P31C-0544
Thermal Conductivity Control of Heat Flow and Effects on Crustal and Surface Compositions and Aqueous Extrusions in Thick Evaporites: Earth and Mars
Thick deposits of evaporitic hydrates can markedly influence heat flow due to the anomalous low thermal conductivity of hydrated salts and the high conductivity of anhydrous salts. The regulation or oscillation of hydration states and generation or freezing of saturated brines can be effected by interactive feedbacks between hydration states, volumes, thermal conductivities, and conductive regimes of hydrated materials. Tens of meters of hydrates can be important on Earth, and hundreds of meters can be crucial on Mars. Brines can exist and saline seeps may occur where none otherwise would. High temperatures beneath hydrate deposits may be attained at comparatively shallow depths even today in cold regions of Mars. Thus, modern and ancient aqueous processes can be linked. On Earth, hydrocarbon maturation and, on Mars and Earth, low-grade metamorphism in the zeolite and prehnite-pumpellyite facies is likely in the upper crust beneath hydrate deposits; zeolites, epidote, chlorite, serpentine, and other minerals are apt to form. Lateral discontinuities or thickenings/thinnings of thick hydrate deposits can cause lateral heat flow and produce surface-propagating zones of increased heat flux, even if the heat flux coming into the base of the deposits is constant. Such "heat leaks" can localize saline springs, gas emissions, and mud volcanism or diapirism, which potentially may be active even today on Mars. Thermal conductivity control of brine migration may transport minor soluble species to specific zones of high near-surface thermal gradient and heat flux. The movement of solutes and water in predictable sites relative to buried evaporitic sequences might result in unique effluorescences and high or low hydration states, depending on circumstances. These may have observable consequences in multispectral and thermal remote sensing and for surface morphologies, such as mud lumps/domes, desiccation cracks, patterns of effluorescence and wetting, and intensities/shapes of near infrared absorption bands due to bound water. Rare evaporite and pegmatite mineralization (e.g., economic borates in the Mojave Desert) that traditionally are attributed to volcanic interactions within evaporitic settings or magmatic/hydrothermal processes (e.g., low-temperature fluorite- bearing veins) might potentially be related to hydrate-associated insulation/heating and zone refining processes in thermally heterogeneous evaporite-hosted groundwater systems. Large bromide fractions in evaporitic halide solid solutions might be generated by fractionations in such systems. With bromine abundances believed to be much higher in the Martian crust than in Earth's, bromides may be an important marker of such fractionations on Mars. The modern viability of Martian habitats may be linked, in places, to this mechanism.
P31C-0545
Gully formation in terrestrial simple craters: Meteor Crater, USA and Lonar Crater, India
Geomorphic features such as gullies, valley networks, and channels on Mars have been used as a proxy to understand the climate and landscape evolution of Mars. Terrestrial analogues provide significant insight as to how the various exogenic and endogenic processes might contribute to the evolution of these martian landscapes. We describe here a terrestrial example from Meteor Crater, which shows a spectacular development of gullies throughout the inner wall in response to rainwater precipitation, snow melting and groundwater discharge. As liquid water has been envisaged as one of the important agents of landscape sculpturing, Meteor Crater remains a useful landmark, where planetary geologists can learn some lessons. We also show here how the lithology and structural framework of this crater controls the gully distribution. Like many martian impact craters, it was emplaced in layered sedimentary rocks with an exceptionally well-developed centripetal drainage pattern consisting of individual alcoves, channels and fans. Some of the gullies originate from the rim crest and others from the middle crater wall, where a lithologic transition occurs. Deeply incised alcoves are well-developed on the soft sandstones of the Coconino Formation exposed on the middle crater wall, beneath overlying dolomite. In general, the gully locations are along crater wall radial fractures and faults, which are favorable locales of groundwater flow and discharge; these structural discontinuities are also the locales where the surface runoff from rain precipitation and snow melting can preferentially flow, causing degradation. Like martian craters, channels are well developed on the talus deposits and alluvial fans on the periphery of the crater floor. In addition, lake sediments on the crater floor provide significant evidence of a past pluvial climate, when groundwater seeped from springs on the crater wall. Caves exposed on the lower crater level may point to percolation of surface runoff and selective discharge through fractures on the crater wall. Similar relationships are seen at Lonar Crater, India. Although these hydrological processes continue at Meteor Crater today, conditions at the crater are much more arid than they were soon after impact, reflecting a climatic shift similar in direction to that inferred for Mars.
