In recent years, evidence of water ice on Mars has rapidly grown. At the poles, the ice is there all year round. And there is good evidence that it can be found just below the surface of latitudes down to 45 degrees, the extent of the southern ice cap in winter.
But the polar regions are cold, ever-changing and hazardous. If humans ever visit Mars, they'll have to first land near the equator, where the red planet is milder and more hospitable.
That may just have become more feasible thanks to the announcement of evidence of water ice beneath the surface of Mars at tropical latitudes as low as 25 degrees.
Mapping water ice deposits on Mars is a tricky business. Most of the water ice we know about is beneath the surface and cannot be seen directly. Its presence is inferred by the thermal characteristics of the areas we can see and measure.
For example, carbon dioxide ice can often only form on the surface if there is a cold layer beneath. The properties of this layer can only be explained by water ice.
The data on which these calculations are based come from cameras on the Mars Express and Mars Reconnaissance Orbiter spacecraft which have been circling the planet since 2004 and 2006 respectively.
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