Complex sentence: However as no life forms outside of Earth have yet been studied or characterized, it is impossible to do a standard probability assessment, and risk assessment has to be carried out by combining knowledge of Earth life with knowledge of Martian geology.
Split into simpler sentences: However as no life forms outside of Earth have yet been studied or characterized, it is impossible to do a standard probability assessment. They concluded that risk assessment has to be carried out by combining knowledge of Earth life with knowledge of Martian geology.