Liquid Water on Mars

4.3.1.1 Fluvial Flows

In 1972, the Mariner 9 probe provided photographs of the surface of Mars showing geomorphologic structures.42,43 These structures pointed to ancient river beds, now drained, and outflow valleys. Four years later, new pictures of the surface of Mars taken by the Viking probes in orbit around Mars (Fig. 4.2) confirmed the presence ofthese geomorphologic structures whose formation seems to imply intense and transitory liquid water flows.44,45

More recently the THEMIS instrument (Thermal Emission Imaging System) of the Mars Odyssey mission made it possible

Figure 4.2. Images of the surface of Mars taken by the Viking probes in orbit around Mars. On the left, "ramified valley" in the area of Nirgal. On the right, "outflow valley". These geomorphologic structures seem to have been formed following catastrophic flows. (Source: NASA and ref. 45).

Figure 4.2. Images of the surface of Mars taken by the Viking probes in orbit around Mars. On the left, "ramified valley" in the area of Nirgal. On the right, "outflow valley". These geomorphologic structures seem to have been formed following catastrophic flows. (Source: NASA and ref. 45).

Figure 4.3. On the left, image in the thermal infrared obtained by the THEMIS instrument onboard the Mars Odyssey probe, in the Fallen Chasma area north of Valles Marineris. On the right, diagram of the fluvial networks. From Mangold, N., Quantin, C., Ansan, V., Delacourt, C. and Allemand, P. Evidence for Precipitation on Mars from Dendritic Valleys in the Valles Marineris Area. Science 2004; 305: 78-81. Reprinted with permission from AAAS.

Figure 4.3. On the left, image in the thermal infrared obtained by the THEMIS instrument onboard the Mars Odyssey probe, in the Fallen Chasma area north of Valles Marineris. On the right, diagram of the fluvial networks. From Mangold, N., Quantin, C., Ansan, V., Delacourt, C. and Allemand, P. Evidence for Precipitation on Mars from Dendritic Valleys in the Valles Marineris Area. Science 2004; 305: 78-81. Reprinted with permission from AAAS.

to identify dendritic valleys in the plateau and the canyons of the Valles Marineris area. The geomorphologic characteristics of these valleys—especially those that present a strong density of ramifications comparable with the networks present on Earth—suggest a formation by streaming due to atmospheric precipitations (rains) (Fig. 4.3).46 The channels and the maturity of the networks may indicate that the flows ofliquid water were permanent over prolonged geological periods and suggest the existence of a hydrological cycle on Mars more than 3 billion years ago.

4.3.1.2 Sedimentary Deposits

The surface of Mars and in particular the southern hemisphere, is riddled with impacts of meteoritic objects. It was suggested that some of them could be temporarily occupied by lakes. The basin type morphology of the impact craters could have supported the formation of a water reservoir and the accumulation of sediments. Images of the surface of Mars show a fine stratification of clear and dark layers on the floors of some of the craters.47-49

Based on comparison with terrestrial morphologies, the arrangement of this type of layers is often characteristic of sedimentary deposits in a lake. These stratifications at the bottom of the craters were thus interpreted as evidence for the past existence of lakes on Mars (Fig. 4.4). Some ofthe deposits have a "delta" morphology that could be an indicator of the supply of liquid water to the lakes by rivers. However, other assumptions were also proposed to explain the formation ofthese structures not involving liquid water: deposits of volcanic lava and/or ashes, or dust brought by the wind.50

In 2004, the panoramic camera (Pancam) of the Opportunity rover, one of the two rovers ofthe Mars Exploration Rover mission, took the first photos ofsedimentary deposits on a planet other than the Earth. These deposits levelled within a few meters ofthe landing probe (Fig. 4.5) in a crater named Eagle51 in the Terra Meridianni area. This discovery for the first time of sedimentary deposits on the martian surface represents a key discovery relating to the question of the presence of liquid water. The deposits appear to be fine intersected stratifications the geometry ofwhich suggests formation in the presence of liquid water.

4.3.1.3 Ancient Ocean?

A highly discussed theory proposes that the low lands of the martian surface was occupied by a vast stretch of liquid water.52-56 According to this theory, an ancient ocean of liquid water or mud could have been formed temporarily in the vast area of the plains of the northern hemisphere, named Oceanus Borealis. This ocean would have been several kilometers deep.57 It was also proposed that the wide impact basins of the southern hemisphere, Hellas Planitia and Argyre Planitia, could have formed inland seas.58,59

One of the instruments of the Mars Global Surveyor probe, the altimetric laser MOLA, provided information on the topography of the surface of Mars. In the zone corresponding to the probable limits of this ocean (Fig. 4.6), this instrument revealed a practically constant altitude for a distance ofseveral hundred kilometers, which could correspond, based on comparison with terrestrial geomorpho-logical structures, to shore lines. However, the mineralogical composition of this area is dominated by the presence of volcanic rocks, which is surprising because the presence of such a quantity of water would have favored the formation of sedimentary deposits, particularly the formation of carbonates, above the volcanic rocks.

