According to a New Scientist article, the Martian Viking and Phoenix landers, sent to Mars to look for signs of life, may have accidentally killed it.
As we all know now, the landers found the Martian soil samples they took totally sterile with no life or even any organic compounds... and that's the problem.
Just a quick aside, 'organic' does not mean 'living.' Organic compounds do not need life to form, but, as far as we know, life cannot exist without them. These chemicals make up all of the amino acids and proteins in our bodies including our DNA, but they are not the only thing that is needed. All 'organic' really means is 'made of carbon.'
We know that asteroids and comets are full of organic material and so many of them impacted the Earth at the time when life began that it's been theorized that they were responsible for seeding the Earth with life to begin with. For its entire history, Mars has been hit by those same asteroids and comets, so it should have evidence of these same organic compounds in its soil... but none were found. It's been a mystery and the mystery might have been solved.
It may be that the Martian soil is laced with a class of chemicals known as perchlorates. Perchlorates are extremely useful because when they heat up, they generate a lot of highly-combustible oxygen. For this reason, they are a chief component in rocket fuel. In effect, perchlorates, when heated, burn up all the organic material around them, leaving virtually no trace of them behind.
And herein lies the irony- in order to do a chemical analysis of the soil samples the landers collected, they had to first heat them up to a high temperature in order to turn them into gas which could then be analyzed. If there is life on Mars, our first contact with it ended up in us killing it.