Di Achille, Gaetano听1听;听Hynek, Brian M.听2
1听Laboratory for Atmospheric and Space Physics, 乐播传媒
2听Department of Geological Sciences, 乐播传媒
Terrestrial marine deltas are among the most typical coastal landforms on Earth. They exhibit a large diversity of morphology as a result of a combination of several factors. Nevertheless, they share the characteristic of being formed approximately at the same elevation all over the planet, that is, the mean global sea level. Analyses of terrestrial deltas and their correlation across the planet have been proven to give clues on worldwide trends and changes of the mean sea level during the geological history of the Earth [1]. Several possible deltas have been recently identified on Mars and, in agreement with the terrestrial paradigm, they could provide a reliable approach to test the occurrence of a martian ocean. Here, we present results from such a test.
By using the global distribution of known martian deltaic deposits and martian valley networks [2] in conjunction with the topography from the Mars Orbiter Laser Altimeter (MOLA) (Fig. 1a), we extracted the elevation values of the apex (maximum water level), delta front (mean highstand), and of the basin floor (minimum water level) for each delta (Fig. 1a-b). These values were used as proxies for the maximum water level excursion (apex elevation - floor elevation) and of the main highstands during the formation of the sedimentary deposits. The morphometric parameters have been plotted as a function of the longitude (Fig. 1c) to detect possible equipotential surfaces indicative of ancient ocean coastlines. The analysis of deltas delineates two main planetwide equipotential surfaces (S1 at -2554 + 174 m and S2 at -2501 + 193 m) that encompass complete topographic enclosures within and along the margins of the northern lowlands of the planet. The inferred levels could represent two paleoshorelines of a past ocean covering the northern hemisphere of Mars during its early history and are generally consistent with the 鈥淎rabia shoreline鈥� previously suggested from geomorphologic and topographic observations [3]. Additionally, the reconstructed ocean boundaries are in agreement with theoretical predictions from thermophysical properties of Mars [3] and with the elevation values that would minimize the deviation of the 鈥淎rabia shoreline鈥� if that would have been caused by motions related to true polar wander [4].
These results support the theory that the northern hemisphere of Mars was covered by an ocean during the early history of the planet and imply that climatic conditions allowed the occurrence of a complex hydrosphere integrating valleys and deltas across the planet. Such a hydrological configuration could have been part of an Earth-like global and lively hydrological cycle providing transient and repeated climate changes in later times.
[1] Stanley, D. G., and G. W., Warne (1994), Worldwide initiation of Holocene marine deltas by deceleration of sea-level rise, Science 265, 228-231.
[2] Hynek, B. M., Beach, M., and Hoke, M., Assessing the History of Water on Mars through Global Analysis of Valley Networks, this workshop.
[3] Clifford, S.M., and T. J. Parker (2001), The evolution of the Martian hydrosphere: implications for the fate of a primordial ocean and the current state of the northern plains. Icarus, 154, 40鈥�79.
[4] Perron, J.T., Mitrovica, J.X., Manga, M., Matsuyama, I., Richards, M.A. (2007). Evidence for an ancient Martian ocean in the topography of deformed shorelines. Nature 447, 840鈥�843.