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Megaripples at Wau-an-Namus, Libya: A new analog for similar features on Mars
Abstract
Transverse Aeolian Ridges (TARs) are curious aeolian megaripples on Mars, but there are few sites identified thus far to study as terrestrial analogs. A new terrestrial analog site for TARs has been found in and around an isolated volcano caldera in the Sahara Desert of Libya. The Wau-an-Namus volcano is potentially active and recognizable from space because of its strong albedo contrast. Wau-an-Namus has large dark aeolian megaripple bedforms, with the same horizontal length scales and overall planform patterns as observed for TARs on Mars. Some new patterns for these TAR-like features also have been found at Wau-an-Namus, such as stripe-like and clustered patterns, which may shed new light on the formation of these features on both planets. Other aeolian features such as sand dunes and dust devil tracks are also evident in this region. This is the first terrestrial analog site with dark megaripple features, similar to some TARs on Mars. The dark halo around Wau-an-Namus, visible from space, represents a unique site to study the formation and evolution of these mysterious features, and thereby further our understanding of the probable recent history of the wind regime on Mars.
... Further, we scoured the literature for suitable GSDs [3][4][5][6]8,[11][12][13][15][16][17][18][20][21][22]25,[31][32][33]38,43,56,65,[83][84][85] . Studies reporting either only two (or even just one) characteristic grain sizes 11,12,17,18,65,[83][84][85] , or with GSDs that did not contain the full coarse-grain peak 8,31 , or with insufficient resolution (grain sizes discretized in class intervals larger or equal than −0.5 · log 2 d) 3,4,13,32 , or with too low (single-digit) counts of grains in the coarse-grain peak 56 , especially in the right tail of the coarse-grain peak, were sorted out. ...
... Further, we scoured the literature for suitable GSDs [3][4][5][6]8,[11][12][13][15][16][17][18][20][21][22]25,[31][32][33]38,43,56,65,[83][84][85] . Studies reporting either only two (or even just one) characteristic grain sizes 11,12,17,18,65,[83][84][85] , or with GSDs that did not contain the full coarse-grain peak 8,31 , or with insufficient resolution (grain sizes discretized in class intervals larger or equal than −0.5 · log 2 d) 3,4,13,32 , or with too low (single-digit) counts of grains in the coarse-grain peak 56 , especially in the right tail of the coarse-grain peak, were sorted out. We also could not use data from ref. 21 , since each sample contained material from several bedforms, and data from ref. 33 , for which we were unsure whether the location of the right margin of the coarse-grain peak originates from the reptation regime or is due to the absence of bigger immobile grains in the sand source. ...
Aeolian sand transport is a major process shaping landscapes on Earth and on diverse celestial bodies. Conditions favoring bimodal sand transport, with fine-grain saltation driving coarse-grain reptation, give rise to the evolution of megaripples with a characteristic bimodal sand composition. Here, we derive a unified phase diagram for this special aeolian process and the ensuing nonequilibrium megaripple morphodynamics by means of a conceptually simple quantitative model, grounded in the grain-scale physics. We establish a well-preserved quantitative signature of bimodal aeolian transport in the otherwise highly variable grain size distributions, namely, the log-scale width (Krumbein phi scale) of their coarse-grain peaks. A comprehensive collection of terrestrial and extraterrestrial data, covering a wide range of geographical sources and environmental conditions, supports the accuracy and robustness of this unexpected theoretical finding. It could help to resolve ambiguities in the classification of terrestrial and extraterrestrial sedimentary bedforms.
... Long believed immobile [8][9][10][11] , recent evidence suggests that TARs may be active over multiyear periods 12 . TARs are morphometrically similar to both ripples and dunes, with symmetric profiles like large wind ripples or small reversing dunes 13,14 , and have wavelengths intermediate between ripples and dunes somewhere on the order of 20-100 m 5,12,[15][16][17][18][19][20] , TARs are larger than wind ripples on Mars or Earth, but smaller than dunes on either planet [21][22][23][24][25][26] with amplitudes generally of a few to tens of meters 5,12,[15][16][17][18][19] Possible terrestrial proxies include gravel megaripples in Argentina 27,28 , megaripples in Iran and Libya [29][30][31] , and reversing dunes in Idaho 32 . Recent work suggests that lower wind pressures on Mars allow megaripples to grow essentially without limit 33 . ...
... At these locations (Fig. 3), hexagonal networked TARs display residual rectangular forms (i.e., relict-identifiable primary/secondary ridges) at the margins of the field ( Supplementary Fig. 5), whereas the converse pattern-rectangular TARs surrounded by hexagonal TARs-was not observed at any location. Previous studies have noted that TARs have an affinity for topographic depressions 2,3,9,30 , and we found that this pattern was especially true for hexagonalnetworked TARs ( Supplementary Fig. 6, Supplementary Table 2). ...
Transverse aeolian ridges – enigmatic Martian features without a proven terrestrial analog – are increasingly important to our understanding of Martian surface processes. However, it is not well understood how the relationships between different ridges evolve. Here we present a hypothesis for the development of complex hexagonal networks from simple linear forms by analyzing HiRISE images from the Mars Reconnaissance Orbiter. We identify variable morphologies which show the presence of secondary ridges, feathered transverse aeolian ridges and both rectangular and hexagonal networks. We propose that the formation of secondary ridges and the reactivation of primary ridge crests produces sinuous networks which then progress from rectangular cells towards eventual hexagonal cells. This morphological progression may be explained by the ridges acting as roughness elements due to their increased spatial density which would drive a transition from two-dimensional bedforms under three-dimensional flow conditions, to three-dimensional bedforms under two-dimensional flow conditions. Transverse aeolian ridges on Mars develop into networked configurations by formation of secondary ridges, reactivation of primary ridges and a transition from 3D to 2D air flow, according to analyses of HiRISE images from the Mars Reconnaissance Orbiter.
