Scientists in the US say that initial data from a new way of scanning Mars has shown up to half of the Red Planet's surface may contain ice.
The Phoenix Mars Mission heads for the Red Planet in August
The new method of scanning for water offers vastly more accurate readings than before, they say.
The data could prove vital for the Phoenix Mars Mission which launches this August and which will put a lander on the surface to dig for ice.
The new data shows wide variation as to how deep below the surface ice exists.
The deposits - far beyond the ice that is known to exist in the planet's North Pole - could be so large that were they to melt, they would deluge the planet in water forming an ocean.
Up until now, scientists had been able to search for water deposits using a spectrometer fixed to the orbiting Mars Odyssey spacecraft.
It is a device that measures gamma rays coming from a planet to detect different materials.
However, only readings that are accurate to within several hundred kilometres can be obtained.
Now Dr Joshua Bandfield of Arizona State University has devised a new method for detecting ice.
By comparing seasonal changes in thermal infrared patterns, detected by the same Odyssey spacecraft, he says he can make readings accurate to within just hundreds of metres.
Dr Bandfield said water ice in terms of surface area would be "probably roughly a third to a half".
Though there is plenty of water ice, the new thermal imaging data also shows that there is considerable variation across the planet in terms of how far down ice can be found.