Why go hunting for signs of life in Mars ? Very soon, Martians will be looking for life on planet Earth. Let us launch a world-wide project to destroy this earth, and all traces of life on this earth. Very soon, you will have Martians visiting this earth. All you have to do is, join hands and do the following ::
* Start by getting all those useful fossil fuels out of the earth to deny them to future generations. Turn one half into non-biodegradable polymers and contaminate oceans with them to damage the food chain at its root. Turn the other half into carbon dioxide and release it into the atmosphere by burning it. The resulting green house effect will lead to a climate catastrophe dealing further damage to the biosphere.
* Dig deeper and pump out all the petroleum. Make bigger cars, sell more of them, and drive around aimlessly.
* Get all low-radioactive uranium isotopes out of the earth and turn it into much more radioactive fission products using nuclear reactors. The resulting waste will bother future generations for millennia. Not just because these elements are very hazardous, but also because some of them can be used to manufacture nuclear weapons.
* Any liquid biological or chemical waste should be dumped into fresh-water springs to banish clean drinking water.
* Speaking of drinking water: Ground water can be contaminated by pumping large amounts of hazardous chemicals into the ground in order to hydraulically fracture underground rock formations. This breaks up deposits of natural gas which can add further contamination to ground water. As an additional plus, it can also cause an earthquake or two and brings plenty of contaminated waste water to the surface (see previous point).
* Drive various species of natural fauna and flora to extinction while overcultivating a few selected others. Those species should be bred for total dependence on humans to make it impossible for them to survive in the wild. The resulting biological imbalance will result in a critically unstable ecosystem which will soon collapse on its own.
You might wonder: how could I convince billions of people to cooperate with such an ambitious plan? Here are a few tips:
* Establish a world-wide economical system which incentivizes short-term gain over long-term sustainability. Design it in a way that those who embody this doctrine become more powerful while those who don’t get as few political influence as possible.
* To prevent the humanity from realizing your plan, distract them from what is happening through a very broad supply of shallow entertainment. Mix it with subliminal messages which manipulate them to crave useless status symbols which can only be obtained by working harder for the destruction of the planet’s ecosphere.
* When there are still people who care about politics after all, distract them by putting topics on the public agenda which are highly emotional but don’t actually matter much for the survival of the human species, like gay marriage, abortion or sexism in video games. Throw them a bone once in a while by making a public statement in one or the other direction in these regards and watch them fight while you continue your plan.
* To prevent humanity from allying against you, follow the good old tradition of “divide and rule”. Separate humanity into easily distinguishable sub-cultures like race or religion. Then make sure they hate each other and enter pointless wars. Sounds hard? You will be surprised how easy it can be to destabilize a whole region just by supporting the right people. When the wars start, the involved people will not only stop bothering you when they think there is a greater threat, these wars will also motivate even more ruthless exploitation of natural resources and stop them from caring much about any collateral damage they cause to the ecology.
In short: continue doing what we already do.