Even if we one day have rovers, balloons and airplanes continuously moving around the surface of Mars, we should never judge a planet by its cover. We suspect that today's desert-like Martian surface covers an aquifer of liquid water at some level deep underground. To pursue our goal of "following the water" in search of signs of life, we must gather radar images of the top layers of crust and eventually send subsurface explorers to look for the water.

Although we do not know much about the subsurface, data from our Viking Missions and Mars Global Surveyor have given us abundant evidence of ancient surface water, including streams and probably ancient oceans. While there is photographic evidence for recent gullies, possibly cut by flowing water, we can see no evidence for liquid water currently at the Martian surface.

We draw some of our theories for a subsurface presence of water by looking at Earth. Even in desert regions, we know that water can be abundant in the subsurface. We also know that Mars is colder than Earth. From this information, we can speculate that the subsurface of Mars may resemble some of the colder parts of Earth. For example, in Antarctica or Iceland, we know that water is stored in a layer of permafrost and beneath that, as liquid groundwater. Even if the ancient surface water on Mars evaporated, there may still be substantial reservoirs of water, in either liquid or frozen form, in the subsurface.

Radar Imaging

Studying the geophysics of the crust is the first step in figuring out what the subsurface is like. The very first subsurface exploration of Mars for NASA will be in partnership with the European Space Agency (ESA). In 2003, ESA will be sending their Mars Express spacecraft to the Red Planet. This spacecraft will be carrying a subsurface sounding instrument that will use a 40-meter (130 feet) antenna to attempt to detect and map subsurface water.

Electric signals will be sent down the antenna, creating low-frequency radar waves. The radar waves will penetrate the Martian surface as deep as five kilometers (three miles) and will be reflected back to the spacecraft by different subsurface features, including water. This will give us a three-dimensional understanding of where and how much water may be distributed in the Martian subsurface. Only with this knowledge can we go to the next stage of exploration, which is drilling.

Subsurface Exploration Technology

If we identify a geophysical signature for water from orbit or surface studies, the next step is to drill in that location. To get to the zone where frozen water--and possible dormant life--might be present, we expect to drill to a depth of 200 meters. Liquid groundwater will be even deeper. That's no easy feat, but it's critical for understanding the possibility of past or present life on Mars and for confirming that water resources are available for future human explorers.

Deep subsurface access on Mars will have unique challenges. First of all, unlike on Earth, we will not be able to use a drill to go through mud, water or probably even gas pressure to carry the cuttings away from the bit. We will need new systems for fluidless drilling. Second, we will need an effective means of keeping the hole open while the drilling is proceeding. On Earth, this is normally done with steel casing, which is very heavy. We are actively seeking alternative ways of doing this on Mars that don't require so much mass. Finally, we will have to develop autonomous control systems that allow the drill to make operational decisions for itself. On Earth we know that drills can get stuck very quickly, so we will need to teach a Mars robotic drill or subsurface explorer how to recognize, avoid and solve problem situations.

Robotic Moles

One device that scientists are considering for underground exploration is a robotic mole. Mimicking the behavior of the small furry Earth-bound creatures that burrow into the ground, robotic moles will drill underground by pulverizing rock and soil, avoiding the need for a complex drill stem. The European Space Agency (ESA) plans to carry the first robotic mole to Mars aboard the Beagle Lander deployed from Mars Express launched in 2003 but this device will have the ability to penetrate less than a meter below the surface. A much more capable mole under development in NASA's technology program, weighing about 20 kilograms, will be capable of drilling hundreds of meters into the ground and possibly deeper at a rate of 10-20 meters a day. Excavated soil will be moved to the back of the mole and a small tube leading to the surface will help alleviate the pressure from the growing mounds of soil. The tube will also be used to send soil samples back to the surface and carry power to the robotic mole.

The moles will look for many things as they drill into the Martian subsurface. Most importantly, they will search for liquid water. If liquid water is found, it could lead to discovery of extinct life or possibly even present-day life. Any indications of life would not likely be bigger than microbes. Even if the moles are unable to find liquid water, scientists hope they will better determine the depth where liquid water might exist by calculating how quickly the ground heats up as the mole goes deeper.

The samples sent up to the surface would be studied for scientific data such as mineral content and oxidation levels of soil in the subsurface. A mole drilling at the polar cap would study the layers of ice which tell the story of its history, much like the rings of a tree reveal many things from its past. All of this data would be clues in the search for ancient, or possibly current, life.