Marsis is a low-frequency radar on board the European Mars Express mission. The instrument can deeply probe the Martian subsurface up to several kilometres below the polar ice caps. By studying the radar reflectivity of the Martian surface (intensity of the first reflected radar echo), a global map of the dielectric constant could be established. It directly depends on the composition and physical characteristics of the subsurface's first tens metres and so on the soil's geological nature.
Northern (a, c) and southern (b) hemisphere map between the poles and 30th parallel in polar stereographic projection. The blue-red colour scale shows the dielectric constants measured by MARSIS. The low values (blue) are better explained by low density materials and/or water ice presence whereas high values (red) result from the presence of dense volcanic materials. The presence of more than 10 % of water (hydrogen equivalent or WEH) in the first metre and the ice stability theoretical limit are both illustrated on the map by outlines. The continuous and dotted lines (figure, lower chart) present the region bounded by the hypothetical ocean's coasts (“Oceanus Borealum”; “Deuteronilus” and “Arabia”).
The topographic measurements performed by Mars Orbiter Laser Altimeter (MOLA) here appear in relief.
The final map shows a low dielectric constant area (low radar reflectivity) in the low plains of the northern hemisphere. “We were immediately very intrigued when we realised that this low dielectric constant area coincided exactly with the hypothetical Martian ocean” Jérémie Mouginot, first author of the article, said. In fact, this area corresponds to a region bounded by hypothetical coasts identified thanks to geomorphological arguments on the photographs taken by the Viking probes and, in a lesser way, thanks to topographic arguments on the altimetric data gathered by Mars Global Surveyor. Nevertheless, this interpretation is still controversial, especially because of the absence of observation of specific chemical and mineralogical compositions in these regions.
The low values of the dielectric constant bring thus a new important argument confirming the last presence of a circumpolar ocean on Mars. Indeed, only two types of materials could explain such low values: very porous sediments or large concentration of water ice. These two possibilities send back to the distant past existence of an important volume of water containing sediments around the Mars' North pole.
When many geophysical methods of remote sensing only probe the uppest levels of the Martian subsurface, the low-frequency radar survey used in this study made the characterisation of the integrated materials' properties possible on layers of several tens meters below the surface. By probing to larger depths it becomes possible to understand the situation several billion years ago, freeing us from the recent climatic disruptions affecting the most superficial geological units. There, the geographic extension of the low dielectric constant area suggests the implication of the deep breaking up channels of the Chryse, Utopia and Arcadia Planitia regions which could have carried large sediment quantities towards the circumpolar plains two to three billion years ago.
The progress of this polar ocean (Oceanum Boreale) remains unknown. The first possibility is that water got lost into space because of atmospheric escape mechanisms. Another possibility is that water is still present on Mars but trapped in the underground within a deep and thick cryosphere.
J. Mouginot12, A. Pommerol13, P. Beck1, W. Kofman1 and S. Clifford4, Dielectric map of the Martian northern hemisphere and the nature of plain filling materials, Geophysical Research Letters 19 janvier 2012
1 Laboratoire de Planétologie de Grenoble, Grenoble, France
2 Department of Earth System Science, University of California, Irvine, California, USA
3 Space Research and Planetary sciences Division, Physikalisches Institut, Universität Bern, Switzerland
4 Lunar and Planetary Institute Houston, Texas, USA.