Odyssey results show a water-rich Mars

It has been evident for several years that there is a significant amount of water on Mars, and although most of that water is currently frozen, in the past much of it might have been liquid.  In February of 2002 NASA’s Mars Odyssey spacecraft was placed into to orbit around Mars and almost immediately set to work mapping the flux of neutrons and gamma rays from the surface of the planet.  One of the main objectives of the Odyssey project is to map the chemical composition of the Martian surface, and in particular to see how much water the might be on Mars.  Neutron and gamma-ray detectors are well-suited for this purpose because they can “see” approximately 1 metre beneath the surface.

Because of its very weak magnetic field and thin atmosphere, the surface of Mars is bombarded with high-energy cosmic rays (mostly protons).  These particles interact with atoms in the atmosphere and on the surface, and they can even penetrate several tens of cm beneath the surface.  The atomic interactions produce neutrons that either escape back out into space, or interact with other atoms to produce gamma rays (very high-energy radiation).  Some naturally radioactive elements on Mars, including uranium, thorium and potassium, also produce gamma ratiation.  The Mars Odyssey spacecraft has a number of different instruments in its GRS package that can measure the flux, wavelengths and energy characteristics of neutrons and gamma radiation and use that information to make inferences about the elemental makeup of the Martian surface and immediate sub-surface.  The GRS instruments are especially good at mapping hydrogen levels, but are also able to map elements such as oxygen, silicon, chlorine, potassium, iron, thorium and uranium.

GRS data from the Odyssey’s mission were published in Science in July (Feldman et al., 2002, Mitrofanov et al., 2002 and Boynton et al., 2002).  Although these data are only preliminary in nature, and represent only one month of measurements, they provide some amazing insights into the distribution of water on Mars.

Firstly, there is a very strong H signal (ie. low neutron flux) in the south polar region (south of 60º).  The north polar region does not show the same levels of H, but that is because this area was covered with a thick layer of CO₂-ice during February and March.  There is also a strong H signal in the northern near-polar region (the blue area in the upper left part of the figure above).

A preliminary interpretation of the GRS data indicates that there is a thin essentially ice-free layer in the upper part of the Martian soil (ranging from less than 50 cm near to the poles to to over 1 m towards the equator), and that the underlying regolith (rocky soil) has 20 to 35% H₂O.  While the GRS instruments cannot "see" below 1 m, other studies of Mars suggest that this regolith might extend to depths of well over 1 km, and that it may contain enough H₂O to provide the equivalent of a global water layer over 500 m deep.

References

Feldman, W. et al., 2002, Global distribution of neutrons from Mars: results from Mars Odyssey, Science, V. 297, p. 75-78 (July 2002).

Mitrofanov, I.  et al., 2002, Maps of subsurface hydrogen from the High Energy Neutron Detector, Mars Odyssey, Science, V. 297, p. 78-81 (July 2002).

Boynton, W. et al., 2002, Distribution of hydrogen in the near surface of Mars: evidence for subsurface ice deposits, Science, V. 297, p. 81-85 (July 2002).

Steven Earle, 2002. Return to Earth Science News