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. Instead we have dramatic outflow channels forming, almost certainly formed by release or melting of ground water, probably triggered by the volcanism associated with the Tharsis bulge, that created the big Northern volcanoes, like Olympus Mons. The quantities of outflow were very large, apparently creating a medium sized, if shallow, ocean in the basin that forms a big portion of the North Polar surroundings of the planet.
The most recent three billion years or so have been much colder and dryer. Water persists, in polar icecaps, dust covered mountain glaciers, and underground in the North, but it's mostly frozen and buried.
So where has all the atmosphere gone? The answer, it seems, is blown away in the solar wind. Unprotected by an Earth style magnetic field, and held rather tenuously by the weak Martian gravity, it just blew off.
Did life have a chance to evolve there during the few comparatively balmy years of Noachian or Hesperian? TDB.