... found in the Lut Desert of Iran (Hugenholtz et al., 2017), Argentinean Puna (Gough et al., 2020;De Silva et al., 2013;Bridges et al., 2015) and the Sahara Desert of Libya (Foroutan et al., 2019). Therefore, the formation and evolution of TARs remain elusive due to the absence of in-situ data. ...
Transverse aeolian ridges (TARs) are enigmatic landforms found across Mars, whose formation mechanism remains largely unknown. China's Tianwen-1 mission, which landed on Mars in 2021, provided extensive data aiding in-depth investigations of TARs. However, manually identifying TARs across large regions of Mars is time-consuming and labor-intensive, making it impossible to complete the entire region's TAR identification. To solve this issue, we propose the AiTARs-Net, an automatic arbitrary-oriented TRA detection network. This model begins by extracting TAR features using an enhanced dimension-aware global-local attention module, which focuses on interactions between spatial and channel features to capture discriminative features of TARs. After that, we employ an anchor-free proposal generation network to produce TAR candidates with arbitrary orientations. The proposal generation network uses a nonaxis-aligned two-variable Gaussian function to model the target as an oriented center heatmap. Then, the oriented bounding box and category information are predicted at the corresponding center position. Finally, we introduce the rotated region-based convolutional neural network to refine the proposals to obtain more accurate TARs' locations and orientations. To assess the efficacy of our proposed method, we built the Martian TARs dataset (M-TARset), an compilation of TARs labeled in six different topographical and morphological types, containing various shapes and illumination scales, to facilitate training and prediction of potential TARs. The experimental results obtained on a Martian TARs dataset and a large-scale TARs extraction at the Zhurong landing site confirm that the proposed framework outperforms the leading generic object extraction methods in accuracy, demonstrating its strong generalization abilities for large-scale TAR detection. The source code and M-TARset are available at https://github.com/PlanetaryScience3510/M-TARset.
... Dust-resistant materials Choosing dust-resistant solar panel materials can reduce the accumulation of dust on the surface. In areas with high levels of dust or pollution, the impact can be even greater [25]. Therefore, it is important to regularly clean the solar PV panels to prevent dust buildup. ...
The effects of dust aerosols on solar panels and people can occasionally extend thousands of kilometres across the atmosphere. The research aims to present the evaluation methods on the impact of dust accumulation on the performance of solar panels in the Libyan climate zone. The study conducted a series of experiments to measure the degradation of solar panel efficiency due to the deposition of dust on the surface of the solar panels. The presented results indicate that the accumulation of dust has a significant negative impact on the performance of solar panels, reducing their efficiency. Furthermore, the study listed the frequency and cleaning methods required to maintain the solar panels' performance over time in this harsh climate. The findings highlight the importance of routine cleaning and maintenance of solar panel installations in dusty and arid regions to ensure optimal performance and maximize energy efficiency
... The Puna represents a useful analog for aeolian features observed on Mars, including PBRs (Hugenholtz et al. 2015) and megaripples covered by coarse-grained particles (Milana 2009;de Silva et al. 2013;Bridges et al. 2015;Zimbelman et al. 2016). Different Earth locations other than the Puna that have been proposed as analogs for megaripples on Mars include Israel (Yizhaq 2005;Yizhaq et al. 2009), Iran (Foroutan & Zimbelman 2016;Hugenholtz & Barchyn 2017), and Libya (Foroutan et al. 2019). The utility of any analog site, including the Puna, is enhanced considerably by conducting field studies at the site that can provide insight and critical data valuable for better understanding aeolian processes on Mars and other planetary surfaces (Lorenz & Zimbelman 2014). ...
Plains covered by gravel-dominated desert pavement in the Puna of Argentina have an aerodynamic roughness height (or length) z 0 of ∼1 cm, likely representing a skimming flow regime above the closely spaced gravel particles. Aerodynamic roughness height locally may transition from that of skimming flow over the gravels to a z 0 that includes the effects of obstacles considerably larger than those of the gravel particles alone. Among large (>60 cm tall) megaripples, z 0 is elevated beyond that of the gravels alone to values of 2–4 cm. These results represent an analog for an improved understanding of the aerodynamics of gravel-dominated desert pavement and megaripples documented by multiple rovers on Mars.
... The discovery of aeolian landforms in the solar system, including the comet 67P/Churyumov-Gerasimenko and Pluto, have increased (and continues to increase) interest in similar wind-related landforms on Earth (Bridges et al., 2015;de Silva et al., 2013;Favaro et al., 2020Favaro et al., , 2021Foroutan & Zimbelman, 2016;Foroutan et al., 2018;Hugenholtz et al., 2015;Jia et al., 2017;Milana, 2009;Telfer et al., 2018;Zimbelman et al., 2009). A better understanding of bedform formation in the presence of transient atmospheres could be an important research topic over the next decade. ...
Aeolian landforms are widespread in our solar system. Understanding the exact nature and processes of formation of these features are challenging tasks necessitating a strong collaboration between scientists with different skills and scientific backgrounds. This paper describes the special issue for the 5th International Planetary Dunes Workshop, which includes 15 research papers and three commentaries. Among the 18 papers included in this collection, 16 cover Martian aeolian science and two Titan aeolian science. The papers presented focus on bedform morphology and dynamics via remote sensing data, modeling, analogs studies and laboratory experiments. Here we put the main results of the papers in their appropriate scientific context and discuss potential future lines of research.
