Dust devil sediment flux on Earth and Mars: Laboratory simulations

Laboratory simulations using the Arizona State University Vortex Generator (ASUVG) were run to simulate sediment flux in dust devils in terrestrial ambient and Mars-analog conditions. The objective of this study was to measure vortex sediment flux in the laboratory to yield estimations of natural dust devils on Earth and Mars, where all parameters may not be measured. These tests used particles ranging from 2 to 2000 μm in diameter and 1300 to 4800 kg m⁻³ in density, and the results were compared with data from natural dust devils on Earth and Mars. Typically, the cores of dust devils (regardless of planetary environment) have a pressure decrease of ∼0.1-1.5% of ambient atmospheric pressure, which enhances the lifting of particles from the surface. Core pressure decreases in our experiments ranged from ∼0.01% to 5.00% of ambient pressure (10 mbar Mars cases and 1000 mbar for Earth cases) corresponding to a few tenths of a millibar for Mars cases and a few millibars for Earth cases. Sediment flux experiments were run at vortex tangential wind velocities of 1-45 m s⁻¹, which typically correspond to ∼30-70% above vortex threshold values for the test particle sizes and densities. Sediment flux was determined by time-averaged measurements of mass loss for a given vortex size. Sediment fluxes of ∼10⁻⁶-10⁰ kg m⁻² s⁻¹ were obtained, similar to estimates and measurements for fluxes in dust devils on Earth and Mars. Sediment flux is closely related to the vortex intensity, which depends on the strength of the pressure decrease in the core (ΔP). This study found vortex size is less important for lifting materials because many different diameters can have the same ΔP. This finding is critical in scaling the laboratory results to natural dust devils that can be several orders of magnitude larger than the laboratory counterparts.     

Evidence for weathering on early Mars from a comparison with terrestrial weathering profiles

Phyllosilicates on Mars are widespread in the ancient crust suggesting the presence of liquid water at the martian surface and therefore warmer conditions during its early history. However, the role of the ancient climate in the alteration process, which produced these phyllosilicates, remains under debate, because similar mineral assemblages can be produced by hydrothermal alteration at depth. This paper focuses on the origin of coincident outcrops of Fe/Mg bearing phyllosilicates and Al-bearing phyllosilicates, which are observed in several regions of Mars. We performed a detailed mineralogical comparison between a section in Nili Fossae, Mars, and a weathering profile located at Murrin Murrin, Western Australia. The Murrin Murrin profile is developed in Archaean serpentinized peridotite massifs over a ∼40 m thick sequence. It has three alteration zones: the serpentine mineral saprolite is found at the bottom, immediately overlain by Fe/Mg-bearing smectites and then Al-bearing phyllosilicates (kaolinite) mixed with iron hydroxides. This example illustrates how Al-dominated minerals can derive from the alteration of initially Al-poor ultramafic rocks by the intense leaching of Mg²⁺. This mineralogical sequence is very similar to that detected locally in Nili Fossae by orbital spectroscopy. By analogy, we propose that the mineral assemblage detected on Mars is the result of long-term weathering, and thus could be the best evidence of past weathering as a direct result of a climate significantly warmer than at present.     

A spectroscopic method for identifying terrestrial biocarbonates and application to Mars

Searching for traces of extinct and/or extant life on Mars is one of the major objectives for remote-sensing and in situ exploration of the planet. In previous laboratory works we have investigated the infrared spectral modifications induced by thermal processing on different carbonate samples, in the form of fresh shells and fossils of different ages, whose biotic origin is easily recognizable. The goal was to discriminate them from their abiotic counterparts. In general, it is difficult to identify biotic signatures, especially when the organisms inducing the carbonate precipitation have low fossilization potential (i.e. microbes, bacteria, archaea). A wide variety of microorganisms are implicated in carbonate genesis, and their direct characterization is very difficult to evaluate by traditional methods, both in ancient sedimentary systems and even in recent environments.In the present work we apply our analysis to problematic carbonate samples, in which there is no clear evidence of controlled or induced biomineralization. This analysis indicates a very likely biotic origin of the aragonite samples under study, in agreement with the conclusion previously reported by Guido et al. (2007) who followed a completely different approach based on a complex set of sedimentary, petrographic, geochemical and biochemical analyses. We show that our method is reliable for discriminating between biotic and abiotic carbonates, and therefore it is a powerful tool in the search for life on Mars in the next generation of space missions to the planet.     

