A global view of Mars in intermediate-energy, or epithermal, neutrons. A low intensity of epithermal neutrons (colored deep blue in the map) provides a unique identification of soil enriched in hydrogen. The view shown here is a Mollweide projection of measurements made during the first week of mapping (February 2002) using the neutron spectrometer. The central meridian in this projection is 0 longitude.The neutron spectrometer was supplied by Los Alamos National Laboratory and is one of the instruments in the gamma-ray spectrometer instrument suite, which was supplied by the University of Arizona in Tucson.

Mars Odyssey’s neutron spectrometer maps water-ice

Scientists last Friday unveiled maps that detail the location of hydrogen, that may indicate water-ice, just below Mars’ surface. The maps are based on data from a neutron spectrometer built at the Laboratory and flown aboard NASA’s Mars Odyssey now in orbit. The data are supported by simultaneous measurements made using the Mars Odyssey’s gamma-ray spectrometer.

Scientists and NASA officials held a press conference at NASA’s Jet Propulsion Laboratory in Pasadena, Calif., to discuss the latest data received from the spacecraft.

Mars Odyssey successfully went into mapping orbit on Feb. 14 and began sending back data. These preliminary data indicated large amounts of hydrogen, a component of water, near the southern pole of Mars and extending up to -60 degrees in latitude.

Two weeks later, data with better statistical precision confirm the previous, preliminary indication that Martian soil south of-60 degrees latitude indeed is very rich in hydrogen, and by inference, water ice, according to Bill Feldman of Space and Atmospheric Sciences (NIS-1), the principal investigator on the design and construction of the neutron spectrometer.

“These data are utterly beautiful,” Feldman said. “I am in heaven.

“The maps generated from the neutron data will vary with the seasons because in the winter, as it is now at the north pole, it snows dry ice over the poles down to about 60 degrees latitude,” said Feldman. “We will be mapping Mars’ surface for well beyond one Martian year so we will get maps of its surface during all of its seasons.” A Martian year lasts 687 Earth days, or about two Earth years. Mars Odyssey’s orbit is such that the entire planet’s surface is sampled in four-degree longitudinal increments weekly.

Some scientists believe at one time a large ocean may have existed near the north pole of Mars. Further evidence suggests that rivers of water may have once flowed across the Martian surface carving channels and flooding plains. The Mars Odyssey neutron and gamma-ray spectrometers are looking not only for existing water on Mars in the form of water-ice, but also are looking for residue, such as chlorine that would indicate locations of sodium chloride, or salt, that could verify that standing and flowing water once existed on the planet.

Locating water on Mars would support theories that the environment once supported – and possibly still does – life. “Our understanding is that the elements necessary to foster and sustain life here on Earth may also exist on Mars,” said Feldman. “If there was standing water on Mars at some point that means that Mars had a warmer climate and there could have been life, but if there was no life in that warmer climate, we have to ask what makes Earth so special.”

Mars Odyssey’s discoveries will help determine where future missions should explore for clues about the planet’s history.

During the spacecraft’s 1,000-day science mission the Laboratory’s neutron spectrometer, from its orbit 250 miles off Mars’ surface, will continue to measure neutrons that leak outward from the upper meter of the Martian soil. It takes 20 minutes for data generated aboard Mars Odyssey to travel the distance to Earth. Scientists will use this data to determine the amount of water on Mars and map the basaltic lava cover, measure the seasonal variation of dry ice snowfall at the poles and help convert data from the gamma-ray spectrometer to determine the quantity and composition of the most abundant elements on the planet.

“Our team also had instruments look for water and analyze soil composition on the moon,” said Feldman. “What we are seeing on Mars is that the variation is much richer than found at the moon – we saw a thermal neutron variation of three on the Moon and we are seeing a variation of 7.5 on Mars. This portends quite a bit of excitement.”

The Mars Odyssey neutron spectrometer looks for neutrons generated when galactic cosmic rays slam into the nuclei of atoms on the planet’s surface, ejecting neutrons skyward with enough energy to reach an orbiting spacecraft. Elements create their own unique distribution of neutron energy – fast, thermal or epithermal – and these neutron flux signatures allow scientists to determine the general distribution of the soil’s elemental composition based on the data received from the instruments.

By looking for a decrease in epithermal neutron flux the scientists can locate hydrogen. Hydrogen in the soil efficiently absorbs the energy from neutrons, reducing their flux in the surface and also the flux that escapes the surface to space where it is detected by the spectrometer. Since hydrogen is most likely in the form of water-ice, the spectrometer can measure directly, a meter deep into the Martian surface, the amount of ground ice and how it changes with the seasons.

Mars Odyssey launched from Cape Canaveral Air Force Station in April 2001 and arrived in Martian orbit in late October.

--Shelley Thompson

A view of the south pole of Mars in intermediate-energy, or epithermal, neutrons. A low intensity of epithermal neutrons (colored deep blue in the map) provides a unique identification of soil enriched in hydrogen. The view shown here is a Lambert azimuthal equal-area projection of the south pole of Mars of measurements made during the first week of mapping (February 2002) using the neutron spectrometer. The neutron spectrometer was supplied by Los Alamos National Laboratory and is one of the instruments in the gamma-ray spectrometer instrument suite, which was supplied by the University of Arizona in Tucson.

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