P31C-0546
Gully-Polygon Interactions, Stratigraphy, and Distribution on Earth and Mars: Thermal Contraction Crack Polygons as Part of Cold-Desert, Near-Surface, Fluvial Systems
The distribution and structure of thermal contraction crack polygons in the South Fork region of Wright Valley are examined in order to document the manner in which patterned ground contributes to the flow of water in cold desert gully systems. Thermal contraction crack polygons span the interface between the atmospheric, permafrost, and hydrological systems in Wright Valley, providing a unique window into the interactions between these agents of geomorphic change. We find that polygonally patterned ground contributes to the generation, transport, and storage of water in the Wright Valley gully systems. Generation is contributed to by the trapping of windblown snow in gully troughs; transport, by changes to the ice-cement table and active layer hydrological properties caused by polygon trough formation; and storage by sorting of colluvium within polygons by grain-size. Further, we find that the styles of patterned ground can serve as an indicator of the degree of fluvial modification of a surface, ranging from nearly unmodified sand-wedge polygons to polygons forming within inactive gully channels. Finally, we find that thermal contraction crack polygons can be used as a stratigraphic element for evaluating the formation sequence of landscape elements. Cross-cutting relationships between gully channels and polygons suggest that gully channels formed in a polygonally patterned surface. Gully fan deposits overprint patterned ground, and are dissected by patterned ground that forms a continuous network across the fan surface contact, suggesting that patterned ground evolution "kept pace" with gully fan aggradation. Taken together, these lines of evidence suggest the continuous presence of permafrost beneath the Wright Valley gullies during the entire period of gully emplacement. These observations provide a baseline for interpreting patterned ground observed on gullied terrains on Mars. Similar relationships between gully alcoves, channels, and fans and patterned ground suggest that many martian gullies formed on patterned surfaces, implying that these gullies may form from the melting of trapped frost and snow under appropriate microclimate conditions.
P31C-0547
Possible Glacial Erosion of Interior Layered Deposit Mounds in Central Candor Chasma
Within Candor Chasma, there are 5 mesas or mounds (Candor and Baetis Mensae and 3 unnamed mensae to the east) of interior layered deposits (ILDs) that encircle the central Chasma low and have v- and wedge-shaped terminations that point toward this basin. Several datasets (Viking, MOLA, MOC, THEMIS, and HRSC) show features, analogous to glacial landforms, deposited on and carved into these ILD flanks and into the dark materials that embay them and also cover the chasma floor (Chapman et al., 2004, 2005). Five canyons (1 west of Candor Mensa and 4 between the five ILD mounds) are observed to terminate at the floor of central Candor Chasma. West of Candor Mensa, canyon 1 shows a pair of side-bounding ridges that may be possible lateral moraines, one on the west side of the southern mensa horn. This canyon is headed by a 3- forked (or pronged) headwall (centered at about lat 6.5° S., long 74°) that is very similar to analog glacial cirques. More erosional features can be observed up slope from this possible cirque. Canyon 2, between Candor and Baetis Mensae, is bound on the sides by possible lateral moraines and can be traced to a possible cirque on the wall between Ophir and Candor Chasmata. Just below this possible cirque (west of Baetis) the canyon contains a narrow central ridge that may be a medial moraine. Canyon 3, east of Baetis Mensa, shows numerous grooves cut into its floor and eroded, stair-stepped ridges of dark material on the canyon terminus at the central Candor Chasma floor. Analogous terrestrial terminal ridges can form as glaciers shrink and recess back from a point of maximum extent, each ridge may be a depositional end moraine that marks a hiatus in the recession process. Canyon 4, centered at about lat 6.5° S., long 71.4° and between the two unnamed ILD mounds east of Baetis Mensa, is a short hanging wall canyon again bound by sets of ridges (lateral moraines?). Canyon 5, between the easternmost ILDs of central Candor Chasma, shows 2 terraces or valley headwalls (centered at about lat 7° S., long 71.1°) that may be cirques within dark floor material. The ILD mounds bounding this canyon have been sheared off at level heights close to that of the possible cirque headwalls. This shearing is well shown on THEMIS image V10551002 and MOC images: E1700142 and E1900200. Between the sheared mounds, both ILD and dark material in the northernmost canyon floor is cut by grooves that are parallel to the canyon. An area of pits, possible kettle holes formed in dark material, is shown best on THEMIS image V11175002. HRSC topographic measurements indicate that many cross-sections along these 5 canyons have u-shaped profiles. Collectively, cirques, level shearing of ILD rock materials, lateral and terminal ridges (moraines), erosional grooves, and u-shaped canyons can not be formed by any other process than ice erosion and glacial processes. That these possible ice-formed features occur in dark and young (Geissler et al., 1990; Lucchitta, 1990) floor materials indicates that the putative ancient glaciers were relatively very young: an unusual find in equatorial Mars that signals a very different climate from that of the present. We are currently using HRSC DEMs to measure the topographic heights of cirques, shear levels, and medial ridges, to estimate a volume of the possible glacial ice cover in the area.
