Rock fractures are indicators of stress release within geologic systems, and fracture morphologies can commonly be used to infer formation conditions. Polygonal fractures are common in isotropic, ...contractional stress regimes such as in rocks exposed at the surface of a planet undergoing thermal cycling or in sedimentary substrates undergoing repeated wetting and drying. Such polygonal fracture systems, on centimeter to decameter scales, have been widely documented on Mars. Utilizing a combination of orbital‐ and ground‐based images, we report a laterally extensive polygonal fracture network that occurs within siliciclastic rocks of the lowermost Siccar Point group, Gale crater, Mars. The Siccar Point group is exposed over approximately 20 km2 in northwest Gale crater, where it unconformably overlies eroded strata of Mount Sharp (Aeolis Mons) and reflects likely aeolian deposition along the lower flanks of Mount Sharp. Images reveal an extensive network of erosionally resistant polygons, approximately 7.5 m across, that exhibit interior angles (i.e., fracture intersections) with modes at 90° and 120°. Polygon morphology indicates that fractures formed during multiple cycles of expansion and contraction, which is attributed to desiccation and subsequent recharge of near‐surface groundwater. The erosional resistance of preserved fractures is inferred to reflect postfracture diagenetic fluid flow along the sub–Siccar Point group unconformity and cementation. Evidence for multiple fluid events in the relatively young strata of the Siccar Point group requires a protracted history of fluid stability in Gale crater.
Plain Language Summary
Deciphering the processes that affect sedimentary rocks after their deposition is critical to understanding the geologic history of a basin. Fractures occur when applied forces exceed the strength of the host material, and we can infer the process by which fracturing occurred by assessing the morphology of fractures. In this study, we analyze a network of polygonal fractures within rocks of the Siccar Point group, a relatively young geologic unit that is exposed over ~20 km2 of Gale crater, the field site of the Curiosity rover. Polygons formed by these fractures are similarly sized and intersect at angles that show dominant modes at 90° and 120°. These observations suggest that fractures formed under conditions of uniform stress and likely result from contractional processes such as climate‐driven wet‐dry cycles. The cementation of fractures by later fluids then imparted erosional resistance that permits these features to be recognized from orbit. The presence of an extensive, fluid‐driven fracture system within aeolian strata highlights the potential complexity of martian climate signals.
Key Points
Siccar Point group strata record extensive fracturing and fluid flow, resulting in a regional network of erosionally resistant polygons
Polygons reflect single to multiple cycles of sediment contraction associated with evaporation and recharge of near surface fluids
The presence of fluid‐related fractures within a predominantly dry aeolian system highlights potential variability in martian climate
Prior to the Mars Global Surveyor (MGS) mission, a very general view had emerged in which Martian surface materials were seen as consisting of a mixture of bright dust, dark sand, and rocks. The ...configuration of these materials and the winds that transport and deposit sand and dust have been thought to be directly linked to the albedo patterns that have been observed on Mars for centuries. High spatial resolution images (1.4–20 m/pixel) obtained by the MGS Mars Orbiter Camera (MOC) between September 15, 1997, and July 4, 1999, provide new information about the physical nature of the windblown material on the Martian surface and show that the pre‐MGS view was much too simple. In addition to bright dust and dark sand, MOC images show evidence of bright sediment that can be transported by saltation (e.g., sand) and dark material that can be transported in suspension (e.g., silt). New views of eolian wind streaks in Daedalia Planum show that part of this region, thought to be mantled by bright dust based upon Viking and Mariner observations, is instead covered by a thin veneer of bright, windblown sand. MOC images of Sinus Sabaeus and parts of Syrtis Major, two regions thought to be sandy based upon Viking era observations, exhibit thick mantles that are inferred to consist of fine‐grained sediment deposited from eolian suspension. Low albedo wind streaks in western Arabia Terra are also dark mantles, and their association with eroded crater floors and megaripples/dunes on these floors suggest that these particular wind streaks are deposits of silt‐sized material that was only briefly suspended before settling to the surface. MOC images also show evidence that some eolian dunes are active on Mars today (i.e., in 1998 and 1999); the evidence for activity is largely based upon wind‐and avalanche‐induced streaks superposed on or eroded into seasonal frost on high‐latitude dune fields. MOC images show that some other dunes are inactive, but the albedo of dunes relative to surrounding terrain is not a good indicator of dune activity because some inactive dunes are not mantled by dust. Inactive dunes are best identified by superposed features such as impact craters, landslide deposits, and yardangs.
