Planet atmospheric compositions are determined by the availability of a gas species, its molecular weight and the mass (or gravity force) of a planet. Both Mercury and the Moon are not massive enough to hold any gas species to form an atmosphere. The observed atmospheric compositions of all other terrestrial planets (Venus, Earth and Mars) are consistent with the calculated lower bounds of the critical mass (CM) for various atmospheric gas species. The proto-atmospheres of Venus, Earth and Mars during accretion should be composed primarily of CO2. The Martian massis significantly smaller than the lower bound of CM for gaseous H2O.Thus, Mars is not capable of retaining H2O in its atmosphere. In terms of today’s atmospheric compositions, the Earth appears to be the only “abnormal” planet in our Solar System. This may suggest that the fate of the Earth might be unique among the terrestrial planets by the fact that the Earth has an over-massive Moon. The capture of the Moon by a giant impact process might produce Earth’s indigenous hot supercritical H2O-CO2 ocean that quickly reacted with feldspar, the most abundant surface mineral, and eventually removed all CO2 from Earth’s proto-atmosphere.