P31C-0548
Field Study of Mars Analog Materials in Spitsbergen (Norway) Using a Portable X-ray Diffraction Instrument
NASA’s Mars Science Laboratory (MSL) is the next major landed Mars mission scheduled for Launch in 2009. MSL is primarily a geological mission intended to assess if past environments on Mars could have supported life. An X-ray diffraction instrument called CheMin is part of the MSL rover science payload. CheMin was developed and is managed by NASA Ames Research Center and the flight system is currently being built at JPL. A miniature portable instrument was developed for NASA ARC by inXitu, Inc. (California) to support the CheMin Science Team with a tool that can easily be deployed on terrestrial Mars analog terrains. The instrument will be used to practice with field mineralogical analysis in preparation for the operational phase of the mission. The instrument is called mini-CheMin for its reduced size (45x32x12cm) and weight (14.5kg) compared to previous CheMin prototypes. Mini-CheMin was deployed in Spitsbergen in August 2007 as part of the science payload of the Arctic Mars Analog Svalbard Expedition (AMASE). The instrument was used for a variety of field tests, including two rover operation simulations. XRD data of sufficient quality for mineral identification and semi-quantitative analysis could be obtained in as little as a few minutes. XRF data, through limited in energy range to 3 - 8 keV, was very useful in restricting the search space for mineral identification with complex samples. In one of the deployment sites, a carbonate rich hot spring, a sample collected and analyzed in situ was found to be composed of mainly calcite with a minor amount of monohydrocalcite. Samples collected from this site and later analyzed with mini-CheMin onboard the expedition ship did not show any monohydrocalcite, the phase having been dehydrated to calcite by conventional laboratory sample preparation methods. This illustrates the benefit of in situ field mineralogical analysis for which samples can be analyzed in their pristine mineralogical makeup.
P31C-0549
Tests for Wet Mechanism of Slope Streaks Formation on Mars.
Slope streaks are forming in some equatorial regions on Mars. They have been mostly interpreted as a result of dry mass wasting of dust. Recently a striking morphological similarity with wet slope streaks in the Antarctic Dry Valleys has been demonstrated. Physical conditions on Mars do not allow the same mechanism. We propose a "wet" mechanism on Mars, which inherits the main feature of the Antarctic slope streaks responsible for their distinctive planforms, namely, shallow subsurface percolation of liquid above the ice table. This mechanism assumes some ice in the shallow subsurface, which had been emplaced under previous wetter climate conditions and currently is undergoing slow desiccation. This icy soil is overlaid by a layer highly enriched in chlorides. On top of this layer, there is a thin layer of dry fine dust. During the warm season, droplets of highly concentrated brines are formed in the salty layer. Sometimes at some places, the droplets coalesce; the liquid percolates downhill, wicks up through the dust layer and dries up. This alters the uppermost dust layer structure, which affects the surface albedo and observable as a dark streak. Further gradual changes of this new surface structure lead to slow brightening of the streak and final fading away. Such a mechanism is at the margin of physical possibility and consistency with observational constraints. Prospective observational tests for "wet" mechanism: (1) Spectral signature of hydrated chlorides in fresh streaks, would make "wet" hypothesis much more probable. (2) Gentle slope of slope streaks, especially of their uppermost parts would strongly favor "wet" mechanism. (3) Observation of slope streak formation during cold seasons, when the day-average surface temperature is below 190 K would reject "wet" mechanism. (4) Observation of a slope streak in the process of formation (a streak that lengthens from earlier to later image) would almost prove "wet" mechanism, at least, almost reject dry avalanche scenario. (5) Absence of a shallow high-thermal-inertia layer would be inconsistent with a "wet" scenario. (6) Geomorphologic observations supporting subsurface ice in the slope streak regions would favor the "wet" mechanism.
