Peer review of: Sublimation of terrestrial permafrost and the implications for ice-loss processes on Mars (2021)

Reviewers' comments:  
 
Reviewer #1 (Remarks to the Author):  
 
The manuscript by Douglas and Mellon combines observations and theory to better constrain the 
magnitude of sublimation on both Earth and Mars. The study is based on observations that were 
made in a tunnel in the permafrost, thanks to the fact that the tunnel was carved 52 years ago, 
providing a timescale sufficiently large to be more significant than lab based studies. Their 
conclusion has strong impact on terrestrial studies on permafrost as well as studies on Mars 
geology. They observe sublimation rates 4 times higher than predicted.  
 
Overall, I think the paper is well done, the sections with terrain data and measurements are well 
detailed and the implications well written. I have only minor comments before publications. The 
paper will be a nice contribution to this topic for multiple communities (Geomorphology, Planetary 
science, Environmental Science).  
 
Minor comments:  
- End of page 8: The effective diffusion coefficient ca be scaled to a value of: 9.9 10^-4 m2/s 
There is a difference of two orders of magnitude compared to Earth (9 10^-6 m2/s). First, this 
difference should be better highlighted. Second, what is the main reason of this difference? Is this 
more P or T? Or both? Or other parameters? Is it at fixed tortuosity. I miss some explanations 
here.  
 
- Middle of page 9: It is mentioned that studies were inconsistent pointing to similar values and up 
to a factor 4x. Is this really so inconsistent? A factor 4 is not two orders of magnitude different and 
so it is not that meaningful. Which also lead to the question: What are the implications of the 
finding of your study, in other words, what differences a factor 4 would make in the understanding 
of landscapes or processes on Earth or Mars? For instance, given the poor knowledge on tortuosity 
of Mars ground, would the factor of 4 be so significant compared to the parameters controlling the 
soil properties?  
 
- End of page 6, it is written that equation 3 can be fit to the observations on Figure 4. It would be 
useful to actually show this curve on the plot (for instance by a thin dashed line).  
 
 
 

Reviewer #2 (Remarks to the Author):  
 
Review:” Sublimation of terrestrial permafrost and the implications for ice-loss processes on Mars” 
by T. A. Douglas and M. T. Mellon.  
 
This manuscript claims the first in situ measurement of the diffusion coefficient of ice-free loess 
and argues for its relevance to interpretation of features on Mars. The justification for the 
exclusive nature of the measurement is the unique permafrost tunnel environment, in which 
sublimation is the dominant erosional process.  
 
The manuscript is well written, of appropriate length, etc. (the only quibble is the repeated use of 
the word relic where relict is intended).  
 
The highlighted findings are (1) a measured diffusion constant of .0905 cm^2/s compared to 
previous measurements of .0735 cm^2/s, and (2) a sublimation rate 4x larger than predicted by 
accepted theory, and (3) implications for interpretation of martian geology. Taking these 
separately:  
 
1) The 23% discrepancy of D relative to prior laboratory measurements is relatively small 
compared to the natural variation in properties. Since the measurement is in situ, this might still 
be worthy of reporting if quantitatively correct but, as the authors note in equation 1, the 
measured value is the product of D ∆N, where ∆N is the difference in water vapor density at the 
ice surface and in the tunnel. As a result, referring to steam tables, the same result would be 
achieved if D were identical to the lab measurement and the ice surface were at -2˚C instead of 
the assumed -4.1˚C (i.e. in equilibrium with the air conditioned tunnel air). As stated in the 
manuscript, the Fairbanks area has a mean annual temperature of -3.4˚C as per a cited 2001 
study, already nearly a degree warmer than assumed. Considering the influence of climate change 
since 2001, as well as the fact that mean air temperature and mean ground temperature aren’t 
necessarily the same, -2˚C is a plausible ice surface temperature. Since evidence that the wall 
temperature is in equilibrium with the air conditioned tunnel air was not provided in the 
manuscript, the new result would have to be considered as within the error bar of the old.  

2) A similar argument applies to the anomalous 4x faster sublimation rate than predicted by 
theory. In that case, the theoretical rate goes as ∆rho/rho (or, since it is nearly isothermal, ∆p/p 
where p is partial water vapor pressure). Since RH is maintained at 91% within the tunnel, the 
measured sublimation rate turns out to be consistent with an ice surface that is only slightly 
warmer than the air conditioned air in the tunnel. To give a specific example; saturation vapor 
pressure (SVP) at -4˚C is ~440 Pa, so at 91% RH, ∆p will be ~40 Pa and p will be 400 Pa. If the 
ice surface is at -1˚C (SVP~560 Pa), ∆p will be 160 Pa, 4x the assumed value. As above, lacking 
quantitative measurement of the ice surface temperature, the measured rate would seem to fall 
within the error bars. Thermodiffusion may also contribute to a larger rate if the wall is warmer 
than the surrounding air.  

3) With respect to realistic diffusion constants for Mars, the authors failed to cite the extensive 
work of Hudson and others (Hudson et al JGR 112 E-5-16 2007; Hudson & Aharonson JGR 113, 
E09008, 2008 and references therein), which provide more insight than the work here on both the 
physics of diffusion under martian conditions and the range of expected value.