Two chief reasons :
low mass = low gravitational attraction.
no magnetic field around Mars ... thus solar wind strips air, so water evaporates faster, and water molecules are also subject to solar wind erosion.
Mars has evidence of erosion by water, including river valleys, channels, and minerals formed in the presence of water. Mars once had liquid water flowing on its surface, shaping the landscape over time.
Mars has water in the form of ice at its poles and possibly underground. Liquid water is scarce on the surface due to the thin atmosphere and low atmospheric pressure. Current estimates suggest that if all the water ice on Mars were to melt, it would cover the planet with a layer about 35 meters deep.
The Rover mission confirmed that Mars once had liquid water on its surface.
NASA photographs of Mars are the best evidence that water has been flowing on Mars recently. During warmer periods in Mars' atmosphere, the photographs show streaks that look like streams. Scientists agree that this is evidence of water on Mars.
Mars is the planet thought to have had liquid water in the past based on various geological features such as canyons, river valleys, and mineral signatures found on the surface. These suggest that Mars likely had a more hospitable environment billions of years ago where liquid water could have existed.
Mars lost its water (or it all froze) fairly recently . . . about 1 million years ago.
Some tresses of water on mars .So we can say that there is water on mars
mars has flowing water on it
Mars has no liquid surface water.
There was water at one time on Mars.
There is no liquid water on Mars.
Water for Mars was created in 2009.
Mars is hot so frozen water can melt in Mars.
Mars does have water, it is just frozen in its ice caps.
Mars has water (Ice) on its polar caps
Yes. Mars use to have water like Earth's. When Mars's core cooled dramatically, the water on Mars froze.
Mars has evidence of erosion by water, including river valleys, channels, and minerals formed in the presence of water. Mars once had liquid water flowing on its surface, shaping the landscape over time.