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.     

Distribution and interplay of geologic processes on Titan from Cassini radar data

The Cassini Titan Radar Mapper is providing an unprecedented view of Titan’s surface geology. Here we use Synthetic Aperture Radar (SAR) image swaths (Ta-T30) obtained from October 2004 to December 2007 to infer the geologic processes that have shaped Titan’s surface. These SAR swaths cover about 20% of the surface, at a spatial resolution ranging from ∼350 m to ∼2 km. The SAR data are distributed over a wide latitudinal and longitudinal range, enabling some conclusions to be drawn about the global distribution of processes. They reveal a geologically complex surface that has been modified by all the major geologic processes seen on Earth - volcanism, tectonism, impact cratering, and erosion and deposition by fluvial and aeolian activity. In this paper, we map geomorphological units from SAR data and analyze their areal distribution and relative ages of modification in order to infer the geologic evolution of Titan’s surface. We find that dunes and hummocky and mountainous terrains are more widespread than lakes, putative cryovolcanic features, mottled plains, and craters and crateriform structures that may be due to impact. Undifferentiated plains are the largest areal unit; their origin is uncertain. In terms of latitudinal distribution, dunes and hummocky and mountainous terrains are located mostly at low latitudes (less than 30°), with no dunes being present above 60°. Channels formed by fluvial activity are present at all latitudes, but lakes are at high latitudes only. Crateriform structures that may have been formed by impact appear to be uniformly distributed with latitude, but the well-preserved impact craters are all located at low latitudes, possibly indicating that more resurfacing has occurred at higher latitudes. Cryovolcanic features are not ubiquitous, and are mostly located between 30° and 60° north. We examine temporal relationships between units wherever possible, and conclude that aeolian and fluvial/pluvial/lacustrine processes are the most recent, while tectonic processes that led to the formation of mountains and Xanadu are likely the most ancient.     

Stability of methane clathrate hydrates under pressure: Influence on outgassing processes of methane on Titan

We have conducted high-pressure experiments in the H₂O-CH₄ and H₂O-CH₄-NH₃ systems in order to investigate the stability of methane clathrate hydrates, with an optical sapphire-anvil cell coupled to a Raman spectrometer for sample characterization. The results obtained confirm that three factors determine the stability of methane clathrate hydrates: (1) the bulk methane content of the samples; (2) the presence of additional gas compounds such as nitrogen; (3) the concentration of ammonia in the aqueous solution. We show that ammonia has a strong effect on the stability of methane clathrates. For example, a 10 wt.% NH₃ solution decreases the dissociation temperature of methane clathrates by 14-25 K at pressures above 5 MPa. Then, we apply these new results to Titan’s conditions. Dissociation of methane clathrate hydrates and subsequent outgassing can only occur in Titan’s icy crust, in presence of locally large amounts of ammonia and in a warm context. We propose a model of cryomagma chamber within the crust that provides the required conditions for methane outgassing: emplacement of an ice plume triggers the melting (if solid) or heating (if liquid) of large ammonia-water pockets trapped at shallow depth, and the generated cryomagmas dissociate surrounding methane clathrate hydrates. We show that this model may allow for the outgassing of significant amounts of methane, which would be sufficient to maintain the presence of methane in Titan’s atmosphere for several tens of thousands of years after a large cryovolcanic event.     

Possible liquid water origin for Atacama Desert mudflow and recent gully deposits on Mars

Evidence of recent gully activity on Mars has been reported based on the formation of new light toned deposits within the past decade, the origin of which remains controversial. Analogous recent light toned gully features have formed by liquid water activity in the Atacama Desert on Earth. These terrestrial deposits leave no mineralogical trace of water activity but rather show an albedo difference due to particle size sorting within a fine-grained mudflow. Therefore, spectral differences indicating varying mineralogy between a recent gully deposit and the surrounding terrain may not be the most relevant criteria for detecting water flow in arid environments. Instead, variation in particle size between the deposit and surrounding terrain is a possible discriminator to identify a water-based flow. We show that the Atacama deposit is similar to the observed Mars gully deposits, and both are consistent with liquid water activity. The light-toned Mars gully deposits could have formed from dry debris flows, but a liquid water origin cannot be ruled out because not all liquid water flows leave hydrated minerals behind on the surface. Therefore, the Mars deposits could be remnant mudflows that formed on Mars within the last decade.     

