For a story, what physiological implications and reactions might occur if this were to happen?

It is my personal belief that the universe is a pretty sterile environment. That life is rare and intelligent life is very rare. Very unlike Star Trek and Star Wars. The premise of my story is that after 300 – 500 years of human space exploration humans have come to believe that they are truly alone.

Then, while searching for earth like planets to colonize and terraform a space crew lands on a likely candidate. The planet is covered with small hard rice size granules. They also detected an odd low-level radiation, which has been previously found on several other planets.

The explorers discovered indisputable evidence that:

1) The planet once had harbored intelligent life

2) That this life was exterminated

3) That the other planets with this radiation were likely also the victims of mass genocide.

What physiological implications and reactions might occur if this were to happen?