The ice covering the south pole of Mars is thick enough that, if it melted, it would flood the entire planet in three metres of water.
The new estimate comes from the scientists examining data delivered by the Mars Express radar instrument.
"The south polar layered deposits of Mars cover an area as wide as a big portion of Europe," said Dr. Jeffrey Plaut in a news release.
In another NASA news release, the size of the ice is described as covering "an area bigger than Texas."
Plaut, a co-principal investigator for the radar and lead author of the study, says the amount of water had been estimated earlier, but "never with the level of confidence this radar makes possible."
Scientists say the ice deposit, about four kilometers thick, sits under a polar cap of white frozen carbon dioxide and water. It's at least 90 per cent frozen water, mixed in with some dust.
Recent reports from Mars have given scientists new insights into the planet's weather and geography, but there are still mysteries.
According to NASA, there is one area with a bright reflection that is still puzzling researchers.
"It resembles what a thin layer of liquid water might look like to the radar instrument, but the conditions are so cold that the presence of melted water is deemed highly unlikely," NASA says.
The latest data was obtained by joint NASA-Italian Space Agency instrument on the European Space Agency's Mars Express spacecraft.
The radar instrument, named the Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS), is also mapping the thickness of similar deposits at the north pole of Mars.
Scientists say the history of water on Mars is key to knowing whether the planet has ever sustained life as we know it.
The study on Mars' south pole appears in the current issue of Science magazine.