Interferometric millimeter observations of water vapor on Mars and comparison with Mars Express measurements

We present interferometric mapping of the 225.9-GHz HDO and 203.4-GHz lines on Mars obtained with the IRAM Plateau de Bure facility (PdBI). The observations were performed during martian year 28 (MY28), at L_s=320.3° for the HDO line, and at L_s=324.3° for the line. The HDO line is detected at the eastern (morning) and western (evening) limbs of the northern hemisphere, corresponding to a water column density in the range 3-6 pr.-μm. The line is not detected, which is compatible with the column densities derived from the HDO line. Quasi-simultaneous far infrared measurements obtained by the Planetary Fourier Spectrometer (PFS) onboard the Mars Express spacecraft confirm our PdBI results, yielding a 5±1 pr.-μm meridionally constant water column abundance.Such a low water abundance during the southern mid-autumn of MY28 does not correspond to the standard martian climatology as observed during the previous years. It was however already retrieved from near-infrared observations performed by the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) onboard the Mars Reconnaissance Orbiter spacecraft [Smith, M.D., Wolff, M.J., Clancy, R.T., Murchie, S.L. 2009. CRISM observations of water vapor and carbon monoxide. J. Geophys. Res. 114, doi: 10.1029/2008JE003288]. Our observations thus confirm that the planet-encircling dust storm that occurred during MY28 significantly affected the martian water cycle. Our observations also demonstrate the usefulness of interferometric submillimeter observations to survey the martian water cycle from ground-based facilities.     

The magnetic field near Mars: A comparison between a hybrid model, Mars Global Surveyor and Mars Express observations

Mars Express (MEX) does not carry its own magnetometer which complicates interpretation of ASPERA-3/MEX ion measurements. The direction of the interplanetary magnetic field (IMF) is especially important because it, among other things, determines the direction of the convective electric field and orientation of the cross tail current sheet and tail lobes. In this paper we present a case study to show the properties of the magnetic field near Mars in a quasi-neutral hybrid (QNH) model at the orbits where the Mars Global Surveyor (MGS) has made measurements, present a method to derive the IMF clock angle by comparing fields in a hybrid model and the direction of the magnetic field measured by MGS by deriving the IMF clock angle. We also use H⁺ ring velocity distribution observations upstream of the bow shock measured by the IMA/ASPERA-3 instrument on board MEX spacecraft. These observations are used to indirectly provide the orientation of the IMF. We use a QNH model (HYB-Mars) where ions are modeled as particles while electrons form a mass-less charge neutralizing fluid. We found that the direct MGS and non-direct IMA observations of the orientation magnetic field vectors in non-crustal magnetic field regions are consistent with the global magnetic field draping pattern predicted by the global model.     

Hydrovolcanic features on Mars: Preliminary observations from the first Mars year of HiRISE imaging

