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Cassini is scheduled to crash land into Saturn later in 2017. The reason stated was that they did not want to risk contaminating any of Saturn's moons.

However, during the Cassini mission, we landed the Huygens probe on Titan. If we're concerned about bacteria onboard Cassini surviving, why are we not concerned that the bacteria may have been on the Huygens probe instead?

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  • $\begingroup$ Probably the lander module was sterilized to some other standard than the orbiter? $\endgroup$
    – jkavalik
    Apr 29, 2017 at 15:34
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    $\begingroup$ No, it was not. $\endgroup$
    – Mark Adler
    Apr 29, 2017 at 23:34

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First off, what's done is done. Don't make it worse.

Second the main concern is Enceladus, not Titan. Enceladus is an ocean world whose ocean regularly and significantly interacts with its surface. Titan may be an ocean world, but it is unlikely that that ocean interacts significantly with the surface.

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  • $\begingroup$ PHChilly's answer quoting the NASA Planetary Protection Office would seem to disagree with that Titan is of lesser importance than Enceladus. In fact, NASA PPO lists Titan before Enceladus, for whatever that is worth (probably insignificant in the grand scheme of things). Do you have a citation for the statement that Titan is less of a concern than Enceladus? $\endgroup$
    – user
    May 5, 2017 at 17:10
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    $\begingroup$ This reference lists Titan landers as Cat II and Enceladus landers as Cat IVa-c. Though interestingly Callisto is not called out at all. $\endgroup$
    – Mark Adler
    May 7, 2017 at 2:58
  • $\begingroup$ So, the actual reason for protecting Titan & Enceladus now, but not then, seems to be that their protection status was upgraded only after Huygens landed. $\endgroup$
    – Felix Dombek
    Sep 14, 2017 at 20:11
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    $\begingroup$ @FelixDombek Correct. The same is true for where Phoenix landed on Mars. We would not be able to do the Phoenix mission today without some level of sterilization of the spacecraft. $\endgroup$
    – Mark Adler
    Sep 14, 2017 at 20:25
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NASA's Planetary Protection office is a good place to start. An excerpt that you might find useful:

"The most stringent requirements are those for Mars, Jupiter’s moons Europa, Ganymede and Callisto, and the Saturn’s moon of Titan and Enceladus due to the great potential for studying the origin, evolution, and distribution of life on these bodies. "

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Enceladus and some of the other icy moons have the possibility of supporting Earth based life forms. Titan, on the other hand is fairly toxic.

Also, they are being paranoid.

However, if we can learn something new before Cassini looses contact, it'll have some value.

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