My favorite but most disheartening is the theory Mars never had a warm past with oceans and running water. Though I think evidence shows there was because we found many river bed sediments with rounded rocks and pebbles which is a sign of brisk water erosion.
(1) Surprise return of Philae lander (2) Potential sign of life methane spotted in Mars meteorites (3) Lightest alien world found around a normal star (4) Rainfall satellite tumbles to Earth (5) Moon's dust cloud comes from tiny collisions (6) Early Mars may have been frigid, not warm
If Mars ever had water, why is it not there now? Also, how would the planet have been "clothed" with water. Would it have occurred during the same build phase of the solar system when earth accumulated it's store of the stuff? In other words, any accumulation of water would have started and ended at more or less the same time as earth. Earth still has the water it gained from early bombardment.
That makes me wonder if - us being the next planet in - any of Mars' lost water streamed here through the void much like the seed pods from Invasion Of The Body Snatchers, or the cylinders from War Of The Worlds. Interesting thought, eh?
Yes, Mars in theory had oceans of water in its infancy like Earth. (Though not as deep) There's water frozen in the polar caps and we don't yet know if there's any frozen or liquid water underground. Mars lost most of it's surface water when the atmosphere was blown away by the solar winds over millions or billions of years.
Earth has a magnetic field generated by the core of the planet which blocks most of the deadly radiation from the Sun. If it wasn't for our hot metallic core, Earth would be as dry and dead as Mars. Mars has no such core or magnetic field to protect it's atmosphere. It's also possible Mars much lower gravity field also contributed to loss of atmosphere. (Mars being almost half the size of Earth.)