Layers in the upper martian crust record a complex history of deposition and erosion. We examined a region between 2°–9°N, 1°–8°W and found that light‐toned, layered, sedimentary rocks in northwest ...Sinus Meridiani also occur in southwest Arabia Terra, where they are mantled by dust. The rocks are divided into four distinct stratigraphic units. Impact craters of diameters 30–60 km are interbedded with the ∼200 m‐thick units, attesting to their antiquity, revealing the presence of temporal unconformities between units, and indicating that the sequence spans considerable time. A crater at 8°N, 7°W, containing hundreds of repeated sedimentary layers, is stratigraphically lower than most of the four‐unit section. The layers in the crater formed in a different environment than the layered material outside the crater. Deciphering the geologic history of Mars requires recognition that the planet has more than a cratered surface; its crust is a cratered volume.
The Mars Science Laboratory Curiosity rover performed coordinated measurements to examine the textures and compositions of aeolian sands in the active Bagnold dune field. The Bagnold sands are ...rounded to subrounded, very fine to medium sized (~45–500 μm) with ≥6 distinct grain colors. In contrast to sands examined by Curiosity in a dust‐covered, inactive bedform called Rocknest and soils at other landing sites, Bagnold sands are darker, less red, better sorted, have fewer silt‐sized or smaller grains, and show no evidence for cohesion. Nevertheless, Bagnold mineralogy and Rocknest mineralogy are similar with plagioclase, olivine, and pyroxenes in similar proportions comprising >90% of crystalline phases, along with a substantial amorphous component (35% ± 15%). Yet Bagnold and Rocknest bulk chemistry differ. Bagnold sands are Si enriched relative to other soils at Gale crater, and H2O, S, and Cl are lower relative to all previously measured Martian soils and most Gale crater rocks. Mg, Ni, Fe, and Mn are enriched in the coarse‐sieved fraction of Bagnold sands, corroborated by visible/near‐infrared spectra that suggest enrichment of olivine. Collectively, patterns in major element chemistry and volatile release data indicate two distinctive volatile reservoirs in Martian soils: (1) amorphous components in the sand‐sized fraction (represented by Bagnold) that are Si‐enriched, hydroxylated alteration products and/or H2O‐ or OH‐bearing impact or volcanic glasses and (2) amorphous components in the fine fraction (<40 μm; represented by Rocknest and other bright soils) that are Fe, S, and Cl enriched with low Si and adsorbed and structural H2O.
Plain Language Summary
The Bagnold dune field is an active sand field with moving dunes and sits between the landing site of the Curiosity rover and rocks of interest higher up on Mount Sharp. When passing through the dune field, we used all of Curiosity's instruments to measure the chemistry, mineralogy, and grain size of sands in the Bagnold dune field in order to figure out where they came from, how the sands are transported, and what volatile materials (like water) lie within Martian soils. We found that the Bagnold sand dunes are very well sorted; no dusty materials are found within them, in stark contrast to soils seen previously with Curiosity and with rovers at other landing sites. We found that the coarser sand grains are enriched in the volcanic minerals olivine and pyroxene, confirming a prediction from orbit that wind‐related activity seems to concentrate these phases. We also found that the dunes were much lower in water and other volatile elements like sulfur and chlorine versus all previous Mars soils. Using a combination of the rover's sieving system and chemical measurement tools, we figured out that two types of materials host water. In the first type of material, common in these sands, water is low in abundance (≤~1%), very tightly bound to the grains, and is not released until temperatures >200°C. In the second type of material, water is higher in abundance (2%) and more easily released by heating. Sieved water‐bearing fine materials may be a useful resource for human explorers.
