Nili Fossae LCROSSMars Society Switzerland
Water
Water was Global Northen Ice CapEvidence for pervasive mud volcanism in Acidalia Planitia
Figure 1 de l'étude de Dorothy Oehler
Figure 2 de l'étude de Dorothy Oehler
CarbonatesDorothy Z. Oehler and Carlton C. Allen; Astromaterials Research and Exploration Science Directorate (NASA-Johnson Space Center). Published by Elsevier in Icarus (August 2010). Synthesis by Pierre Brisson. This synthesis is an interpretation, as faithful as possible, of the scientific publication referred to. You are advised to read it directly in case of need. Geographic setting : Centered at about 55°N, the Acidalia Planitia area constitutes, farther off Chryse Planitia, the exit towards Vastitas Borealis of a large embayment at the mouth of the valleys coming from the tectonic faults of Valles Marineris, or from the base of the Tharsis platform (Kasei Vallis). The altitude of Acidalia Planitia (- 4000 to -5100 m) is lower than that of Chryse (- 3200 to -4000 m). These two “planitia” correspond to two impact basins filled with sediments. They overlap slightly.
Figure 3 de l'étude de Dorothy Oehler
Structures: Approximately 40.000 high albedo mounds , dot the southern part of the impact basin of Acidalia Planitia, covering an area of approximately 70.000 km ² in a crescent shape. Their diameter vary from 300 meters to a few km; their height, from a few tens of meters up to 180 meters. They have a soft and round form. They are located on giant polygonal plates and more particularly upon their rims. They are denser and smaller in the center of the crescent. The rims of the polygons are less observable in the center (blurred). The study relates to approximately 45% of the area and to approximately 18.000 mounds.
Figure 3 de l'étude de Dorothy Oehler
Data and instruments: The study used the “MarsGIS” DVD of the US Geological Survey, compiling the data from many instruments aboard Martians orbiters, particularly the MOLA altimeter (aboard the Mars Global Surveyor orbiter), the infra-red night observations (“IR”) and visible light day observations (“VIS”) of the THEMIS spectrometer (aboard the Mars Odyssey orbiter); the CRISM spectrometer (aboard Mars Reconnaissance Orbiter, “MRO”), the HiRISE and CTX cameras (aboard MRO). Visual Characteristics: The mounds albedo is definitely higher than that of the surrounding rocks. These mounds have a dome shape with a more or less flat caldera on top. Surface seems to have a soft texture contrasting with that of surrounding rocks, as if spread from the top. There are concentric wrinkles on the tops, and aprons at the foot, which often extend on plane surfaces and sometimes dip into the flat ditches delimiting the desiccation plates.
Figure 2 de l'étude de Dorothy Oehler
Spectrometric observations: Surprisingly, the CRISM spectrometer does not clearly differentiate the mounds from their environment. However they seem to contain more iron and more crystalline iron oxides while the plains contain more basalt but neither olivine nor pyroxene (they have subdued mafic features). No evaporite, phyllosilicate (clay) or hydrated sulfate, were noted. They would have evidenced a strong metamorphism by water and heat. During the night, using infra-red light, the mounds appear very dark. Their thermal inertia is slightly weaker than that of the surrounding rocks. Interpretations : Mineralogic Interpretation: The albedo is not enough to characterize the chemical difference between the soil in the plain on the one hand and the mounds on the other. The spectrometers indicate slightly different characteristics between them even though they have a very similar albedo, which suggests that the coating of the mounds was enriched with ferrous oxides. Perhaps the cold climate prevented clay formation. It is also possible that the external coating of the mounds hides rocks having undergone a more important transformation. Phyllosilicates (clays) could have been carried from Mawrth Vallis down to Acidalia, but their concentration might be too weak to be detectable from the Martian orbiters. Cryoturbation (alternation of freezing and thaw) or other processes could have blurred the mafic signature of the whole area (the “ma””f”ic rocks, rich in magnesium and iron, are rocks coming from the mantle of the planet). Geographic/Geologic Interpretation: The shapes of the mounds, the appearance of their “coating” are typical of “mud volcanism” on Earth. From the layout of the place it is possible that there was, during cataclysmic floods (abundant volumes of water, high speed of flood), a tearing off of surface layers of the highlands at the foot of the Tharsis volcanoes, and a brutal evacuation of muds by the large channels of Valles Marineris, or Kasei Vallis, in the basin of Chryse Planitia, then in that of Acidalia Planitia. Muddy water could then decant their heaviest elements in the first basin and their lighter elements in the second one (Acidalia). The thickness of the deposited sediments could have been enormous and the pressure plus heat which resulted from it could have created optimal conditions for a sudden surge of the hotter and more liquids layers through the cold, dry, heavy surface layers, i.e. mud volcanism. Age of the Mounds : The age of the mounds is estimated at 3 down to 1,75 billion years, since they are younger than the soil of the plains of Vastitas Borealis to which Acidalia Planitia is contiguous. They are also younger than the desiccation polygons on which they are sometimes laid and to the delimiting ditches onto which mud was sometimes spread. On the other hand, the number of craters rejects them into a rather remote past. Analysis : These structures have geological, physical, mineralogical, and morphological characteristics compatible with a phenomenon similar to terrestrial mud volcanism. On Mars this process would have just had particular characteristics fitting a naturally different geological and climatic history. Alternatively to a simple abundant sedimentation followed by a rather fast drying up of Acidalia Planitia, the huge number of the mounds coupled with their broad spreading, can also reflect a single event, complementary and caming after the flood, which would have triggered an eruption through multiple vents in the whole area. This event could match the loss of excessive mass by sublimation of an extensive body of cold water positioned on Acidalia and to tectonic readjustments resulting from this loss of mass. Hydrothermal /tectonic pressures coming, after sedimentation, from Tharsis, causing the dissociation of gas hydrates (“clathrates”) contained in the sediments. These various factors could have also combined between themselves. Implications for Life : Mud volcanism provides a mechanism for carrying up the sediments from important depths to the surface, without major transformation. Such sediments could have contained chemical biomarkers, mineral biosignatures or structural remains of past life deposited by the large flood of Acidalia coming from the highlands or by indigenous, endolithic, micro-organisms which could have thrived (and remained?) in microhabitats, rich in fluids under surface, or in the conduits through which the water spurts transited. The fact that “life bearing” clays were not detected is disappointing because it implies that water undoubtedly did not interact sufficiently with the rocks in this place (cataclysmic nature of the floods and temperature too cold). That should however not be an obstacle: as noted, the external surface of mud coating and spreading around the domes, worn out by time, might hide an internal variety of rocks which the spectrometers cannot detect. Missions on the ground would be obviously essential. Conclusions : None of the previous landings on Mars were done in areas where putative mud volcanism has been identified. The authors of the study propose the mounds of Acidalia to be considered as a new class of exploration targets. Pierre Brisson