WASHINGTON (AFP) -- Voyagers must dig to uncover hidden water reservoirs on Mars, said a new study that debunks a hunch the water and CO2 in its ancient atmosphere were blown away by solar winds.

Scientists had theorized that Mars, once likely wet but now dry, was the victim of a catastrophic event that drained the planet or that solar winds eroded its formerly dense atmosphere over time.

But data collected by the European Mars Express spacecraft showed only a small amount of water vapors and carbon dioxide in its atmosphere has escaped the red planet over the past 3.5 billion years.

Stas Barabash of the Swedish Institute of Space Physics and Jean-Andre Sauvaud of the Center for Space and Radiation Study in France calculated that only 0.2 to 4.0 millibars (a unit of pressure) of CO2 and a few centimeters of water could have been lost due to solar winds during this period.

The co-authors of the study, published in the journal Science, concluded that huge water reserves and carbon dioxide deposits must be hidden beneath the surface of Mars.

American roving robots Opportunity and Spirit have verified that Mars once contained water and carbon dioxide in its atmosphere.

Sedimentary deposits, traces of rivers, and dried river beds indicate intense water activity in its past.

Surface water is now absent and only low levels of water vapors in the planet's atmosphere remain.

Some frozen water was found on the surface in a handful of craters and in polar regions.

But, the small quantities found were not enough to explain geological evidence of a possible bygone ocean and rivers, leading researchers to want look underground for answers to the mystery.