P43D-01
Mars habitability: epochs, processes, remnants
Following the OMEGA/Mars Express pioneering discovery of hydrated phyllosilicates in a variety of sites spread over the ancient terrains, an impressive set of results, combining refined observations by OMEGA/Mars Express and CRISM/MRO, relevant contextual images by HRSC/Mars Express and HiRiSe/MRO, modeling and simulations, has been acquired. They confirm the initial hypothesis that these minerals record an ancient era during which water might have remained liquid over extended durations, which led to naming this era: phyllosian. It is remarkable that 30 years after the Mariner and Viking images of Mars as a desolated and inhospitable world, a new vision is emerging, for Mars to have possibly hosted environmental conditions favoring the presence of a key ingredient of potential astrobiological relevance. If ever Mars was habitable, it happened during the phyllosian. A variety of coupled information, from compositional, geomorphological and magnetic data, enables to assess the processes that took place, and their chronology. The observations that we will discuss include the specific composition identified, in relation to the stratification observed; the scarcity of phyllosilicates within the cratered terrains where mafics dominate; the large depth of deposits in a variety of sites, in particular within Mawrth Vallis; the distribution of remnant magnetization, with almost none within the large impacts and along the volcanic features. As a consequence, it will be shown that this era is tightly limited in time. It seems to have ended prior to the heavy bombardment to have ceased: the Noachian expanded long after the phyllosian ended. This habitable era could have been enabled by the maintenance of an active dynamo: its drop triggered its end, given the properties of the early Sun, by the lost of suitable atmospheric conditions for surface water to remain stable as a liquid. Up to now, neither with the Moon or any other body, we did not have means to study the complex history of inner planets along their first hundreds of millions years, at a time when the early bombardment took place, while maintaining habitable conditions which, at least on the Earth, led to the emergence of life. Did the impact rate steadily decreased, or did it drop a number of times, up to late events some 3.8/3.9 billion years ago, offering quiet epochs with biologically active standing water reservoirs in place? Mars is unique in offering means to study this era, as its history preserved sites recording this era with potentially biorelics still in place. The occurrence of phyllosilicate-rich sites offers the exciting perspective to address scientifically the question of the emergence of life elsewhere than on the Earth, and to decipher the processes that drove diverging evolutionary pathways for Mars and the Earth.
P43D-02
Giant impacts on early Mars and the cessation of the Martian dynamo
Mars currently has no global dynamo-driven magnetic field, but strong crustal fields [1] indicate that a global field existed in the past. Surface observations [2,3] indicate that 15 giant impacts (leaving basins >1000 km in diameter) occurred over a period of 100 Ma at about the same time the global magnetic field disappeared. Furthermore, the youngest basins in the sequence (e.g. Hellas and Argyre) are all demagnetized, suggesting that a dynamo operated early, but stopped in the mid-Noachian [4]. Giant impacts can deliver significant amounts of heat to the interior [5], reducing the rate of core cooling. Recent dynamo simulations suggest that if the Martian dynamo was subcritical, a decrease in core-mantle boundary (CMB) heat flow of as little as 1% would be sufficient to shut down the global magnetic field, but that an increase in heat flow of ~25% above the subcritical level is needed in order to restart the dynamo [6]. Thus, if the dynamo fails once, it may not return even if the original core cooling rate is restored. Here we investigate a possible link between the giant impacts during the early to mid-Noachian and the contemporaneous cessation of the Martian dynamo. We determined the heating of the Martian mantle due to impactors [5] that formed the 20 largest exposed and buried impact basins with diameters greater than 1000 km [2]. At model times corresponding to the Hartmann-Neukum ages of the impact basins [3] we introduced this impact heating into 3D spherical convection models [7] of the Martian mantle and examined the effect on the CMB heat flow. We find that impact heating associated with the larger basins (D > 2000 km) can cause the global heat flow at the CMB to decrease significantly (>10%) and even briefly go negative for the largest basins (D > 3000 km). We suggest that such a reduction in core heat flow may have led to the cessation of the Martian dynamo. Utopia is the largest and the oldest demagnetized impact basins. We suggest that impact heating associated with the Utopia impact could permanently shut down the Martian magnetic field if the dynamo was subcritical at that time. The loss of the global magnetic field would have exposed the atmosphere to stripping and sputtering by the solar wind, resulting in atmospheric loss over time. Such loss may be responsible for the global shift in climate from wet (Phyllosian era) to drier, more acidic conditions (Theikian era) which has been inferred from mineralogical results from the OMEGA experiment [8], to have occurred < 200 Ma after the dynamo died. The atmospheric loss and climate change may have serious consequences for planetary habitability during the mid-Noachian. [1] Acuña, M.H. et al. (2001), JGR, 106, 23,403-23,418. [2] Frey, H. V. (2008), GRL 35, L13023. [3] Lillis, R. J. et al. (2008), GRL 35, L14203. [4] Arkani-Hamed, J. and D. Boutin (2004), JGR, 109, E03011. [5] Watters, W. A. et al. (2008), JGR 113, in press. [6] Kuang, W. et al. (2008) GRL 35, L14204. [7] Zhong, S. et al. (2000), JGR 105, 11,063-11,082. [8] Bibring, J. F. et al. (2006), Science 312, 400-404.
