Mars has been populated by all kinds of creatures for much longer than we have been on earth. The reason why we had to keep it all a secret was that we know how dangerous many of those species are and we cannot allow them to ever walk on our planet. The scariest thing is that they are able to adapt to our environment without any issues. The order already brought one of them to earth and it broke loose for 2 days in Norway.
We managed to contain it after a while but it was a very difficult task and it was actually able to refuel energy by draining our power lines. They feed from something very similar to man made electricity, but our energy source seems to make them even stronger than theirs. We cannot allow an invasion and this is why we attacked the planet in 1978 as a warning for them. So far they have stayed away.