Christopher Shinohara believes we're a good half-century away from sending astronauts to Mars. But before we can even consider such a venture, he said, scientists must learn more about the red planet: its climate, radiation levels, geologic history, and evidence for water or life-sustaining environments, either present or past. As project manager of the 2001 Mars Odyssey Gamma Ray Spectrometer, Shinohara helped bundle a complex three-part sampling apparatus into the Odyssey spacecraft—an instrument that, with 473 days of Mars orbit racked up as of February 8, had given researchers their clearest picture yet of the presence of water on the red planet.
As Mars Odyssey continued to circle our planetary neighbor, continued to collect and beam back data on energetic gamma rays and sluggish neutrons and millirads of radiation, Shinohara and colleague Heather Enos visited Penn State to deliver the third of the six 2003 Frontiers of Science lectures. Enos is business operations and outreach manager for the Odyssey Gamma Ray Spectrometer; both she and Shinohara work in the Lunar and Planetary Laboratory (LPL) at the University of Arizona in Tucson.
Shinohara has been laboring on Mars missions for almost 13 years. It was his task to integrate LPL's gamma sensor head with a high energy neutron detector head made by the Space Research Institute, Moscow, Russia, along with a neutron spectrometer constructed by Los Alamos National Laboratory in New Mexico. Together, the three instruments comprise the Odyssey Gamma Ray Spectrometer.
Leading up to the April 7, 2001, launch, Shinohara and his fellow team members faced many challenges. They had to shield their instruments from damaging cosmic rays during Odyssey's 200-day, 17,000-mile-per-hour interplanetary cruise. (En route, radiation detectors in the Mars Radiation Environment Experiment, with which the spectrometer shared payload space, recorded radiation levels twice those facing astronauts on board the International Space Station.) They needed to protect the devices during that problematic stage known as Mars insertion, in which the spacecraft, firing its rocket engine to decelerate, goes skipping along like a flat stone on the outer surface of the Martian atmosphere until it slows down enough to be captured by the planet's gravitational pull: to prevent over-heating during this period, the instruments were sheathed in gold-foil multilayering insulation. After a Mars orbit was achieved, it was tightened over a three-month period by occasional rocket-braking burns. Once the craft reached a stable orbit, the LPL's gamma sensor head went periscoping out at the end of a six-meter boom, designed to keep the spacecraft's own gamma ray emissions from confusing the detector.
At every step of assembly and testing, the instruments had to be kept clean and uncontaminated—both to avoid generating spurious data during the mission and to guard against contaminating the Martian environment should the probe inadvertently end up on the planet's surface. In Arizona, Shinohara's team worked in a special room that was cleaner than any hospital operating room in Tucson, he said. The timeline was short: 34 months from when the LPL signed a contract to when it delivered the finished spectrometer to Kennedy Space Center. After NASA installed the device in the spacecraft, Lockheed Martin tested the combined payload in a thermal vacuum chamber, its walls filled with liquid nitrogen, which duplicated the cold temperatures and low pressures of space.
The heart of the LPL gamma ray detector, Shinohara explained, is a high-purity germanium crystal measuring 6.5 centimeters in diameter by 6.5 centimeters in length (6.5 centimeters is a little over 2.5 inches). Charged with 3,000 volts of electricity, the crystal sits in its housing at the end of the boom waiting to get hit by high-energy ionizing photons and charged particles emanating from the surface of Mars. The electric charge generated by each impact is amplified, filed as data, and sent back to Earth.
When struck by radiation from space, elements in the Martian landscape undergo minute reactions, giving off gamma rays of differing energy levels. Odyssey, hurtling overhead some 200 miles up, maps the gamma rays, which, said Shinohara, can be thought of as invisible light. The spectrometer separates the rays into a spectrum, revealing how much of a given element is present in a target area. The spectrometer sorts out and measures hydrogen, oxygen, sodium, magnesium, aluminum, silicon, chlorine, potassium, calcium, iron, thorium, and uranium.
Shinohara's supervisor at the University of Arizona, planetary scientist William Boynton, is in charge of the gamma ray spectrometer. After analyzing early Odyssey data, Boynton and his colleagues concluded that vast amounts of water lie frozen at and below ground level at Mars's south pole. Although scientists had theorized that water ice existed at the Martian poles, no one had demonstrated its presence: Odyssey's sensors did not sample the water directly, but they did detect large amounts of hydrogen—and water, most scientists agree, is the only hydrogen-bearing compound that could conceivably exist in such abundance on the red planet. Boynton and his colleagues reported their results in three articles in the July 5, 2002, issue of Science, with a colorful data-driven image of Mars on the cover.
For years, scientists have suggested that Mars was not always the freeze-dried habitat that it appears to be today: it must have immense stores of water somewhere, water that once carved the planet's surface. The Odyssey data suggest that much of that water is now frozen into the polar ice caps and permafrost. Poleward from about 60 degrees of Mars latitude, wrote Boynton and twelve co-authors in one of the Science articles, lies terrain rich in hydrogen—probably water ice buried beneath a layer of hydrogen-poor soil. As far as we know, water is a necessity of life, and its presence on Mars helps shore up the position of scientists who suspect that life once existed on the planet. Advocates of interplanetary travel see the waters of Mars as a source of hydrogen that could someday be turned into fuel, to bring astronauts back to earth following a Mars landing or to propel them farther into the solar system.
If all goes well, Mars Odyssey should continue sending back data until August 2004. In addition to the Gamma Ray Spectrometer, a Thermal Emission Imaging System takes pictures using infrared as well as visible light. These high-resolution images let scientists map the planet's surface in ever greater detail. The images provide clues to Mars's mineralogy and geologic history. And some day in the not-too-distant future, they may help mission planners pick out hazard-free landing sites to set down landers: robotic craft and ones carrying astronauts.