We provide an overview of features indicative of the interaction between water and lava and/or magma on Mars as seen by the High Resolution Imaging Science Experiment (HiRISE) camera during the Primary Science Phase of the Mars Reconnaissance Orbiter (MRO) mission. The ability to confidently resolve meter-scale features from orbit has been extremely useful in the study of the most pristine examples. In particular, HiRISE has allowed the documentation of previously undescribed features associated with phreatovolcanic cones (formed by the interaction of lava and groundwater) on rapidly emplaced flood lavas. These include “moats” and “wakes” that indicate that the lava crust was thin and mobile, respectively [Jaeger, W.L., Keszthelyi, L.P., McEwen, A.S., Dundas, C.M., Russel, P.S., 2007. Science 317, 1709-1711]. HiRISE has also discovered entablature-style jointing in lavas that is indicative of water-cooling [Milazzo, M.P., Keszthelyi, L.P., Jaeger, W.L., Rosiek, M., Mattson, S., Verba, C., Beyer, R.A., Geissler, P.E., McEwen, A.S., and the HiRISE Team, 2009. Geology 37, 171-174]. Other observations strongly support the idea of extensive volcanic mudflows (lahars). Evidence for other forms of hydrovolcanism, including glaciovolcanic interactions, is more equivocal. This is largely because most older and high-latitude terrains have been extensively modified, masking any earlier 1-10 m scale features. Much like terrestrial fieldwork, the prerequisite for making full use of HiRISE’s capabilities is finding good outcrops.     

Modeling aqueous perchlorate chemistries with applications to Mars

NASA’s Phoenix lander identified perchlorate and carbonate salts on Mars. Perchlorates are rare on Earth, and carbonates have largely been ignored on Mars following the discovery by NASA’s Mars Exploration Rovers of acidic precipitated minerals such as jarosite. In light of the Phoenix results, we updated the aqueous thermodynamic model FREZCHEM to include perchlorate chemistry. FREZCHEM models the Na-K-Mg-Ca-Fe(II)-Fe(III)-Al-H-Cl-Br-SO₄-NO₃-OH-HCO₃-CO₃-CO₂-O₂-CH₄-Si-H₂O system, with 95 solid phases. We added six perchlorate salts: NaClO₄·H₂O, NaClO₄·2H₂O, KClO₄, Mg(ClO₄)₂·6H₂O, Mg(ClO₄)₂·8H₂O, and Ca(ClO₄)₂·6H₂O. Modeled eutectic temperatures for Na, Mg, and Ca perchlorates ranged from 199 K (−74 °C) to 239 K (−34 °C) in agreement with experimental data.We applied FREZCHEM to the average solution chemistry measured by the Wet Chemistry Laboratory (WCL) experiment at the Phoenix site when soil was added to water. FREZCHEM was used to estimate and alkalinity concentrations that were missing from the WCL data. The amount of is low compared to estimates from elemental abundance made by other studies on Mars. In the charge-balanced solution, the dominant cations were Mg²⁺ and Na⁺ and the dominant anions were , and alkalinity. The abundance of calcite measured at the Phoenix site has been used to infer that the soil may have been subject to liquid water in the past, albeit not necessarily locally; so we used FREZCHEM to evaporate (at 280.65 K) and freeze (from 280.65 to 213.15 K) the WCL-measured solution to provide insight into salts that may have been in the soil. Salts that precipitated under both evaporation and freezing were calcite, hydromagnesite, gypsum, KClO₄, and Mg(ClO₄)₂·8H₂O. Epsomite (MgSO₄·7H₂O) and NaClO₄·H₂O were favored by evaporation at temperatures >0 °C, while meridianite (MgSO₄·11H₂O), MgCl₂·12H₂O, and NaClO₄·2H₂O were favored at subzero temperatures. Incongruent melting of such highly hydrated salts could be responsible for vug formation elsewhere on Mars.All K⁺ precipitated as insoluble KClO₄ during both evaporation and freezing simulations, accounting for 15.8% of the total perchlorates. During evaporation, 35.8% of perchlorates precipitated with Na⁺ and 48.4% with Mg²⁺. During freezing, 58.4% precipitated with Na⁺ and 24.8% with Mg²⁺. Given its low eutectic temperature, the existence of Mg(ClO₄)₂ in either case allows for the possibility of liquid brines on Mars today. FREZCHEM also showed that Ca(ClO₄)₂ would likely not have precipitated at the Phoenix landing site due to the strong competing sinks for Ca as calcite and gypsum. Overall, these results help constrain the salt mineralogy of the soil. Differences between evaporites and cryogenites suggest ways to discriminate between evaporation and freezing during salt formation. Future efforts, such as sample return or in situ X-ray diffraction, may make such a determination possible.     