... Other proposed mechanisms to explain the formation of TARs are surface creep of coarse-grained particles (Bourke et al., 2003;Hugenholtz et al., 2017;Zimbelman, 2010) or the deposition, induration, and erosion of dominantly dust-sized particles (Geissler, 2014). Although initially without good terrestrial analogs, coarse-grained moderate-scale aeolian bedforms (2-250 m wavelength, 1-4 m tall) were recently identified in deserts of Iran, Libya, and Argentina that could provide analogs to at least some martian TARs (de Silva et al., 2013;Foroutan and Zimbelman, 2016;Foroutan et al., 2019). Generally, these features have only been studied via orbital images, although it is possible that that the Curiosity rover traversed a small TAR in Gale crater (Zimbelman and Foroutan, 2020). ...
Across planetary surfaces, aeolian bedforms are unique and useful records of sustained interaction between wind and granular materials. The first extraterrestrial dunes were found on Mars and martian dunes studies have since yielded improved estimations of dune sand size(s) and saltation threshold wind speeds, refined dune morphology models that enable improved prediction of past/present winds, and present-day sediment flux rates that contribute to erosional processes. However, new data are needed to address critical questions, such as about the sources/ages of martian dune sand, estimation of the martian sediment budget, and the connections between surface wind distributions and sediment lofting/transport rates.
... TARs within large fields of asymmetrical dunes as in this study). Many studies have identified TAR proxies on Earth: gravel megaripples in Argentina (de Silva et al., 2013;Hugenholtz et al., 2015), megaripples in Iran and Libya (Foroutan & Zimbelman, 2016;Foroutan et al., 2019;Gough et al., 2020;, and reversing dunes in Idaho (Zimbelman & Scheidt, 2014), but a definitive terrestrial analogue has not been determined. ...
Here we present new observations of two different interactions between aeolian ridges and boulder fields on Mars that provide insight into past wind conditions. First, an analysis of ridge and boulder interactions at two test sites in Proctor Crater and an additional site ~430 km to the northeast shows that ridge geometry can be affected by changes in surface roughness elements generated by boulder fields. Second, a detailed examination of some of the boulder fields found that individual boulders can generate multi‐armed ‘wakes’ that have no clear proxy on Earth. The ridge/boulder dynamics suggest that transverse aeolian ridges (TARs) acted as roughness during their development, indicating that they formed at a length‐scale larger than wind ripples. The boulder wakes seem to represent an unusual interaction between flow separation and pre‐exiting ridges; why this pattern is not observed on Earth remains uncertain.
... The landing area is covered by aeolian bedforms which are normally referred to as transverse aeolian ridges (TARs) or megaripples S4 and S5) (Balme et al., 2008(Balme et al., , 2017Berman et al., 2018;Bhardwaj et al., 2019;Foroutan et al., 2018;Foroutan & Zimbelman, 2016;Hugenholtz et al., 2017;Zimbelman, 2010). Two types of TARs have been previously detected in the landing site: regular TARs (up to a few meters in height) within impact craters and other topographic depressions and 15-25 cm-tall mini-TARs found on the surrounding plains (Balme et al., 2017) (Fig. S4). ...
Wind‐formed features are abundant in Oxia Planum (Mars), the landing site of the 2022 ExoMars mission, which shows geological evidence for a past wet environment. Studies of aeolian bedforms at the landing site were focused on assessing the risk for rover trafficability, however their potential in recording additional climatic fluctuations has not been explored. Here we show that the landing site experienced multiple climatic changes in the Amazonian, which are recorded by an intriguing set of ridges that we interpret as Periodic Bedrock Ridges (PBRs). Clues for a PBR origin result from ridge regularity, defect terminations and the presence of preserved megaripples detaching from the PBRs. PBR orientation differs from superimposed transverse aeolian ridges pointing toward a major change in wind regime. Our results provide constrains on PBR formation mechanisms and offer indications on paleo winds that will be crucial for understanding the landing site geology.
... Transverse aeolian ridges (TARs), first identified in Mars Orbiter Camera (MOC) imagery, are bright relict linear aeolian features [1][2][3]. While TARs are common on Mars, their role in past Martian sediment cycles is poorly understood [2][3][4][5][6][7], and no exact proxy on Earth has been identified [8][9][10][11][12][13][14][15][16][17]. Improved understanding of TARs requires a significant amount of data on their distribution, morphologies, and development. ...
Transverse aeolian ridges (TARs) are unusual bedforms on the surface of Mars. TARs are common but sparse on Mars; TAR fields are small, rarely continuous, and scattered, making manual mapping impractical. There have been many efforts to automatically classify the Martian surface, but they have never explicitly located TARs successfully. Here, we present a simple adaptation of the off-the-shelf neural network RetinaNet that is designed to identify the presence of TARs at a 50-m scale. Once trained, the network was able to identify TARs with high precision (92.9%). Our model also shows promising results for applications to other surficial features like ripples and polygonal terrain. In the future, we hope to apply this model more broadly and generate a large database of TAR distributions on Mars.
... Graduate work at the University of Waterloo provided her the means to apply her mapping technique to automated documentation of crevasses on glaciers (Foroutan et al., 2019a), but her fascination with TARs did not diminish even while she was studying terrestrial features. She started an electronic correspondence with photographer George Steinmetz, who had published spectacular photos in National Geographic of features that looked very much like TARs, which led to publication of another TAR analog site in Libya (Foroutan et al., 2019b). She had applied her mapping technique to large samples of both TARs on Mars and the analog features in the Lut Desert as the final chapter of her MSc thesis, and we were working on revising this material for publication when tragedy struck. ...
... Such a similarity is evident in Figure 10b reinforcing the existence of a relationship between dune migration/fluxes and megaripple migration/fluxes at the dune field scale. Thus, our results support previous interpretations of megaripples/TARs as analogues to terrestrial megaripples where coarse grains are displaced by rolling/creep caused by saltating sand impactors [Lapotre et al., 2016a;Foroutan et al., 2018]. This coupling between active sand and megaripple migration seems to be valid even locally, where only the megaripple portions that are directly in contact with the dune sand are moving (Figure 9 and animations S10, S11). ...
