Even if rovers, balloons, and airplanes continuously move around and near the surface of Mars one day, we should never judge a planet by its cover. Today's desert-like Martian surface likely hides the presence of water below ground. To "follow the water" to where it is today, we must go beneath the surface of the planet with subsurface explorers.

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.

First Subsurface Exploration: Radar and Robotic "Moles"

The very first subsurface exploration of Mars for NASA will be in partnership with the European Space Agency (ESA) in their Mars Express mission. This spacecraft carries a subsurface radar instrument that will use a 40-meter (130-foot) antenna 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 data will give us a three-dimensional understanding of where and how much water may be distributed in the Martian subsurface.

A lander on Mars Express called Beagle 2 will also carry the first 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. Beagle 2's mole will only have the ability to penetrate less than a meter (less than 3 feet) below the surface.

A much more capable mole is under development in NASA's technology program. Weighing about 20 kilograms (44 pounds), it will be capable of drilling hundreds of meters (hundreds of yards) into the ground and possibly deeper at a rate of 10-20 meters (33 - 66 feet) a day. Excavated soil would be moved to the back of the mole and a small tube leading to the surface would help alleviate the pressure from the growing mounds of soil. The tube would also send soil samples back to the surface and carry power to the robotic mole. The samples sent up to the surface would be studied for scientific data such as mineral content and oxidation levels of subsurface soil. A mole drilling at the polar cap would study the layers of ice that tell the story of its history, much like the rings of a tree reveal many things from its past. All of this data would provide clues in the search for ancient, or possibly current, life.

Drilling Below the Surface

Once we know in more detail where the water lies, the next step is to drill in those locations. To get to the zone where frozen water--and possible dormant life--might be present, we will probably need to drill to a depth of 200 meters (656 feet). 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 proceeds. On Earth, this task is normally done with steel casing, which is very heavy. Engineers are actively seeking alternative ways that don't require us to send heavy equipment to Mars given the expense. Finally, we will have to develop systems that allow the drill to make operational decisions for itself. On Earth, drills can get stuck very quickly, so a Mars robotic drill or subsurface explorer must know how to recognize, avoid, and solve problems on its own.