Bright dust devil tracks on Earth: Implications for their formation on Mars

We report on the first observations of bright dust devil tracks (BDDTs) on Earth, observed in the Turpan depression desert in northwestern China, where raindrop impacts on sand surfaces form aggregates of sand, silt and clay resulting in rough surface textures, which are destroyed by passages of dust devils leading to smooth surface textures within the tracks. The differences in photometric properties between the track and outside the tracks cause the albedo differences leading to the formation of BDDTs and similar processes might lead to BDDTs on Mars in areas with thick dust covers.     

Rosetta-Alice observations of exospheric hydrogen and oxygen on Mars

The European Space Agency’s Rosetta spacecraft, en route to a 2014 encounter with comet 67P/Churyumov-Gerasimenko, made a gravity assist swing-by of Mars on 25 February 2007, closest approach being at 01:54 UT. The Alice instrument on board Rosetta, a lightweight far-ultraviolet imaging spectrograph optimized for in situ cometary spectroscopy in the 750-2000 Å spectral band, was used to study the daytime Mars upper atmosphere including emissions from exospheric hydrogen and oxygen. Offset pointing, obtained five hours before closest approach, enabled us to detect and map the H i Lyman-α and Lyman-β emissions from exospheric hydrogen out beyond 30,000 km from the planet’s center. These data are fit with a Chamberlain exospheric model from which we derive the hydrogen density at the 200 km exobase and the H escape flux. The results are comparable to those found from the Ultraviolet Spectrometer experiment on the Mariner 6 and 7 fly-bys of Mars in 1969. Atomic oxygen emission at 1304 Å is detected at altitudes of 400-1000 km above the limb during limb scans shortly after closest approach. However, the derived oxygen scale height is not consistent with recent models of oxygen escape based on the production of suprathermal oxygen atoms by the dissociative recombination of .     

CO2 Snow on Mars and Early Earth: Experimental Constraints

Greenhouse warming due to carbon dioxide atmospheres may be responsible for maintaining the early Earth's surface temperature above freezing and may even have allowed for liquid water on early Mars. However, the high levels of CO₂ required for such warming should have also resulted in the formation of CO₂ clouds. These clouds, depending on their particle size, could lead to either warming or cooling. The particle size in turn is determined by the nucleation and growth conditions. Here we present laboratory studies of the nucleation and growth of carbon dioxide on water ice under martian atmospheric conditions. We find that a critical saturation, S=1.34, is required for nucleation, corresponding to a contact parameter between solid water and solid carbon dioxide of m=0.95. We also find that after nucleation occurs, growth of CO₂ is very rapid, and we report the growth rates at a number of supersaturations. Because growth would be expected to continue until the CO₂ pressure is lowered to its vapor pressure, we expect particles larger than those being currently suggested for the present and past martian atmospheres. Using this information in a microphysical model described in a companion paper, we find that CO₂ clouds are best described as “snow,” having a relatively small number of large particles.     

Lidar atmospheric measurements on Mars and Earth

The LIDAR instrument operating from the surface of Mars on the Phoenix Mission measured vertical profiles of atmospheric dust and water ice clouds at temperatures around −65 °C. An equivalent lidar system was utilized for measurements in the atmosphere of Earth where dust and cloud conditions are similar to Mars. Coordinated aircraft in situ sampling provided a verification of lidar measurement and analysis methods and also insight for interpretation of lidar derived optical parameters in terms of the dust and cloud microphysical properties. It was found that the vertical distribution of airborne dust above the Australian desert is quite similar to what is observed in the planetary boundary layer above Mars. Comparison with the in situ sampling is used to demonstrate how the lidar derived optical extinction coefficient is related to the dust particle size distribution. The lidar measurement placed a constraint on the model size distribution that has been used for Mars. Airborne lidar measurements were also conducted to study cirrus clouds that form in the Earth’s atmosphere at a similar temperature and humidity as the clouds observed with the lidar on Mars. Comparison with the in situ sampling provides a method to derive the cloud ice water content (IWC) from the Mars lidar measurements.     

