Climatic simulations directed by astronomers from Paris Observatory and researchers from IPSL Planetology Departement (Paris VI) and published in the journal "Nature", show that this ice may come from an ancient reservoir of equatorial ice created during high obliquity episodes on Mars but which became unstable during the more recent episodes of low obliquity. This study has permitted to illustrate the existence of glacial cycles on Mars even more severe than on Earth.

Even if the presence of ice caps has been observed on the Mars poles for more than three centuries, the arrival of Mars Global Surveyor and Mars Odyssey spacecrafts in 1996 and 2001 has permitted to show that important quantities of ice (more than 70% of volume) had undoubtedly also been present in the top two meters of martian high latitudes (Figure 1).

It seemed difficult to explain the existence of such a quantity of ice so nearby the surface: more than half a milimeter of water frost is currently laying down during autumn and winter at high latitudes. Nevertheless, this ice cap sublimates completely at the end of spring.

This ice was proposed to be resulted from a slow diffusion of water between the Martian regolith and the atmosphere but the in situ measurements of porosity from Viking spacecrafts have shown that the regolith can not contain any ice with such a concentration. The study directed by the researchers from Paris Observatory and IPSL suggests that the solution may come from astronomical forcing of Martian climates.

For almost thirty years, sedimentary and ice cores have confirmed that the variations of the insolation received on the Earth's surface resulting from slow changes of the orbit and the Earth's obliquity had given rise to glacial/interglacial periods.

However, the martian obliquity variations are chaotic and much more significant than on Earth. The Martian obliquity has indeed varied between 25 deg and 45 deg during the 5-10 Ma time intervall and between ~15 deg and 35 deg during the last 5 Ma, with a "periodicity" close to 120 000 years.

A climatic 3-dimensional model of General Martian Circulation developed by the team of Fran�ois Forget (IPSL, Paris VI) and simulating faithfully the current seasonal cycle of water has been used to determine the path of Martian ice through these large variations.

These simulations have brought the intense latitudinal redistribution of Martian ice to light. When the obliquity overpasses 35 deg (compared to the current average value which is of ~25.19 deg), the summer insolation becomes too strong to maintain the stability of the current Northern cap which provokes a quick atmospheric transfer of ice towards the equatorial high topography region of Tharsis (Arsia, Pavonis, Ascraeus et Olympus Montes).

Remarkably, these summits sides present morphological traces which may be the result of the recent presence of glaciers. When the obliquity is below the current value, the equatorial ice becomes unstable and is carried not only to the polar zones but also to the high latitudes of the both hemispheres. The latitudinal distribution of stable ice obtained is then very close to the Mars Odyssey observations, illustrating a severe martian ice age.

How this ice can be preserved? As it is currently observed on Mars, ice is expected to be co-deposited with dust. When ice begins to sublimates, a dust lag is forming and prevents some ice from complete sublimation at every cycle so as to permit a "regular" forming of sedimentary meters-thick and ice-rich layers. These deposits are visible at high latitudes and more spectacularly in the polar caps.

The ice observed by Mars Odyssey would also be the mark of an ancient Martian glacial age (probably inferior to 5 Ma), covered nowadays with a thin cover of dry layer. If this is true, there must be some ice not only on the top meters but on hundreds of meters depth. The radars MARSIS and SHARAD respectively aboard Mars Express (in progress) and Mars Reconnaissance Orbiter which is forseen to be launched in 2005 will probably brought additional constraints on these underground reservoirs.

Recent ice-rich deposits formed at high latitude on Mars by sublimation of unstable equatorial ice during low obliquity Levrard, B., Forget, F., Montmessin, F. and Laskar, J., Nature, 28 octobre 2004. Long term evolution and chaotic diffusion of the insolation quantities of Mars. Laskar, J., Correia, A., Gastineau, M., Joutel, F., Levrard, B., Robutel, P.: 2004, Icarus, 170, 343-364.

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