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On 15th September 2017, the Cassini probe will be destroyed by crashing it into Saturn. This will be done in order to eliminate the unlikely risk of it crashing into and forward contaminating one of Saturn's moons with micro-organisms originating from Earth.

Why have such concerns not prevented us from landing probes on other bodies such as Mars, the Moon, and Titan? Why are we selective about which bodies we don't want to risk contaminating and which we are willing to take the risk with?

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – called2voyage ♦
    Commented Apr 28, 2017 at 15:29

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Early Mars probes were heavily sterilized. Further studies have shown that Mars's surface isn't habitable, at least not in most areas, and thus a low risk approach has been developed, where certain sensitive parts are carefully controlled (Usually robotic arms), and missions to certain interesting places (Such as locations where liquid water is suspected) are more carefully monitored, but overall, the restrictions are much looser.

There are 4 key bodies (Aside from Earth) that have interesting conditions that might harbor life in our solar system. These are Mars, Europa, Titan, and Enceladus. These are all known to have liquids at or near their surfaces, although how much is debatable. Mars has been studied well enough to know it doesn't possess any risk except underneath the surface, so we aren't as worried about it.

Specifically, there are 5 major categories of planetary protection objects of interest, where Category 1 has very low chance of life, and Category 5 is a sample return mission. The Wikipedia article linked provides further details.

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  • $\begingroup$ I decided to accept this rather than the more upvoted (and still very good) answer as the link explaining protection categories and associated sterilization strategies is very useful in answering the question. $\endgroup$
    – JBentley
    Commented Oct 24, 2017 at 17:19
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The deciding factor is if the solar body has a chance of harboring life (or having done so in the past). Scientists don't want to run the risk of false positives: detecting life on another planet, only to find that we're the ones who brought it there.

So probes sent to Mars are sterilized as best we can, while we don't sterilize probes to bodies that are definitely dead (the Moon, or Saturn). Titan and several other moons have conditions that could possibly have led to life existing there, so we avoid contaminating them.

This has been agreed in the Outer Space Treaty. For NASA's implementation, see the Planetary Protection web site.

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – called2voyage ♦
    Commented Apr 28, 2017 at 15:11
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    $\begingroup$ Your answer says the Moon is definitely dead. How can we say the moon is definitely dead and Mars isn't? $\endgroup$
    – corsiKa
    Commented Apr 28, 2017 at 17:43
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    $\begingroup$ @corsiKa while definitely might be a bit strong, current theory makes it extremely unlikely that life every existed or could have existed on the Moon. If it does, it so different that anything Earth based could not be confused with it. $\endgroup$
    – James Jenkins
    Commented Apr 28, 2017 at 18:06
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    $\begingroup$ ...also, we're fairly sure Earth life (from contaminations) will not develop there - so if any other life exists, it won't be endangered. The only thing we're spoiling is tests for presence of Earth-like life, which we're pretty sure would turn out negative anyway. If we crashed a probe with a kilogram of moist soil from siberia into Europa, chances are more than good in a hundred years its ocean would be bustling with microorganisms... $\endgroup$
    – SF.
    Commented Apr 28, 2017 at 18:40
  • $\begingroup$ @corsiKa - You are looking at this as an either-or, dead or life-bearing, proposition. That's not the criteria. The criteria is whether or not it can said with a near certainty that a solar system body is dead. Just because we cannot answer in the affirmative to "is body X dead, to a very high degree of certainty" does not mean that the body does harbors life. It merely means the body has a remote chance of harboring life. $\endgroup$
    – David Hammen
    Commented Apr 28, 2017 at 22:42

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