The Odyssey spacecraft, now settling into orbit around Mars, will be hunting for water - the key nutrient for past or present life. But the spacecraft can neither see nor smell its quarry.
How will Odyssey scientists know when they have found it?
This part of the mission is an exercise in indirect detection. Instead of looking for water itself, a suite of instruments will look for gamma rays and neutrons emitted by hydrogen. The Odyssey team assumes that hydrogen at or near the Martian surface probably will be locked in water molecules. It's the "H" in "H2O."
Some elements are naturally radioactive. For others, such as hydrogen, cosmic rays from space slam into atomic nuclei and kick off neutrons. Some of these can reach the spacecraft. Neutrons also interact with nuclei to stimulate emission of gamma rays, which are electromagnetic radiation more energetic even than X-rays.
Each chemical element emits neutrons or gamma rays with characteristic energies. These stand out in the spectrum of neutron and gamma-ray energies the spacecraft measures. They're like name badges saying "I'm thorium," "I'm iron," or in the case of the grand prize - "I'm hydrogen."
This trick of finding water by way of hydrogen wouldn't work for a spacecraft orbiting Earth. Our atmosphere blocks most cosmic rays. The Martian atmosphere is too thin to stop them.
Odyssey's Gamma Ray Spectrometer payload is designed to make the most of what the atmosphere lets through.
The gamma-ray detector from the University of Arizona in Tucson is supported by a spectrometer from the Los Alamos (New Mexico) National Laboratory that measures a range of neutron energies and a high-energy neutron detector from Russia's Space Research Institute.
The gamma-ray flux from Mars is so weak, Odyssey will have to add data for several hundred days for scientists to have confidence they are detecting hydrogen.
Even then, their conclusion would be questionable if based on the gamma-ray data alone. Backed up by independent detections from the neutron instruments, their confidence should be secure.
The hydrogen gamma rays come only from the top few inches of Martian soil. The neutrons come from layers down to a little more than 3 feet.
Taken together, these different types of data will let scientists draw up a global map of the distribution of near-surface water, most likely ice. They also should track the waxing and waning of polar ice sheets on Mars.