Solar models indicate that the Sun was 30% fainter 4 billion years ago. However, Mars has ancient river channels which imply that it had a warmer climate. This can be explained if Mars had about 1 bar of CO₂ early in its history, creating a massive greenhouse effect which permitted rain and rivers. An alternate explanation would be if the river channels represent a single catastrophic release of water from the interior that is not permanently locked up as ice.