Multitemporal observations of identical active dust devils on Mars with the High Resolution Stereo Camera (HRSC) and Mars Orbiter Camera (MOC)

Active dust devils were observed in Syria Planum in Mars Observer Camera - Wide Angle (MOC-WA) and High Resolution Stereo Camera (HRSC) imagery acquired on the same day with a time delay of ∼26 min. The unique operating technique of the HRSC allowed the measurement of the traverse velocities and directions of motion. Large dust devils observed in the HRSC image could be retraced to their counterparts in the earlier acquired MOC-WA image. Minimum lifetimes of three large (avg. ∼700 m in diameter) dust devils are ∼26 min, as inferred from retracing. For one of these large dust devil (∼820 m in diameter) it was possible to calculate a minimum lifetime of ∼74 min based on the measured horizontal speed and the length of its associated dust devil track. The comparison of our minimum lifetimes with previous published results of minimum and average lifetimes of small (∼19 m in diameter, avg. min. lifetime of ∼2.83 min) and medium (∼185 m in diameter, avg. min. lifetime of ∼13 min) dust devils imply that larger dust devils on Mars are active for much longer periods of time than smaller ones, as it is the case for terrestrial dust devils. Knowledge of martian dust devil lifetimes is an important parameter for the calculation of dust lifting rates. Estimates of the contribution of large dust devils (>300-1000 m in diameter) indicate that they may contribute, at least regionally, to ∼50% of dust entrainment by dust devils into the atmosphere compared to the dust devils <300 m in diameter given that the size-frequency distribution follows a power-law. Although large dust devils occur relatively rarely and the sediment fluxes are probably lower compared to smaller dust devils, their contribution to the background dust opacity by dust devils on Mars could be at least regionally large due to their longer lifetimes and ability of dust lifting into high atmospheric layers.     

Thermal contraction crack polygons on Mars: A synthesis from HiRISE, Phoenix, and terrestrial analog studies

Thermal contraction crack polygons are complex landforms that have begun to be deciphered on Earth and Mars by the combined investigative efforts of geomorphology, environmental monitoring, physical models, paleoclimate reconstruction, and geochemistry. Thermal contraction crack polygons are excellent indicators of the current or past presence of ground ice, ranging in ice content from weakly cemented soils to debris-covered massive ice. Relative to larger topographic features, polygons may form rapidly, and reflect climate conditions at the time of formation—preserving climate information as relict landforms in the geological record. Polygon morphology and internal textural characteristics can be used to distinguish surfaces modified by the seasonal presence of a wet active layer or dry active layer, and to delimit subsurface ice conditions. Analysis of martian polygon morphology and distribution indicates that geologically-recent thermal contraction crack polygons on Mars form predominantly in an ice-rich latitude-dependent mantle, more likely composed of massive ice deposited by precipitation than by cyclical vapor diffusion into regolith. Regional and local heterogeneities in polygon morphology can be used to distinguish variations in ice content, deposition and modification history, and to assess microclimate variation on timescales of ka to Ma. Analyses of martian polygon morphology, guided by investigations of terrestrial analog thermal contraction crack polygons, strongly suggest the importance of excess ice in the formation and development of many martian thermal contraction crack polygons—implying the presence of an ice-rich substrate that was fractured during and subsequent to obliquity-driven depositional periods and continually modified by ongoing vapor equilibration processes.     