P31C-0550
Terrestrial Analogue and Origin of the Spiral Troughs in Martian Polar Ice Caps
The characteristics and origin of spiral troughs in the Martian polar ice caps have been studied over three decades and it has been claimed that there is no terrestrial analogue of its kind. Several hypotheses have been proposed to explain the origin of the spiral troughs. However, it remains unclear how the spiral troughs form and what controls their spacing, orientation, curvature and spiral-out senses. To understand the formation and origin of spiral troughs on Mars, we first introduce and investigate its terrestrial analogue of vortex fracture systems developed and found in Earth's crust, though not in Earth's polar ice caps. One such pattern is found in Inner Mongolia of China. It consists of five arcuate fractures arranged in a spiral pattern. A second such pattern showing spiral or vortex principal stress trajectories is located in the Western Alpine arc with the center of the spiral stress field located around Torino, Italy. We propose a model of differential rotation between the permanently frozen inner part, and the movable outer part, of the Martian polar ice cap to explain the origin of spiral troughs. Observation of the Martian polar ice caps suggests this model. This model is further supported by physical modeling using a brittle-coating, by an analytic solution employing elasticity theory, and by finite element numerical modeling. Based on our preliminary study, the proposed new model explains the formation of spiral troughs in both the northern and southern polar ice caps with spiral arrangements displaying different spiral-out senses. The new model infers an increase in the rotational velocity of Mars during the past hundred thousands of years.
P31C-0551
Which processes form the volcanic sands on Mars?
Volcanic sands are common at the surface of Mars. They are usually of basaltic composition. Occurrence of sands, mostly recognized as dark dune fields include numerous impact craters in the southern hemisphere [1], several volcanic provinces such as Cerberus and Syrtis Major[2], several impact craters in the northern hemisphere, the large basins (Hellas and Argyre), Valles Marineris, and the poles [3]. In most cases, the sands are of basaltic composition [2,4], at the exception of the polar dunes which are made of sulfates [3]. It is interesting to note that dunes have been found on the Hesperian volcanic plateau of Systis Major, while they are not reported on Tyrrhena Terra, a volcanic province similar in age and morphology to Syrtis. It seems thus that the formation of sand from volcanic material is not systematic and thus requires particular conditions. These different situations which will be presented raise the following questions. When did these volcanic sands form in the Martian history? Did they result from a long-standing and slow process operating in the present cold conditions or did they result from several episodes associated for instant to climate changes? We review several mechanisms which could account for the formation of volcanic sand on Mars from the volcanic material. In particular, we focus on the role of cold-climate processes from an analysis of terrestrial analogs in Iceland. In this case, the advance and retreat of glaciers over a recent erupted shield volcano associated with the strong catabatic winds have resulted in the rapid formation (less than few thousands years) of large volumes of sands. [1] Fenton, L. K. (2005), Potential sand sources for the dune fields in Noachis Terra, Mars, J. Geophys. Res. 110, E11004, doi :10.1029/2005JE002436. [2] Vaucher et. al, in revision for Icarus [3] Langevin et. al, (2005), Science, 307, 1584-1586 [4] Poulet F., Mangold N. and Erard S. (2003), Astron. & Astrophys. 412, L19-L23.
