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Spoilers for The Last Jedi, this banner is to just make sure any previews don’t have the spoilers pop up. Note: all quotes are para-phrased.

During the final battle on Crait, when they begin the battle, a rebel climbs out of the trench and scans the First Order forces landing on the planet.

One soldier takes up some of the white powder on their finger, licks it and says “It’s salt.”

Was there any relevance to there being a layer of salt? Has this been commented on by any officials? Why could they not simply have used any other white powder or something else?

Did it possibly have any part in:

Allowing Luke to display himself there as an image, while still on Ahch-To?

Essentially the question is why did they point out the salt? I apologise if this was previously unclear.

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It could be snow instead of salt, that's irrelevant. The point is that it's a very thin white layer that is wiped clean by simply walking on it.

When the Resistance leader walks on it, he reveals the red underneath. But when Luke moves his feet while facing down Kylo Ren, the salt stays and the red underneath isn't exposed! It's a subtle hint that he's not actually there in physical form.

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    But his projection is clearly physical in some sense: he can affect Kylo's lightsaber, and the dice that he projects persist for a while and are physically interacted with. – Adamant Dec 18 '17 at 6:32
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    He actually never makes contact with Kylo's lightsaber (or Kylo himself). He dodges around for a while, and the first swing that would hit him completely goes through him. But you're right about the dice - Leia touches his hand and then holds the dice. We already know Luke/Leia have a strange connection from being twins and being connected by the force - I assumed she was the only one who could touch him, and perhaps she even realized that she was only touching him through his force projection. (Kylo can pick up the dice for the same reason - he's Leia's son). – LevenTrek Dec 18 '17 at 6:38
  • I don’t think that’s it - while familial ties could have something to do with it, I don’t see any reason they should be. Rather, I think Luke can choose how tangible his projection is. If he doesn’t focus on making himself weigh something, he won’t weigh anything. If he wants to project himself physically, he can - it’s really just Force fields and light in the end. – Adamant Dec 18 '17 at 6:40
  • That is 100% entirely possible. The dice is definitely a big hole in the "non-physical form" idea - but he does dodge every lightsaber swing, and doesn't move the salt, so he clearly chose to be non-physical for some parts. Too bad he didn't explain it to us before becoming a force ghost. Perhaps it's just beyond us mortals... the force is just a mystery to us. It surrounds us and penetrates us, but doesn't live in us like it lives in Rey :-) – LevenTrek Dec 18 '17 at 6:48
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    @Adamant what do you mean he can affect his lightsaber? He's never shown interacting with Kylo's lightsaber. – Edlothiad Dec 18 '17 at 7:06
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I think that it is to make clear to the audience that it is not snow. Visually there is no way to know what it is, which is why the resistance fighter tastes it.

Audiences know of snow planets in the star wars saga, I would imagine that there were enough comments in production about it being 'snow' that they felt the need to show it was something different. Not snow above a layer of frozen blood which was my first impression.

With regards to the question in spoiler, I don't think the planet surface had any bearing on it. The phenomena happened both inside, with a normal surface, and outside, on the salt.

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    But what is the relevance of pointing out the salt? That is the question at heart. Considering the white powder could've been anything. There must be a reason for salt. – Edlothiad Dec 14 '17 at 20:06
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    @Edlothiad It could easily be that they just wanted to point out that it isn't like Hoth (ice and snow desert) and salt was the only thing that came to mind for them. – Thomas Dec 15 '17 at 12:11
  • @Thomas bear in mine the answer was completely re-written 30 minutes ago invalidating my comment. – Edlothiad Dec 15 '17 at 12:12
  • Yes, I put in edit comment that I had changed it but didn't mention it in the answer. Sorry. – Jeremy French Dec 15 '17 at 12:42
  • "Not snow above a layer of frozen blood which was my first impression" - wow... I have no idea where that idea came from... I feel like maybe you should talk to somebody about the fact was that your first thought was a sea of frozen blood... ;-) – Chris Dec 26 '17 at 21:32
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These are just my thoughts, and I can't find anything explicitly and/or officially stated anywhere to back them up, but here's why I believe they chose to use salt for the surface of Crait. IMHO, it mostly has to do with visual effects choices made for the planet and its fauna.

  1. From a pseudo-scientific visual effects perspective, it couldn't have been snow in order to produce the spectacular red plumes behind the V-4X-D Ski Speeders as they raced across the surface. Snow (being composed of frozen water in crystallized form) would have, over time, gone through a cycle of melting and refreezing, which would have compacted the red mineral beneath it into more of a clay, which would (most likely) not have the appropriate physical properties for making such a large cloud of pulverized "dirt" by simply skiing across it.
  2. While the white covering could potentially have been sand, a planet comprised of mostly sand (e.g., Tatooine, Geonosis, etc.) probably would not produce a creature with the crystalline look of the vulptex (crystal wolf). Salt is a much more likely "base" for producing creatures with crystalline features. Additionally, sand would generally not produce the same reflective properties as salt, nor would it be as "slippery". Even though it isn't expressly pointed out in the movie, according to Wookieepedia's description of Crait's salt flats:

The white salt-covered surface reflected the planet's sunlight, which would be blinding at midday. The sodium-containing salt also made the planet surface slippery, hard to travel; with gusts of wind sending up flumes of white salt.

Of course, they needed something that would shift under the characters' feet as they moved across it in order to "subtly" give away Skywalker's secret during his battle with Kylo Ren. With snow and sand excluded for the reasons above, salt seems to be the next most logical choice. As I said, these are just my speculations, but they seem to be the most reasonable explanation for why salt was specifically chosen in this case.

As for why they made a point of mentioning it (in answer to the actual question you asked), if the filmmakers were following the same line of reasoning as I've outlined here, this may have been done in an effort to explain the points I made above.

EDIT: I found the following article from Earther.com that explains some of the "actual" science that could potentially be in play on the surface of a planet like Crait. Titled "The Amazing Earth Science Behind The Last Jedi's New Mineral World", the article talks about the possibilities that could lead to the red color just underneath that layer of white salt:

Malaska [Mike Malaska, a planetary scientist at NASA’s Jet Propulsion laboratory] could think of a few potential explanations.

For one, it’s possible that we’re looking at a very thin coating of salt, atop some sort of bright red soil. “When salts and dirts are mixed and wetted, the salts can dissolve, then get wicked up through the dirt and evaporate, making a nice white crystal evaporite on top,” Malaska explained.

The salt’s crimson color could also be the result of radiation damage—halite (table salt) can turn dark red-brown, and sylvite (potassium chloride) can turn purple when radiation bombardment causes defects in the minerals’ crystal structures. In this case, recrystallization of a thin, upper salt layer has reset the crystal lattice, but the radiation damage—and bright colors that accompany it—are visible right beneath the surface.

Finally, Crait’s crimson salt streaks could be the result of some weird alien microbes.

Plenty of salt-tolerant microbes live in salars on Earth, taking advantage of the ample sunlight and abundant chemical energy available in the minerals. But they have to be UV radiation-tolerant, a feat which certain bugs, like the pink cyanobacteria in Death Valley’s salt flats, accomplish with brightly-colored pigments. Perhaps Crait’s microbes have evolved a red pigment to protect them from the harsh, alien sun.

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The fact that it is salt comes up during the "duel" between Luke and Kylo Ren. When Ren slides his foot on the ground, salt dust rises. When Luke does it, no dust (and maybe marking) is made, hinting at the fact that Luke is not actually there. So the salt is relevant in that regard.

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    But it didn't have to be salt, it could've been snow, or sand or anything. – Edlothiad Dec 18 '17 at 20:33

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