Barchan dune asymmetry: Observations from Mars and Earth

Barchan dune asymmetry refers to the extension of one barchan limb downwind. It is a common dune form on Earth and also occurs on Mars and Titan. A new classification of barchan limbs is presented where three types of limb morphology are identified: linear, kinked and beaded. These, along with other dune-scale morphological signatures, are used to identify three of the causes of barchan asymmetry on Mars: bi-directional winds, dune collision and the influence of inclined topography.The potential for specific dune asymmetric morphologies to indicate aspects of the formative wind regime on planetary surfaces is shown. For example, the placement of dune limbs can indicate the general direction and relative strength of formative oblique winds; an extreme barchan limb length may indicate a long duration oblique wind; a kinked limb may be evidence of the passage of a storm; beaded limbs may represent surface-wave instabilities caused by an increase in wind energy parallel to the dune. A preliminary application of these signatures finds evidence for bi-modal winds on Mars. However, these and other morphological signatures of wind direction and relative strength should be applied to planetary landforms with caution as more than one process (e.g., bi-modal winds and collision) may be operating together or sequentially on the dunefield. In addition, analysis should be undertaken at the dunefield scale and not on individual dunes. Finally, morphological data should be acquired from similar-scale dunes within a dunefield.In addition to bi-modal wind regimes on Mars, the frequent parallel alignment of the extended barchan limb to the dune suggests that dune collision is also an important cause of asymmetry on Mars. Some of the more complex dunefield patterns result from a combination of dune collision, limb extension and merging with downwind dunes.Dune asymmetric form does not inhibit dune migration in the Namib Desert or on Mars. Data from the Namib suggest that dune migration rates are similar for symmetric and asymmetric dunes. Further modeling and field studies are needed to refine our understanding of the potential range of limb and dune morphologies that can result from specific asymmetry causes.     

Global-scale near infrared variability on Mars: Analysis of 2003 Mars opposition observations from HST/NICMOS

We have used HST/NICMOS to observe approximately 57% of the martian surface in 7 narrow band filters (0.97, 1.08, 1.13, 1.66, 1.90, 2.12, and 2.15 μm) during the 2003 opposition (Ls∼250°) and at a resolution of ∼12 km/pixel. Principal components analysis (PCA) of the dataset has identified regional variability on scales of hundreds of kilometers associated with differences in the near-infrared spectrum of Mars. Visualization of the data in principal component space has allowed us to identify spectral endmembers associated with the south polar cap, the classic bright terrains, northern Syrtis Major, southern Syrtis Major, Tyrrhena Terra, and Acidalia Planitia. The two Syrtis Major endmembers and the Tyrrhena Terra endmember differ in their absolute reflectivities but have the same spectral shape at wavelengths longer than 1.6 μm. The Acidalia endmember is distinct from the other dark terrain endmembers because it exhibits a strong negative near-IR spectral slope. Comparisons with spectral library measurements cannot provide unique constraints on the surface mineralogy for these sparsely-sampled spectral data. However, the observed spectral variations between Tyrrhena Terra and Syrtis Major are consistent with variations in iron- and sulfur-bearing minerals, and the relatively strong negative spectral slope in the spectrum of Acidalia is consistent with the presence of hydrated alteration products. Additional comparison with previous NICMOS observations taken in 1997 at Ls∼150° indicate that the average near-IR spectral slope of the Acidalia region is more negative during the late northern fall than during the mid northern summer. This may indicate seasonal variations in the presence of either adsorbed water or re-hydrated minerals in the regolith of Acidalia.     

Geomorphic knobs of Candor Chasma, Mars: New Mars Reconnaissance Orbiter data and comparisons to terrestrial analogs