P31C-0552
True Polar Wander Due to Surface Mass Loading on Mars: Interaction between Rotation and Deformation through Pole Tide
Time variation of a pole location is calculated regarding a case of large-scale true polar wander due to surface mass loading on Mars. In this calculation, both cases with and without effect of pole tide are investigated. Through comparison between them, the effect of the pole tide on the time variation of the pole location is discussed. As a conclusion, this calculation quantitatively indicates that the pole tide stabilizes the pole location over much longer time scale than that of relaxation. On the other hand, it also implies that the effect of the pole tide is negligibly small in a case of longer term variation than the delay by this stabilization.
P31C-0553
Zero Obliquity Studies of Heat Transport in the Martian Paleo-climate
Global-mean climate models of the Martian atmosphere have predicted that early in Martian history, and for a range of initial total CO2 inventories, the atmosphere heat transport would be insufficient to prevent the formation of year-round CO2 polar caps. As a consequence of cap formation, the atmosphere would collapse to a vapor pressure, or cap-buffered, state. If Mars were trapped in a collapsed state for most of its planetary history, the amount of time available for physical and chemical weathering would be, as a result, greatly limited. Predictions of atmospheric collapse in the extant global-mean climate models involves representation of an inherently three-dimensional, time varying process—heat transport—in terms of a single, globally uniform parameterization. This parameterization is unavoidably the weakest link in any low-order (0-D and 1-D) atmospheric evolution model, though its proper representation is only of critical importance when the atmosphere is near a significant transition, such as the threshold for collapse. Using a global climate model, MarsWRF, we investigate the details of the three-dimensional, time varying heat transport at the threshold for atmospheric collapse. To definitively address whether pole-ward atmospheric heat transport can, alone, prevent collapse, the most illuminating experiment is one at 0° obliquity. In this situation, solar heating near the poles tends to zero, and condensation cannot be prevented in the absence of transport, regardless of the atmospheric thickness and greenhouse effect. This investigation allows us to determine the validity of the heat transport parameterizations used by global-mean climate models, particularly with regard to atmospheric collapse.
P31C-0554
The Evolution of the Past Atmosphere of Mars
Past evolution of terrestrial planets is difficult to study due to the lack of data and in situ measurements. However, it is necessary in order to explain some of the features that are observed by recent missions. This work is meant to show that even with the few data we have and by using simple straightforward models, it is possible to have some answers about the evolution of Mars during the last 3 Gyr. We study possible states of the past Martian atmosphere consistent with present observation through a simple evolution model based on realistic outgassing scenarios and atmospheric loss. We focus on CO2 as the most likely main gas present in the atmosphere at that time and involved in large scale and long term processes. Volcanic degassing is obtained through the use of results from numerical model analysis that yielded the evolutions of crust production rates (Breuer et al., 2003 and 2006, Manga et al., 2006). By evaluating the contents of the lavas, the amount of volatiles that are released can be estimated through different scenarios. The mechanisms leading to the loss of the Martian atmosphere are all thought to be part of the atmospheric escape rather than some surface reservoirs such as carbonate formation as no carbonate has been found on Mars to this day. Atmospheric escape is due to non thermal processes (involving solar emissions) as opposed to thermal processes (such as hydrodynamic escape). We used measurements from ASPERA and Mars Express and models from Chassefière et al. (2006) to estimate the amount of lost atmosphere. Thus we obtain evolutions of the CO2 pressure that are consistent with the present state of the atmosphere. It first appears that a present-day crustal production of at least 0.01 to 0.1 km3/year is needed for the atmosphere to be at steady state. Moreover our models provide us with a rough constraint on the CO2 contents of the Martian mantle. It seems it should be lower than 200ppm in order to fit with present-day conditions. Higher concentrations would lead to thicker atmospheres due to intense release of gases by late volcanism. We also witness around 3 Gyr ago a rapid loss of the primary (and primordial) atmosphere due to atmospheric escape. It is finally found that for most of the scenarios (we investigate a wide range of mantle compositions and atmospheric escape models) the present atmosphere is of volcanic origin and has been created between 1 Gyr and 1.5 Gyr ago. If the volcanic activity and the degassing are intense enough then the atmosphere can even be entirely secondary and as young as 500 Myr, meaning that the present Martian atmosphere can be very young.