The 3D Direct Simulation Monte Carlo Study of Europa’s Gas Plume

Round 1

Reviewer 1 Report

This paper analyzes different plume cases and compares the resulting plume characteristics among each other and with observations. The paper highlights the importance of including collisions in plume modeling, as these noticeably influence the observable characteristics (e.g., plume height). In addition, the paper presents density and column density profiles that will be extremely useful in future in situ plume observations, both in planning and analysis thereof.

The paper is very well written, presents a clear structure, and a well-comprehensible discussion and conclusion. The methodology is sound as are the results and the conclusions. This paper provides an important step in the direction of plume modeling, showing how properties change with changing plume characteristics, but also by showing that collisions are important to include.

I have only a few minor comments that I noticed. I am starting with the general comments:

• the authors often use 'undersurface'. I think 'subsurface' is the term commonly used
• the bins in some of the colorbars are somewhat odd. I think that this is a result of the covered range and the chosen number of bins. E.g., for the knudsen number, regimes are often thought to change at 0.01, 0.1, 1, and 10, for example. I would find reading the plots easier if they were presented in bins that are integer exponents of 10.
• Some of the text is phrased as if it is clear that the observed plumes are originating in the subsurface ocean. This is not the case, though. Please read carefully through the text and ensure that whenever this is discussed it is worded the right way.
• In addition, the extent of the structures in the different parameters is different. E.g. for case 1, the extent of the area where the trans temp is different from space is smaller than the extent where the Knudsen number deviates from space (similar statements are true for other features). I assume that this is an effect of the chosen color bins, but it seems a little odd.
• You never introduce the missions (or the JUICE acronym). Please introduce these and provide references.

For the specific comments, I noticed the following:

• Abstract:
• You talk about 'the various case studies', but these have not yet been introduced. So please either write 'various case studies' (without the 'the') or introduce the case studies first.
• Introduction:
• You don't mention the Hubyrighs paper. I know that Jia wrote a reply to this paper, but Huybrighs wrote also a note to that note. Please at least mention the Huybrighs study.
• You write that the plumes 'likely' originate from the subsurface water. Please check the references to see if the observers actually state so.
• You write '(3) the water molecule observed'. An 's' is missing.
• When you talk about derived column density, please indicate whether it is transversal order radial column density that was observed, as this influences how they compare.
• You write 'The durations of Europa’s plume eruptions are unknown, either, '. Please either write 'The durations of Europa’s plume eruptions are not known, either, ' or 'The durations of Europa’s plume eruptions are also unknown, '
• Materials and Methods:
• When listing the code features please add an 'and' before the last feature.
• You mention how much the plume height and width change when modifying vent parameters, but you don't provide the information on how much these were varied. It's difficult to assess if the change is a lot or not without knowing how much the parameters were changed.
• Results:
• You state that in case 3 gamma is not close to zero, but I can't see this. There are dark blue regions, so why do you exclude case 3 from this statement?
• I appreciate the number and column density plots. Could you add in one of the previous plots circles at the given altitudes, though? It is difficult to assess where in the previous plots these altitudes lie.
• Also, for the column density, can you please add a sketch showing the two viewing angles. The setup is not entirely clear to me.
• Discussions:
• When talking about the observed plumes, please mentioned that all observations are likely of different plumes and that the observed parameters thus don't have to agree.
• In the last paragraph, you talk about the plumes carrying along ocean water. Again, this does not necessarily have to be true as we don't know the plumes' sources.
• Summary:
• Not all speeds are supersonic.
• Also, in the second to last sentence: The measurements can only shed light on the subsurface ocean environment if this is where the plumes originate.

Author Response

We highly appreciate the reviewer's effort to make this manuscript better.

All the changes made corresponding to each point are already shown in the revised manuscript (please see "Track Changes"). 

Reviewer 2 Report

This paper is an interesting study by means of mathematical modeling of the plume structure near the surface of Jovian moon - Europa. The authors had developed the 3D DSMC model toinvestigate the physical processes and space structure of the water plumes observed at south pole of  Europa. Results of this modeling study will be useful for the interpretation of the future measurements .

I have a few comments and questions cocerning this manuscript. These comments are placed in the attached manuscript file.

When these comments and suggestions will be taken into account the manuscript can be recommended for publishing in the Universe journal.

Comments for author File: Comments.pdf

Author Response

We highly appreciate the reviewer's effort to make this manuscript better.

Some changes made corresponding to some questions are already shown in the revised manuscript (please see "Track Changes"). 

Some answers to the specific questions, not written in the manuscript, are replied in the following.

1. The boundary conditions at the moon’s surface:

The low temperature (< 120K) (Ashkenazy, 2019) on the surface of Europa suggest that the water molecules will mostly re-condense when they collide with the boundary of Europa's surface. Therefore, the simulation particles in the model will be removed when they collide with the surface, and the accordingly backward flux is not considered.

2. What is a characteristic macroscopic length L used to calculate Kn?

The Europa's radius (1560 km) is used as a characteristic macroscopic length, L. It is good to investigate the global structure of Europa’s large plumes. Since it is a constant value through the whole plume, the Kn plots (Figure 1) represent the actual variations of dynamical and kinetic scales for the region under study.

3. The ratio between radial and tangential (transverse) temperatures:

This ratio is not analyzed in the current work. Instead, we choose to study the local thermal equilibrium between internal (i.e. rotational) and kinetic (i.e. translational) temperatures which would be helpful for the future study of radiative transfer process for multiple-wavelength remote sensing observations.