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Watermap Mars1
Credits: NASA/JPL-Caltech/ASU

The map above shows underground water ice on Mars, it's from the article NASA's Treasure Map for Water Ice on Mars.
The blue and purple represent water ice less than 30 centimeters (one foot) below the surface, and the black zones indicate areas where a landing spacecraft would sink into fine dust.

..; the outlined box (on the map) represents the ideal region to send astronauts for them to dig up water ice.

(emphasis added by me)

But (part of) the albedo map of Mars below from MARS Global Data Sets shows that this so-called "ideal region" is rather bright in general.

Albedo map Mars

From Mars Global Surveyor Thermal Emission Spectrometer experiment: Investigation description and surface science results (page 23,848):

Therefore it is likely that the surface rocks and sands in the dark regions are less coated by bright dust than those seen at any site visited to date. It has been suggested that Mars dark regions are likely areas of active removal of bright dust by sand saltation or other means [Thomas 1984; Christensen 1988]. Landing sites in dark regions might provide an opportunity to directly sample exposed, dust-free rocks [Edgett et all., 1994].

So it might be that the "ideal region" in the outlined box will be rather dusty, despite not enough "to sink a spacecraft". And Martian dust is poisonous, containing perchlorate, so digging for the water ice would be better done without dust.

So to what extent will the presence of dust in a region with water ice close to the surface, be a reason to look elsewhere for better conditions ?

For example, there are two regions next to each other, one rather dusty with water ice 20 cm below the surface and the other almost free of dust but with water ice 30 cm below its surface.
What will be the choice, and with what reasons ?

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    $\begingroup$ Is dust anywhere on Mars sink-into-able? $\endgroup$ – ikrase Aug 23 at 18:55
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    $\begingroup$ Yes and no. Drop something some of the large dunes, leave for 100 years and it might well "sink". But not within a mission timeframe. Certainly not in a "where did I put my spaceship" sense. $\endgroup$ – user2702772 Aug 24 at 7:07
  • $\begingroup$ Martian dust is not notably poisonous, and spacecraft aren't going to sink into it. $\endgroup$ – Christopher James Huff Aug 24 at 13:54
  • $\begingroup$ @ChristopherJamesHuff hub.jhu.edu/magazine/2018/winter/… $\endgroup$ – Cornelisinspace Aug 24 at 14:18
  • $\begingroup$ @Cornelisinspace What of it? That article only mentions effects on instruments and solar power. It makes no claims of toxicity or other hazards to spacecraft. $\endgroup$ – Christopher James Huff Aug 24 at 14:22
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Water is extremely useful on Mars. You can use it to generate oxygen directly via electrolysis and the hydrogen byproduct can be reacted with carbon dioxide to make even more water. We drink water. If they want to come home, water+carbon dioxide+power can produce methane and oxygen. That's the most likely rocket fuel to get home.

On the other hand, dust is everywhere. You can't avoid it - the place has planet wide dust storms. More dust, less dust, you have to deal with the dust either way.

But water you're going to need by lots. Hundreds of cubic meters. Stick with the water.

Edit to add:. Dust will interfere with almost all missions to Mars. Not having enough water will end missions.

Edit to add: To answer the 20cm Vs 30cm question. Consider gardening on Earth. How much harder is it too dig a 30cm hole, Vs 20cm? Not all that hard. In any near surface ice bed the local variation in depth will be more than 10cm. The real question will be "Where is ice within 1m of the surface".

Once you've dug down to ice, as can put in a simple air lock, pump in CO2 and prevent the ice from subliming away. If it's more than 1m down... Look at the problems Insight has had digging at all.

You must have water, dust is so much less important. You can travel to features you're interested in. You can't realistically dig water up from 70m down or carry tonnes for kilometers.

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    $\begingroup$ But would it not be important to have dust as little as possible ? Martian dust is poisonous ! $\endgroup$ – Cornelisinspace Aug 23 at 19:19
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    $\begingroup$ Well, yes and no. Most of it is iron oxides and titanomagnetite. Not great, wouldn't want to eat, but not Poisonous. I think you're thinking about perchlorates, which are a problem with the soil. But this is a soluble problem - literally - they're very soluble in water. Wash soil with water, distill the water, rinse and repeat. Puns intended. Or heat it to 500 degrees or so, perchlorates decompose into salts and oxygen. Unless you're thinking something else? $\endgroup$ – user2702772 Aug 23 at 21:03
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    $\begingroup$ Very nice answer! Since no good deed goes unpunished, I've just asked Will MOXIE get clogged or chemically degraded by martian dust? $\endgroup$ – uhoh Aug 23 at 22:36
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    $\begingroup$ @Cornelisinspace That paper backs up what I said about the iodine issues, and covers some approaches to removing perchlorates. It doesn't invalidate anything I've said. Perchlorates are mildly toxic with effects that can be directly mitigated with iodine supplements, present in small concentrations, do not bio-accumulate, are unstable and easily decomposed by heat or chemical/biological action, and they are highly soluble and easily washed out of soil. They're a minor detail to living on Mars. $\endgroup$ – Christopher James Huff Aug 24 at 15:21
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    $\begingroup$ Particularly note what I said about bio-accumulation. What manages to get absorbed without reacting with something gets excreted in the urine, with a serum half-life in humans of 6-8 hours. As a reactive oxidizer, it will quickly find some reducing agent to react with in water recycling or agricultural systems, and likely won't survive to reach a human a second time. What you need to be worried about in closed systems like this are things like heavy metals or persistent organic pollutants that could just build up over time and concentrate at the top of the food chain...the humans. $\endgroup$ – Christopher James Huff Aug 24 at 15:32

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