Many geomorphological structures thus seem to indicate the presence of water in the liquid state on the surface of Mars since the beginning of its history. They also suggest that water could have been a permanent presence over a time scale of several hundred million years.

4.3.1.4 Mineralogical Records

In year 2000, the TES instrument (Thermal Emission Spectrometer) of the Mars Global Surveyor probe made it possible for the first time to identify mineralogical clues favoring the presence of liquid water: grey or crystalline hematite in areas like Sine Meridianni, Valles Marineris and the bottom of the crater Aram Chaos (Fig. 4.7)60,61 and carbonates in the dust particles of Mars.62 However, the origin of these carbonates so far remains unexplained.

In 2004, two missions, the NASA Mars Exploration Rovers (MER) and the ESA Mars Express (MEx), revealed new records of minerals formed in the presence of liquid water. In addition to providing the first images of sedimentary deposits, the instruments onboard both MER rovers identified sulphates. On Earth, these minerals have the property of being formed only in the presence of liquid water.51-63-64

However, these sulphates can form very quickly, making it unnecessary that the water must remain stable over prolonged geological periods: these sulphates can indeed settle out when the water evaporates. The OMEGA instrument (Observatoire pour la Minéralogie, l'Eau, les Glaces et l'Activité) onboard the Mars Express probe made it possible to highlight not only outcrops ofsulphates salts65-67 but also outcrops of clays.65,68 Both of them share the same property of being formed only in the presence of liquid water. However, the presence ofclays carries another important piece ofinformation: their formation most probably required an interaction between liquid water and the silicated rocks ofthe Mars crust over long geological periods (Fig. 4.8). The presence of clays at the bottom of the Holden crater was confirmed very recently by the CRISM instrument (Compact Reconnaissance Imaging Spectrometer for Mars) onboard the Mars Reconnaissance Orbiter.69

Figure 4.4. On the left, sedimentary deposits at the bottom of the Holden crater in the Arabia Terra area. On the right, alluvial deposits laid out in a delta. These structures seem to point to an environment of lakeside deposits, which would be an indicator of the presence of liquid water. (Source: ref. 49).

Figure 4.4. On the left, sedimentary deposits at the bottom of the Holden crater in the Arabia Terra area. On the right, alluvial deposits laid out in a delta. These structures seem to point to an environment of lakeside deposits, which would be an indicator of the presence of liquid water. (Source: ref. 49).

4.3.1.5 Water History on Mars

Considering all these geomorphologic and mineralogical data, it appears increasingly obvious that liquid water was indeed present at the beginning of Mars' history potentially in a durable way as long as the conditions necessary to its maintenance were established. Several geological periods based on the history of water on Mars were proposed to fit the description of the two principal types of Mars rocks that required the presence of liquid water for formation: clays and sulphates.70 Clays were detected in the supposedly oldest surfaces, particularly where wind erosion or the impact of a meteoritic object revealed ancient deposits. The sulphates, although present in surfaces older than 3 billion years, seem more recent than the clays. Consequently, during the first 700 million years of the history of Mars, the planet probably knew several distinct episodes when liquid water was more or less abundant (Fig. 4.9):

• The first period, the "phyllosian," duringwhich liquid water was probably abundant at the surface ofthe planet—probably deep and able to alter the surfaces over the long durations needed to form clays. It should be noted that this period probably coincided with the presence ofa magnetic field able to attenuate the loss of the Mars atmosphere, which would correspond, at the maximum, to a period of 500 million years.71,72

• The second period, the "theiikian," during which the environment would have become drier, more acidic and then favourable to sulphate salts deposition with possibly great episodes of volcanic activity on the surface of Mars.70 This period would coincide with the loss of the magnetic field.

• A third period, the "siderikian," between 3.8 billion years ago and the present: the conditions on the surface of Mars would have been degraded considerably and liquid water disappeared (except for some very short hypothetical episodes of catastrophic45 or more classical73 flows). Mars was then moving to the current environment we know to-day. This period would be dominated by the very slow weathering of the surface of Mars that led to the formation of anhydrous ferric oxides.

Figure 4.5. One of the first color pictures taken by the Pancam camera onboard the Opportunity rover. This image shows sedimentary deposits which level on the internal slopes of the Eagle crater at the bottom of which Opportunity landed. The instruments on the rover detected in particular the presence of sulphates which would be evidence for an aqueous environment. (Credit: NASA).

Figure 4.6. Possible ancient shore lines in the northern hemisphere in Arabia (A) and Deuteronilus (D). Reprinted by permission from Macmillan Publishers Ltd, Nature 447,840-843, copyright 2007.
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