General comments:

Dear Nikolaus Kuhn, Federica Trudu and (associate) editor,

I have read the manuscript with great interest. Experiments in low g are rare and are therefore a very valuable addition and of great significance to the planetary surface processes community. I am convinced the new data will be appreciated, as is this new experimental setup to acquire grain settling data during parabolic flights. I am looking forward to more data with different grain sizes in the future.

The scientific quality of the manuscript is good. A similar methodology was already used in previous publications by the authors, and this manuscript clearly describes why and what had to be improved for this flight (L69-74). As with all experiments, the authors, and I, see room for improvement, but this is no reason not to publish this work. I think with some moderate changes this paper will be a great addition to ESurf. The authors have especially paid much attention to the uncertainties in the measurements, which I appreciate. However, I think more space could be spent on a hypothesis or explanation why the predictions by the Feguson and Church relation deviate from the measurements. In my opinion, the process description is minimal, and therefore the results are less convincing. If one cannot find an explanation in processes, one keeps searching for errors.

The manuscript was easy to read, understand, and is concise. In my specific comments I mention some concerns about the structure (most importantly parts of the Results and discussion I consider methodology) and address some confusing sentences. I think overall, the manuscript could benefit from naming the experiments 1 to 12 and clearly distinguishing between the planned experiment and what was measured, e.g., how many particles, preferentially in one Table (possibly replacing Table 1). This would clarify some statements and the content of other Tables.

Overall, there are two major things I am missing in this manuscript: 1) reference experiments with Earth gravity, and 2) an overview graph of settling velocity vs gravity containing al the measured data of individual grains (with uncertainties) and predictions by Feguson and Church. These points will be further addressed in the specific comments section.

Specific comments:

Abstract

1. The first sentence of the abstract (L10) is very general. There is no explanation (here or in the introduction) why it is essential.
2. “models” (L20) is not defined here and therefore the sentence is unclear. There are many types of models. Please specify you mean the Ferguson and Church relation.
3. “underestimate” (L21), by reading the rest of the manuscript I understand this follows from the data and previous experiments, however, without context I would think terrestrial inputs leads to overestimation of settling (because gravity might not be correctly included), not underestimation. This thought process should be explained or previous experiments should be referenced.

Introduction

1. “Drag depends on the size, density and velocity of the particle,” (L33) And shape, right? This seems to be the case according to Ferguson and Church (2004).
2. Similar to the abstract, specify that with “models” (L56) you refer to the Ferguson and Church relation. Just “models” is too broad.
3. Similar to the abstract, I do not understand why you assume a potential for “underestimation of sedimentation velocity on Mars” (L57). I have trouble following your thought process without having read about the previous experiments or results. Lower settling velocity for Mars due to lower gravity, like Ferguson and Church predict, makes sense to me. Please explain why you think it could have been an underestimation. Without context, the opposite would make more sense to me. If you calibrate everything on Earth, you might underestimate the gravity effect, so overestimate the settling velocity.
4. Same issue, “underprediction” (L64), underprediction by? Feguson and Church? Compared to Earth?
5. “Drag values derived on Earth” (L65). Do you mean using the same drag value for Earth and Mars does not work? Or is the predicted drag based on Mars gravity by Furguson and Church does not work?
6. L69-L74, very clear why these experiments were conducted.

Materials and Methods

1. The methods (section 2.1) are very clear.
2. Figure 2: The figure is quite clear. But I do not understand why it was not made to scale, which seems like it would be an improvement. Or would that make certain elements too small?
3. Table 2: the caption of Table 2 is very unclear as it refers to Chamber number, but the reader has no information about which experiments was performed in which chamber. There is mention of a 1 particle lunar experiment and a 3-particle experiment, which are not mentioned in Table 1. The naming in the left column of the Table is in my opinion also unclear.
4. Table 2: I am missing the data of reference experiments on Earth. This seems valuable information and an extra data point in terms of gravity and certainly in terms of validation.

Results and Discussion

1. “Some samples got stuck as they moved from the upper valve to the lower ball valve” (L171). Did this not happen during your tests with Earth gravity?
2. Parts of the result section should be transferred to the methods section. L171-188 in section 3.1 and L214-223 section 3.2 was not measured or discovered by the authors. I am also unsure if the error determination should be in the result/discussion section. This could also be methodology or separate discussion.
3. “a group of three particles (Sample 1 and Sample 1/3 to 3/3) has been detected.” (L189-190). Clarify that this was due to a problem in the experiments. Also, clarify what you mean with Sample 1, 1/3 and 3/3. These names were not defined. Is it related to number of particles, sample number or something else? Consider naming your experiments or samples 1 to 12 and indicate their planned and measured particles to avoid confusion.
4. In my opinion it is valuable to create a graph of settling velocity (terminal fall velocity) over gravity which includes all data points of individual particles, uncertainties, and Earth experiments. In this case the reader can decide for themselves if the uncertainty is good or bad. This graph can also contain the prediction by Ferguson and Church.
5. “Uncertainty of the position data” (L201). Despite that I think the uncertainties are reasonable, one aspect of uncertainty was not mentioned. Due to the viewing angle of the gopro and the distance between the particle and the ruler, the particle can appear in a different location. If the particle is close to the ruler the uncertainty is smaller than when the particle is closer to the gopro. The distance travelled might look larger than in reality due to the viewing angle. This could lead to overestimation of the calculated settling velocity.
6. “It is plausible to hypothesize that there was a slowdown, due to particle interaction” (L234-235) Earlier you argue this is not the case.
7. L237: Can you further expand on why you argue that fluid status is the cause of the difference between predicted and measured values? Why is it not and how should it be included in the Ferguson and Church relation?
8. I think it would be a good idea to also vary particle size in the future. I think it is more important information that tests with 1, 5 or 10 particles, which is in al cases likely to few particles for significant hindering to occur anyway.
9. Section 3.2: I am really interested to see if your tests on the ground with Earth gravity compare 100% with the predictions. If you cannot show this, it seems impossible to me to prove that the chosen values for C1 and C2 were inaccurate. These are calibration parameters. For glass spheres the difference should be minimal, but still, it is worth showing you can reproduce the predicted values with your experiments.

Conclusions

1. L251-252, see previous comment, I am not convinced there is no distortion. The curvature due to the lens might be removed, but the issue of the viewing angle remains. If the particle is far away from the ruler and closer to the camera, it might appear to be at a different height then in reality. I am not sure, but I think this distortion is increased by the air-water transition.

Technical corrections:

Abstract

1. “Once operational, it will be …” (L12) Is it not operational now? This sentence should not be future tense.
2. “… with settling particles forming a sediment” (L15) Particles are sediment. This part of the sentence should be rephrased or removed.
3. Remove “that” (L20)

Introduction

1. References should be merged to 1 set of brackets (L30): (Yin & Koch, 2007; Hagemeier et al., 2021). This should be corrected throughout the paper.
2. “(see equations (1) and (2))” (L45) Brackets in brackets here are unnecessary.

Materials and Methods

1. “is” (L108) replace by “was”
2. “while density” (L136) replace by for example “with densities ranging”.
3. “planned measurements” (L141). Past tense, probably a remnant of a research proposal. Replace by for example “the experiments”. If the planned and executed experiments are different, please specify.
4. Punctuation problem at L167
5. Figure 5: I only see 4 particles.

Results and Discussion

1. Capitalise “discussion” in section title.

Author has presented the findings in a systematic manner.

The experimentation is essentially done on spherical particles. The author is requested to narrate how it can be extended to the non-spherical particles.

Design of instrument has been described in excellent and lucid manner.