Is there an explanation for the absence of any sign that would demonstrate the eight retro rockets in the image below were really working? It seems that no gas was getting out of the nozzles.

enter image description here

The sky crane of Perseverance rover with the engines apparently stopped

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    $\begingroup$ One plausible indication that they were working is that the rover didn't crash into the ground. $\endgroup$ – Aaron Lavers Feb 24 at 0:39
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    $\begingroup$ ... and in 50 years of time, people will discuss whether the Mars Landing was a hoax, craftily orchestrated by special effects and CGI. $\endgroup$ – Dohn Joe Feb 24 at 7:45
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    $\begingroup$ The funny thing is that the NASA CGI generated imagery has bright orange flames coming out of the engines. $\endgroup$ – Michael Feb 24 at 18:57
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    $\begingroup$ The reason for that is that there are no people on mars to brain control with chemtrails so they didn't add chemtrail cocktail to the propellant. $\endgroup$ – Stian Yttervik Feb 25 at 10:43
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    $\begingroup$ @Michael I've worked on developing many, many simulations of space vehicles. We have a derogatory name for those flames: we call them "management plumes" or "PR plumes". $\endgroup$ – David Hammen Feb 27 at 4:44

If you watch the coverage on YouTube, they explain hydrazine rockets burn clear. This is supported by the dust flying everywhere when it gets closer to the ground.​

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    $\begingroup$ @azot - The physics of rocket thrusters means that you want the pressure of the exhaust to be close to ambient, and the temperature to be low for maximum energy extraction (some vacuum thruster operations are visible because the exhaust condenses). Given the Martian atmosphere is pretty close to a vacuum anyway an interesting question what the actual pressure and temperature actually was. $\endgroup$ – GremlinWranger Feb 23 at 9:40
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    $\begingroup$ they claimed that they had test footage on earth that showed the same thing but i haven't seen it. They explained people were curious why animators made plumes coming from the rockets during simulations showing people what it would look like, when both on earth and mars its clear. It was just a mix up and something that wasn't communicated. $\endgroup$ – necroncryptek Feb 23 at 9:42
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    $\begingroup$ I think Matt Walace also mentions that if you look closely at the rim of the nozzle, you can see a faint pink glow that indicates its burning. Other than that, yes the exhaust is quite clear and now that we have seen it, the animations will follow suit. Exhaust-plume interaction was interesting to look at and that is certainly nobody could have guessed beforehand. $\endgroup$ – OrangeDurito Feb 23 at 10:08
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    $\begingroup$ @azot this answer explains that these are hydrazine monopropellant thrusters, you can't compare them to a launch vehicle's engines; way different size and scale and power, and totally different chemical mechanisms. Here's an old, strange video, they have to dim the lights and use a black background to see anything: youtube.com/watch?v=9DqtWjZOVfY In the engine it decompose(s) into ammonia, nitrogen gas, and hydrogen gas and that gas escapes the nozzle. There's no flame. $\endgroup$ – uhoh Feb 24 at 4:34
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    $\begingroup$ @azot Note that hydrazine decomposition is often visible on earth because the exhaust is mostly a hot plume of nitrogen gas and hydrogen gas. While they are both invisible gasses, as soon as the hot hydrogen mixes with atmospheric oxygen, it tends to catch fire and burn with a dim orange flame. On Mars, the atmosphere is carbon dioxide, so the hydrogen can't burn. $\endgroup$ – Darth Pseudonym Feb 25 at 19:25

It would be rather surprising if the exhaust from these engines was visible. The engines use hydrazine ($\mathrm{N_2 H_4}$) monopropellant, and the reactions involved (catalysed by iridium) are

$$ \begin{aligned} \mathrm{N_2 H_4} &\rightarrow \mathrm{N_2 + 2H_2} &&\text{exothermic}\\ \mathrm{3N_2 H_4} &\rightarrow \mathrm{4NH_3 + N_2} &&\text{very exothermic, $\mathrm{NH_3}$ is ammonia}\\ \mathrm{4NH_3 + N_2H_4} &\rightarrow\mathrm{3N_2 + 8H_2} &&\text{endothermic, makes more gas} \end{aligned} $$

So there are two important things here: firstly these reactions are catalysed: hydrazine decomposes like this only in the presence of an iridium catalyst; and secondly the products of these reactions are colourless gases: hydrogen, and nitrogen, probably some ammonia and probably some unconsumed hydrazine.

We also know, from this paper (that copy wants you to pay for it, but there is a free copy here), that the exit temperature for MSL (aka Curiosity, same landing system presumably slightly lower thrust though), is about $217\,\mathrm{K}$. That's $-56^{\circ}\mathrm{C}$. If you put your hand in the plume you're going to freeze (and probably have all the skin removed from it since the exit velocity is extremely supersonic). In fact I think that the plume temperature is lower than the ambient temperature at the time of landing.

The reason it's cold is some combination of the last, endothermic, reaction, and the adiabatic expansion of the gases as they leave the engine, I think.

So we can put all of this together:

(1) The reactions take place in the presence of an iridium catalyst, which is inside the engines. This means in particular that all of the exothermic reactions are taking place inside the engine (and presumably a lot of the heat produces is then going to drive the endothermic reactions). And, indeed, parts of the engines get very hot and you can see this. The exhaust products are not still reacting because they're not near the catalyst any more.

(2) The reaction products are colourless gases. There's no soot or other awful crud from partial combustion as you get with, say, kerosene rocket engines. There may be some small amount of crud which has come from the inside of the engine, but if the engines are working well this will be a tiny amount of material.

