Complex sentence: Phoenix also made isotopic measurements of the carbon and oxygen atoms in the atmospheric CO 2 in the atmosphere which show that the oxygen has exchanged chemically with some liquid on the surface, probably water, in the recent geological past.
Split into simpler sentences: Phoenix also made isotopic measurements of the carbon and oxygen atoms in the atmospheric CO 2 in the atmosphere. These measurements show that the oxygen has exchanged chemically with some liquid on the surface, probably water, in the recent geological past.