Mercury surface composition: Integrating petrologic modeling and remote sensing data to place constraints on FeO abundance

A significant opaque component in Mercury’s crust is inferred based on albedo and spectral observations. Previous workers have favored iron-titanium bearing oxide minerals as the spectrally neutral opaque. A consequence of this hypothesis is that Mercury’s surface would have a high FeO content. An array of remote sensing techniques have not provided definitive constraints on the FeO content of Mercury’s surface. However, spectral observations have not detected a diagnostic 1 μm absorption band and have thus limited the FeO in coexisting silicates to <2 wt.% FeO. In this paper, we assess equilibrium among oxide and silicate minerals to constrain the distribution of iron between opaque oxides and silicates under a variety of environmental conditions. Equilibrium modeling is favored here because the geologic process that produced Mercury’s low-albedo intermediate terrain must have occurred globally, which favors a common widespread igneous process. Based on our modeling, we find that iron-rich ilmenite cannot occur with silicates that do not display a 1 μm absorption feature unless plagioclase abundances are high. However, such high plagioclase abundances are precluded by Mercury’s low albedo. Incorporating equilibrium crystallization modeling with spectral and albedo constraints we find the iron abundance of Mercury’s intermediate terrain is ?10 wt.% FeO. This intermediate iron composition matches constraints provided by visible albedo and total neutron absorption observed by MESSENGER. In fact, the total neutron absorption of mixtures of oxide, plagioclase, olivine and pyroxene for the oxide abundances estimated for Mercury, favor Mg-rich members of the ilmenite-geikielite solid-solution series. This work offers compositional constraints for Fe, Ti, and Mg that will be testable by various MESSENGER instrument data sets after it begins its orbital mission.     

Possible impact melt and debris flows at Tooting Crater, Mars

Candidate examples of impact melt flows and debris flows have been identified at Tooting crater, an extremely young (<2 Myr), 29 km diameter impact crater in Amazonis Planitia, Mars. Using HiRISE and CTX images, and stereo-derived digital elevation models derived from these images, we have studied the rim and interior wall of Tooting crater to document the morphology and topography of several flow features in order to constrain the potential flow formation mechanisms. Four flow types have been identified; including possible impact melt sheets and three types of debris flows. The flow features are all located within 2 km of the rim crest on the southern rim or lie on the southern interior wall of the crater ∼1500 m below the rim crest. Extensive structural failure has modified the northern half of the crater inner wall and we interpret this to have resulted in the destruction of any impact melt emplaced, as well as volatile-rich wall rock. The impact melt flows are fractured on the meter to decameter scale, have ridged, leveed lobes and flow fronts, and cover an area >6 km × 5 km on the southern rim. The debris flows are found on both the inner wall and rim of the crater, are ∼1-2 km in length, and vary from a few tens of meters to >300 m in width. These flows exhibit varying morphologies, from a channelized, leveed flow with arcuate ridges in the channel, to a rubbly flow with a central channel but no obvious levees. The flows indicate that water existed within the target rocks at the time of crater formation, and that both melt and fluidized sediment was generated during this event.     

Super-resolution of THEMIS thermal infrared data: Compositional relationships of surface units below the 100 meter scale on Mars

The Thermal Emission Imaging System (THEMIS) has provided the highest spatial resolution (100 m/pixel) thermal infrared (TIR) data of the surface of Mars to date. These data have enabled the discovery of many small-scale compositional units and helped to better constrain surface processes operating at these scales. However, with higher-resolution visible (VIS) instruments revealing smaller-scale surficial differences, there exists a need to detect and map compositional variability using TIR data at scales below 100 m. Because it is unlikely there will be a higher-resolution TIR instrument sent to Mars in the near future, creative image processing techniques commonly classified under the umbrella of “super-resolution” can be employed to improve or enhance the spatial resolution of the THEMIS TIR data. These approaches typically integrate another higher-resolution dataset and can either be qualitative for visual appeal, quantitative for data accuracy, or some combination of both. The super-resolution approach presented here produces enhanced TIR images that are radiometrically accurate, but also visually appealing. For the technique to be successfully applied, multi- to hyper-spectral data from two different spectral regions are required (e.g., the THEMIS TIR and VIS datasets). The focus here is to introduce this new super-resolution methodology and demonstrate its ability using existing THEMIS IR and VIS data. The quartzofeldspathic deposit in northern Syrtis Major was selected because of the spectral variability detected using the original IR resolution data and to better constrain the relationship between the small-scale surface morphology and areal extent of the deposit as well as its formation process. Despite being associated with the central peaks of two craters, the results here show no positive correlation between the small rocky outcrops and the quartzofeldspathic unit. A gradational contact exists between the unit and basaltic sands within the intercrater eolian material. The super-resolution approach offers an alternative approach to traditional sub-pixel deconvolution identification and provides a higher-resolution IR dataset for thermophysical and spectral analysis on Mars.     

