Carl Sagan, NASA (IMAGE) Georgia Institute of Technology Caption Famed late NASA astronomer Carl Sagan first hypothesized that the reason early Earth stayed warm although the sun shone dimly had to do with a greenhouse effect involving a gas mixture different from that in Earth's atmosphere today. Credit NASA-JPL Usage Restrictions NASA press handout for use with applicable stories License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.