Where Have All the Flowers Gone?
So far as we know, Mars never had any flowers, but it did have water, and fair amounts of it, at least for a while. My current favorite MOOC is Mike Brown's Caltech Solar System class via Coursera. The first third of the course is devoted to Mars, and it's fascinating. The oldest terrain on Mars, called the Noachian, shows evidence of rain, flowing streams, and lakes. There are also lots of craters, suggesting that this may have occurred during the so-called Late Heavy Bombardment. Not a whole lot of rain, but something comparable to the deserts and semi-deserts of modern Earth. Obviously this would have required a warmer and wetter Mars than that of today. How could it have been that warm, especially with the Sun perhaps 70% as bright as today? The most plausible answer is greenhouse gas, probably mostly CO2, supplemented by some water vapor. After a few hundred millions of years, by the Hesperian period, evidence of rain goes away, but evidence of water doesn't...