ScienceDaily (Nov. 18, 2008) — An international team of scientists who analyzed data from the Gamma Ray Spectrometer onboard NASA's Mars Odyssey
reports new evidence for the controversial idea that oceans once covered about a third of ancient Mars.
We compared Gamma Ray Spectrometer data on potassium, thorium and iron above and below a shoreline believed to mark an ancient ocean that
covered a third of Mars' surface, and an inner shoreline believed to mark a younger, smaller ocean,
said University of Arizona planetary
geologist James M. Dohm, who led the international investigation.
Our investigation posed the question, Might we see a greater concentration of these elements within the ancient shorelines because water and
rock containing the elements moved from the highlands to the lowlands, where they eventually ponded as large water bodies?
Dohm said.
This 3D map superimposes gamma-ray data from Mars Odyssey's Gamma-Ray
Spectrometer onto topographic data from the laser altimeter onboard the Mars Global Surveyor. The red arrow indicates the shield volcanoes of Elysium
rise in northern Mars, seen obliquely to the southeast. Blue-to-violet colors at the Elysium rise and highlands stretching to the foreground of the
map mark areas poor in potassium. Red-to-yellow colors mark potassium-rich sedimentary deposits in lowlands below the Mars Pathfinder landing site
(PF) and Viking 1 landing site (V1). (Credit: Image courtesy of University of Arizona)
Full article
GRS: The Gamma Ray Spectrometer
The gamma ray spectrometer has measured the abundance and distribution of many elements of the periodic table, including hydrogen, silicon, iron,
potassium, thorium, and chlorine. Knowing what elements are at or near the surface gives detailed information about how Mars has changed over time. To
detmine the elemental makeup of the martian surface, the experiment uses gamma ray spectrometer and two neutron detectors.
A word from GRS' lead scientist
marsprogram.jpl.nasa.gov...
This map is based on gamma rays from the element hydrogen on Mars.
Image Credit: NASA/JPL/UA
How GRS Works
When exposed to cosmic rays (charged particles in space that come from the stars, including our sun), chemical elements in soils and rocks emit
uniquely identifiable signatures of energy in the form of gamma rays. The gamma ray spectrometer looks at these signatures, or energies, coming from
the elements present in the Martian soil.
By measuring gamma rays coming from the martian surface, it is possible to calculate how abundant various elements are and how they are distributed
around the planet's surface. Gamma rays, emitted from the nuclei of atoms, show up as sharp emission lines on the instrument's spectrum. While the
energy represented in these emissions determines which elements are present, the intensity of the spectrum reveals the elements concentrations. The
spectrometer has added significantly to the growing understanding of the origin and evolution of Mars and the processes shaping it today and in the
past.
marsprogram.jpl.nasa.gov...
How are gamma rays and neutrons produced by cosmic rays? Incoming cosmic rays--some of the highest-energy particles--collide with atoms in the
soil. When atoms are hit with such energy, neutrons are released, which scatter and collide with other atoms. The atoms get " excited" in the
process, and emit gamma rays to release the extra energy so they can return to their normal rest state. Some elements like potassium, uranium, and
thorium are naturally radioactive and give
off gamma rays as they decay, but all elements can be excited by collisions with cosmic rays to produce gamma rays. The HEND
and Neutron Spectrometers on GRS directly detect scattered neutrons, and the Gamma Sensor detects the gamma rays.
How GRS Helps Detect Water
By measuring neutrons, it is possible to calculate the abundance of hydrogen on Mars, thus inferring the presence of water.
The neutron detectors are sensitive to concentrations of hydrogen in the upper meter of the surface. Like a virtual shovel "digging into" the
surface, the spectrometer allows scientists to peer into this shallow subsurface of Mars and measures the amount of hydrogen that exists there. Since
hydogen is most likely present in the form of water ice, the spectrometer is able to measure directly the amount of permanent ground ice and how it
changes with the seasons.
