little bit more quantitative? 
But,  I  recongize  that  the  authors  are  probrably  right  by  stating:  “Transient  liquids  are  therefore  an 
unlikely and unnecessary hypothesis.” 

Fig. 2: All the data collected in these plots are nadir observation? No distribution of incident angles (to 
be sure we are observing a distribution of incident angles)? Please be clear. Please show (with another 
colour?) observations which are  considered  “anomalous”. The “peak” observed in  Cassini  data is not 
really in accordance with AO/GBT data since for Cassini with have a peak position around ~23°, while 
for AO/GBT we get a position higher by a few degrees, please put ticks every degree. 

l. 170-185: 
The author clearly identify the “anomalous reflections” only at this point of the text … in my opinion 
they should be identify before. 

> In Fig. 2 please put anomalous reflections measurements in another color. 
 
l. 87-196: 
“Titan  has  dunes  analogous  to  sand  dunes  on  Earth  except  that  the  grains  are  likely  composed  of 
hydrocarbons  (tholins)  rather  than  silicates  (e.g.,  ref.  15  ).  Dune  fields  cover  millions  of  square 
kilometers  on  Titan,  primarily  in  its  tropical  region,  and  are  one  of  Titan’s  main  terrain  units.  The 
AO/GBT  specular  reflections  are  from  Titan’s  tropical  region.  From  this  cursory  perspective,  it  is 
tempting to relate the specular reflections to the dunes or interdune areas (which are brighter than the 
dunes  in  Cassini  altimetry  observations  16  ).  On  closer  inspection,  however,  there  is  no  correlation. 
Seven  AO/GBT  locations  are  clearly  dune/interdune  regions  in  RADAR  images  and  their  AO/GBT 
maximum-NRCS range from 0.56-1.09. From Figure 2, these NRCS values are not anomalous and do 
not  include  the  specular  reflections  of  interest.  Thus,  the  dune  fields  are  decidedly  ruled-out  as  the 
source of the AO/GBT specular reflections.” 

> Very well, but why dune fields should be considered as very efficient reflectors? Since they are pretty 
rough  …  a  priori  they  should  be  bad  reflector!  What  is  the  underlying  physical  process/scenario 
assumed by the authors? 

Fig.  3:  is  the most convincing argument  up to  this point. The correlation  between AO/GBT relection 
and 5 microns bright zones identified by MacKenzie et al. is striking. 

l. 226-227: How the radar brightness of these possible paleolakes/paleoseas compares with the radar 
brightness of river dry beds proposed by Le Gall et al. (2010)? 

l. 254: “The dielectric constant of a surface depends on its composition (and other parameters) … ”: 
what are these “other parameters”? Chemical composition is not the main parameter? 

l. 266-267: “There are two noteworthy exceptions to the strong correlation of specular AO/GBT 
observations to Hotei/Tui. The first has a subradar point at ≈47°W, 13°S, … “ : please show this point 
with a label, an arrow … in one map. 

l. 275-275: “The second exception has a subradar point at ≈163°W, 24°S and was acquired on 
11/3/2000.” : idem, label in a figure/map. 

l. 299-304: “Using Titan as a solar system ground-truth in the search for oceans on exoearths suggests 
that,  to  identify  liquids  by  specular  reflections,  a  stringent  definition  of  specular  should  be  used.  We 
recommend  a  definition  based  on  the  coherence  of  the  reflected  electromagnetic  waves  rather  than 
definitions based on combinations of relative brightness, incidence angle, location, and/or polarization; 
reflections that are specular in the latter senses are quasi-specular.” 

>  For  “exo-ocean-planet”,  the  integration  time  of  an  electromagnetic  signal  could  be  quite  long 
compared  to  timescale  associated  to  waves,  this  could  significantly  alter  the  coherence  of  the  signal. 
But,  the author are right encouraging scientific discussions about  what  could  be the best  criterion  for 
the identification of specular reflection. 


Reviewer #3 (Remarks to the Author): 
 
I do think the results presented in this manuscript would be of interest to the planetary 
community. However, to improve the impact factor, I recommend a rewrite of the manuscript 
to clean up some problems. The main issue is the way the manuscript is written – it is highly 
disconnected to a point that it is difficult to read. Grammatically, the manuscript is fine. 
However, due to the disjointed nature of the writing, the manuscript is very hard to follow 
and therefore not as easy to interpret. For example, consider the sentence starting with 
“Thick haze and abundant methane…” on lines 31 and 32. The sentence is out of place and has nothing to do with the sentences around it. My guess would be that the author is trying to tell a story about the aerosol, which obscures 
Titan’s lower atmosphere and surface at visible wavelengths – this is justification for going 
to longer wavelengths. But, this is well known and as written, it is highly disconnected and 
out of place, which is true for much of the manuscript. It almost reads like random thoughts 
were placed together, which borders on being incoherent. This type of disconnect in the 
writing appears throughout the entire manuscript. 
 

The structure of the manuscript is also rather odd. It begins with a two paragraph Abstract, 
followed by a very long Result section with no subsections, and ends with a Discussion 
section. As written, I’m not convinced that ground-based AO/GBT observations are indeed 
specular. This impact factor is simply not in the as-written manuscript. A major rewrite is 
needed especially if published in a higher quality journal. 
 
 
In the Results section, when you call out Fig. 1, make sure to state in the text (just like you 
do in the figure caption) that these are maps from the Cassini RADAR instrument. Stating 
just a “Cassini map” is careless. Also, when were the RADAR swaths acquired? I would add 
the time and Cassini flybys in the text and captions, and also do the same for the VIMS data 
shown. 
 
 
If you are going to dive into four very long explanations as to how Figure 1 is different than 
Figure 17 in Black et al., then you need to first introduce what Black et al. was reporting. 
Instead, I’d add a subsection and discuss how this work effort compares to other efforts. 
And, make sure to show more figures, especially talk to the ones you keep referring to in 
other publications (like those of Black et al.). 
 
 
To improve the manuscript, I would really rethink the figures. Fig. 1B doesn’t add much. Fig. 
2 B and D don’t add much either – A and C carry the impact factor. Do you really need Fig. 
3A since you have Fig. 1A? Fig. 3B,C, and D are really good.