By Paul Rincon
BBC News Online science staff, in Paris
Mars Express has detected an area of high water vapour over a region of the Red Planet called Arabia Terra.
Mars Express will soon use its radar to hunt for water below the surface
The finding seems to confirm earlier data from the Mars Odyssey spacecraft that the region was "wet".
Vapour was also found to be enhanced in the Tharsis region of Mars. High water vapour areas seem to match bright patches, when Mars is seen from space.
"Increased water vapour correlates with higher albedo on the surface," said mission scientist Dr Dmitri Titov.
The Arabia Terra finding agrees with a map of water-equivalent hydrogen on the Red Planet compiled by Nasa's Mars Odyssey probe. The map is thought to be an indicator of water reservoirs lurking beneath the surface.
Arabia Terra was found to be one of the most significant "reservoirs" of subsurface hydrogen.
This water may mostly be stored as water-ice, although scientists say there could still be stores of liquid water on the planet.
The Marsis radar instrument aboard Mars Express is designed to look for liquid water and ice beneath the Martian surface, but its deployment has been delayed until later in the year.
The Tharsis results were presented at the 35th Committee on Scientific Research (Cospar) scientific assembly here in Paris, France.
The data comes from the probe's Planetary Fourier Spectrometer science instrument, which has also confirmed the presence of methane in Mars' atmosphere.
Tharsis is the equatorial region on Mars that contains some of the planet's great volcanoes, such as Olympus Mons.
Arabia Terra is a 5,000km-wide swathe of cratered terrain to the east and north of Tharsis.