The initial responses of hot liquid water released under low atmospheric pressures: Experimental insights

Experiments have been performed to simulate the shallow ascent and surface release of water and brines under low atmospheric pressure. Atmospheric pressure was treated as an independent variable and water temperature and vapor pressure were examined as a function of total pressure variation down to low pressures. The physical and thermal responses of water to reducing pressure were monitored with pressure transducers, temperature sensors and visible imaging. Data were obtained for pure water and for solutions with dissolved NaCl or CO₂. The experiments showed the pressure conditions under which the water remained liquid, underwent a rapid phase change to the gas state by boiling, and then solidified because of removal of latent heat. Liquid water is removed from phase equilibrium by decompression. Solid, liquid and gaseous water are present simultaneously, and not at the 611 Pa triple point, because dynamic interactions between the phases maintain unstable temperature gradients. After phase changes stop, the system reverts to equilibrium with its surroundings. Surface and shallow subsurface pressure conditions were simulated for Mars and the icy satellites of the outer Solar System. Freezing by evaporation in the absence of wind on Mars is shown to be unlikely for pure water at pressures greater than c. 670 Pa, and for saline solutions at pressures greater than c. 610 Pa. The physical nature of ice that forms depends on the salt content. Ice formed from saline water at pressures less than c. 610 Pa could be similar to terrestrial sea ice. Ice formed from pure water at pressures less than c. 100 Pa develops a low thermal conductivity and a ‘honeycomb’ structure created by sublimation. This ice could have a density as low as c. 450 kg m⁻³ and a thermal conductivity as low as 1.6 W m⁻¹ K⁻¹, and is highly reflective, more akin to snow than the clear ice from which it grew. The physical properties of ice formed from either pure or saline water at low pressures will act to reduce the surface temperature, and hence rate of sublimation, thereby prolonging the lifespan of any liquid water beneath.     

Evaporation effects on the formation of martian gullies

In order to investigate the formation of martian gullies and the stability of fluids on Mars, we examined about 120 gully images. Twelve HiRISE images contained a sufficient number of Transverse Aeolian Ridges (TARs) associated with the gullies to make the following measurements: overall gully length, length of the alcove, channel and apron, and we also measured the frequency of nearby TARs. Six of the 12 images examined showed a statistically significant negative correlation between overall gully length (alcove, channel and apron length) and TAR frequency. Previous experimental work from our group has shown that at temperatures below ∼200 K, evaporation rate increases by about an order of magnitude as wind speed increases from 0 to ∼15 m/s. Thus the negative correlations we observe between gully length and dune frequency can be explained by formation at temperatures below ∼200 K where wind speed/evaporation is a factor governing gully length. In these cases evaporation of the fluid carving the gully was a constraint on their dimensions. Cases where there is no correlation between gully length and TAR frequency, can be explained by formation at temperatures >200 K. The temperatures are consistent with Global Circulation Model and Thermal Emission Spectrometer (TES) data for these latitudes. The temperatures suggested by these trends are consistent with the fluid responsible for gully formation being a strong brine, such as Fe₂(SO₄)₃ which has a eutectic temperature of ∼200 K. We also find that formation timescales for gullies are 10⁵-10⁶ years.     

Flow patterns of lobate debris aprons and lineated valley fill north of Ismeniae Fossae, Mars: Evidence for extensive mid-latitude glaciation in the Late Amazonian

A variety of Late Amazonian landforms on Mars have been attributed to the dynamics of ice-related processes. Evidence for large-scale, mid-latitude glacial episodes existing within the last 100 million to 1 billion years on Mars has been presented from analyses of lobate debris aprons (LDA) and lineated valley fill (LVF) in the northern and southern mid-latitudes. We test the glacial hypothesis for LDA and LVF along the dichotomy boundary in the northern mid-latitudes by examining the morphological characteristics of LDA and LVF surrounding two large plateaus, proximal massifs, and the dichotomy boundary escarpment north of Ismeniae Fossae (centered at 45.3°N and 39.2°E). Lineations and flow directions within LDA and LVF were mapped using images from the Context (CTX) camera, the Thermal Emission Imaging Spectrometer (THEMIS), and the High Resolution Stereo Camera (HRSC). Flow directions were then compared to topographic contours derived from the Mars Orbiter Laser Altimeter (MOLA) to determine the down-gradient components of LDA and LVF flow. Observations indicate that flow patterns emerge from numerous alcoves within the plateau walls, are integrated over distances of up to tens of kilometers, and have down-gradient flow directions. Smaller lobes confined within alcoves and superposed on the main LDA and LVF represent a later, less extensive glacial phase. Crater size-frequency distributions of LDA and LVF suggest a minimum (youngest) age of 100 Ma. The presence of ring-mold crater morphologies is suggestive that LDA and LVF are formed of near-surface ice-rich bodies. From these observations, we interpret LDA and LVF within our study region to result from formerly active debris-covered glacial flow, consistent with similar observations in the northern mid-latitudes of Mars. Glacial flow was likely initiated from the accumulation and compaction of snow and ice on plateaus and in alcoves within the plateau walls as volatiles were mobilized to the mid-latitudes during higher obliquity excursions. Together with similar analyses elsewhere along the dichotomy boundary, these observations suggest that multiple glacial episodes occurred in the Late Amazonian and that LDA and LVF represent significant reservoirs of non-polar ice sequestered below a surface lag for hundreds of millions of years.