Here we go.
“We found life on Mars but we might have been the ones that sent it there...”
I am 100% convinced they absolutely already know there is life there, and NOT life we sent, but are afraid of announcing it, and will make up whatever it takes to avoid it!
I actually have a Mars meteorite, it’s a piece of a rock discovered, I believe, in Antarctica.
It’s very, very possible, even extremely likely, that some of the meteor/asteroid impacts on Earth in the past have sent material far enough up and away to eventually get to Mars.
Mars is much lighter than Earth, and it has very little atmosphere. A rock can be struck from the surface of Mars by another meteorite, it can attain escape velocity (5.0 km/s) without burning up in the atmosphere, and it can reach the next planet.
The same process on Earth would require a massive meteor strike. They are very rare. But let's assume one happened and some rocks were ejected from the Earth's surface. To escape the Earth's gravity they need to be accelerated up to 11.2 km/s (more than double that of Mars.) Rocks don't have a rocket engine in them, so they have to start at even higher speed at the surface to compensate for losses in atmosphere. Atmospheric losses at those speeds will be exactly what a Shuttle experienced during landing - it will be a wall of super hot plasma. It will have a good chance of killing all life forms on the surface. If the rock is small then it will be hot inside too; if the rock is large then it may remain cold - but it really takes a lot of energy to throw it up into the sky.
Another problem is that the levels of energy needed to launch a rock to Mars are likely to vaporize the rock on the spot. Nuclear bombs kicked up mushroom clouds, but those clouds were nothing but fine dust because few, if any, rocks are strong enough to survive the initial shockwave. You need a lot of initial energy to lift the rock all the way to orbit so that it leaves the atmosphere with escape velocity for that altitude. Even if you can come up with such energy, it will crumple the rock into dust. That's why we use powered spaceflight; rockets deliver acceleration not for milliseconds but for five to ten minutes, subjecting occupants and equipment to manageable G levels.
Here is yet another reason. You want a meteor to come down to the surface at a good speed - one that is enough to strike stones back into space. The Martian atmosphere is thin, so this is possible - incoming meteors are not losing much energy, and they are not burning up in the process. On Earth most meteors don't even make it to the surface - and those that do simply arrive at their terminal velocities which are pretty low. They don't carry enough leftover energy to shoot debris back into space.
Earth had experienced some serious asteroid impacts in the past; it is remotely possible that a few rocks escaped Earth. But Earth and Mars are far apart, and space is big. You want a steady source of meteorites from the source planet if you want at least a few to reach the other planet.