Aeolian megaripples, with 5‐ to 50‐m spacing, are abundant on the surface of Mars. These features were repeatedly targeted by high‐resolution orbital images, but they have never been observed to move. Thus, aeolian megaripples (especially the bright‐toned ones often referred as Transverse Aeolian Ridges—TARs) have been interpreted as relict features of a past climate. In this report, we show evidence for the migration of bright‐toned megaripples spaced 1 to 35 m (5 m on average) in two equatorial areas on Mars indicating that megaripples and small TARs can be active today. The moving megaripples display sand fluxes that are 2 orders of magnitudes lower than the surrounding dunes on average and, unlike similar bedforms on Earth, can migrate obliquely and longitudinally. In addition, the active megaripples in the two study areas of Syrtis Major and Mawrth Vallis show very similar flux distributions, echoing the similarities between dune crest fluxes in the two study areas and suggesting the existence of a relationship between dune and megaripple fluxes that can be explored elsewhere. Active megaripples, together with high‐sand flux dunes, represent a key indicator of strong winds at the surface of Mars. A past climate with a denser atmosphere is not necessary to explain their accumulation and migration.
... However, several interpretations [2,20] envisage them as granule ripples under a surface layer of mm-sized particles, too coarse to be moved by the present-day Martian winds [17]. There are several reported possible terrestrial analogs for Martian TARs, such as the megaripples in Argentina [21] and in the Lut Desert of Iran [19,22], megaripples at Wau-an-Namus, Libya [23], and reversing sand dunes in Idaho, USA [24]. TARs have generally brighter or similar albedo as the surrounding terrain and can occur singularly or as large contiguous stretches [10,22]. ...
Aeolian processes are believed to play a major role in the landscape evolution of Mars. Investigations on Martian aeolian landforms such as ripples, transverse aeolian ridges (TARs), and dunes, and aeolian sediment flux measurements are important to enhance our understanding of past and present wind regimes, the ongoing dust cycle, landscape evolution, and geochemistry. These aeolian bedforms are often comprised of loose sand and sharply undulating topography and thus pose a threat to mobility and maneuvers of Mars rovers. Here we present a first-hand account of the distribution, morphologies, and morphometrics of TARs in Oxia Planum, the recently selected ExoMars 2020 Rover landing site. The gridded mapping was performed for contiguous stretches of TARs within all the landing ellipses using 57 sub-meter high resolution imaging science experiment (HiRISE) scenes. We also provide the morphological descriptions for all types of TARs present within the landing ellipses. We use HiRISE digital terrain models (DTMs) along with the images to derive morphometric information for TARs in Oxia Planum. In general, the average areal TAR coverage was found to be 5.4% (±4.9% standard deviation), increasing from west to east within the landing ellipses. We report the average TAR morphometrics in the form of crest-ridge width (131.1 ± 106.2 m), down-wind TAR length (17.6 ± 10.1 m), wavelength (37.3 ± 11.6 m), plan view aspect ratio (7.1 ± 2.3), inter-bedform spacing (2.1 ± 1.1), slope (10.6 • ± 6.1 •), predominant orientations (NE-SW and E-W), and height (1.2 ± 0.8 m). While simple TARs are predominant, we report other TAR morphologies such as forked TAR, wavy TAR with associated smaller secondary ripples, barchan-like TAR, networked TAR, and mini-TARs from the region. Our results can help in planning the rover traverses in terms of both safe passage and scientific returns favoring aeolian research, particularly improving our understanding of TARs.
The outcomes of extraterrestrial exploration, mostly on Mars, have revealed a series of dune features that challenge traditional interpretations. A comprehensive examination of the morphological characteristics and types of Martian dunes has revealed what originally appeared to be unique Martian aeolian bedforms (Dong et al. in Adv Earth Sci 35(9):902–911, 2020a; Adv Earth Sci 36(2):125–138, 2021). However, close examination of these aeolian landforms has revealed that most of these so-called unique Martian aeolian bedforms also exist on Earth, but that they have been long neglected by researchers. A growing number of studies have shown that these neglected landforms have vital research value because of the insights they provide on the formation mechanisms of aeolian landforms.
Aeolian landforms provide valuable insights into the planetary surface environment and its evolutionary history. In this study, the formation and evolution of megaripples in the Qaidam Basin and their relationship with the development environment are analyzed. By quantifying the wind environment, morphology, grain‐size distribution, sedimentary structure, and luminescence age of megaripples, we propose for the first time that there are multiple megaripple evolution modes. Investigation revealed that three evolution modes were responsible for forming megaripples in different equilibrium states: transient, stable, and metastable. Well‐sorted coarse sand grains accumulate on ridges and overlay poorly sorted fine sand grains to form transient megaripples. Stable megaripples have alternating sedimentary bedding of coarse and fine sand grains. Metastable megaripples have a secondary ripple formation on the surface. Throughout their formation, coarse and fine sand grains undergo regrouping. The response of coarse grains to the change in wind speed lags behind that of fine grains. This process controls the erosion and accumulation of megaripples and affects their size and sedimentary structures. The evolution mode, scale, and sedimentary structure of megaripples are influenced by the grain‐size range under the same wind conditions. The luminescence ages of the coarse‐grained megaripple sediments are less than 700 years. This study provides a fresh perspective on the coexistence of various sand ripples and transverse aeolian ridges found on Mars.