Key Points
Because of ongoing aeolian activity, the Bagnold dunes consist of well‐sorted sands and lack the finer grains typical of Martian soils
Dune sands are chemically distinct with elevated Si, Mg, and Ni and lower H2O, S, and Cl relative to all previously measured Martian fines
Two distinct, water‐/OH‐bearing amorphous components are identified: Fe‐, S‐, and Cl‐rich material in dust and Si‐rich material in the sands
The first billion years of Martian geologic history consisted of surface environments and landscapes dramatically different from those seen today, with flowing liquid water sculpting river channels ...and ponding to form bodies of water. However, the hydro‐climatic context, the frequency, and the duration under which these systems existed remain uncertain. Addressing these fundamental questions may improve our understanding of early Mars climate. Here, we reconstruct a long‐lived archive consisting of an array of fluvial systems inside the Antoniadi crater––one of the largest lake basins on Mars (9.58 × 104 km2). We found that the fluvial activity occurred throughout four major intermittent active intervals during the Late Noachian to Early Amazonian (∼3.7 to >2.4 Ga). This resulted in at least two major lakes, which formed during periods of markedly increased surface runoff production. The record of these four riverine phases is preserved in fluvial ridges, valley networks, back‐stepping or down‐stepping fan‐shaped landforms, and terrace‐like formations within an outlet canyon. These morphologies point to lake‐controlled base‐level fluctuations suggestive of episodic precipitation‐fed surface runoff punctuated by intermittent catastrophic floods that were capable of breaching crater‐lake rims and incising outlet canyons. Fluvial‐deposit thickness, junction angles of channels, and lake morphometry suggest that riverine systems lasted at least 103–106 years and episodically occurred under primarily arid and semi‐arid climates. These findings place new regional constraints on the fluvial frequency, longevity, and climatic regime of one of the largest Martian lakes, thereby bolstering the hypothesis that episodic warming likely punctuated the planet's early history.
Plain Language Summary
The planet Mars is now a vast desert. However, geologic evidence points to radically different kinds of landscapes in the past, with precipitation‐fed ancient rivers and lakes. As a consequence, questions have been raised about the climatic and environmental contexts that persisted during the formation of these hydrological records. Here, we have used high‐resolution remotely sensed data to constrain the volumes, frequency, and periodicity of an array of water‐formed landforms inside one of the largest lake systems on Mars that occupy the Antoniadi crater. We demonstrate that the Antoniadi crater was intermittently wet, hosting multiple rivers and at least two main bodies of standing water between 3.7 and 2.4 Ga. The morphometries of the lake and river systems imply that they episodically survived between a few thousand and 1 million years under arid climates. These findings make Antoniadi an interesting site for future Mars exploration dedicated to the potential ancient habitability of Mars because of such long‐lived fluvial history.
Key Points
Antoniadi crater is the site of an ancient lake that was punctuated by locally and regionally wet conditions between 3.7 and 2.4 Ga
Antoniadi crater likely records at least four episodes of surface runoff
The river and lake systems at Antoniadi were probably active for 103–106 years, supporting long‐lived fluvial activity under arid climates
We assessed dust coverage on the Mars Science Laboratory Mars Hand Lens Imager (MAHLI) and Alpha Particle X-Ray Spectrometer (APXS) calibration targets from 14 MAHLI images acquired at <100 μm/pixel, ...between Mars Science Laboratory Curiosity rover sols 34 and 2248, in order to place constraints on dust accumulation and removal on two endmember orientations (vertical and horizontal). Dust coverage was estimated by (1) determining reflectance ranges for image pixels covered by dust; (2) using the ratio of calibrated MAHLI red band data to blue band data as a proxy for the concentration of dust on the calibration target; and (3) manually counting dust-covered pixels in representative areas of the MAHLI calibration target. The results of each method are consistent within uncertainties, but the reflectance method provided the most efficient and effective way to measure dust cover on each target. Mean and median dust coverage is ~4.7% and 3.9% for the MAHLI target, 9.4% and 8.9% for the APXS target, and 51.9% and 63.4% for the REMS UV sensor. Maximum dust coverage (during the dust storm) is 49% and 42% for the MAHLI and APXS calibration targets respectively, and 80% for the REMS UV sensor. In modeling dust accumulation and removal, the best fit for the MAHLI and APXS targets is one that assumes 2–4% dust removal per sol and 20–40% efficiency in collection, while for the REMS UV sensor, it is one that assumes near 0 removal and 7% of nominal accumulation. Results indicate that the vertically-mounted MAHLI and APXS targets accumulated less dust overall during the mission than horizontally-mounted hardware such as the REMS UV sensor; this was true even during the 2018 global dust event. In addition, while the vertical orientation did not protect the targets from dust deposition during that event, dust removal following the event was more effective on these targets than on horizontally-mounted hardware. Because conditions cannot be monitored continuously, these studies cannot fully discriminate among the potential causes of this dust removal. However, the results suggest that vertical mounting is a reasonable dust mitigation strategy for hardware for which short-term dust accumulation is not a risk factor.