High Resolution Imaging Science Experiment (HiRISE) imagery and digital elevation models of the Candor Chasma region of Valles Marineris, Mars, reveal prominent and distinctive positive-relief knobs amidst light-toned layers. Three classifications of knobs, Types 1, 2, and 3, are distinguished from a combination of HiRISE and Thermal Emission Imaging System (THEMIS) images based on physical expressions (geometries, spatial relationships), and spectral data from Compact Reconnaissance Imaging Spectrometer for Mars (CRISM). Type 1 knobs are abundant, concentrated, topographically resistant features with their highest frequency in West Candor, which have consistent stratigraphic correlations of the peak altitude (height). These Type 1 knobs could be erosional remnants of a simple dissected terrain, possibly derived from a more continuous, resistant, capping layer of pre-existing material diagenetically altered through recrystallization or cementation. Types 2 and 3 knobs are not linked to a single stratigraphic layer and are generally solitary to isolated, with variable heights. Type 3 are the largest knobs at nearly an order of magnitude larger than Type 1 knobs. The variable sizes and occasional pits on the tops of Type 2 and 3 knobs suggest a different origin, possibly related to more developed erosion, preferential cementation, or textural differences from sediment/water injection or intrusion, or from a buried impact crater. Enhanced color HiRISE images show a brown coloration of the knob peak crests that is attributable to processing and photometric effects; CRISM data do not show any detectable spectral differences between the knobs and the host rock layers, other than albedo. These intriguing knobs hold important clues to deducing relative rock properties, timing of events, and weathering conditions of Mars history.     

First ENA observations at Mars: Solar-wind ENAs on the nightside

We present measurements with an Energetic Neutral Atom (ENA) imager on board Mars Express when the spacecraft moves into Mars eclipse. Solar wind ions charge exchange with the extended Mars exosphere to produce ENAs that can spread into the eclipse of Mars due to the ions' thermal spread. Our measurements show a lingering signal from the Sun direction for several minutes as the spacecraft moves into the eclipse. However, our ENA imager is also sensitive to UV photons and we compare the measurements to ENA simulations and a simplified model of UV scattering in the exosphere. Simulations and further comparisons with an electron spectrometer sensitive to photoelectrons generated when UV photons interact with the spacecraft suggest that what we are seeing in Mars' eclipse are ENAs from upstream of the bow shock produced in charge exchange with solar wind ions with a non-zero temperature. The measurements are a precursor to a new technique called ENA sounding to measure solar wind and planetary exosphere properties in the future.     

Study of terrestrial fossils in phyllosilicate-rich soils: Implication in the search for biosignatures on Mars

The search for biosignatures in the soil of Mars is a major objective of the planet exploration, but the detection of such structures is not straightforward due to the degradation of the organic material. In a previous work our group has analyzed the spectral reaction to thermal processing of biomineral carbonate samples including fresh and fossil shells. We found that two terrestrial fossil shells collected from clay deposits preserve their biomineral characteristics much better than coeval fossils not embedded in clay layers.In the present paper we extend our analysis to a set of fossils found in three different terrestrial clay deposits. Our results confirm that the level of degradation may be much lower than the standard values if fossils are surrounded by clay minerals. As a result these fossils have a spectroscopic response to thermal treatment which make them much more distinguishable from their abiotic counterparts than coeval fossils not collected from clay deposits. This result implies that the phyllosilicates regions recently discovered on Mars may represent very interesting environments that can provide conditions favorable to preserve evidence of biomarkers, and hence can be regarded as good candidate locations for their detection.     

Polygonal cracks in bedrock on Earth and Mars: Implications for weathering

Polygonal crack systems with domal microrelief imaged by the Mars Exploration Rover (MER) Opportunity show remarkable similarity to terrestrial crack systems developed on outcrop surfaces. Study of Jurassic Navajo Sandstone surfaces show development of crack systems in relatively isotropic host rock as a result of tensile weathering stresses. These terrestrial analogs are utilized to understand potential weathering processes on Mars.     

Power law of dust devil diameters on Mars and Earth

Estimates from visual surveys of the frequency of dust devils, even at terrestrial localities known for their abundance, vary by some four orders of magnitude, making a quantitative hazard assessment difficult. Here I show (1) that new high-quality observations from Mars fit a power law size distribution, (2) that such a power law population can unify the discrepant terrestrial surveys, and (3) that the populations on the two planets appear similar.