تعد دراسة التصحر وقياس التدهور في الخصائص الحيوية، وتحديد درجاته من الدراسات المهمّة اليوم؛ لما تعانيه العديد من دول العالم من تفاقم مشكلة التصحر، وزيادة معدلات خطورته على حياة الإنسان ومستقبله القريب. من هنا جاءت هذه الدراسة لتتناول موضوع التصحر بمنطقة سهل بنغازي، وقياس حجم التدهور الحاصل، إلى جانب قياس درجاته، والوقوف على الأسباب التي ساعدت على تطور المشكلة بمنطقة الدراسة. واعتمدت الدراسة على تقنيات الاستشعار عن بعد ونظم المعلومات الجغرافية في تحليل البيانات، من خلال الاعتماد على صورتين فضائيتين في فترات مختلفة (1984 ،2019)؛ لرصد حجم المشكلة، ومعرفة أسبابها، وتحديد أكثر المناطق المعرضة للتصحر. حيث اعتُمِدَ على مجموعة من المعايير التي تحدد درجات التصحر، ومنها: (نموذج الارتفاع الرقمي DEM_ درجة الحرارة_ الأمطار_ التدخلات البشرية متمثلة في العمران_ زحف الرمال_ الأراضي الملحية_ النبات)، بالإضافة إلى تحديد درجات خطورة التصحر من خلال تحليل الصور الفضائية لمنطقة الدراسة.
We provide a critical review of research paradigms for classifying intermediate‐scale aeolian bedforms on Mars and the new terminology that has emerged. The systematic classification of bedforms has always been challenging and debated, and no paradigmatic knowledge organization system exists beyond general agreement on the importance of a distinction between ripples and dunes. The diverse aeolian landscapes of Mars have introduced further topics and challenges to these debates. We argue that Martian aeolian geomorphology's knowledge organization system for intermediate‐scale bedforms is preparadigmatic and that consensus over terminology and their definitions has not been established in the literature. A preparadigmatic science can be functional only if scientists operating in the discipline provide precise, falsifiable definitions or use abductive logic. Drawing on evidence and examples from the literature, we argue that the replacement of the conventional abductive paradigm used in bedform classification with inductive logic has created an emerging disciplinary paradigm based on scientific hesitancy and a dependence on complex inductive research structures, epitomized by the concepts of ‘transverse aeolian ridges’ (TARs) and ‘large Martian ripples’ (LMRs). We show that TAR and, increasingly, LMR are inductive constructs that have been popularized despite them causing significant confusion. Notably, we highlight how the terms are irreconcilably used as both a class of bedform and as a non‐genetic placeholder term for bedforms. Suggestions for moving beyond the need for TAR and LMR are provided, focusing on a return to more direct and local hypothesis‐driven research inspired by W.M. Davis's notion of outrageous geological hypotheses. Recent debate surrounding bedforms in Gale crater is presented as an example of the productivity of such an approach, and it is recommended that TAR and LMR no longer be used.
In this chapter the concentration is upon a series of sites which demonstrate the importance of structures and of rock types for determining the nature of desert landscapes. The entries are divided into three groups: inselbergs, circular structures, and sandstone landscapes. Inselberg landscapes are discussed from Namibia, Australia; circular structures from Aorounga Crater, Chad; Arkenu, Libya; Gross Brukarros, Namibia; Iranian salt domes; the Richât Structure, Mauritania; and Waw an Namus, Libya. Sandstone landscapes are described from the Arches region of Utah, USA; Djado and Mangueni, Niger; Gilf Kebir, Egypt; and Wadi Rum, Jordan.KeywordsStructuresInselbergsImpact cratersVolcanoesSalt domesArchesSandstones
Aeolian bedforms are the signatures of wind interaction with unconsolidated, granular surface materials. Transverse aeolian ridges (TARs) are widely distributed on Mars but their formation remains enigmatic. China's Zhurong rover explored four crescent-shaped TARs, with two horns generally facing south, during the first 107 sols in southern Utopia Planitia, Mars. Rover images show that these bedforms have distinct light and dark variations on their surfaces that likely result from the combination of a bimodal distribution of particle sizes and the crust formed by the accumulation of aeolian dust. Two of these bedforms exhibit erosional forms on their west sides, where megaripples facing in a direction different from that of the crescentic bedforms they disrupt were created by more recent winds from the northeast. Differing erosional configurations of each of these bedforms in close proximity to each other are probably related to the angle between the bedform crest and the wind direction, and may further suggest that erosion of TARs starts from their two flanks. Secondary ridges of TARs widely recognized on Mars could be megaripples formed during this erosion process. At the Zhurong landing site, TARs degraded into megaripples, suggesting that they might share similar formation and evolution mechanisms there and elsewhere on Mars.
The enigmatic transverse aeolian ridges (TARs), with distinct morphology and albedo, are among the key geological features investigated by China's Tianwen-1 Zhurong rover on southern Utopia Planitia, Mars. Their morphologies and morphometrics are investigated through high-resolution imaging science experiment (HiRISE) orthoimage and Digital Terrain Model (DTM) products. A total of 5089 TARs are identified, with barchan TARs being predominant (97.6%). Morphometric analysis shows these TARs to be small and symmetrical aeolian landforms, with an average crest–ridge lengths of 33.9 ± 20.5 m, profile widths of 9.4 ± 3.8 m, profile heights of 0.4 ± 0.4 m, profile-height-width ratios of 0.04 ± 0.02, and profile symmetry ratios of −0.01 ± 0.13. In-situ observations from the Navigation and Terrain Camera (NaTeCam) show the crests of the TARs to be dark and sharp, while the flanks are interlaced by dark and bright materials. Close-up Multispectral Camera (MSCam) images reveal the TARs to be coated by granules of ∼1.5 mm in diameter. Given the morphometric characteristics and the presence of coating granules, the TARs in the landing area could be categorized as megaripples. Buffered crater counting (BCC) technique-derived absolute model age (AMA) reveals the formation time, or the last active period of the TARs, could be as recent as 1 Ma in the Late Amazonian. The morphometrics and direction of the horns of the barchan TARs suggest the winds for the formation of TARs blew mostly from the north. During the spring-summer transition period (Ls: 50°–93°), the Mars Climate Station (MCS) had recorded local bimodal winds in the landing area, with the speed of the northerly wind in the afternoon being a little stronger than the speed of the southerly wind in the morning. These observations are consistent with the wind fields described in the Mars Climate Database (MCD), which imply the northerly winds during the northern winter season to be responsible for the net sediment transport to the south. Two TARs observed in-situ with secondary NW-SE trending crest-ridges indicate that forked TARs might form given sufficient time (i.e., in the order of millions of years) under modern wind conditions, i.e., the TARs may be currently reworked, if only extremely weakly.