•Three methods give accurate estimates for martian dust coverage.•Horizontal surfaces are subject to mechanisms that deposit but rarely remove dust.•Vertical surfaces are subject to processes that deposit but also remove dust.•Vertical mounting means less dust accumulation long-term.•Such a configuration does not fully protect hardware from dust.
Mars Global Surveyor Mars Orbiter Camera images that were targeted specifically to observe locations where published accounts argue for the presence of landforms created by the interaction of a large ...body of water with Martian topography fail to reveal any evidence to support the hypothesis that the northern lowlands were once the site of oceans or seas. Given the difficulty of identifying ancient coastlines on Earth from orbital and aerial photography in the absence of field work, this result does not preclude the possibility that Mars once had large standing bodies of water on its surface, but calls into question shorelines previously proposed.
The Curiosity rover discovered fine-grained sedimentary rocks, which are inferred to represent an ancient lake and preserve evidence of an environment that would have been suited to support a martian ...biosphere founded on chemolithoautotrophy. This aqueous environment was characterized by neutral pH, low salinity, and variable redox states of both iron and sulfur species. Carbon, hydrogen, oxygen, sulfur, nitrogen, and phosphorus were measured directly as key biogenic elements; by inference, phosphorus is assumed to have been available. The environment probably had a minimum duration of hundreds to tens of thousands of years. These results highlight the biological viability of fluvial-lacustrine environments in the post-Noachian history of Mars.
Polar processes can be sensitive indicators of global climate, and the
geological features associated with polar ice caps can therefore indicate
evolution of climate with time. The polar regions on ...Mars have distinctive
morphologic and climatologic features: thick layered deposits, seasonal CO
2 frost caps extending to mid latitudes, and near-polar residual frost
deposits that survive the summer. The relationship of the
seasonal and residual frost caps to the layered deposits has been poorly constrained, mainly by the limited spatial resolution of the available data.
In particular, it has not been known if the residual caps represent simple
thin frost cover or substantial geologic features. Here we show that the residual
cap on the south pole is a distinct geologic unit with striking collapse and
erosional topography; this is very different from the residual cap on the
north pole, which grades into the underlying layered materials. These findings
indicate that the differences between the caps are substantial (rather than
reflecting short-lived differences in frost cover), and so support the idea
of long-term asymmetry in the polar climates of Mars.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Sedimentary rocks at Yellowknife Bay (Gale crater) on Mars include mudstone sampled by the Curiosity rover. The samples, John Klein and Cumberland, contain detrital basaltic minerals, calcium ...sulfates, iron oxide or hydroxides, iron sulfides, amorphous material, and trioctahedral smectites. The John Klein smectite has basal spacing of ~10 angstroms, indicating little interlayer hydration. The Cumberland smectite has basal spacing at both ~13.2 and ~10 angstroms. The larger spacing suggests a partially chloritized interlayer or interlayer magnesium or calcium facilitating H2O retention. Basaltic minerals in the mudstone are similar to those in nearby eolian deposits. However, the mudstone has far less Fe-forsterite, possibly lost with formation of smectite plus magnetite. Late Noachian/Early Hesperian or younger age indicates that clay mineral formation on Mars extended beyond Noachian time.
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