Map of Mars' northern (a, c) and southern (b) hemispheres between the 30° latitude and the poles, displayed in polar stereographic projection. The blue-red colour scale represents the dielectric constants measured by MARSIS. The low values (blue) are best explained by low-density materials and/or the presence of water ice, while high values (red) indicate the presence of dense volcanic materials. The contour-line indicating the presence of more than 10% water (water equivalent hydrogen or WEH) in the first metre and the one giving the theoretical stability limit of ice are also drawn on these maps. The solid and hash lines (lower figure) show the area delimited by the shoreline of the hypothetical ocean ("Oceanus Borealum"; "Deuteronilus" and "Arabia").
Mars Orbiter Laser Altimeter (MOLA) topographical measurements are shown in shaded-relief.
By studying data accumulated over 5 years by the MARSIS radar on board Mars Express, an international scientific team led by the Institut de Planétologie et d'Astrophysique from Grenoble recently achieved a significant advance in the debate about the geological nature of the circumpolar lowlands of Mars' northern hemisphere.
MARSIS is a low frequency sounding radar on board the European Mars Express mission. This instrument has the ability to deeply probe the Martian subsurface, down to a few kilometres under the polar icecaps. It is by studying Mars' surface radar reflectivity (the intensity of the first reflected radar echo) that a global map of the dielectric constant could be drawn. This constant directly depends on the composition and physical properties of the first tens of metres of the subsurface and thus on the geological nature of the terrain.
The resulting map shows an area of low dielectric constant (low radar reflectivity) in the northern hemisphere lowlands. "We were immediately struck and intrigued when we realized that this low dielectric constant area corresponded to the position of the possible Martian ocean" confided Jérémie Mouginot, the leading author of the article. This area indeed corresponds to a region delimited by a possible shoreline, identified by the geomorphological features revealed in Viking probe pictures and, to a lesser extent, by topographical features from Mars Global Surveyor altimetry data. This interpretation has nonetheless always been disputed, particularly because no special chemical or mineralogical compositions were observed in these areas.
The low dielectric constant values are a strong new argument in favor of the past presence of a circum-polar ocean on Mars. Only two kinds of materials can explain such low values: either very porous sedimentation deposits or a large concentration of water ice. These two possibilities infer the existence in a distant past of a large water mass charged with sediments around Mars' north pole.
While many geophysical remote-sensing methods only probe the most superficial layers of Mars' subsurface, the low-frequency radar sounding used in this study is able to characterize the properties of materials several tens of metres below the surface. By sounding deeper, it becomes possible to trace the course of events that took place several billion years ago while avoiding recent climate disturbances that affect the more superficial geological units. In this case, the geographical extension of the low dielectric constant area suggests the involvement of the major debacle channels of the Chryse, Utopia and Arcadia Planitia areas, which could have transported great quantities of sediments towards the circumpolar lowlands two to three billion years ago.
The fate of this polar ocean (Oceanum Boreale) is still unknown. One possibility is that the water was lost to space as it escaped through the atmosphere. Another is that water is still present on Mars, trapped underground in a deep thick cryosphere.