Granule ripples are widespread on Earth and are a fundamental component of dryland sedimentary systems, but the relationship between their wavelength and surface particle size is unclear; in addition, their sedimentary bedding and granule ripple development are incompletely understood. In this study, we examined the relationship between granule ripple development, morphology, and the surface grain‐size distribution. We measured the morphology and particle‐size distribution of granule ripples in China's Kumtagh Desert, and found an exponential relationship between ripple wavelength and average crest particle size, which contradicts previous results. We hypothesize the ripples resulted from creep of coarse particles caused by saltation impacts, accompanied by decreasing bed elevation, leading to crest migration and increased crest height. The size composition and wind regime determined the movement mode of particles, which controlled ripple development and size, but dry–wet and freeze–thaw cycles also contributed. We established a new conceptual model for the development of large granule ripples. Sand ripples and small granule ripples develop similarly, but large ripples did not form from merger of smaller ripples. We report for the first time that particles between 2.3 and 4.7 mm in diameter were missing in the ripple crests (confirmed in wind tunnel experiments). This gap affects the particle movement mode. Since the threshold wind speed for particle saltation at 4.7 mm approaches the regional maximum wind speed, quantifying the missing particles may permit prediction of regional maximum wind speeds, and has practical significance for identifying wind conditions elsewhere, including the surface of Mars.
Considering that aeolian sand ripples are formed primarily by creeping particles caused by wind-driven saltation sand particles, we obtain a formulation for determining the height of saturated aeolian sand ripples by incorporating the reptation fluxes with previous experimental results on migration velocities of sand ripples. Based on existing observational results of terrestrial sand ripples on Earth's surface, it estimates that the wavelength of aeolian sand ripples on Mars is generally up to several meters. This implies a possibility that there is another sand ripple on Mars similar in scale to Transverse Aeolian Ridges (TARs) at some time when surface saltation was prevalent. Moreover, perhaps part of the widely observed TARs is the degradation of saltation sand ripples, whose formation is intimately related to saltation and reptation of sand particles. While the other two types of ripple-like morphologies (plain ripples and crater ripples) found by Opportunity Rover are essentially not. Further, we propose that the main factor controlling the scale feature of Martian sand ripples is the intense particle-bed collision process.
Aeolian sand transport is a major process shaping landscapes on Earth and on diverse celestial bodies. Conditions favoring bimodal sand transport, with fine-grain saltation driving coarse-grain reptation, give rise to the evolution of megaripples with a characteristic bimodal sand composition. Here, we derive a unified phase diagram for this special aeolian process and the ensuing nonequilibrium megaripple morphodynamics by means of a conceptually simple quantitative model, grounded in the grain-scale physics. We establish a well-preserved quantitative signature of bimodal aeolian transport in the otherwise highly variable grain size distributions, namely, the log-scale width (Krumbein phi scale) of their coarse-grain peaks. A comprehensive collection of terrestrial and extraterrestrial data, covering a wide range of geographical sources and environmental conditions, supports the accuracy and robustness of this unexpected theoretical finding. It could help to resolve ambiguities in the classification of terrestrial and extraterrestrial sedimentary bedforms.
تعد دراسة التصحر وقياس التدهور في الخصائص الحيوية، وتدك درجاته من الدراسات المهمة اليوم لما تعانيه العديد من دول العالم من تفاقم مشكلة التصحر، وزادة معدلات خطورته على حياة الإنسان ومستقبله القرب.
من هنا جاعت هذه الدراسة لتتناول موضوع التصحر بمنطقة سهل بنغازي، وقياس حجم الدهور الحاصل، إلى جانب قياس درجاته، والوقوف على الأسباب التي ساعت على تطور المشكلة بمنطقة الدراسة.
واعتمدت الدرسة على تقنيات الاستشعار عن بعد ونظم المعلومات الجغرافية في تطيل البيانات، من خلال الاعتماد على صورتين فضائيتين في فتزت مختلفة (1984 ،2019) لرعد حجم المشكلة، ومعرفة أسبابها، وتحديد أكثر المناطق المعرضة لتصحر. حث اعتمد على مجموعة من المعايير التي تحدد درجات التصحر، ومنها: (نموذج الارتفاع الرقمي DEM_ درجة الحرر:_ الأمطار_ التدخلات البشرة متمثلة في العمر_زحف الرمال_ الأرضي الملحية_ النبات)، بالإضافة إلى تحديد درجات خطورة التصحر من خلال تطيل الصور الفضائية لمنطقة الدراسة.
