Contamination of other worlds is a major source of concern, particularly when we’re going someplace that might be habitable for some form of life. All spacecraft (and this includes satellites, telescopes, and rovers) have all of their pieces go through some pretty serious decontamination before launch, and most are assembled in a very high test clean room. (The last time I was near a clean room of this caliber, we weren’t allowed in the corridor that eventually led to the clean room, to try and keep airborne particles to a minimum).
Part of this clean assembly is for the good of the craft itself. Discovering that somehow, dust settled on the mirror of your brand new telescope would be a nightmare scenario; this means that your telescope is seriously underperforming, to the detriment of all the science that it might have been able to do. And you certainly don’t want grit making its way into the moving parts, which could jam the operation of those parts. If the part that needs to move is critical to the success of the mission (say, for instance, it’s your communications antenna), a stuck part could cripple the entire mission. So keeping everything as clean as possible gives the craft itself the best possible chance at operating the way it was intended.
If your spacecraft has a gas chromatograph or a mass spectrometer on it, both of which are instruments designed to sample the molecular properties of a gas, then you have to be extra careful. If you have such a sensitive metric of the chemical composition of air or vaporized soil, any kind of contamination left from Earth on the tray you’re using to hold the gas or dust will contaminate your measurement, and you’ll wind up measuring the contaminant instead of what you really want it to be sampling (Mars, for instance). This has actually been an issue in the past; the Viking landers on Mars made this kind of measurement of some of the soil and found some unusual chemicals in the soil in 1976. Unfortunately for the soil measurement, it was found that you could get this reading either by having Mars be doing some interesting chemistry, or if there was still some residual cleanser on the soil tray.
With some later measurements by the Phoenix lander in 2008, it’s possible that the ‘interesting chemistry’ interpretation is actually the correct one, but until some more measurements are made on the exact isotopes of the atoms involved in the reading, it’ll be hard to rule out the cleanser contamination explanation. Curiosity, on the other hand, seems to have found genuine complex molecules from Mars air and vaporized rock, but the scientists working on the team could only say this after determining that the results of the experiment on different rocks were sufficiently different to rule out contamination. (If you thought the signal you were getting was from a single contaminant on your tray, it would produce a similar signal no matter which rocks you were inspecting.)
Landers so far have been sitting in places that are pretty dry and reasonably flat; the flat part is just because it gives us the best possible chance at actually landing without smashing our spacecraft on an unexpected ledge. The dry tends to go along with the flat; by and large our best observations of material flowing on Mars has been along steep slopes, where it is too dangerous to land. Avoiding those places for the purposes of avoiding contaminating any water reservoirs is done out of an abundance of caution. It’s hard to guarantee that we don’t have any cleanser products left on our rovers, in spite of our best efforts to send as clean a craft as possible. And if there is some form of extreme life out there on Mars, we’d like to not kill it accidentally.
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