Complex sentence: The pits in the crater-fill deposits have not been observed within lunar or terrestrial craters and appear to be unique to crater-fill deposits in only the freshest and best-preserved Martian craters.
Split into simpler sentences: The pits in the crater-fill deposits have not been observed within lunar or terrestrial craters. They are unique to crater-fill deposits in only the freshest and best-preserved Martian craters.