Ripples, transverse aeolian ridges (TARs), and dark‐toned sand sheets and dunes are common aeolian bedforms on the Martian surface. They are important for understanding the nature of present‐day Martian sediments and regional aeolian processes. Here we present a case study investigation of ripples, TARs, a dark‐toned sand sheet, and dunes in an unnamed—but well‐covered by remote sensing datasets—crater in Terra Sabaea, Mars, to consider their nature and possible origin scenarios. Repeat high‐spatial resolution images show only minor albedo changes among the dark‐toned sand dunes but no obvious changes in the ripples and TARs. Visible and infrared spectra show that the megaripples and TARs are pyroxene‐bearing, while the dark‐toned sheet and dunes are olivine‐bearing. Thousands of TARs are superimposed on the crater walls, and they have a similar composition as the bedrock exposed around the central pit, suggesting that some percentage of the sediment composing the TARs may be locally derived. Megaripples have a similar composition as TARs, suggesting they may share a similar origin. Dark‐toned sand sheets and sand dunes show a different composition from the substrate of the crater, plus bedform orientations indicative of a dominant, north‐northwest wind, indicating that some of these dark sands might have been blown in from outside of the crater. Alternatively, the sand in the megaripples, TARs, sand sheets, and dunes could share a common source, and some or even all of them could be recycled from the weathering and erosion of the sand‐bearing clastic rocks exposed in the crater walls.
Mars exhibits diverse surface changes at all latitudes and all seasons. Active processes include impact cratering, aeolian sand and dust transport, a variety of slope processes, changes in polar ices, and diverse effects of seasonal CO2 frost. The extent of surface change has been surprising and indicates that the present climate is capable of reshaping the surface. Activity has important implications for the Amazonian history of Mars: understanding processes is a necessary step before we can understand their implications and variations over time.
Transverse aeolian ridges (TARs) are poorly understood relict aeolian Martian surface features. Processes that create TARs are not well-constrained, and understanding their formation is complicated since they appear to share some features of ripples, megaripples, and dunes. While some evidence of multi-stage TAR formation has been documented in Nirgal Vallis, here we present additional evidence for this process at nine locations on Mars using cratering superposition between different ridge morphologies. Most occurrences of multistage evolution will not preserve the precise series of cratering and formation events documented here, which potentially means that this formative process may have been more common than even these new widespread observations suggest. This formative process can help determine the relative similarity of TARs to ripples, megaripples and dunes. Based on our observations, we conclude that primary TAR forms are most like megaripples, and that subsequent ridges formed like aqueous ripple spurs.
Transverse aeolian ridges (TARs) are poorly-understood bedforms unique to the surface of Mars. While various TAR morphologies have been documented and described in the literature, there is confusion about some aspects of TAR formation and evolution. It is an open question as to whether different TAR morphologies represent disparate formative processes, or represent different evolutionary stages. Further, the relationship between the currently unclassified “feathered” TAR morphology and other well-described morphologies is currently undocumented. Here, we use basic geomorphological principles to provide evidence from Nirgal Vallis that more complex TAR morphologies have evolved in multiple stages, and that feathered TARs are actually an unrecognized intermediate morphology related to networked TARs. Identifying feathered TARs as precursors to networked TARs provides evidence for TAR-field complexity being related to progressive evolution from simpler TAR morphologies, rather than to multiple formative mechanisms.
Extensive evidence of landform-scale martian geomorphic changes has been acquired in the last decade, and the number and range of examples of surface activity have increased as more high-resolution imagery has been acquired. Within the present-day Mars climate, wind and frost/ice are the dominant drivers, resulting in large avalanches of material down icy, rocky, or sandy slopes; sediment transport leading to many scales of aeolian bedforms and erosion; pits of various forms and patterned ground; and substrate material carved out from under subliming ice slabs. Due to the ability to collect correlated observations of surface activity and new landforms with relevant environmental conditions with spacecraft on or around Mars, studies of martian geomorphologic activity are uniquely positioned to directly test surface-atmosphere interaction and landform formation/evolution models outside of Earth. In this paper, we outline currently observed and interpreted surface activity occurring within the modern Mars environment, and tie this activity to wind, seasonal surface CO2 frost/ice, sublimation of subsurface water ice, and/or gravity drivers. Open questions regarding these processes are outlined, and then measurements needed for answering these questions are identified. In the final sections, we discuss how many of these martian processes and landforms may provide useful analogs for conditions and processes active on other planetary surfaces, with an emphasis on those that stretch the bounds of terrestrial-based models or that lack terrestrial analogs. In these ways, modern Mars presents a natural and powerful comparative planetology base case for studies of Solar System surface processes, beyond or instead of Earth.
On Mars, large aeolian ripples with wavelengths typically 1–3 m but lacking very coarse sand at crests have been encountered by rovers and observed from orbit. These bedforms have no terrestrial counterpart and several hypotheses for origins have been proposed. This work reports results of Computational Fluid Dynamics (CFD) experiments with ANSYS Fluent under terrestrial and Martian boundary layer conditions, using the k − ω SST turbulence model to evaluate shear stress along a topographic profile of large Martian ripples at different boundary layer wind speeds. Results indicate that, compared with Earth conditions: (1) boundary‐layer flow along large ripples under Martian conditions is less turbulent due to higher kinematic viscosity; (2) reverse‐flow vortex regions from crests at ripple lee flanks are larger; and (3) shear stresses at crests of large ripples are relatively low, so ripple flattening is less likely at high wind speeds. These results indicate Martian ripples formed by the saltation impact splash mechanism should be less constrained by shear stress effects limiting growth of exposed ripple crests, because the low‐density Martian atmosphere applies relatively low wind‐related shear stress to ripple surfaces. Other origins for the large Martian ripples are not excluded, however. On Earth, very large ripples with crests unprotected by very coarse grains do not develop due to higher wind‐related shear stresses.
