Subsurface Water on Mars
By Bill O’Neill
The news is just too exciting for me not to write about it. The initial results from the Mars Odyssey spacecraft indicate that a lot of water (ice) lies just beneath large expanses of the Martian surface.
Mars Odyssey was launched April 7th last year, and six and half months (292 million miles) later it began to spiral into a two-hour orbit to survey Mars for the next two years. Its instruments were turned on at the end of February and, within a month (before their sensitivity was enhanced in June), the data astonished investigators, some of whom had waited 15 years to see it.
Measurements of the neutron fluxes emitted from Mars in various energy regimes, along with spectra of gamma-ray emissions (induced by neutron capture reactions) make it possible to map the distribution of hydrogen to a depth of about one meter in the Martian soil. Analysis and modeling of the first month’s data, reported in the form of three publications authored primarily by scientists at U. Arizona, Los Alamos Nat. Lab. and the Institute for Space Science in Moscow, were submitted to the journal Science May 8th, peer-reviewed, and e-published May 31st. The papers are still only available on the Web, but until June 27th anyone can access the full text at www.scienceonline.org. It’s a free opportunity to check out the internet version of the AAAS Journal.
In this space, and with my limited knowledge, I can only summarize what can be digested in detail at that web site, and one can obtain more information about the methods through links at NASA’s, www.mars.jpl.nasa.gov .
The suite of three instruments, collectively known as the Gamma-Ray Spectrometer (GRS), had previously been included in the Mars Observer spacecraft, but it stopped working 3 days after entering Mars orbit in 1993. GRS consists of a Neutron Spectrometer, a High Energy Neutron Detector and a Gamma Ray Subsystem. Cosmic rays striking the Martian atmosphere and surface give rise to neutrons by various nuclear reactions. The neutrons then lose energy by collision with surrounding nuclei and, in the process, excite some nuclei to emit gamma rays.
Neutrons with relatively low (thermal) energy can be captured by nuclei, which in turn generate novel gamma rays.
These processes reflect the chemical composition of the surface and atmosphere, since different elements possess different cross sections for capture and vary in their ability to moderate neutrons. The composition of the layer of rocky debris that covers Mars (the regolith) will affect both neutron and gamma ray fluxes, but they differ with respect to their depth dependence. Epithermal and thermal neutrons can reflect processes 2-3 times deeper than gamma rays. Hydrogen is an especially effective moderator of neutrons due to its mass being near that of a neutron. The gamma emission line of hydrogen, resulting from capture of thermal neutrons, was strong in the spectrum from the Martian south polar region (where it was late summer), but nearly absent in mid-latitudes.
Over most of the planet that hydrogen gamma signal was weak, but increased as the pole was approached, in parallel with a declining epithermal neutron flux – an independent indicator of high H levels. Relating these data to the distribution of H in the upper meter of surface requires computer modeling, for which a two-layer model appears to best fit the data. (It was assumed that the distribution of all elements other than H was similar to that measured by the Alpha Proton X-ray Spectrometer on the little Pathfinder robot, and water was assumed to be the only reasonable host molecule containing the hydrogen atoms.)
These initial analyses suggest a thin (few inches) upper layer with water content of about 1% near – 40°latitude, doubling by –6o°, and a lower layer (at least 3 feet deep) containing 35±15% water by weight (40-73% ice by volume, substantially higher ice content than terrestrial permafrost). A similar H-rich region was observed in the north, although it was obscured near the pole by its seasonal frozen CO2 cap.
Earlier evidence indicates the regolith could exceed a kilometer in thickness, suggesting that the subsurface ice detected by Mars Odyssey’s GRS may be only the tip of an iceberg, or a vast frozen water table, that could host life and also prove vital for human exploration of Mars, and beyond.