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Probing the Martian Underground

Publié le 1 février, 2009 | Pas de commentaires
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The climate on planet Mars is too cold and too dry to house water in liquid form. Images of Martian topology, however, reveal river beds and water gullies, suggesting that large amounts of liquid water once shaped the landscape. Even the estimated amount of water molecules in the planet’s polar ice caps is well short of the amount of water necessary to create the river beds on Mars. To unravel the history of water, the hunt for water reservoirs has gone underground with instruments that were first developed by nuclear physicists.

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Giant canyons and networks of gullies mark Mars’ land surface, resembling the liquid water landscape features here on Earth. Martian topology thus bears historical traces of large quantities of water, now gone. Today, Mars’ cold, dry climate prevents any liquid water from existing on the surface (1). The average atmospheric water content is only about one thousand of that on Earth and the Martian mean global temperature is approximately -77 degrees Celsius – well below the freezing point of water. Although water ice exists in the polar regions of Mars, the quantity is negligible compared to the size of the oceans that once carved the Martian landscape. Since water is no longer present on the surface of the planet, scientists are now looking underground for answers. Ongoing mapping of the mineral content in the Martian subsurface could reveal possible traces of water on Mars. Finding out what happened to all that water could help shed light onto the history of the Martian climate, and more broadly, Martian life forms. Indeed, if the planet was wetter and warmer, it may have been more hospitable to microorganisms.

Exploring the underground

Scientists are gathering information about Martian soil composition to better understand the history of water on Mars. Areas with high subsurface concentrations of hydrogen and water soluble minerals can lead to discoveries of large underground water reservoirs. To understand this concentration of different elements in the soil, we must necessarily look at incoming and outgoing particles.

Cosmic particles originate from various sources in the universe, including the sun, remote stars and particle collisions with interstellar gas (3). They are present just about everywhere in the universe and strike anything that comes their way.

In the case of Mars, incoming cosmic particles face a thin atmosphere and a very weak magnetic field. While on Earth, our magnetic field and dense atmosphere slows down the incoming particles, the low disturbance on Mars allows them to reach the ground with original speed. At such high speeds, incoming particles interact with the atoms about one meter underground (4). The interaction produces outgoing particles, gamma-rays and neutrons, that carry energies that are unique to the soil composition. These specific gamma-ray and neutron energies help to extract concentrations of different elements in the Martian soil. By studying these particles scientists can explore hidden reservoirs and the history of Mars’ climate.

Catching recoiling particles

The exploration uses the satellite 2001 Mars Odyssey and particle detectors, originally developed for nuclear physics experiments. The detectors take the role of a camera to capture images of Mars. The Odyssey project is part of NASA’s Mars Exploration Program, designed to make robotic explorations of Mars (5). The satellite has been orbiting Mars since October 2001 at a height near 400 kilometers. On board the orbiter are three experiments, one of which is called the Gamma Rays Spectrometer (GRS). The purpose of GRS is to map the Martian subsurface soil composition, including the contents of hydrogen and water soluble minerals. Given that the GRS data are sensitive to chlorine concentrations, the chlorine data may confirm past water reservoirs that collected salt.

The GRS system is equipped with three detector types that initially were developed for Nuclear physics experiments (4). These detectors are capable of capturing incident recoiling gamma-rays and neutrons from cosmic particle collisions in the Martian subsurface. The GRS instruments also measure the neutron and gamma-ray energies and therefore provide the neutron and gamma-ray intensity for different particle energies. Given the unique energy levels for different elements, the GRS data are used to deduce the abundance and ratios of different elements in the subsurface. The 2001 Mars Odyssey orbits the entire planet, and thus provides a global map of Mars’ soil composition.

By 2002, less than a year after the orbiter had arrived at Mars, the Odyssey experiments had gathered enough data to provide an almost complete global map of the hydrogen content in the top meter of the Martian soil (6). Assuming that hydrogen is present in the form of water molecules, these results confirmed vast amounts of water ice buried beneath the surface in the polar regions. In January 2007, the GRS experiment released maps of hydrogen, silicon, chlorine, potassium, iron, and thorium concentrations in the Martian subsurface in the low and mid-latitude regions (2). These data helped narrow down hypotheses of different processes that affected the planet.

Future investigations into Mars’ past

The GRS experiment has provided detailed mapping of the water content in the Martian soil, continues to gather data and to publish improved resolutions. The GRS data are analyzed by scientists who test theories of how different processes have evolved on the planet. There are other satellites orbiting Mars that conduct experiments to gather data of the subsurface. One of those is the Mars Express Orbiter which is part of the European Space Agency’s effort to monitor the planet. The Express orbiter is equipped with radar instruments that are designed to measure the distribution of liquid and frozen water down to five kilometers below the surface. The Express data could lead to exciting discoveries of underground water reservoirs, providing a window into the evolution of Mars’ landscape and perhaps even ancient forms of life.

References

(1)Jakosky, Bruce M., and Michael T. Mellon. “Water on Mars.” Physics Today 57. 6 (2004): 71-76.
(2) Boynton, W. V. et al. “Concentration of H, Si, Cl, K, Fe, and Th in the low- and mid-latitude regions of Mars.” Journal of Geophysical Research 112. E12S99 (2007).
(3) Yao, W. M. et al. “Review of Particle Physics.” Journal of Physics G33. (2007). 20 August 2008 <http://pdg.lbl.gov/>.
(4)Gamma Ray Spectrometer Home Page. 31 Jan. 2008. The University of Arizona. 20 August 2008 <http://grs.lpl.arizona.edu/home.jsp>.
(5)NASA’s Mars Exploration Program Home Page. 20 August 2008. NASA. 20 August 2008 <http://mars.jpl.nasa.gov/>.
(6) Bell, Jim. “Tip of the Martian Iceberg?” Science 297. 7 (2002): 60-61.; Mitrofanov, I. et al. “Maps of Subsurface Hydrogen from the High Energy Neutron Detector, Mars Odyssey.” Science 297. 7 (2002): 78-81.; Boynton, W. V. et al. “Distribution of Hydrogen in the Near Surface of Mars: Evidence for Subsurface Ice Deposits.” Science 297. 7 (2002): 81-85.

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