Complex sentence: The consensus view is that although the risk of harmful back contamination is likely to be zero, the risk is not demonstrably zero: consequently, any returned samples from Mars must be treated as potentially biohazardous until scientists determine that the returned samples are safe.
Split into simpler sentences: The consensus view is that the risk of harmful back contamination is very likely to be zero. Nevertheless, returned samples from Mars will be treated as potentially biohazardous until scientists can determine that the returned samples are safe.