They wouldn't tell us if there was life. They always crap on any evidence of it.
Join Date : September 2010. Still boldly going forward because Cryptic didn't install reverse.
Warning : I am currently unsupervised. Anyone who knows me will inform you that this will most likely lead to trouble. =3
Technically speaking, it's impossible for Earth-style life to exist on Mars for the following reasons:
1: Mars has no viable atmosphere. While there is a thin layer of carbon dioxide, perhaps with trace amounts of nitrogen and oxygen, there is not enough to sustain anything larger than perhaps a couple small bacterial colonies.
2: Radiation. Mars is continuously blasted by charged particles which emanate from the sun (the "solar wind") and because Mars does not have a magnetic field, these particles proceed to the planet's surface unimpeded. Therefore, anything on the planet will slowly become irradiated over time.
3: Lack of just about everything. If this was a rat or lizard, what would it eat? Energy has to come from somewhere, so unless this creature is capable of photosynthesis there is no way for it to obtain energy from an external source.
Also, nitrogen generally only exists in a planet's atmosphere. Nitrogen is essential to the construction of DNA. With almost no atmosphere, there is nowhere for anything on Mars to draw nitrogen from.
The thing to remember, the images we get are heavily processed - they go through extensive automated processing and most that are released to the public also have a manual photoshop pass.
The raw images are pretty boring to look at, they're all grayscale and there's multiple separate ones taken with different filters, and there's a LOT of other data with them that isn't resolved into the final image, and anything standing out (different reflective properties, different spectrum, motion, thermal emissions, etc) would show up in that data.
And they're not taken simultaneously - a single still takes several seconds to snap. Anything that moves won't line up and will be immediately obvious. This is why you see a LOT of photoshopping in Cassini shots, if there's multiple objects in view, they won't line up in all three filters and you'll have to move one of them to line up.
So here's NASA's evidence that it's a rock:
Gives off no heat*
Looks like a rock*
Is surrounded by rocks*
Has the same optical properties as surrounding rocks*
Has the same composition as surrounding rocks*
*-gathered from raw scientific data collected by advanced sensor systems on site
And the counterevidence offered by conspiracy theories:
Is that... I guess that could be a tail if you look at it a certain way.**