P43D-03
Noachian Megabreccia on Mars
Megabreccia consists of randomly oriented angular blocks, many larger than 1 m diameter, which forms
suddenly in energetic environments such as impact events. Megaregolith is a related term for
unconsolidated material resulting from heavy cratering, but megabreccia may be indurated. We have, to
date, identified megabreccia in more than 50 locations on Mars from HiRISE images, generally in the form of
indurated bedrock. It is commonly found in the central uplifts of large craters in or near Noachian (>3.8
Ga) terrains, near the rims of large basins such as Isidis, and in deep exposures such as the floor of Uzboi
Valles and in parts of Valles Marineris. Well-exposed rock outcrops are required to identify megabreccia, in
particular from the diversity of colors and textures indicating diverse lithologies. CRISM has identified
hydrated minerals, especially clays, in many of these locations and perhaps alteration will be found in nearly
all of the deposits once the data are acquired and analyzed, but they also appear to contain unaltered clasts.
These may be among the very oldest rocks exposed on the surface of Mars, dating back to the time of
heavy bombardment. In some cases megabreccia with relatively small (1-5 m) clasts probably formed by post
Noachian cratering, particularly when found in the pitted and ponded material filling the crater floors, which
may be analogous to suevite. The indurated megabreccia with large (>10 m) blocks is only found in
locations consistent with deep bedrock, such as the central uplifts, although further brecciation and alteration
may have occurred in the crater that exposed the rocks. It has long been assumed that Mars has a ~2 km
thick layer of porous and permeable megaregolith, but we suggest that it may be largely cemented by melt
and hydrothermal alteration. Heavy bombardment of the ice-rich crust of Mars could have produced a very
different surface layer than on the dry Moon. Life on Earth may have begun during the period of heavy
bombardment, but the most ancient rocks are heavily metamorphosed. Mars preserves a much better record
of the environmental effects of heavy bombardment into a water-rich crust. Another reason to study the
oldest outcrops in that the younger Noachian may have been relatively dry. At least 3 of the 7 remaining
candidate Mars Science Laboratory (MSL) landing sites (Nili Fossae, Holden, and Eberswalde) would provide
access to Noachian megabreccia; we may already have a sample of this type of deposit in ALH84001.
http://hirise.lpl.arizona.edu
P43D-04 INVITED
Diversity of Mineralogy and Occurrences of Phyllosilicates on Mars
Minerals and their occurrences tell us about the chemistry, pressure,and temperatures of past environments, and the possible conditions for past habitability. To date, a fair number of phyllosilicates and other minerals have been detected on Mars (e.g., Poulet et al., Nature v438,p623, 2005; Mustard et al., Nature, v454, p309, 2008; Bishop et al., Science, V321, p830, 2008, and references therein). Minerals and amorphous materials detected and mapped over large areas include kaolinite/halloysite, montmorillonite, Fe/Mg-smectite, nontronite, saponite, chlorite, opal/hydrated glass, illite, muscovite, magnesite, prehnite, olivine, high- and low-calcium pyroxene, hematite, jarosite, alunite, kieserite, gypsum, coquimbite or ferricopiapite, possible szomolnokite and others yet to be identified. Phyllosilicate minerals are generally seen associated with Noachian outcrops and are thought to result from aqueous alteration, perhaps over sustained periods. Poly- and mono-hydrated Mg-sulphates appear to have been formed after the phyllosilicates. The patterns and occurrences of minerals so far mapped do not appear to show classic hydrothermal systems as have been observed on Earth (e.g., Yellowstone, Wyoming, and Cuprite, Nevada). Prehnite, previously identified on Mars as scapolite, a low-temperature phyllosilicate commonly found in mafic volcanics on Earth, appears widespread on Mars, often in association with Fe/Mg-smectite or chlorite. Phyllosilicates are observed in local outcrops, but occur regionally, generally indicating the effects of a common alteration process during the Noachian epoch. The discovery of mineralogies indicating both acidic and alkaline environments using CRISM and OMEGA data show that conditions were locally diverse. If the environments for the regional phyllosilicate deposits are found to be hostile to past habitability, perhaps studying the smaller mineralogically diverse areas may prove more fruitful. This talk will review the minerals and their diversity, and geologic environments on Mars and how they compare to terrestrial environments.
