Two lines of evidence from the current robotic explorations of Mars add to less tenuous ones that the planet is really wet – icy to be precise.  One is mineralogical.  Spectroscopy of the surface being slowly trundled across by a NASA rover, shows abundant signs of the hydrated, iron-potassium sulphate jarosite, which probably can only form under wet conditions.  When it was precipitated is not known with certainty, but it occurs in layered sediments that contain structures that clearly point to transport in and deposition from surface water.  The time when liquid water could exist at the surface probably goes back to the earliest events on Mars, tied to the famous canyons and more recently discovered dendritic drainage patterns.  The other evidence stems from even more remote sensing, that captures short-wavelength infrared radiation emitted by the Sun and reflected from the Martian surface.  Ices of water and carbon dioxide have distinct and unique reflected spectra, because of the different ways in which they absorb a small proportion of solar radiation.  Results from the OMEGA instrument aboard the European Space Agency’s Mars Express satellite show that the south polar region contains as much as 15% water ice mixed with solid CO₂ (Bibring, J-P et al. 2004.  Perennial water ice identified in the south polar cap of Mars.  Nature, v. 428, p. 627-630).