Frozen CO2 Gale Crater Atmosphere SGM2010 Dust Page 2 Page 2Oceanus Borealis (Page 1)
If we look at the MOLA topographic map of Mars (the "Mars Orbiter Laser Altimeter" of "Mars Global Surveyor"), the heterogeneity of the surface of the planet is obvious: the Northern Hemisphere is very different from the Southern one. This is called the "crustal dichotomy" of Mars. Much of the northern hemisphere (about 40% of the surface of the planet) is indeed a vast depression (5,000 or 6,000 meters below datum) centered on the North Pole, with a flat and smooth surface, the Southern Hemisphere is much more rugged (cratered). Since this area (Vastitas Borealis) is lower than the other and completely flat and since we know that there was water on Mars, anybody’s straightforward reaction is to think it had to be the catchment basin of an ocean that we could call "Oceanus Borealis" and of which we today see the dried bed.
However, appearance can be misleading and should not led to accept this conclusion without carefully considering what geology has to say: Spectrometers aboard various orbiters (including CRISM and OMEGA) found hydrated rocks (whose chemical nature had been transformed by water) everywhere across the surface of the Southern highlands (clays, sulfates and some carbonates) but they did not find anything of the kind in the Northern Lowlands. Since the surface of the Highlands is the oldest, as it is the most cratered, it is necessary to admit the fact that water was permanently stable on the surface of Mars only during the earliest period of its history and that the rocks that we can see today on the surface of the Northern plains are not those of an ocean floor.
Crédit Image: NASA MOLA Science Team
In Jérémy Mouginot et al. "Dielectric map of the Martian Northern Hemisphere and the nature of plain filling material", Geophysical Research Letters, Doi:10.1029/2011GLO50286,2012
Two studies of the underground of the Northern hemisphere support this reasoning. In Mouginot et al. (1) published in January 2012, written from data collected by MARSIS, we can read that over one hundred meters, layers of sediments display a low di-electricity. This tells us that thick sediments were deposited in abundant water. As this process did not result in any chemical weathering of rocks, it can be inferred that the conditions of pressure, temperature and time that would have existed in the bottom of a perennial ocean, were not met. In John Carter et al., published in June 2010 (2), we can read that hydrated rocks identical to those of the Southern hemisphere were found within several deep craters of the Northern hemisphere, among which the Lyot Crater (particularly described in this document). This is the evidence that the rocks under the sediments were chemically altered by water, prior to the deposition of these sediments.
Hydrated Minerals Exposed at Lyot, Northern Mars (June 25, 2010) Lyot spans 236 kilometers in diameter, centered at 50.5 degrees north latitude, 29.3 degrees east longitude. Image Credit: NASA/ESA/JPL-Caltech/JHU-APL/IAS
Voir suite page suivante. Références : (1)-Jérémie Mouginot (1) et al. :Dielectric map of the Martian northern hemisphere and the nature of plain filling materials ; Geophysical Research Letter, Vol. 39, published 19th January 2012. (1) Institut de Planétologie et d’Astrophysique de Grenoble. (2)-John Carter et al. Detection of hydrated silicates in crustal outcrops in the Northern plains of Mars; Science Magazine; Science 25 June 2010:Vol. 328 no. 5986 pp. 1682-1686DOI:10.1126/science.1189013. (3)-Cédric Gillmann (1) et al. : The Long Term effects of volcanism and atmospheric escape on the evolution of Mars surface conditions . Earth and Planetary Science Letters, 5 Feb 2011. (1) Institute für Geophysik, Dept of Earth Sciences; ETH Zürich, Switzerland. -Jean-Pierre Bibring; Institut d’Astrophysique Spatiale d’Orsay, L’exploration spatiale de Mars : tout commence; conférence du 24 janvier 2012 ; Les Mardi de l’espace des sciences, au Champs Libres à Rennes, Partenariat Université de Rennes 2 / CREA -Grégor Golabek (1) / Tobias Keller (1) et al. Modeling of Core formation, onset of Mantle convection and Crust formation on Mars, 6 March 2009. (1) Institute für Geophysik, Basel, Switzerland. -Bethany Ehlmann et al.: Subsurface water and clay mineral formation during the early history of Mars. Nature 479, 53-60 (3 nov. 2011) doi:10.1038/nature10582; published online 2 Nov 2011.
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