P43D-05 INVITED
Orbital Identification of Carbonate-Bearing Rocks on Mars
Carbonate is an expected weathering product of basalt in an aqueous environment and an atmosphere with CO2. Its absence in rocks examined by numerous orbiting and landed instruments and its presence as only a very minor phase in Martian meteorites and dust has been proposed to imply that either ancient Mars" surface was not persistently wet, pCO2 was not high, or conditions were too acidic to permit significant carbonate formation and preservation. Here we report a regional rock layer with near-infrared spectral characteristics consistent with the presence of magnesium carbonate, which has been identified and mapped in the Nili Fossae region by the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM). Carbonate is identified by strong, correlated absorption features at 2.3 and 2.5 micron whose position and width correspond uniquely to those of magnesite. Subtle absorptions at 3.4 and 4.0 microns appear to be present, although the identified spectral class lacks strong or definitive 3-4 micron features characteristic of some laboratory carbonate samples. The relative weakness of these bands is likely due to the presence of water (also indicated by a 1.9 micron absorption) and intermixed minerals in the carbonate-bearing rocks along with instrumental artifacts in this spectral region. The carbonate-bearing unit is positioned stratigraphically above the Fe/Mg smectite clay- bearing unit but below a mafic cap unit in a position similar to that of a regional olivine-bearing unit. The carbonate likely formed during the Noachian or early Hesperian via alteration of olivine by either hydrothermal fluids or near-surface water. The Nili Fossae carbonate-bearing unit is relatively thin and thus would not have sequestered large quantities of CO2. Based on OMEGA global mapping and reconnaissance with CRISM observations, this spectroscopic unit is not observed in significant outcrops elsewhere on Mars. With the possible exception of a carbonate component in transported sediments within Jezero crater, we find no evidence of classic bedded sedimentary carbonates resembling those on Earth. Instead, our results are consistent with carbonates having formed in response to unique local conditions, which require both ultramafic rocks and their substantial interaction with water to generate Mg carbonate in sufficient abundance to enable detection from orbit at a resolution of tens of meters per pixel. The existence of carbonate suggests neutral to alkaline conditions at the time of its formation, consistent with those implied by accompanying clays. Because aqueous activity in the Nili Fossae region extended into the Hesperian era, persistence of carbonate to the present indicates that acidic weathering conditions, proposed to be characteristic of Hesperian Mars, did not destroy these carbonates and thus did not dominate all aqueous environments.
P43D-06
Phyllosilicates in Mawrth Vallis: Implications for a Past Aqueous Environment
Mawrth Vallis contains the largest exposure of phyllosilicates on Mars, as observed by OMEGA and CRISM. OMEGA identified nontronite and montmorillonite as the most common phyllosilicates in the Mawrth Vallis region, and CRISM has confirmed and refined this observation with the further identification of kaolinite and hydrated silica [1-3]. In addition, saponite has been identified in one image within a crater. Two main phyllosilicate units occur in the Mawrth Vallis region: the lowermost unit is nontronite-bearing and is unconformably overlain by an Al-phyllosilicate-bearing unit composed of montmorillonite plus hydrated silica, with a thin layer of kaolinite plus hydrated silica at the top. These two units are capped by a spectrally- unremarkable unit [4, 5]. Spectra retrieved near the boundary between the nontronite and Al-phyllosilicate units exhibit a strong positive slope from 1-2 μm, likely from a ferrous component within the rock. This component could be ferrous olivine, troilite/phyrrhotite, or glauconite, for example, and indicates either rapid deposition or reducing conditions, sometimes supplied on Earth by microorganisms. Each of the phyllosilicate minerals identified form only under hydrolytic conditions, indicating a regional wet period in the Noachian. It is likely the two units formed by separate events; however, the processes are similar so we address them together. The large areal extent, stratigraphy, mineralogy, and observed layering of the two primary units suggest that they may be formed in a manner similar to bentonite: volcanic ash of different composition deposited into a body of water and then altered. This suggests the presence of a stable ground water table and extensive volcanism near Mawrth Vallis in order to produce the volume of phyllosilicates observed here. Other potential formational processes include: i) sediment deposition into a marine or lacustrine basin, ii) pedogenesis, or iii) hydrothermal alteration due to an overlying hot impact melt sheet (Al-phyllosilicate unit only). Clay deposits in marine basins typically reflect nearby continental conditions, and the species and volumes of phyllosilicates identified indicate a warm-wet climate. Pedogenesis in a warm-wet environment will produce laterites with alteration horizons of kaolinite and other Al-phyllosilicates and nontronite in the lowermost horizons. Impacts can mobilize frozen water as well as liquid water and the mineral assemblage formed is the same for a given temperature and water to rock ratio, so the climate cannot be determined if the Al-phyllosilicates were formed by an impact melt sheet hydrothermal system. It seems likely that these phyllosilicates formed in a warm-wet climate and are another indicator of a more hospitable environment in Mars' past. [1]Bibring, J.-P., et al. (2006) Science, 312, 400-404. [2]Poulet, F., et al. (2005) Nature, 438, 632-627. [3]Bishop, J. L., et al. (2008) Science, 321, 830-833. [4]Michalski, J. R., et al. (2007) Geology, 35, 951-954. [5]Wray, J. J., et al. (2008) GRL, 35, L12202.
