[polak]

So is Venus (or was, technically). All 3 are widely accepted to have had liquid oceans of water at some point. Mars lost it's due to the lack of a Magnetic Field while Venus lost it's to Rapid heating of it's atmosphere due to water vapor (A much stronger greenhouse gas than Co2) from the evaporating ocean most likely caused by the early sun increasing in brightness as it matured.

Earth still has it's ocean but there was a time with it was frozen solid.