Before humans arrive on Mars and especially before any planned presence there, we need to know it life presently exists or ever existed.  That is to protect the evidence from Earth organisms. The article, NASA Mars Sample Return Options, suggests that evidence of life has been found which is deceptive. The chances that complex carbon molecules would exist near the surface are close to zero. They would likely be destroyed by the strongly oxidizing regolith and UV radiation prevalent on the surface.

 

If life existed or continues to exist, it would be far underground where heat and liquid water could still exist. The primary reason for the sample return is to study the geological history of the planet. The motivation to do this before human presence is probably driven by politics more than science. Our government wants to be first and may be willing to spend many billions of dollars to that end.

According to this article, Scientists discover ‘sunken worlds’ hidden deep within Earth’s mantle that shouldn’t be there. It seems possible that these ancient pieces of Earth’s crust, estimated to be 4 billion years old, coincide with the proposed collision of the theorized planet Theia that happened around the same time. The combined mass of the two planets would have resulted in a partially molten mass before the Moon was spun off. It seems logical that shattered crust pieces would end up deep within Earth’s mantle.