P43D-07
Diverse Habitable Environments Characterized with Mars Reconnaissance Orbiter and Mars Express Data
The regional mineralogy from OMEGA and high-resolution mineralogy from CRISM are showing that the Noachian crust contains a diverse suite of hydrated minerals resulting from the alteration of the primary igneous and volcanic crust. Nevertheless, the most common phyllosilicates dating from this era are Mg/Fe smectite clays indicating moderate to low temperature (<200 C) formation. In contrast, mineral assemblages indicative of high temperature metamorphism or extensive hydrothermal systems are absent or rare. The precise characterization of the potentially habitable environments represented by these mineral deposits is hindered by the complex stratigraphy resulting from impacts, up to basin-forming. Megabreccia has been observed in a number of Noachian terrains and may date from this period of heavy bombardment. Preserved in the megabreccia are blocks that retain layered sequences suggesting that sedimentary processes were widely present in the Noachian. During the waning stages of the Noachian, extensive valley network formation and chains of crater lakes are the possible last stages of processes that dominated the surface during the Noachian. In the Nili Fossae region this period of transition is well preserved and provides a window into the Noachian habitable environments. The Noachan crust is well exposed showing megabreccia containing layered blocks rich in phyllosilicate as well as blocks of unaltered rock. These units are the window to the period of abundant phyllosilicate formation to determine if phyllosilicate formation was driven by weathering near or at the surface, or was dominantly due to alteration by hydrothermal processes in the shallow crust. Following the formation of the Isidis basin, widespread gradation facilitated by water transported abundant material to topographic lows, filling craters and graben. Valley network formation followed, resulting in the transport and deposition of phyllosilicate minerals in the crater lake delta in Jezero Crater. Concurrent with this era of valley network activity is the formation of a near-surface deposit rich in kaolinite accompanied perhaps by the the formation of a thin carbonate unit. By the time of the emplacement of the Hesperian Syrtis Major lavas, processes of alteration were either absent or declining. Thus this region of Mars captures critical components of the Noachian crust relevant to habitability: unaltered, altered and layered blocks in megabreccia; water-borne deposits filling the floors of craters and graben; valley networks and a crater lake with deltas; and regional deposits of kaolinite and carbonate. Ongoing work will further refine these relationships and seek to extend these analyses to other regions.
P43D-08 INVITED
Assessing the Nature, Distribution and Duration of Noachian Habitable Environments
Life as we know it indicates that habitable environments must simultaneously provide liquid water, conditions favorable for the assembly of complex organic molecules, nutrients for cellular constituents, and sources of energy that can sustain metabolism. These requirements must be met simultaneously and perhaps at least intermittently over geologically long periods. Recent missions indicate that liquid water probably persisted in the shallow subsurface during the Noachian. Surface waters occurred at least locally as indicated by geomorphologic features observed by orbiters and by analyses of Meridiani bedrock by MER Opportunity. Atmospheric precipitation and springs probably sustained surface waters at least intermittently during the Noachian. However whether lakes or a northern ocean persisted for long periods remains controversial. Theoretical models and surface geomorphology indicate that extensive groundwater probably persisted for geologically long times. Observations of spectral features of methane in the atmosphere indicate that groundwater might exist even today. Perhaps most challenging for achieving habitable conditions on Mars has been the requirement that liquid water and biologically useful sources of energy occur simultaneously. Because liquid water has been unstable at the martian surface for most of its history, both solar energy and liquid water have probably not been simultaneously available to sustain photosynthesis for most of Mars' history. Fortunately microorganisms also can obtain energy from iron and sulfur redox reactions in the absence of light. MER Spirit found evidence that iron oxidation might have occurred simultaneously with the aqueous alteration of rocks. MEX, MRO and MER documented extensive sulfates that might have derived at least in part from volcanic sulfur emissions oxidized under aqueous conditions. Perhaps most challenging for Mars exploration will be to visit sites where habitable environments once persisted and where their evidence has been well preserved. On Earth, phyllosilicates, carbonates, silica and evaporite minerals have retained remarkable records of our early biosphere. MEX, MRO and MER have revealed sites where such minerals occur and therefore perhaps also where evidence of ancient habitable environments has been preserved.