THOR: Tracing Habitability, Organics, and Resources

Finding water deep below the surface of Mars is the goal of THOR, a mission proposed for NASA's Mars Scout 2011 program by researchers at Arizona State University's Mars Space Flight Facility, the Jet Propulsion Laboratory, and Lockheed Martin Space Systems. THOR stands for Tracing Habitability, Organics, and Resources.

THOR focuses on water because if Mars ever had life, water was an essential ingredient. As scientists seek habitable environments where Martian life could survive, their hunt begins by tracing places where water is now or has been in the past.

Martian water definitely exists today. Although the Sun's warmth has dried the equatorial zone to depths of hundreds of feet, Mars has polar caps made of ice that are visible from Earth even in backyard telescopes. In addition, the Mars Odyssey orbiter detected abundant water (in the form of ice) only a few inches below the surface in areas closer to the poles than about 60 degrees latitude north and south. Finally, recent findings by Mars Global Surveyor indicate liquid water has flowed in some mid-latitude gullies in the last few years.

In 2007, NASA will send a Mars Scout mission, Phoenix, to land at a high northern latitude. Phoenix will scrape away the surface soil, chip off some of the underlying ice, and analyse it for biological ingredients.

But this leaves much of Mars unexamined — in particular, the wide middle-latitude bands, between about 40 and 60 degrees (north and south). Here liquid water is thought to exist, at least periodically, down to the current day. But such water deposits, perhaps only a few yards underground, are not reachable with current lander and rover technology.

So THOR will get at the water a different way.

The mission plan calls for targeting two solid-copper impactors, weighing 1,200 kilograms (2,600 pounds) and 450 kg (990 lbs), on promising sites. These will excavate craters to expose subsurface ice or water. Under solar warmth, the exposed water will turn into vapor, making an unmistakable signature visible to sensors on the THOR orbiter overhead.

As it studies how the two artificial craters evolve, THOR will continue its study of Mars' habitability by surveying the Martian atmosphere for trace gases of possible biological origin. These include methane, which has been detected by earlier spacecraft and from Earth.

THOR is the first mission that aims to probe the Martian subsurface in the middle latitudes. This is perhaps the only environment on today's Mars where water ice can both exist and periodically melt.

As THOR explores regions that are difficult or impossible for other kinds of missions to reach, it will answer fundamental Mars science questions about the planet's past and present habitability, its organics, and its resources.