In Scandia Cavi on Mars, barchans migrating over a field of transverse aeolian ridges (TARs) leave behind distinctive trails (“wakes”) comprising both TARs undergoing exhumation and coarse-grained ripples being shed from the barchans. With distance upwind from the barchans, the combined pattern of these bedforms coarsens and defect density decreases, thus appearing to mature with exposure time. We present results of morphological analyses of the wake bedform crestlines using HiRISE images, seeking to determine how the wake pattern reflects TAR growth and pattern development. TARs interact with each other, exhibiting defect repulsions and possible lobe extensions, indicating that these bedforms have migrated in the past, despite the lack of identifiable change in overlapping images spanning 9.5 years. Mapping one wake in detail, we found that the TAR pattern is not affected by superposing ripples. However, the ripples undergo many interactions, first with one another, and later (with distance upwind) with the underlying TARs. Near the dune, many ripples laterally link, growing in length, and they preferentially form along TAR crests, resulting in small bedform repulsions and longer superposing ripples. Most of these ripples will be consumed by the TARs, an as-yet unreported growth dynamic for TARs that is consistent with the work of others, who have found a continuum between TARs and the meter-scale ripples that form on dunes. Constructing a DTM, orthorectifying HiRISE images, and measuring dune migration rates places the timescale of ripple absorption by TARs in a wake at several thousand years, with the first ∼1,000 years dominated by lateral linking of ripples. Assuming that TAR growth is accomplished entirely through dune burial and subsequent ripple consumption, we estimate a lower limit age of the TARs, and by extension, the dune field, to be ∼270 kyr.
Rover data can provide important constraints on the possible origin of wind‐related martian features that have been termed “Transverse Aeolian Ridges” (TARs). The large aeolian bedform traversed by Curiosity at Dingo Gap has a planform and albedo similar to members of nearby TAR fields, so that the crossing at Dingo Gap revealed the surface characteristics of a small TAR. Mars Hand Lens Imager images documented 1–2 mm diameter, well‐rounded particles on the Dingo Gap bedform, along with numerous images obtained from the Mastcam, Hazcam, and Navcam science and engineering cameras on the rover. These images confirm that this feature (and, by extension, all small TARs) can be considered a megaripple whose surface is coated by rounded particles that are too large to be set into motion through saltation, but they may be induced to roll (creep) along the surface by the impact of saltating sand grains (a characteristic of megaripples on Earth). The images also confirm that a bimodal wind was present here, a condition that could have contributed to the symmetrical profile of the bedform. The bearing strength at Dingo Gap was better than that of the Purgatory megaripple that trapped Opportunity for several weeks, so that all megaripples should not be automatically presumed to represent a trafficability hazard for a rover.
Aeolian processes are the dominant geological activity on Mars under current environmental conditions, and decoding the surface features on Mars is important for determining its history. Transverse Aeolian Ridges (TARs) on Mars are mysterious aeolian features originally considered to be unique to Mars. The origin, formation process and sediment sources for TARs remain uncertain, but using high-resolution satellite images and studying terrestrial analogs are important tools for examining these features. TAR-like features in the Lut desert of Iran are excellent analogs for exploring the diversity and dimensions of TAR-like features, in order to shed light on the most likely origin of TARs on Mars. We used a semi-automatic mapping methodology to document TAR-like bedforms in the Lut desert. Morphometry of more than 2 million TAR-like features in the Lut study area were automatically extracted for this study, along with manual measurements of ∼2000 features using photoclinometry in order to document heights. High Resolution Imaging Science Experiment (HiRISE) digital terrain models (DTMs) were used to obtain measurements for ∼2000 Martian TARs, in order to compare similar data sets for the morphometry of both Martian TARs and the Lut analogs. We analyzed measured physical attributes in order to explore physical conditions that may have contributed to their formation. A multi-temporal analysis of the Lut TAR-like features used satellite images spanning an 8-year time period. Four groups of TAR-like features in the Lut Desert were identified by their morphometry and their response to wind conditions.
Numerous of lava rise plateaus and tumuli developed in the medial and distal portions of the Al-Halaq al Kabir lava flow field during the last (Holocene) eruption in the central part of the Al Haruj Volcanic Province (AHVP). These inflation structures provide important data on the lava-emplacement mechanism in this part of the Al Haruj region where the widespread occurrence of tumuli and lava rise plateaus are good indicators of the mode of emplacement. We report the results of detailed measurements of the maximum and minimum diameters in plan-view of 551 lava-rise plateaus and 289 tumuli from the distal part of the Al-Halaq al Kabir lava field using ArcGIS and field observations. Tumuli and lava rise plateaus may be divided into subpopulations according to abrupt changes in the scaling exponents on log-log plots of their frequency versus diameter. Using the estimated stiffness (Young's modulus) of the basaltic rocks in the study area of 10–34 GPa, numerical and analytical results show that theoretical maximum tensile stresses in the inflated upper (solidified) crustal layers of the developing tumuli and lava rises at tens of mega-pascals (MPa). This theoretical stress is orders of magnitude higher than typical in-situ tensile strengths of rocks (0.5–9 MPa) and thus high enough to rupture the crustal layers of the lava rises and tumuli. The high tensile stresses are generated during crustal doming driven by a magmatic overpressure of only about 1 MPa. Our results partly explain the abundance of tension fractures at the surface of the Al-Halaq al Kabir lava flow field. Tumuli observed in the area are remarkably similar in morphology and aspect (height/width) ratios to the flow-lobe tumuli in Holocene lava flow fields in Iceland, suggesting an analogous mechanics of formation. There appears to be a coincidence between the age of an initial volcanism in the AHVP (7.9–5.3 Ma) and a local fauna and flora disappearance in the As-Sahabi area, NE Sirt Basin, during Messinian-Zanclean time (7–5 to 5 Ma). We speculate that the volcanism may have had a local negative environmental impact and contributed to the reported decline in the fauna and flora in the area.
HiRISE images of small sand dunes and megaripples in Gamboa and Matara craters on Mars raise intriguing questions regarding the relationship between sand ripples and megaripples. The observations suggest that there might be a progressive transition from sand ripples to megaripples, something worthy of consideration by the community as analysis of HiRISE images continues.
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