On the topic of ''grabby aliens'', humanity better not ever expands to outer space and does not (in that case necessarily ultimately unethically and in vain) try to colonize space. There are (still) space missions set up in which the plan is to land on the moons Enceladus, Europa, Ganymede, Titan, and Callisto. Those are in their contamination risk profile very different from Mars. They have been chosen as targets in part because scientists speculate based on reasonable assumptions that life may exist on them more than on other celestial bodies, which in turn means that if there is no life present on some of them, then if it were accidentally carried to them, it would be more likely to stay there. And this means that just a single contamination event can be a point of no return from which on microbes rapidly spread exponentially there for the whole future of any such moon and risking such forwards-contamination is not tolerable.