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.
These cameras clearly show that CO2 forms on polar facing slopes throughout the year at tropical latitudes.
Mathieu Vincendon at Brown University in Rhode Island and buddies have created a detailed model of the thermal budgets involved to explain how this ice forms. Their conclusion is that it is only possible if there is a cold layer that helps to store and release heat two or three metres beneath the surface.
Two materials match the thermal properties of this layer: water ice or ordinary bedrock.
However, Vincedon and co say that the distribution of CO2 ice patches around Mars rules out the possibility that bedrock is responsible . To explain this distribution the bedrock layer would need to be uniformly buried in longitude but increasingly buried more deeply with latitude, they say.
Furthermore, such a layer of bedrock, although stretching across vast swathes of the planet, is not visible in images from space. That it has never been revealed by processes such as erosion or meteor impacts makes its existence unlikely.
The only other option is that there is water ice just beneath the surface of Mars at tropical latitudes.
That’s good news for the next generation of robotic explorers. One of the candidate landing sites for the Mars Science Laboratory, due for launch later this year, is the Holden Crater at a latitude of 26 degrees south.
This is right next to an area that Vincedon and co say has vast reserves of subsurface water ice. And that means the Mars Science Laboratory could get its robotic paws on the stuff within the next couple of years.