(3) The exhaust plume is cold: tens of degrees below the freezing point of water.

So what you are looking at is a plume of colourless gases at a temperature somewhat below ambient. And, not surprisingly, you can't see it.

In fact, I'd expect that the best chance of seeing the plumes would be to see any residual ammonia and hydrazine condensing in them. I presume that the engines burn cleanly enough that not enough of these is getting into the plume to be visible.

One interesting thing is that I would expect that, on Earth, plumes from these engines probably would be visible. I'd expect this because a plume of very cold gas would rapidly mix with the air, and a lot of water vapour in the air would condense out in the now-much-colder mixture. So you'd get 'vapour trails' – plumes of, not water vapour, but an aerosol condensed from water vapour, or in other words fog. This doesn't happen on Mars because the atmosphere is extremely dry compared to Earth's.

As a note: this question is clearly at least tangentially related to yet another 'we didn't go to x' conspiracy theory. How likely does it seem that, if people were to stage such an imagined conspiracy, they would forget to put the nice bright rocket plumes on their faked footage? Because, to me, it seems not likely at all.

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    $\begingroup$ @azot Because it's an animation; an artist's impression. It's not a physically accurate animation. Many artistic depictions of the Apollo ascent stage showed bright engines flames, but in reality, the exhaust was basically transparent. $\endgroup$ – Star Man Feb 25 at 4:45
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    $\begingroup$ @tbrookside: I suspect that the people doing the animation knew (or knew who to ask, and asked). I also suspect that they knew that if they didn't put in Dramatic Flamy Rockets (TM) they knew that conspiracy theorists would have a field day. Sadly it was not possible to add DFRs to the real thing, or I'm sure they would have. $\endgroup$ – tfb Feb 25 at 13:09
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    $\begingroup$ @azot This was answered in the press conference. The animation looked weird without visible plumes, so they added those as artistic license (although they went a bit overboard with the effect, IMO). This is simply a case of Reality is Unrealistic (insert standard TVTropes memetic hazard warning). $\endgroup$ – JohannesD Feb 25 at 14:21
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    $\begingroup$ "why did they make an animation (for presentation purposes) showing white hot flames with red hot tails getting out of the nozzles?" For the same reason they added dramatic music. $\endgroup$ – ceejayoz Feb 25 at 21:28
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    $\begingroup$ "How likely does it seem that [...] they would forget to put the nice bright rocket plumes on their faked footage?" Because they done goofed when they faked the Apollo ascent stage video, and now they have to continue this ridicule myth of transparent exhaust to stay consistent (just kidding, of course 😂) $\endgroup$ – armand Feb 26 at 5:49

If you look very closely at the video you'll find that the top-right engine is glowing red inside because, while hydrazine burns clear, the engine chamber is still 1000 °C hot.

This has been explained also by Scott Manley on Youtube where you can see the footage:

  • $\begingroup$ That engine could be glowing red or its color could be influenced by the general magenta hue of the image. $\endgroup$ – azot Feb 24 at 8:36
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    $\begingroup$ @azot I don't think so. If it was the hue of the image you'd see both top-right engines glow, not just one (they are basically in the same orientation respective to the camera, they receive the same light from the surroundings), and I believe during hover only 4 engines are lit. Also it's not magenta at all, is red-hot. There are other dark spots right around there but they do not show the "magenta hue of the image" which you'd expect if that color was just the "hue of the image". Finally the location is consistent with the hottest parts of the engine. $\endgroup$ – GACy20 Feb 24 at 9:17

@armand's comment:

You can compare with the footage of the Apollo lunar module ascent stage departure: the engine produces no visible flame

Indeed! Here is a screenshot from the Smithsonian National Air and Space Museum video "Apollo 17 Liftoff from Moon - December 14, 1972", found in How did NASA get the video camera on the Moon to track the LM ascent stage, considering the substantial delay? See also the excellent answers to Exit film of moon landing departure and this version of the video found in Gizmodo's How NASA Captured This Iconic Footage Of Apollo 17 Leaving The Moon

Apollo 17 Liftoff from Moon - December 14, 1972

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    $\begingroup$ @azot space.stackexchange.com/q/28172/6944 $\endgroup$ – Organic Marble Mar 2 at 23:28
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    $\begingroup$ @azot Look into the issue more deeply. In fact, it's the technology necessary for faking the Apollo missions that we don't have. Run into the arms of con men just to fancy yourself as "not a sheeple"... That's so sad, man. $\endgroup$ – armand Mar 3 at 0:52
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    $\begingroup$ @azot yeah, but how do you fake the radio transmissions that were not encrypted and closely monitored by the whole socialist block ? How do you fake the mirrors and other measuring instruments that were left there ? How do you fake the pictures taken of the various descent modules by competitor space agencies with strong incentives to debunk an american hoax ? How do you fake the trajectories of dust grains moved by Apollo members walking on camera, which match free fall under moon gravity but impossible to reproduce on Earth ? Don't just cherry pick only what favors your conclusion. Sad. $\endgroup$ – armand Mar 3 at 3:16
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    $\begingroup$ Didn't know this movie. Now I wanna watch it, thanks. But if this is your standard for "credible" I understand why you believe a lot of bad theories... $\endgroup$ – armand Mar 3 at 3:21
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    $\begingroup$ @armand hoaxers live for this kind of exchange; they remain elusively close to seeming convincable and/or redeemable, but then lead you on to the next "mystery" and one after that ad nauseam. It's a game and you are the toy. This is the nature of trolling. $\endgroup$ – uhoh Mar 3 at 6:43

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